Chemical Formula: Fe7Si8O22(OH)2 Name Origin: Named for Louis Emmanuel Gruner (1809-1883), Swiss-French chemist who analyzed the mineral.
Grunerite is a mineral of the amphibole group of minerals with formula Fe7Si8O22(OH)2. It is the iron endmember of the grunerite-cummingtonite series. It forms as fibrous, columnar or massive aggregates of crystals. The crystals are monoclinic prismatic. The luster is glassy to pearly with colors ranging from green, brown to dark grey. The Mohs hardness is 5 to 6 and the specific gravity is 3.4 to 3.5.
It was discovered in 1853 and named after Emmanuel-Louis Gruner (1809–1883), a Swiss-French chemist who first analysed it.
Physical Properties
Cleavage: {110} Perfect, {???} Distinct Color: Ashen, Brown, Brownish green, Dark gray. Density: 3.4 – 3.5, Average = 3.45 Diaphaneity: Translucent to opaque Fracture: Sub Conchoidal – Fractures developed in brittle materials characterized by semi-curving surfaces. Hardness: 5-6 – Between Apatite and Orthoclase Luminescence: Non-fluorescent. Luster: Vitreous – Pearly Streak: colorless
The arrow indicates a large articulation facet of a cervical rib on a fossil cervical vertebra of a woolly mammoth of the Natural History Museum Rotterdam. Credit: Joris van Alphen
Researchers recently noticed that the remains of woolly mammoths from the North Sea often possess a ‘cervical’ (neck) rib — in fact, 10 times more frequently than in modern elephants (33.3% versus 3.3%). In modern animals, these cervical ribs are often associated with inbreeding and adverse environmental conditions during pregnancy. If the same factors were behind the anomalies in mammoths, this reproductive stress could have further pushed declining mammoth populations towards ultimate extinction.
Mammals, even the long-necked giraffes and the short-necked dolphins, almost always have seven neck vertebrae (exceptions being sloths, manatees and dugongs), and these vertebrae do not normally possess a rib. Therefore, the presence of a ‘cervical rib’ (a rib attached to a cervical vertebra) is an unusual event, and is cause for further investigation. A cervical rib itself is relatively harmless, but its development often follows genetic or environmental disturbances during early embryonic development. As a result, cervical ribs in most mammals are strongly associated with stillbirths and multiple congenital abnormalities that negatively impact the lifespan of an individual.
Researchers from the Rotterdam Museum of Natural History and the Naturalis Biodiversity Center in Leiden examined mammoth and modern elephant neck vertebrae from several European museum collections. “It had aroused our curiosity to find two cervical vertebrae, with large articulation facets for ribs, in the mammoth samples recently dredged from the North Sea. We knew these were just about the last mammoths living there, so we suspected something was happening. Our work now shows that there was indeed a problem in this population,” said Jelle Reumer, one of the authors on the study published today in the open access journal PeerJ.
The incidence of abnormal cervical vertebrae in mammoths is much higher than in the modern sample, strongly suggesting a vulnerable condition in the species. Potential factors could include inbreeding (in what is assumed to have been an already small population) as well as harsh conditions such as disease, famine, or cold, all of which can lead to disturbances of embryonic and fetal development. Given the considerable birth defects that are associated with this condition, it is very possible that developmental abnormalities contributed towards the eventual extinction of these late Pleistocene mammoths.
The peer-reviewed study, entitled “Extraordinary incidence of cervical ribs indicates vulnerable condition in Late Pleistocene mammoths” was authored by Jelle Reumer of the Rotterdam Museum of Natural History and Clara ten Broek and Frietson Galis of Naturalis Biodiversity Center (Leiden).
Note : The above story is based on materials provided by PeerJ.
The contours of the Earth’s crust are influenced by the high temperatures deep within the Earth’s mantle, according to a new study published in Science. A team of researchers, led by Brown University, demonstrated that those temperature differences control the elevation and volcanic activity along mid-ocean ridges, the colossal mountain ranges that line the ocean floor.
Forming at the boundaries of tectonic plates, mid-ocean ridges circle the globe like seams on a baseball. Magma from deep within the Earth rises up to fill in the cracks between the plates as they move apart, creating fresh crust on the ocean floor as it cools. This new crust is thicker in some places than others, forming ridges with widely varying elevations. In some parts of the world, these ridges are deep in the ocean, miles beneath the surface. In other places such as Iceland, the ridge tops are exposed above the ocean’s surface.
“These variations in ridge depth require an explanation,” said Colleen Dalton, assistant professor of geological sciences at Brown. “Something is keeping them either sitting high or sitting low.”
The research team discovered that the “something” was the temperature of the rocks deep below the Earth’s surface.
At depths extending below 250 miles, the team was able to show that mantle temperatures along the ridges vary by as much as 250 degrees Celsius by analyzing the speeds of seismic waves generated by earthquakes. They found that, in general, higher points on the ridges are associated with higher temperatures, while lower points are associated with cooler temperatures. One unsurprising finding of this study is that volcanic hot spots along the ridges — such as volcanoes near Iceland, and the islands of Ascension and Tristan da Cunha — all sit above warm spots in the mantle.
“It is clear from our results that what’s being erupted at the ridges is controlled by temperature deep in the mantle,” Dalton told Brown University’s Kevin Stacey. “It resolves a long-standing controversy and has not been shown definitively before.”
The mid-ocean ridges function as a window to the interior of the planet for geologists by providing clues about the properties of the mantle below.
A thicker crust is suggested by a higher ridge elevation, indicating that a larger volume of magma erupted at the surface. The new study explains that this excess magma could have been caused by very hot temperatures in the mantle. The fact that hot mantle material is not the only way to produce excess magma, however, presents a challenge to this theory. The amount of melt is also controlled by the chemical composition of the mantle. Some rock compositions melt at lower temperatures, allowing for a larger volume of molten rock. Because of this, it has been unclear for the last several decades whether mid-ocean ridge elevations are caused by variations in the temperature of the mantle or variations in the rock composition of the mantle.
Dalton’s team introduced two additional data sets to help them distinguish between these two possible scenarios.
One data set was the chemistry of basalts, the rock that forms from the solidification of magma at the mid-ocean ridge. Basalt compositions can vary greatly depending on the temperature and composition of the mantle material from which they’re derived. To create this data set, the researchers analyzed almost 17,000 basalts formed along mid-ocean ridges worldwide.
Seismic wave tomography made up the second data set. During earthquakes, seismic waves pulse through the rock of the crust and the mantle. Scientists measure the velocity of those waves to gather data about the characteristics of the rocks through which they passed. “It’s like performing a CAT scan of the inside of the Earth,” Dalton added. Temperature has a great effect on seismic wave speeds, with waves propagating more quickly in cooler rocks than in hotter ones.
By comparing the seismic data from hundreds of earthquakes to data on elevation and rock chemistry from the ridges, the team found correlations which revealed that temperatures deep in the mantle varied between 1,300 and 1,550 degrees Celsius underneath about 38,000 miles of ridge terrain. “It turned out,” said Dalton, “that seismic tomography was the smoking gun. The only plausible explanation for the seismic wave speeds is a very large temperature range.”
The results demonstrated that as mantle temperatures fall, so too do ridge elevations. The hottest point beneath the ridges was found to be near Iceland — also the site of the ridges’ highest elevation — while the lowest temperatures were found near the lowest point, an area of very deep and rugged seafloor known as the Australian-Antarctic discordance in the Indian Ocean.
There has been a long-standing debate in the scientific community about whether a mantle plume — a vertical jet of hot rock originating from deep in the Earth — intersects the mid-ocean ridge in Iceland. The findings of this study provide strong support for this theory, as well as for mantle plumes being the culprit for the excess magma volume in all regions with above-average temperatures near volcanic hot spots.
The Earth’s mantle does not sit still, despite being made of solid rock. It is constantly undergoing convection, where material from the depths of the Earth churns towards the surface and back again.
“Convection is why we have plate tectonics and earthquakes,” Dalton said. “It’s also responsible for almost all volcanism at the surface. So understanding mantle convection is crucial to understanding many fundamental questions about the Earth.”
There are two main factors in the mechanism of convection: variations in the composition of the mantle and variations in its temperature. Dalton says that their findings point to temperature as a primary factor in how convection is expressed on the surface.
“We get consistent and coherent temperature measurements from the mantle from three independent datasets,” Dalton said. “All of them suggest that what we see at the surface is due to temperature, and that composition is only a secondary factor. What is surprising is that the data require the temperature variations to exist not only near the surface but also many hundreds of kilometers deep inside the Earth.”
