September 2015

How Geologists Determined The Way That Mountains Formed

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<tr><td class="tr-caption" style="text-align: center;"><div class="wp-caption-text"><span style="font-size: xx-small;">Mount Everest, view from Kala Patthar (5.700  m), with most important geological formations added. Image by Uwe Gille  (2005), used under creative commons license.</span></div></td></tr>
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Asked why he wanted to climb Mount Everest, George Mallory famously responded “because it´s there.” But there was also a more practical reason – “for the stone from the top for geologists.” <br />
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This wasn’t a jest – geologists were intensely interested in Everest. When Mallory, together with Sandy Irvine, attempted the ascent June 6, 1924, mountaineer and geologist Noel Ewart Odel – the last person to see the two alive – was collecting rocks at the base of the summit era of the 8,848 metres (29,029 ft) a.s.l. high mountain. <br />
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Other geological evidence, especially fossils of organisms, collected by Noeal Ewart demonstrated that “the summit of Mt. Everest is marine limestone.” Geologist and author John McPhee pointed out this geological contradiction – the highest point on earth was once an ancient seafloor.<br />
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The discovery of shells and remains of marine organism high on mountains was not completely new for the time. Renaissance artist and naturalist Leonardo da Vinci had recognized shells embedded in the rocks of the “mountains of Parma and Piacenza,” arguing correctly that the flood described in the Bible could not transport and deposit undamaged shells so high on the mountains. Da Vinci speculated that parts of earth´s crust had instead collapsed into large subterranean cavities filled with water. The displaced water would itself push other parts of the crust up. This mechanism, thought da Vinci, could explain the distribution of mountains and valleys. <br />
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Unfortunately, da Vinci never published these observations and speculations. Thus, many European naturalists continued to consider the Biblical Flood as a good explanation for the observed geological oddities found in mountains. The catastrophic flood and its strong currents, they thought, would redeposit sediments and fold and disrupt the newly formed layers of rocks.<br />
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</div>Physician Johann Jakob Scheuchzer, one of the first naturalists to publish about the Alps, used examples of large-scale folds observed in the Swiss Alps as evidence for the veracity of the Biblical account of the Flood. Sedimentary rocks are widespread in plains and valleys and it´s easy to observe how rivers and occasionally floods erode, transport and deposit new sediments. However, in mountains the situation is more difficult to explain. Naturalists, like Scheuchzer, couldn’t really determine from where the water needed to cover the highest peaks, came from or disappeared to after the Flood. <br />
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<tr><td style="text-align: center;"><a href="https://2.bp.blogspot.com/-eUhkWIgaW0I/V2FMlrIr_CI/AAAAAAAAIso/cZQoJ7vqagsZOeI3h-H9s6dLrWixITFTwCLcB/s1600/Helvetic%2BNappes%2Bof%2BSwitzerland.%2B%2528Credit%2BKurt%2BSt%25C3%25BCwe%2Bvia%2Bimaggeo.egu.eu%2529.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="426" src="https://2.bp.blogspot.com/-eUhkWIgaW0I/V2FMlrIr_CI/AAAAAAAAIso/cZQoJ7vqagsZOeI3h-H9s6dLrWixITFTwCLcB/s640/Helvetic%2BNappes%2Bof%2BSwitzerland.%2B%2528Credit%2BKurt%2BSt%25C3%25BCwe%2Bvia%2Bimaggeo.egu.eu%2529.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><div class="wp-caption-text"><span style="font-size: x-small;"><span style="font-family: inherit;">Helvetic Nappes of Switzerland. (Credit: Kurt Stüwe, via imaggeo.egu.eu)</span></span></div></td></tr>
</tbody></table><div class="separator" style="clear: both; text-align: center;"></div>In the 18th and 19th centuries, geologists started to accurately map the distribution of rocks and disposition of their layers. Additionally, volcanoes were better understood and were now considered important geological forces. Soon, geologists recognized that in mountain ranges – or at least in the Alps – layered sedimentary rocks surrounded a central core of undifferentiated rocks, which were formed by slow cooling of intruded lava-like rocks. <br />
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German geologist Leopold von Buch was convinced that mountains formed like a bubble on earth´s crust: magma from earth´s mantle pushes up, displaces and folds the Earth’s crust, and finally forms a mountain. Von Buch´s “crater of elevation” theory became very popular at the time and was shared by most European geologists. French geologist Elie de Beaumont, adopting von Buch´s theory, could seemingly use it to explain even the complex geology of the Alps. <br />
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Beaumont explained that different tilted layers of sediments, as found in the Alps, were formed by periodic “magmatic” pulses. In a first phase, the horizontally deposited sediments were tilted by the intrusion of a large magmatic core. In a later phase, the already tilted layers become even steeper and then new layers – partly formed by the erosion of older layers and partly as the mountain-forming process increasingly involves a larger area – start to tilt. However, British geologists later showed that this theory couldn’t work as proposed. If a mountain formed around a single bubble of magma, the all the layers should be disposed like in an onion. But ther the strata in the Alps were tilted chaotically and often even folded. <br />
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A new theory – the Contracting Earth theory – was later formulated by the American geologist James Dwigth Dana. This theory explained mountains and continents as products of a cooling and subsequently shrinking earth. Like an old and dry apple, the theory explained, the shrinking surface of Earth would develop fissures (basins) and wrinkles (mountains). <br />
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Following on this work, Austrian Geologist Eduard Suess published in his multi-volume work Das Antlitz der Erde this hand-colored map, the different colors of which show the preserved old “cores of crust” surrounded by the younger basins today filled with oceans. Curiously, he suggested that the deep-sea trenches found along the borders of the Pacific are zones where the seafloor is pushed under the continents.<br />
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The Contracting Earth theory could explain the immense forces needed to crack and fold rocks. But it failed to explain the irregular distribution of mountains on Earth. According to the theory,  features of the Earth’s crust should be distributed randomly on the surface of the cooling and uniformly shrinking planet. However, even a short glimpse on a map or globe shows that mountain ranges are not randomly distributed, but rather form long chains, like the Alp, Caucasus, or Himalayas; or are instead found along one side of a continent, like the Rocky Mountains or the Andes, but not on the other side. <br />
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<u><span style="font-size: x-small;"><span style="font-family: inherit;">SEARLE, M. (2013): Colliding Continents: A geological exploration of the Himalaya. Oxford University Press: 438</span></span></u><br />
<div class="contrib-byline-author"><a class="link ng-binding" href="http://www.forbes.com/sites/davidbressan/" rel="nofollow" target="_blank">David Bressan</a> <span class="author-comma ng-scope">,</span></div><u><span style="font-size: x-small;"><span style="font-family: inherit;"> </span></span></u> </div>