Dalton says that the findings will be useful for future research using seismic waves because the temperature readings as indicated by seismology were backed up by the other datasets. This allows them to be used to calibrate seismic readings for places where geochemical samples aren’t available, allowing scientists to estimate temperature deep in the Earth’s mantle all over the globe.
Note : The above story is based on materials provided by April Flowers for redOrbit
Chemical Formula: Ca3Al2(SiO4)3 Locality: Isle of Mull, Scotland. Name Origin: Grossular is from the Latin grossularia meaning “gooseberry.” Hessonite is from the Greek hesson, meaning “slight” in reference to the smaller specific gravity.
Grossular or grossularite is a calcium-aluminium mineral species of the garnet gemstone group with the formula Ca3Al2(SiO4)3, though the calcium may in part be replaced by ferrous iron and the aluminium by ferric iron. The name grossular is derived from the botanical name for the gooseberry, grossularia, in reference to the green garnet of this composition that is found in Siberia. Other shades include cinnamon brown (cinnamon stone variety), red, and yellow.
Physical Properties
Color: Brown, Colorless, Green, Gray, Yellow. Density: 3.42 – 3.72, Average = 3.57 Diaphaneity: Transparent to subtranslucent Fracture: Sub Conchoidal – Fractures developed in brittle materials characterized by semi-curving surfaces. Hardness: 6.5-7.5 Luminescence: Fluorescent, Short UV=pink, Long UV=orange. Luster: Vitreous – Resinous Streak: brownish white
Subdivision of the Jurassic system according to the IUGS, as of July 2012.
The Late Jurassic is the third epoch of the Jurassic Period, and it spans the geologic time from 161.2 ± 4.0 to 145.5 ± 4.0 million years ago (Ma), which is preserved in Upper Jurassic strata. In European lithostratigraphy, the name “Malm” indicates rocks of Late Jurassic age. In the past, this name was also used to indicate the unit of geological time, but this usage is now discouraged to make a clear distinction between lithostratigraphic and geochronologic/chronostratigraphic units.
Subdivisions
The Late Jurassic is divided into three ages, which correspond with the three (faunal) stages of Upper Jurassic rock:
Tithonian (150.8 ± 4.0 – 145.5 ± 4.0 Ma)
Kimmeridgian (155.7 ± 4.0 – 150.8 ± 4.0 Ma)
Oxfordian (161.2 ± 4.0 – 155.7 ± 4.0 Ma)
Paleogeography
During the Late Jurassic epoch, Pangaea broke up into two supercontinents, Laurasia to the north, and Gondwana to the south. The result of this break-up was the spawning of the Atlantic Ocean. However, at this time, the Atlantic Ocean was relatively narrow.
Note : The above story is based on materials provided by Wikipedia
Subdivision of the Jurassic system according to the IUGS, as of July 2012.
The Middle Jurassic is the second epoch of the Jurassic Period. It lasted from 176 to 161 million years ago. In European lithostratigraphy, rocks of this Middle Jurassic age are called the Dogger. This name in the past was also used to indicate the Middle Jurassic epoch itself, but is discouraged by the IUGS, to distinguish between rock units and units of geological time.
Paleogeography
During the Middle Jurassic epoch, Pangaea began to separate into Laurasia and Gondwana, and the Atlantic Ocean formed. Tectonic activities are active on eastern Laurasia as the Cimmerian plate continues to collide with Laurasia’s southern coast, completely closing the Paleo-Tethys Ocean. A subduction zone on the coast of western North America continues to create the Ancestral Rocky Mountains.
Life forms of the epoch
Marine life
During this time, marine life (including ammonites and bivalves) flourished. Ichthyosaurs, although common, are reduced in diversity; while the top marine predators, the pliosaurs, grew to the size of killer whales and larger (e.g. Pliosaurus, Liopleurodon). Plesiosaurs became common at this time, and metriorhynchid crocodilians first appeared.
Terrestrial life
New types of dinosaurs evolved on land (including cetiosaurs, brachiosaurs, megalosaurs and hypsilophodonts).
Descendants of the therapsids, the cynodonts were still flourishing along with the dinosaurs even though they were shrew-sized; none exceeded the size of a badger. A group of cynodonts, the trithelodonts were becoming rare and eventually became extinct at the end of this epoch. The Tritylodonts were still common though. Mammaliformes, who evolved from a group of cynodonts were also rare and less significant at this time. It was at this epoch that the “true” mammals evolved.
Flora
Conifers were dominant in the Middle Jurassic. Other plants, such as ginkgoes, cycads, and ferns were also common.
Note : The above story is based on materials provided by Wikipedia
Subdivision of the Jurassic system according to the IUGS, as of July 2012.
The Early Jurassic epoch (in chronostratigraphy corresponding to the Lower Jurassic series) is the earliest of three epochs of the Jurassic period. The Early Jurassic starts immediately after the Triassic-Jurassic extinction event, 199.6 Ma (million years ago), and ends at the start of the Middle Jurassic (175.6 Ma).
Certain rocks of marine origin of this age in Europe are called “Lias” and that name was used for the period, as well, in 19th century geology.
Origin of the name Lias
There are two possible origins for the name Lias: the first reason is it was taken by a geologist from an English quarryman’s dialect pronunciation of the word “layers”; secondly, sloops from north Cornish ports such as Bude would sail across the Bristol Channel to the Vale of Glamorgan to load up with rock from coastal limestone quarries (lias limestone from South Wales was used throughout North Devon/North Cornwall as it contains calcium carbonate to fertilise the poor quality Devonian soils of the West Country); the Cornish would pronounce the layers of limestone as ‘laiyers’ or ‘lias’.
Massive cliffs in Zion Canyon consist of Lower Jurassic formations, including (from bottom to top): the Kayenta Formation and the massive Navajo Sandstone.
Geology
There are extensive Liassic outcrops around the coast of the United Kingdom, in particular in Glamorgan, North Yorkshire and Dorset. The ‘Jurassic Coast’ of Dorset is often associated with the pioneering work of Mary Anning of Lyme Regis. The facies of the Lower Jurassic in this area are predominantly of clays, thin limestones and siltstones, deposited under fully marine conditions.
Lias Group strata form imposing cliffs on the Vale of Glamorgan coast, in southern Wales. Stretching for around 14 miles (23 km) between Cardiff and Porthcawl, the remarkable layers of these cliffs, situated on the Bristol Channel are a rhythmic decimetre scale repetition of limestone and mudstone formed as a late Triassic desert was inundated by the sea.
There has been some debate over the actual base of the Hettangian stage, and so of the Jurassic system itself. Biostratigraphically, the first appearance of psiloceratid ammonites has been used; but this depends on relatively complete ammonite faunas being present, a problem that makes correlation between sections in different parts of the world difficult. If this biostratigraphical indicator is used, then technically the Lias Group — a lithostratigraphical division — spans the Jurassic / Triassic boundary.
Life
Ammonites
During this period, ammonoids, which had almost died out at the end-of-Triassic extinction, radiated out into a huge diversity of new forms with complex suture patterns (the ammonites proper). Ammonites evolved so rapidly, and their shells are so often preserved, that they serve as important zone fossils. There were several distinct waves of ammonite evolution in Europe alone.
Marine reptiles
The Early Jurassic was an important time in the evolution of the marine reptiles. The Hettangian saw the already existing Rhaetian ichthyosaurs and plesiosaurs continuing to flourish, while at the same time a number of new types of these marine reptiles appeared, such as Ichthyosaurus and Temnodontosaurus among the ichthyosaurs, and Eurycleidus, Macroplata, and Rhomaleosaurus among the plesiosaurs (all Rhomaleosauridae, although as currently defined this group is probably paraphyletic). All these plesiosaurs had medium-sized necks and large heads. In the Toarcian, at the end of the Early Jurassic, the thalattosuchians (marine “crocodiles”) appeared, as did new genera of ichthyosaurs (Stenopterygius, Eurhinosaurus, and the persistently primitive Suevoleviathan) and plesiosaurs (the elasmosaurs (long-necked) Microcleidus and Occitanosaurus, and the pliosaur Hauffiosaurus).
Terrestrial animals
On land, a number of new types of dinosaurs – the heterodontosaurids, scelidosaurs, stegosaurs, and tetanurans – appeared, and joined those groups like the coelophysoids, prosauropods and the sauropods that had continued over from the Triassic. Accompanying them as small carnivores were the sphenosuchian and protosuchid crocodilians. In the air, new types of pterosaurs replaced those that had died out at the end of the Triassic. While in the undergrowth were various types of early mammals, as well as tritylodont mammal-like reptiles, lizard-like sphenodonts, and early Lissamphibians.
Note : The above story is based on materials provided by Wikipedia
The Jurassic is a geologic period and system that extends from 201.3± 0.6 Ma (million years ago) to 145± 4 Ma; from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic Era, also known as the Age of Reptiles. The start of the period is marked by the major Triassic–Jurassic extinction event. Two other extinction events occurred during the period: the Late Piensbachian/Early Toarcian event in the Early Jurassic, and the Late Tithonian event at the end; however, neither event ranks among the ‘Big Five’ mass extinctions. The Jurassic is named after the Jura Mountains within the European Alps, where limestone strata from the period was first identified.
By the beginning of the Jurassic, the supercontinent Pangaea had begun rifting into two landmasses, Laurasia to the north and Gondwana to the south. This created more coastlines and shifted the continental climate from dry to humid, and many of the arid deserts of the Triassic were replaced by lush rainforests. On land, the fauna transitioned from the Triassic fauna, dominated by both dinosauromorph and crocodylomorph archosaurs, to one dominated by dinosaurs alone. The first birds also appeared during the Jurassic, having evolved from a branch of theropod dinosaurs. Other major events include the appearance of the earliest lizards, and the evolution of therian mammals, including primitive placentals. Crocodylians made the transition from a terrestrial to an aquatic mode of life. The oceans were inhabited by marine reptiles such as ichthyosaurs and plesiosaurs, while pterosaurs were the dominant flying vertebrates.
Etymology
The chronostratigraphic term “Jurassic” is directly linked to the Jura Mountains. Alexander von Humboldt recognized the mainly limestone dominated mountain range of the Jura Mountains as a separate formation that had not been included in the established stratigraphic system defined by Abraham Gottlob Werner, and he named it “Jurakalk” in 1795. The name “Jura” is derived from the Celtic root “jor”, which was Latinised into “juria”, meaning forest (i.e. “Jura” is forest mountains).
Divisions
The Jurassic period is divided into the Early Jurassic, Middle, and Late Jurassic epochs. The Jurassic System, in stratigraphy, is divided into the Lower Jurassic, Middle, and Upper Jurassic series of rock formations, also known as Lias, Dogger and Malm in Europe. The separation of the term Jurassic into three sections goes back to Leopold von Buch. The faunal stages from youngest to oldest are:
During the early Jurassic period, the supercontinent Pangaea broke up into the northern supercontinent Laurasia and the southern supercontinent Gondwana; the Gulf of Mexico opened in the new rift between North America and what is now Mexico’s Yucatan Peninsula. The Jurassic North Atlantic Ocean was relatively narrow, while the South Atlantic did not open until the following Cretaceous period, when Gondwana itself rifted apart. The Tethys Sea closed, and the Neotethys basin appeared. Climates were warm, with no evidence of glaciation. As in the Triassic, there was apparently no land near either pole, and no extensive ice caps existed.
The Jurassic geological record is good in western Europe, where extensive marine sequences indicate a time when much of the continent was submerged under shallow tropical seas; famous locales include the Jurassic Coast World Heritage Site and the renowned late Jurassic lagerstätten of Holzmaden and Solnhofen. In contrast, the North American Jurassic record is the poorest of the Mesozoic, with few outcrops at the surface. Though the epicontinental Sundance Sea left marine deposits in parts of the northern plains of the United States and Canada during the late Jurassic, most exposed sediments from this period are continental, such as the alluvial deposits of the Morrison Formation.
The Jurassic was a time of calcite sea geochemistry in which low-magnesium calcite was the primary inorganic marine precipitate of calcium carbonate. Carbonate hardgrounds were thus very common, along with calcitic ooids, calcitic cements, and invertebrate faunas with dominantly calcitic skeletons (Stanley and Hardie, 1998, 1999).
The first of several massive batholiths were emplaced in the northern Cordillera beginning in the mid-Jurassic, marking the Nevadan orogeny. Important Jurassic exposures are also found in Russia, India, South America, Japan, Australasia and the United Kingdom.
The late Jurassic Morrison Formation in Colorado is one of the most fertile sources of dinosaur fossils in North America.
In Africa, Early Jurassic strata are distributed in a similar fashion to Late Triassic beds, with more common outcrops in the south and less common fossil beds which are predominated by tracks to the north. As the Jurassic proceeded, larger and more iconic groups of dinosaurs like sauropods and ornithopods proliferated in Africa. Middle Jurassic strata are neither well represented nor well studied in Africa. Late Jurassic strata are also poorly represented apart from the spectacular Tendeguru fauna in Tanzania. The Late Jurassic life of Tendeguru is very similar to that found in western North America’s Morrison Formation.
Fauna
Aquatic and marine
During the Jurassic period, the primary vertebrates living in the sea were fish and marine reptiles. The latter include ichthyosaurs, who were at the peak of their diversity, plesiosaurs, pliosaurs, and marine crocodiles of the families Teleosauridae and Metriorhynchidae. Numerous turtles could be found in lakes and rivers.
In the invertebrate world, several new groups appeared, including rudists (a reef-forming variety of bivalves) and belemnites. Calcareous sabellids (Glomerula) appeared in the Early Jurassic. The Jurassic also had diverse encrusting and boring (sclerobiont) communities, and it saw a significant rise in the bioerosion of carbonate shells and hardgrounds. Especially common is the ichnogenus (trace fossil) Gastrochaenolites.
During the Jurassic period, about four or five of the twelve clades of planktonic organisms that exist in the fossil record either experienced a massive evolutionary radiation or appeared for the first time.
On land, large archosaurian reptiles remained dominant. The Jurassic was a golden age for the large herbivorous dinosaurs known as the sauropods—Camarasaurus, Apatosaurus, Diplodocus, Brachiosaurus, and many others—that roamed the land late in the period; their mainstays were either the prairies of ferns, palm-like cycads and bennettitales, or the higher coniferous growth, according to their adaptations. They
were preyed upon by large theropods, such as Ceratosaurus, Megalosaurus, Torvosaurus and Allosaurus. All these belong to the ‘lizard hipped’ or saurischian branch of the dinosaurs. During the Late Jurassic, the first Avialans, like Archaeopteryx, evolved from small coelurosaurian dinosaurs. Ornithischian dinosaurs were less predominant than saurischian dinosaurs, although some, like stegosaurs and small ornithopods, played important roles as small and medium-to-large (but not sauropod-sized) herbivores. In the air, pterosaurs were common; they ruled the skies, filling many ecological roles now taken by birds. Within the undergrowth were various types of early mammals, as well as tritylodonts, lizard-like sphenodonts, and early lissamphibians.
The rest of the Lissamphibia evolved in this period, introducing the first salamanders and caecilians.
The arid, continental conditions characteristic of the Triassic steadily eased during the Jurassic period, especially at higher latitudes; the warm, humid climate allowed lush jungles to cover much of the landscape. Gymnosperms were relatively diverse during the Jurassic period. The Conifers in particular dominated the flora, as during the Triassic; they were the most diverse group and constituted the majority of large trees.
Extant conifer families that flourished during the Jurassic included the Araucariaceae, Cephalotaxaceae, Pinaceae, Podocarpaceae, Taxaceae and Taxodiaceae. The extinct Mesozoic conifer family Cheirolepidiaceae dominated low latitude vegetation, as did the shrubby Bennettitales. Cycads were also common, as were ginkgos and Dicksoniaceous tree ferns in the forest. Smaller ferns were probably the dominant undergrowth. Caytoniaceous seed ferns were another group of important plants during this time and are thought to have been shrub to small-tree sized. Ginkgo plants were particularly common in the mid- to high northern latitudes. In the Southern Hemisphere, podocarps were especially successful, while Ginkgos and Czekanowskiales were rare.
In the oceans, modern coralline algae appeared for the first time.
Note : The above story is based on materials provided by Wikipedia
Chemical Formula: Pb9As4S15 Locality: In the Excelsior mine, Cerro de Pasco, Peru, in large crystals. Name Origin: For Louis Carly Graton (1880-1970), Professor of Economic Geology, Harvard University, Cambridge, Massachusetts, USA.
Gratonite is a lead-arsenic sulfosalt mineral, with the chemical composition Pb9As4S15. Gratonite was discovered in 1939 at the Excelsior Mine, Cerro de Pasco, Peru. It is named in honor of geologist L. C. Graton (1880–1970), who had a long-standing association with the Cerro de Pasco mines.