How Geologists Determined The Way That Mountains Formed

Mount Everest, view from Kala Patthar (5.700 m), with most important geological formations added. Image by Uwe Gille (2005), used under ...

Scientists simulate Earth's middle crust to understand earthquakes

Earhquakes, earth 9:10 PM

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Researchers have for the first time been able to measure a material's resistance to fracturing from various types of tectonic motions in the Earth's middle crust, a discovery that may lead to better understanding of how large earthquakes and slower moving events interact. <br />
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The University of Texas Institute for Geophysics (UTIG), research unit of the Jackson School of Geosciences, spearheaded the discovery. The study was published in the September edition of Nature Geoscience. <br />
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Scientists conducted the research using Carbopol, a gel-like substance that can simulate the characteristics of rock formations in the Earth's middle crust because it is simultaneously brittle and malleable. <br />
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Researchers performed shear tests on the Carbopol, where a portion of the material is pulled one direction and a portion is pulled in the opposite direction. This is similar to what happens to rock formations in the middle crust during earthquakes or slow-slip events, a type of tectonic movement that resembles an earthquake but happens over a much longer period of time. <br />
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Previously, nearly all research into such movements of the Earth's crusts was done by measuring tectonic movement using GPS readings and linking these findings with friction laws. Those observations did not address how rock behaves when it softens under heat and pressure. <br />
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"It is not really clear how slow-slip events interact with earthquakes, whether they can trigger earthquakes or it's the other way around - that earthquakes trigger slow-slip events," said Jacqueline Reber, the study's lead author who performed this research as postdoctoral fellow at UTIG, and who is now an assistant professor at Iowa State University. <br />
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The research also adds insight into middle crust strain transients, temporary stress on surrounding rock that's caused by tectonic motion. <br />
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"By understanding the mechanics of strain transients a little bit better, we eventually hope to get better insight into how they relate to big, catastrophic earthquakes." <br />
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Unlike slow slips events, earthquakes - or stick-slip events - occur when surfaces quickly alternate between sticking to each other and sliding over each other. <br />
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"While earlier studies focused mostly on frictional behavior as an explanation for strain transients we focus in our work on the impact of rheology (how a material flows under stress), especially when it is semi-brittle," said Reber. <br />
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The semi-brittle middle crust can be compared to a candy bar made of nuts and caramel. The nuts represent the brittle rock. The caramel represents the ductile rock. <br />
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Researchers exposed Carbopol, in which the ratio between brittle and ductile parts determines how much stress it can take before being permanently deformed or breaking, to forces created by a simple spring-powered shearing apparatus. Lower yield stress induced the Carbonol to imitate hotter, more viscous rock from deeper in the Earth's crust by making it more ductile; at higher yield stress it imitated cooler, more brittle rock. <br />
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The tests showed viscous deformation and constant creep movement at lower yield stress and slip-stick behavior at higher yield stress. This highlights the importance of a material's often complex properties for determining the manner and speed it will respond to stress. <br />
<br /><u><i><span style="font-size: x-small;"><span style="font-family: inherit;">Provided by: University of Texas at Austin</span></span></i></u><br />
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Scientists simulate Earth's middle crust to understand earthquakes