Physical Properties
Cleavage: None Color: Lead gray, Dark lead gray. Density: 6.22 Diaphaneity: Opaque Fracture: Brittle – Generally displayed by glasses and most non-metallic minerals. Hardness: 2.5 – Finger Nail Luster: Metallic Streak: black
An idealized simulation showing how plate-tectonic boundaries (including complex ones) emerge because of inherited damage following a shift in plate-tectonic driving forces; in this case the shift is similar to that which caused the Emperor-Hawaiian Bend in the Pacific. Credit: David Bercovici
(Phys-org) —Two researchers, David Bercovici of Yale University and Yanick Ricard with the University of Lyon, have together used mathematical modeling to help explain how it was that our planet came to have tectonic plates and why they behave the way they do today. In their paper published in the journal Nature, the pair have explained how they performed mathematical analysis on observations of rocks today and combined data taken from prior theories to come up with a new model that might reflect Earth’s history during the first billion years of its existence.
The surface of planet Earth is covered in several moving plates which scientists believe is one of the major reasons that life was able to evolve and survive. But how the plates came to exist and wound up behaving the way they do today, has been mostly a mystery. In this new effort, the model built by the researchers offers one possible explanation.
It all comes down to grains of rock, they suggest—the smaller the grain, the weaker the rock. One type of rock, known as mylonite has been found to exist at every plate boundary on the planet, suggesting it has something to do with plate tectonics—when found, it is generally deformed with very small grains.
Bercovici and Ricard’s model starts with the idea that very early Earth was covered in hot mushy material. As that material began to cool, some parts or regions would cool faster or slower than others—the cooler parts would then naturally sink. Prior studies have shown that when such types of material sink, the surface becomes slightly deformed. With rock, scientists already known that deformations lead to weaker rock and subsequently smaller grains—and weaker rock would of course lead to additional deformations which suggests a feed-back loop would have evolved. On a planet-wide scale that would mean that weak zones would form at boundaries giving rise to the evolution of tectonic plates. Over billions of years, as the planet continued cooling, the result would be plate tectonics.
The same model can be used to explain why other planets in our solar system didn’t develop plates, and thus the conditions necessary for the development of life. Venus, for example, the numbers suggest, was simply too hot. If pre-plates developed, the deformations would heal due to the high temperatures, preventing the development of actual plates and possibly an atmosphere conducive to life.
An idealized simulation showing how plate-tectonic boundaries (including complex ones) emerge because of inherited damage following a shift in plate-tectonic driving forces; in this case the shift is similar to that which caused the Emperor-Hawaiian Bend in the Pacific. Credit: David Bercovici
Abstract:
The initiation of plate tectonics on Earth is a critical event in our planet’s history. The time lag between the first proto-subduction (about 4 billion years ago) and global tectonics (approximately 3 billion years ago) suggests that plates and plate boundaries became widespread over a period of 1 billion years. The reason for this time lag is unknown but fundamental to understanding the origin of plate tectonics. Here we suggest that when sufficient lithospheric damage (which promotes shear localization and long-lived weak zones) combines with transient mantle flow and migrating proto-subduction, it leads to the accumulation of weak plate boundaries and eventually to fully formed tectonic plates driven by subduction alone. We simulate this process using a grain evolution and damage mechanism with a composite rheology (which is compatible with field and laboratory observations of polycrystalline rocks1, 2), coupled to an idealized model of pressure-driven lithospheric flow in which a low-pressure zone is equivalent to the suction of convective downwellings. In the simplest case, for Earth-like conditions, a few successive rotations of the driving pressure field yield relic damaged weak zones that are inherited by the lithospheric flow to form a nearly perfect plate, with passive spreading and strike-slip margins that persist and localize further, even though flow is driven only by subduction. But for hotter surface conditions, such as those on Venus, accumulation and inheritance of damage is negligible; hence only subduction zones survive and plate tectonics does not spread, which corresponds to observations. After plates have developed, continued changes in driving forces, combined with inherited damage and weak zones, promote increased tectonic complexity, such as oblique subduction, strike-slip boundaries that are subparallel to plate motion, and spalling of minor plates.
The Yellowstone River winds through the Hayden Valley in Yellowstone National Park, Wyoming, June 9, 2013. Credit: Reuters/Jim Urquhart
(Reuters) – Yellowstone National Park assured guests and the public on Thursday that a super-volcano under the park was not expected to erupt anytime soon, despite an alarmist video that claimed bison had been seen fleeing to avoid such a calamity.
Yellowstone officials, who fielded dozens of calls and emails since the video went viral this week following an earthquake in the park, said the video actually shows bison galloping down a paved road that leads deeper into the park.
“It was a spring-like day and they were frisky. Contrary to online reports, it’s a natural occurrence and not the end of the world,” park spokeswoman Amy Bartlett said.
Assurances by Yellowstone officials and government geologists that the ancient super-volcano beneath the park is not due to explode for eons have apparently done little to quell fears among the thousands who have viewed recent video postings of the thundering herd.
Commentary with one of the clips by a self-described survivalist wearing camouflage, dark sunglasses and a black watch cap suggests the wildlife exodus may be tied to “an imminent eruption here at Yellowstone.”
The 4.8 magnitude earthquake that struck early Sunday near the Norris Geyser Basin in the northwest section of Yellowstone, which spans 3,472 square miles of Wyoming, Montana and Idaho, caused no injuries or damages and did not make any noticeable alterations to the landscape, geologists said.
Though benign by seismic standards, it was the largest to rattle Yellowstone since a 4.8 quake in February 1980 and it occurred near an area of ground uplift tied to the upward movement of molten rock in the super-volcano, whose mouth, or caldera, is 50 miles long and 30 miles wide.
But neither the quake, the largest among hundreds that have struck near the geyser basin in the last seven months, nor the uplift suggest an eruption sooner than tens of thousands of years, said Peter Cervelli, associate director for science and technology at the U.S. Geological Survey’s Volcano Science Center in California.
“The chance of that happening in our lifetimes is exceedingly insignificant,” said Cervelli, a scientist with the Yellowstone Volcano Observatory.
Cervelli said the area of uplift that scientists have been tracking since August is rising at a rate of between 10 centimeters (4 inches) and 15 centimeters a year. Geologists who tracked uplift in the same area from 1996 to 2003 also saw elevated seismic activity, he said.
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Note : Note : The above story is based on materials provided by Dan Whitcomb and Steve Orlofsky to Reuters
Chemical Formula: C Locality: Ticonderoga, New York. Madagascar and Ceylon. Name Origin: From the Greek, graphein, “to write.”
Graphite is made almost entirely of carbon atoms, and as with diamond, is a semimetal native element mineral, and an allotrope of carbon. Graphite is the most stable form of carbon under standard conditions. Therefore, it is used in thermochemistry as the standard state for defining the heat of formation of carbon compounds. Graphite may be considered the highest grade of coal, just above anthracite and alternatively called meta-anthracite, although it is not normally used as fuel because it is difficult to ignite.
Occurrence
Graphite occurs in metamorphic rocks as a result of the reduction of sedimentary carbon compounds during metamorphism. It also occurs in igneous rocks and in meteorites. Minerals associated with graphite include quartz, calcite, micas and tourmaline. In meteorites it occurs with troilite and silicate minerals. Small graphitic crystals in meteoritic iron are called cliftonite.
According to the United States Geological Survey (USGS), world production of natural graphite in 2012 was 1,100,000 tonnes, of which the following major exporters are: China (750 kt), India (150 kt), Brazil (75 kt), North Korea (30 kt) and Canada (26 kt). Graphite is not mined in the United States, but U.S. production of synthetic graphite in 2010 was 134 kt valued at $1.07 billion.
Physical Properties
Cleavage: {0001} Perfect Color: Iron black, Dark gray, Black, Steel gray. Density: 2.09 – 2.23, Average = 2.16 Diaphaneity: Opaque Fracture: Sectile – Curved shavings or scrapings produced by a knife blade, (e.g. graphite). Hardness: 1.5-2 – Talc-Gypsum Luminescence: Non-fluorescent. Luster: Sub Metallic Magnetism: Nonmagnetic Streak: black
Chemical Formula: (Fe2+,Mn,Ca)3(PO4)2 Name Origin: Named after its locality at Grafton, New Hampshire, USA.
Graftonite is an iron(II), manganese, calcium phosphate mineral with formula: (Fe2+,Mn,Ca)3(PO4)2. It forms lamellar to granular translucent brown to red-brown to pink monoclinic prismatic crystals. It has a vitreous luster with a Mohs hardness of 5 and a specific gravity of 3.67 to 3.7.
It was first described from its type locality of Melvin Mountain in the town of Grafton, in Grafton County, New Hampshire.
Physical Properties
Cleavage: {010} Perfect Color: Brown, Pink, Dark brown, Reddish brown. Density: 3.67 – 3.7, Average = 3.68 Diaphaneity: Translucent Fracture: Uneven – Flat surfaces (not cleavage) fractured in an uneven pattern. Hardness: 5 – Apatite Luminescence: Non-fluorescent. Luster: Vitreous – Greasy Streak: pale pink
This image shows the dorsal view of Fuxianhuia protensa. The three-inch-long fossil was found in sediments dating from the Cambrian Period 520 million years ago in what today is the Yunnan province in China. Parts of the gut are visible as dark stains along the animal’s midline. Credit: Xiaoya Ma
In 520 million-year-old fossil deposits resembling an ‘invertebrate version of Pompeii,’ researchers have found an ancestor of modern crustaceans revealing the first-known cardiovascular system in exquisitely preserved detail
An international team of researchers from the University of Arizona, China and the United Kingdom has discovered the earliest known cardiovascular system, and the first to clearly show a sophisticated system complete with heart and blood vessels, in fossilized remains of an extinct marine creature that lived over half a billion years ago. The finding sheds new light on the evolution of body organization in the animal kingdom and shows that even the earliest creatures had internal organizational systems that strongly resemble those found in their modern descendants.
“This is the first preserved vascular system that we know of,” said Nicholas Strausfeld, a Regents’ Professor of Neuroscience at the University of Arizona’s Department of Neuroscience, who helped analyze the find.
Being one of the world’s foremost experts in arthropod morphology and neuroanatomy, Strausfeld is no stranger to finding meaningful and unexpected answers to long-standing mysteries in the remains of creatures that went extinct so long ago scientists still argue over where to place them in the evolutionary tree.
The 3-inch-long fossil was entombed in fine dustlike particles – now preserved as fine-grain mudstone – during the Cambrian Period 520 million years ago in what today is the Yunnan province in China. Found by co-author Peiyun Cong near Kunming, it belongs to the species Fuxianhuia protensa, an extinct lineage of arthropods combining advanced internal anatomy with a primitive body plan.
“Fuxianhuia is relatively abundant, but only extremely few specimens provide evidence of even a small part of an organ system, not even to speak of an entire organ system,” said Strausfeld, who directs the UA Center for Insect Science. “The animal looks simple, but its internal organization is quite elaborate. For example, the brain received many arteries, a pattern that appears very much like a modern crustacean.”
In fact, Strausfeld pointed out, Fuxianhuia’s vascular system is more complex than what is found in many modern crustaceans.
“It appears to be the ground pattern from which others have evolved,” he said. “Different groups of crustaceans have vascular systems that have evolved into a variety of arrangements but they all refer back to what we see in Fuxianhuia.”
“Over the course of evolution, certain segments of the animals’ body became specialized for certain things, while others became less important and, correspondingly, certain parts of the vascular system became less elaborate,” Strausfeld said.
Strausfeld helped identify the oldest known fossilized brain in a different specimen of the same fossil species, as well as the first evidence of a completely preserved nervous system similar to that of a modern chelicerates, such as a horseshoe crab or a scorpion.
“This is another remarkable example of the preservation of an organ system that nobody would have thought could become fossilized,” he said.
In addition to the exquisitely preserved heart and blood vessels, outlined as traces of carbon embedded in the surrounding mineralized remains of the fossil, it also features the eyes, antennae and external morphology of the animal.
Using a clever imaging technique that selectively reveals different structures in the fossil based on their chemical composition, collaborator Xiaoya Ma at London’s Natural History Museum was able to identify the heart, which extended along the main part of the body, and its many lateral arteries corresponding to each segment. Its arteries were composed of carbon-rich deposits and gave rise to long channels, which presumably took blood to limbs and other organs.
“With that, we can now start speculating about behavior,” Strausfeld explained. “Because of well-supplied blood vessels to its brain, we can assume this was a very active animal capable of making many different behavioral choices.”
Researchers can only speculate as to why the chemical reactions that occurred during the process of fossilization allowed for this unusual and rare kind of preservation, and as to why only select tissues were preserved between a few rare and different specimen.
“Presumably the conditions had to be just right,” Strausfeld said. “We believe that these animals were preserved because they were entombed quickly under very fine-grained deposits during some kind of catastrophic event, and were then permeated by certain chemicals in the water while they were squashed flat. It is an invertebrate version of Pompeii.”
Possibly, only one in thousands of fossils might have such a well-preserved organ system, Strausfeld said.
At the time Fuxianhuia crawled on the seafloor or swam through water, life had not yet conquered land.
“Terrible sand storms must have occurred because there were probably no plants that could hold the soils,” Strausfeld said. “The habitats of these creatures must have been inundated with massive fallouts from huge storms.”
Tsunamis may also be the cause for the exceptional preservation.
“As the water withdraws, animals on the seafloor dry,” Strausfeld said. “When the water rushed back in, they might become inundated with mud. Under normal circumstances, when animals die and are left to rot on the seafloor, they become unrecognizable. What happened to provide the kinds of fossils we are seeing must have been very different.”
Note : Note : The above story is based on materials provided by University of Arizona
Subdivision of the Triassic system according to the IUGS, as of July 2012.
The Early Triassic is the first of three epochs of the Triassic Period of the geologic timescale. It spans the time between 252.2 ± 0.5 Ma and 247.2 Ma (million years ago). Rocks from this epoch are collectively known as the Lower Triassic, which is a unit in chronostratigraphy. The Early Triassic is the oldest epoch of the Mesozoic Era and is divided into the Induan and Olenekian ages.
The Lower Triassic series is coeval with the Scythian stage, which is today not included in the official timescales but can be found in older literature. In Europe, most of the Lower Triassic is composed of Buntsandstein, a lithostratigraphic unit of continental red beds.
The Permian-Triassic extinction event spawned the Triassic period. The massive extinctions that ended the Permian period and Paleozoic era caused extreme hardships for the surviving species. Many types of corals, brachiopods, molluscs, echinoderms, and other invertebrates had completely disappeared. The most common Early Triassic hard-shelled marine invertebrates were bivalves, gastropods, ammonites, echinoids, and a few articulate brachiopods. The most common land animal was the small herbivorous synapsid Lystrosaurus.
Early Triassic faunas lacked biodiversity and were relatively homogenous throughout the epoch due to the effects of the extinction, ecological recovery on land took 30M years. The climate during the Early Triassic epoch (especially in the interior of the supercontinent Pangaea) was generally arid, rainless and dry and deserts were widespread however the poles possessed a temperate climate. The relatively hot climate of the Early Triassic may have been caused by widespread volcanic eruptions which accelerated the rate of global warming and possibly caused the Permian Triassic extinction event.
Smithian-Spathian extinction
Until recently the existence of an extinction event about 3 million years following the end-Permian extinctions was not recognised, possibly because there were few species left to go extinct. However, studies on conodonts have revealed that temperatures rose in the first 3 million years of the Triassic, ultimately reaching sea surface temperatures of 40 °C in the tropics around 249 million years ago. Large and mobile species disappeared from the tropics, and amongst the immobile species such as molluscs only the ones that could cope with the heat survived; half the bivalves disappeared. On land the tropics were practically devoid of life.
Big, active animals only returned to the tropics, and plants recolonised on land when temperatures returned to normal around 247 million years ago.
Subdivision of the Triassic system according to the IUGS, as of July 2012.
In the geologic timescale, the Middle Triassic is the second of three epochs of the Triassic period or the middle of three series in which the Triassic system is divided. It spans the time between 247.2 Ma and ~235 Ma (million years ago). The Middle Triassic is divided into the Anisian and Ladinian ages or stages.
Formerly the middle series in the Triassic was also known as Muschelkalk. This name is now only used for a specific unit of rock strata with approximately Middle Triassic age, found in western Europe.
During this time there were no flowering plants, but instead there were ferns and mosses. Small dinosaurs began to appear like Nyasasaurus.
Note : The above story is based on materials provided by Wikipedia
Subdivision of the Triassic systemaccording to the IUGS, as of July 2012.
The Late Triassic is in the geologic timescale the third and final of three epochs of the Triassic period. The corresponding series is known as the Upper Triassic. In the past it was sometimes called the Keuper, after a German lithostratigraphic group (a sequence of rock strata) that has a roughly corresponding age. The Late Triassic spans the time between ~235 Ma and 201.3 ± 0.2 Ma (million years ago). The Late Triassic is divided into the Carnian, Norian and Rhaetian ages.
Many of the first dinosaurs evolved during the Late Triassic, including Plateosaurus, Coelophysis, and Eoraptor.
Paleogeography and tectonics
Africa shared Pangea’s relatively uniform fauna which was dominated by theropods, prosauropods and primitive ornithischians by the close of the Triassic period. Late Triassic fossils are found throughout Africa, but are more common in the south than north.
The boundary separating the Triassic and Jurassic marks the advent of an extinction event with global impact, although African strata from this time period have not been thoroughly studied. In the area of Tübingen (Germany), a Triassic-Jurassic bonebed can be found, which is characteristic for this boundary.
Note : The above story is based on materials provided by Wikipedia
The Triassic /traɪˈæsɪk/ is a geologic period and system that extends from about 250 to 200 Ma (252.2 ± 0.5 to 201.3 ± 0.2 million years ago). It is the first period of the Mesozoic Era, and lies between the Permian and Jurassic periods. Both the start and end of the period are marked by major extinction events. The Triassic was named in 1834 by Friedrich von Alberti, after the three distinct rock layers (tri meaning “three”) that are found throughout Germany and northwestern Europe—red beds, capped by marine limestone, followed by a series of terrestrial mud- and sandstones—called the “Trias.”
The Triassic began in the wake of the Permian–Triassic extinction event, which left the Earth’s biosphere impoverished; it would take well into the middle of the period for life to recover its former diversity. Therapsids and archosaurs were the chief terrestrial vertebrates during this time. A specialized subgroup of archosaurs, dinosaurs, first appeared in the Late Triassic but did not become dominant until the succeeding Jurassic. The first true mammals, themselves a specialized subgroup of Therapsids also evolved during this period, as well as the first flying vertebrates, the pterosaurs, who like the dinosaurs were a specialized subgroup of archosaurs. The vast supercontinent of Pangaea existed until the mid-Triassic, after which it began to gradually rift into two separate landmasses, Laurasia to the north and Gondwana to the south. The global climate during the Triassic was mostly hot and dry, with deserts spanning much of Pangaea’s interior. However, the climate shifted and became more humid as Pangaea began to drift apart. The end of the period was marked by yet another major mass extinction, wiping out many groups and allowing dinosaurs to assume dominance in the Jurassic.
Dating and subdivisions
The Triassic is usually separated into Early, Middle, and Late Triassic Epochs, and the corresponding rocks are referred to as Lower, Middle, or Upper Triassic. The faunal stages from the youngest to oldest are:
)During the Triassic, almost all the Earth’s land mass was concentrated into a single supercontinent centered more or less on the equator, called Pangaea (“all the land”). From the east a vast gulf entered Pangaea, the Tethys sea. It opened farther westward in the mid-Triassic, at the expense of the shrinking Paleo-Tethys Ocean, an ocean that existed during the Paleozoic. The remaining shores were surrounded by the world-ocean known as Panthalassa (“all the sea”). All the deep-ocean sediments laid down during the
Triassic have disappeared through subduction of oceanic plates; thus, very little is known of the Triassic open ocean. The supercontinent Pangaea was rifting during the Triassic—especially late in the period—but had not yet separated. The first nonmarine sediments in the rift that marks the initial break-up of Pangaea—which separated New Jersey from Morocco—are of Late Triassic age; in the U.S., these thick sediments comprise the Newark Group. Because of the limited shoreline of one super-continental mass, Triassic marine deposits are globally relatively rare, despite their prominence in Western Europe, where the Triassic was first studied. In North America, for example, marine deposits are limited to a few exposures in the west. Thus Triassic stratigraphy is mostly based on organisms living in lagoons and hypersaline environments, such as Estheria crustaceans.
Africa
At the beginning of the Mesozoic Era, Africa was joined with Earth’s other continents in Pangaea. Africa shared the supercontinent’s relatively uniform fauna which was dominated by theropods, prosauropods and primitive ornithischians by the close of the Triassic period. Late Triassic fossils are found throughout Africa, but are more common in the south than north. The boundary separating the Triassic and Jurassic marks the advent of an extinction event with global impact, although African strata from this time period have not been thoroughly studied.
South America
At Paleorrota geopark, located in Rio Grande do Sul, Brazil, the Santa Maria Formation and Caturrita Formations are exposed. In these formations, one of the earliest dinosaurs, Staurikosaurus, as well as the mammal ancestors Brasilitherium and Brasilodon have been discovered.
Climate
The Triassic climate was generally hot and dry, so that typical deposits are red bed sandstones and evaporites. There is no evidence of glaciation at or near either pole; in fact, the polar regions were apparently moist and temperate, a climate suitable for forests and vertebrates, including reptiles. Pangaea’s large size limited the moderating effect of the global ocean; its continental climate was highly seasonal, with very hot summers and cold winters. The strong contrast between the Pangea supercontinent and the global ocean triggered intense cross-equatorial monsoons.
Although the Triassic may have mostly been a dry period, at least at tropical and subtropical latitudes in the Tethys and surrounding lands, evidence exists that it was punctuated by several episodes of increased rainfall. Sediments and fossils suggestive of a more humid climate are known from the Anisian to Ladinian of the Tethysian domain, and from the Carnian and Rhaetian of a larger area that includes also the Boreal domain (e.g., Svalbard Islands), the North American continent, the South China block and Argentina.
The best studied of such episodes of humid climate, and probably the most intense and widespread, was the Carnian Pluvial Event
Life
Life restaurations of some Triassic plants
Three categories of organisms can be distinguished in the Triassic record: holdovers from the Permian-Triassic extinction, new groups which flourished briefly, and other new groups which went on to dominate the Mesozoic world.
Flora
On land, the holdover plants included the lycophytes, the dominant cycads, ginkgophyta (represented in modern times by Ginkgo biloba) and glossopterids. The spermatophytes, or seed plants came to dominate the terrestrial flora: in the northern hemisphere, conifers flourished. Glossopteris (a seed fern) was the dominant southern hemisphere tree during the Early Triassic period.
Marine fauna
In marine environments, new modern types of corals appeared in the Early Triassic, forming small patches of reefs of modest extent compared to the great reef systems of Devonian times or modern reefs. Serpulids appeared in the Middle Triassic. Microconchids were abundant. The shelled cephalopods called ammonites recovered, diversifying from a single line that survived the Permian extinction. The fish fauna was remarkably
uniform, reflecting the fact that very few families survived the Permian extinction. There were also many types of marine reptiles. These included the Sauropterygia, which featured pachypleurosaurs and nothosaurs (both common during the Middle Triassic, especially in the Tethys region), placodonts, and the first plesiosaurs; the first of the lizardlike Thalattosauria (askeptosaurs); and the highly successful ichthyosaurs, which appeared in Early Triassic seas and soon diversified, some eventually developing to huge size during the late Triassic.
Terrestrial fauna
Important groups of terrestrial fauna, newly in the triassic period or achieving a new level of evolutionary success during the triassic include:
Temnospondyls
One of the largest and most important groups of early amphibians, temnospondyls originated during the Carboniferous but flourished during the Triassic. They lived across the globe, ranged in size from tiny to enormous and prospered both on land and in the water. The largest temnospondyls, such as the Mastodonsaurus were up to 13ft in length.
Rhynchosaurs
These barrel-gutted herbivores thrived for only a short period of time, becoming extinct about 220 million years ago, but they were exceptionally abundant and were the primary large herbivores in many ecosystems. They sheared plants with their beaks and several rows of teeth on the roof of the mouth.
Phytosaurs
One of the most familiar of the crocodile-line archosaurs, phytosaurs prospered during the late Triassic. These long-snouted and semiaquatic predators resemble living crocodiles and probably had a similar lifestyle, hunting for fish and small reptiles around the water’s edge. However this resemblance is only superficial and is a prime-case of convergent evolution.
Aetosaurs
Like the phytosaurs, aetosaurs were exceptionally common during the last 30 million years of the late Triassic but died out at the Triassic-Jurassic extinction. Aetosaurs were heavily armored and resembled giant military tanks. Most aetosaurs were herbivorous, and fed on low-growing plants but some may have eaten meat.
Rauisuchians
The rauisuchians, yet another group of crocodile-line archosaurs, were the keystone predators of most Triassic terrestrial ecosystems. Over 25 species have been found, and include giant quadripedal hunters, sleeks bipedal omnivores, and lumbering beasts with deep sails on their backs. They probably occupied the large-predator niche later filled by theropods.
Theropods
Perhaps the most recognizable group of dinosaurs, the therapods include predators such as the Tyrannosaurus, Allosaurus, and Velociraptor. Theropods first evolved in the Triassic period but did not evolve into giant sizes until later, during the Jurassic. Most Triassic theropods, such as the Coelophysis, were only several feet long and hunted small prey in the shadow of the giant Rauisuchians.
Cynodonts
Cynodonts are a large group that includes true mammals. The first cynodonts evolved in the Permian, but many groups prospered during the Triassic. Their characteristic mammalian features included hair, a large brain, and upright posture. Many were small but some were enormous and filled a large herbivore niche before the evolution of sauropodomorph dinosaurs.
The Permian-Triassic extinction devastated terrestrial life. Biodiversity rebounded with the influx of pioneer organisms, but these were short lived. Diverse communities with complex trophic structures took 30 million years to reestablish.
Temnospondyl amphibians were among those groups that survived the Permian-Triassic extinction, some lineages (e.g. trematosaurs) flourishing briefly in the Early Triassic, while others (e.g. capitosaurs) remained successful throughout the whole period, or only came to prominence in the Late Triassic (e.g. plagiosaurs, metoposaurs). As for other amphibians, the first Lissamphibia, characterized by the first frogs, are known from the Early Triassic, but the group as a whole did not become common until the Jurassic, when the temnospondyls had become very rare. Other survivors were the Chroniosuchia and Embolomeri, more closely related to amniotes than temnospondyls, but went extinct after some million years.
Archosauromorph reptiles—especially archosaurs—progressively replaced the synapsids that had dominated the Permian, although Cynognathus was a characteristic top predator in earlier Triassic (Olenekian and Anisian) Gondwana, and both kannemeyeriid dicynodonts and gomphodont cynodonts remained important herbivores during much of the period. By the end of the Triassic, synapsids played only bit parts. During the Carnian (early part of the Late Triassic), some advanced cynodont gave rise to the first mammals. At the same time the Ornithodira, which until then had been small and insignificant, evolved into pterosaurs and a variety of dinosaurs. The Crurotarsi were the other important archosaur clade, and during the Late Triassic these also reached the height of their diversity, with various groups including the phytosaurs, aetosaurs, several distinct lineages of Rauisuchia, and the first crocodylians (the Sphenosuchia). Meanwhile the stocky herbivorous rhynchosaurs and the small to medium-sized insectivorous or piscivorous Prolacertiformes were important basal archosauromorph groups throughout most of the Triassic.
Among other reptiles, the earliest turtles, like Proganochelys and Proterochersis, appeared during the Norian (middle of the Late Triassic). The Lepidosauromorpha—specifically the Sphenodontia—are first known in the fossil record a little earlier (during the Carnian). The Procolophonidae were an important group of small lizard-like herbivores.
In the Triassic, archosaurs displaced therapsids as the dominant amniotes. This “Triassic Takeover” may have contributed to the evolution of mammals by forcing the surviving therapsids and their mammaliaform successors to live as small, mainly nocturnal insectivores; nocturnal life may have forced the mammaliaforms to develop fur and higher metabolic rates.
Coal
At the start of the Triassic period coal is noticeable by geologists today as being absent throughout the world. This is known as the “coal gap” and can be seen as part of the Permian–Triassic extinction event.Sharp drops in sea level across the Permo Triassic boundary may be the proper explanation for the coal gap. However, theories are still speculative as to why it is missing. During the preceding Permian period the arid desert conditions contributed to the evaporation of many inland seas and the inundation of these seas, perhaps by a number of tsunami events that may have been responsible for the drop in sea level. This due to the finding of large salt basins in the southwest United States and a very large basin in central Canada.
Immediately above the boundary the glossopteris flora was suddenly largely displaced by an Australia wide coniferous flora containing few species and containing a lycopod herbaceous under story. Conifers also became common in Eurasia. These groups of conifers arose from endemic species because of the ocean barriers that prevented seed crossing for over one hundred million years. For instance, Podocarpis was located south and Pines, Junipers, and Sequoias were located north. The dividing line ran through the Amazon Valley, across the Sahara, and north of Arabia, India, Thailand, and Australia. It has been suggested that there was a climate barrier for the conifers. although water barriers are more plausible. If so, something that can cross at least short water barriers must have been involved in producing the coal hiatus. Hot climate could have been an important auxiliary factor across Antarctica or the Bering Strait, however. There was a spike of fern and lycopod spores immediately after the close of the Permian. In addition there was also a spike of fungal spores immediately after the Permian-Triassic boundary. This spike may have lasted 50,000 years in Italy and 200,000 years in China and must have contributed to the climate warmth.
An event excluding a catastrophe must have been involved to cause the coal hiatus due to the fact that fungi would have removed all dead vegetation and coal forming detritus in a few decades in most tropical places. In addition, fungal spores rose gradually and declined similarly along with a prevalence of woody debris. Each phenomenon would hint at widespread vegetative death. Whatever the cause of the coal hiatus must have started in North America approximately 25 million years sooner.
Lagerstätten
Triassic sandstone near Stadtroda, Germany
The Monte San Giorgio lagerstätte, now in the Lake Lugano region of northern Italy and Switzerland, was in Triassic times a lagoon behind reefs with an anoxic bottom layer, so there were no scavengers and little turbulence to disturb fossilization, a situation that can be compared to the better-known Jurassic Solnhofen limestone lagerstätte. The remains of fish and various marine reptiles (including the common pachypleurosaur Neusticosaurus, and the bizarre long-necked archosauromorph Tanystropheus), along with some terrestrial forms like Ticinosuchus and Macrocnemus, have been recovered from this locality. All these fossils date from the Anisian/Ladinian transition (about 237 million years ago).
Triassic-Jurassic extinction event
The Triassic period ended with a mass extinction, which was particularly severe in the oceans; the conodonts disappeared, as did all the marine reptiles except ichthyosaurs and plesiosaurs. Invertebrates like brachiopods, gastropods, and molluscs were severely affected. In the oceans, 22% of marine families and possibly about half of marine genera went missing according to University of Chicago paleontologist Jack Sepkoski.
Though the end-Triassic extinction event was not equally devastating everywhere in terrestrial ecosystems, several important clades of crurotarsans (large archosaurian reptiles previously grouped together as the thecodonts) disappeared, as did most of the large labyrinthodont amphibians, groups of small reptiles, and some synapsids (except for the proto-mammals). Some of the early, primitive dinosaurs also went extinct, but other more adaptive dinosaurs survived to evolve in the Jurassic. Surviving plants that went on to dominate the Mesozoic world included modern conifers and cycadeoids.
What caused this Late Triassic extinction is not known with certainty. It was accompanied by huge volcanic eruptions that occurred as the supercontinent Pangaea began to break apart about 202 to 191 million years ago 40Ar/39Ar dates,forming the CAMP, one of the largest known inland volcanic events since the planet cooled and stabilized. Other possible but less likely causes for the extinction events include global cooling or even a bolide impact, for which an impact crater containing Manicouagan Reservoir in Quebec, Canada, has been singled out. At the Manicouagan impact crater, however, recent research has shown that the impact melt within the crater has an age of 214±1 Mya. The date of the Triassic-Jurassic boundary has also been more accurately fixed recently, at 201.3 ± 0.2 Mya. Both dates are gaining accuracy by using more accurate forms of radiometric dating, in particular the decay of uranium to lead in zircons formed at the impact. So the evidence suggests the Manicouagan impact preceded the end of the Triassic by approximately 10±2 Ma. Therefore it could not be the immediate cause of the observed mass extinction.
The number of Late Triassic extinctions is disputed. Some studies suggest that there are at least two periods of extinction towards the end of the Triassic, between 12 and 17 million years apart. But arguing against this is a recent study of North American faunas. In the Petrified Forest of northeast Arizona there is a unique sequence of latest Carnian-early Norian terrestrial sediments. An analysis in 2002 found no significant change in the paleoenvironment. Phytosaurs, the most common fossils there, experienced a change-over only at the genus level, and the number of species remained the same. Some aetosaurs, the next most common tetrapods, and early dinosaurs, passed through unchanged. However, both phytosaurs and aetosaurs were among the groups of archosaur reptiles completely wiped out by the end-Triassic extinction event.
It seems likely then that there was some sort of end-Carnian extinction, when several herbivorous archosauromorph groups died out, while the large herbivorous therapsids— the kannemeyeriid dicynodonts and the traversodont cynodonts—were much reduced in the northern half of Pangaea (Laurasia).
These extinctions within the Triassic and at its end allowed the dinosaurs to expand into many niches that had become unoccupied. Dinosaurs became increasingly dominant, abundant and diverse, and remained that way for the next 150 million years. The true “Age of Dinosaurs” is the Jurassic and Cretaceous, rather than the Triassic.
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Global perspective: satellites are often the only form of monitoring for remote or inaccessible volcanoes. This view shows some of the recent episodes of deformation seen at volcanoes in Europe and Africa. Credit: Image courtesy of University of Bristol
ESA’s Sentinel satellite, due for launch on April 3rd, should allow scientists to test this link in greater detail and eventually develop a forecast system for all volcanoes, including those that are remote and inaccessible.
Volcano deformation and, in particular, uplift are often considered to be caused by magma moving or pressurizing underground. Magma rising towards the surface could be a sign of an imminent eruption. On the other hand, many other factors influence volcano deformation and, even if magma is rising, it may stop short, rather than erupting.
Dr Juliet Biggs and colleagues in the School of Earth Sciences, with collaborators from Cornell, Oxford and Southern Methodist University, looked at the archive of satellite data covering over 500 volcanoes worldwide, many of which have been systematically observed for over 18 years. Satellite radar (InSAR) can provide high-resolution maps of deformation, allowing the detection of unrest at many volcanoes that might otherwise go unrecognised. Such satellite data is often the only source of information for remote or inaccessible volcanoes.
The researchers applied statistical methods more traditionally used for medical diagnostic testing and found that many deforming volcanoes also erupted (46 per cent). Together with the very high proportion of non-deforming volcanoes that did not erupt (94 per cent), these jointly represent a strong indicator of a volcano’s long-term eruptive potential.
Dr Biggs said: “The findings suggest that satellite radar is the perfect tool to identify volcanic unrest on a regional or global scale and target ground-based monitoring.”
The work was co-funded by the UK Centre for Observation and Modelling of Earthquakes, Volcanoes and Tectonics (COMET) and STREVA, a research consortium aimed at finding ways to reduce the negative consequences of volcanic activity on people and their assets.
“Improving how we anticipate activity using new technology such as this is an important first step in doing better at forecasting and preparing for volcanic eruptions,” said STREVA Principal Investigator, Dr Jenni Barclay.
Co-author Professor Willy Aspinall added: “Global studies of volcano deformation using satellite data will increasingly play a part in assessing eruption potential at more and more volcanoes, especially in regions with short historical records or limited conventional monitoring.”
However, many factors and processes, some observable, but others not, influence deformation to a greater or lesser extent. These include the type of rock that forms the volcano, its tectonic characteristics and the supply rate and storage depth of magma beneath it. Thus, deformation can have different implications for different types of volcanoes. For volcanoes with short eruption cycles, the satellite record typically spans episodes that include both deformation and eruption, resulting in a high correlation between the two. For volcanoes with long eruption cycles, the satellite record tends to capture either deformation or eruption but rarely both.
In the past, radar images of the majority of the world’s volcanoes were only acquired a few times a year, but seismological data indicate that the duration of unrest before an eruption might be as short as only a few days.
Dr Biggs said: “This study demonstrates what can be achieved with global satellite coverage even with limited acquisitions, so we are looking forward to the step-change in data quantity planned for the next generation of satellites.”
The European Space Agency is planning to launch its next radar mission, Sentinel-1 in early April. This mission is designed for global monitoring and will collect images every six to twelve days. Using this, scientists should be able to test the causal and temporal relationship with deformation on much shorter timescales.
Professor Tim Wright, Director of COMET, added: “This study is particularly exciting because Sentinel-1 will soon give us systematic observations of the ups and downs of every volcano on the planet. For many places, particularly in developing countries, these data could provide the only warning of an impending eruption.”
Note : The above story is based on materials provided by University of Bristol.
Temperature, not chemistry. What appears on the surface correlates with temperature deep in the Earth. Higher ridge elevation indicates a hotter mantle — as in Iceland, above, which also appears to sit atop a mantle plume, a vertical jet of hot rock originating from deep in the Earth. Credit: Allison Gale/University of Wisconsin
Scientists have shown that temperature differences deep within Earth’s mantle control the elevation and volcanic activity along mid-ocean ridges, the colossal mountain ranges that line the ocean floor. The findings, published April 4 in the journal Science, shed new light on how temperature in the depths of the mantle influences the contours of Earth’s crust.
Mid-ocean ridges form at the boundaries between tectonic plates, circling the globe like seams on a baseball. As the plates move apart, magma from deep within Earth rises up to fill the void, creating fresh crust as it cools. The crust formed at these seams is thicker in some places than others, resulting in ridges with widely varying elevations. In some places, the peaks are submerged miles below the ocean surface. In other places — Iceland, for example — the ridge tops are exposed above the water’s surface.
“These variations in ridge depth require an explanation,” said Colleen Dalton, assistant professor of geological sciences at Brown and lead author of the new research. “Something is keeping them either sitting high or sitting low.”
That something, the study found, is the temperature of rocks deep below Earth’s surface.
By analyzing the speeds of seismic waves generated by earthquakes, the researchers show that mantle temperature along the ridges at depths extending below 400 kilometers varies by as much as 250 degrees Celsius. High points on the ridges tend to be associated with higher mantle temperatures, while low points are associated with a cooler mantle. The study also showed that volcanic hot spots along the ridge — volcanoes near Iceland as well as the islands of Ascension, Tristan da Cunha, and elsewhere — all sit above warm spots in Earth’s mantle.
“It is clear from our results that what’s being erupted at the ridges is controlled by temperature deep in the mantle,” Dalton said. “It resolves a long-standing controversy and has not been shown definitively before.”
A CAT scan of Earth
The mid-ocean ridges provide geologists with a window to the interior of Earth. The ridges form when mantle material melts, rises into the cracks between tectonic plates, and solidifies again. The characteristics of the ridges provide clues about the properties of the mantle below.
For example, a higher ridge elevation suggests a thicker crust, which in turn suggests that a larger volume of magma was erupted at the surface. This excess molten rock can be caused by very hot temperatures in the mantle. The problem is that hot mantle is not the only way to produce excess magma. The chemical composition of the rocks in Earth’s mantle also controls how much melt is produced. For certain rock compositions, it is possible to generate large volumes of molten rock under cooler conditions. For many decades it has not been clear whether mid-ocean ridge elevations are caused by variations in the temperature of the mantle or variations in the rock composition of the mantle.
To distinguish between these two possibilities, Dalton and her colleagues introduced two additional data sets. One was the chemistry of basalts, the rock that forms from solidification of magma at the mid-ocean ridge. The chemical composition of basalts differs depending upon the temperature and composition of the mantle material from which they’re derived. The authors analyzed the chemistry of nearly 17,000 basalts formed along mid-ocean ridges around the globe.
The other data set was seismic wave tomography. During earthquakes, seismic waves are sent pulsing through the rocks in the crust and mantle. By measuring the velocity of those waves, scientists can gather data about the characteristics of the rocks through which they traveled. “It’s like performing a CAT scan of the inside of the Earth,” Dalton said.
Seismic wave speeds are especially sensitive to the temperature of rocks. In general, waves propagate more quickly in cooler rocks and more slowly in hotter rocks.
Dalton and her colleagues combined the seismic data from hundreds of earthquakes with data on elevation and rock chemistry from the ridges. Correlations among the three data sets revealed that temperature deep in the mantle varied between around 1,300 and 1,550 degrees Celsius underneath about 61,000 kilometers of ridge terrain. “It turned out,” said Dalton, “that seismic tomography was the smoking gun. The only plausible explanation for the seismic wave speeds is a very large temperature range.”
The study showed that as ridge elevation falls, so does mantle temperature. The coolest point beneath the ridges was found near the lowest point, an area of very deep and rugged seafloor known as the Australian-Antarctic discordance in the Indian Ocean. The hottest spot was near Iceland, which is also the ridges’ highest elevation point.
Iceland is also where scientists have long debated whether a mantle plume — a vertical jet of hot rock originating from deep inside Earth — intersects the mid-ocean ridge. This study provides strong support for a mantle plume located beneath Iceland. In fact, this study showed that all regions with above-average temperature are located near volcanic hot spots, which points to mantle plumes as the culprit for the excess volume of magma in these areas.
Understanding a churning planet
Despite being made of solid rock, Earth’s mantle doesn’t sit still. It undergoes convection, a slow churning of material from the depths of the Earth toward the surface and back again.
“Convection is why we have plate tectonics and earthquakes,” Dalton said. “It’s also responsible for almost all volcanism at the surface. So understanding mantle convection is crucial to understanding many fundamental questions about the Earth.”
Two factors influence how that convection works: variations in the composition of the mantle and variations in its temperature. This work, says Dalton, points to temperature as a primary factor in how convection is expressed on the surface.
“We get consistent and coherent temperature measurements from the mantle from three independent datasets,” Dalton said. “All of them suggest that what we see at the surface is due to temperature, and that composition is only a secondary factor. What is surprising is that the data require the temperature variations to exist not only near the surface but also many hundreds of kilometers deep inside the Earth.”
The findings from this study will also be useful in future research using seismic waves, Dalton says. Because the temperature readings as indicated by seismology were backed up by the other datasets, they can be used to calibrate seismic readings for places where geochemical samples aren’t available. This makes it possible to estimate temperature deep in Earth’s mantle all over the globe.
That will help geologists gain a new insights into how processes deep within Earth mold the ground beneath our feet.
Note : The above story is based on materials provided by Brown University.