Researchers have for the first time been able to measure a material's resistance to fracturing from various types of tectonic mot...

Find Your Birthstones And Get Interesting Facts About Them II

gemstones 3:38 PM

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<span style="font-family: Georgia, "Times New Roman", serif;"><a href="https://www.geologyin.com/2015/10/birthstones-meaning-significance-by.html" rel="nofollow" target="_blank">The first part: Birthstones’ Meaning & Significance by Month</a></span><br />
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<span style="color: blue;"><span style="font-family: "georgia" , "times new roman" , serif;">JULY</span></span></h4>
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Rubies, the birthstone of July, are considered the king of gems and represent love, health and wisdom. It was believed wearing a fine red Ruby bestowed good fortune on its owner. A Ruby is the most valuable gemstone and its value increases based on its color and quality.<br />
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<b><span style="font-family: "georgia" , "times new roman" , serif;">Geological Information:</span></b><br />
A ruby is a pink to blood-red colored gemstone, a variety of the mineral corundum (aluminium oxide). The red color is caused mainly by the presence of the element chromium. Its name comes from ruber, Latin for red. Ruby is considered one of the four precious stones, together with sapphire, emerald and diamond.<br />
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<span style="color: blue;"><span style="font-family: "georgia" , "times new roman" , serif;">AUGUST</span></span></h4>
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Peridot, the birthstone of August, carries a lime green color and is one of only a few gemstones that exists in only one color. Peridot is believed to instill power and influence as well as bring the wearer magical powers and properties that protect against nightmares.<br />
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<b><span style="font-family: "georgia" , "times new roman" , serif;">Geological Information:</span></b><br />
Peridot is gem-quality olivine. Olivine is a silicate mineral with the formula of <span style="font-family: "times" , "times new roman" , serif;">(Mg,Fe)<sub>2</sub>SiO<sub>4</sub></span>. Peridot is one of the few gemstones that occur in only one color: an olive-green. The intensity and tint of the green, however, depends on the percentage of iron in the crystal structure, so the color of individual peridot gems can vary from yellow, to olive, to brownish-green.<br />
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<span style="color: blue;"><span style="font-family: "georgia" , "times new roman" , serif;">SEPTEMBER</span></span></h4>
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Sapphire, the birthstone of September, is most desired in its pure, rich blue color but is present in almost every color including pink, yellow and green. In the Middle Ages the gem was believed to protect those close to you from harm and also represented loyalty and trust.<br />
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<span style="color: blue;"><span style="font-family: "georgia" , "times new roman" , serif;">Geological information: </span></span></h4>
Sapphire is a typically blue gemstone variety of the mineral corundum, an aluminium oxide. Although blue is the best-known sapphire color, they occur in other colors, including gray and black, and they can be colorless. A pinkish orange variety of sapphire is called <b>padparadscha</b>. <br />
Formula: <span style="font-family: "times" , "times new roman" , serif;">Al<sub>2</sub>O<sub>3</sub></span><br />
System: Trigonal<br />
Hardness: 9<br />
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<span style="color: blue;"><span style="font-family: "georgia" , "times new roman" , serif;">OCTOBER</span></span></h4>
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October also has two birthstones; Tourmaline and Opal. Tourmaline is a favorite gemstone for many because it’s available in a rainbow of beautiful colors. Opal gemstones are truly unique because each individual gem is adorned with a one-of-a-kind color combination.<br />
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<b><span style="font-family: "georgia" , "times new roman" , serif;">Geological Information:</span></b><br />
Tourmaline is a crystalline boron silicate mineral compounded with elements such as aluminium, iron, magnesium, sodium, lithium, or potassium. Tourmaline is classified as a semi-precious stone and the gemstone comes in a wide variety of colors.<br />
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Opal is a hydrated amorphous form of silica; its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Because of its amorphous character, it is classed as a mineraloid, unlike the other crystalline forms of silica, which are classed as minerals. It is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, marl, and basalt. Opal is the national gemstone of Australia.<br />
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<span style="color: blue;"><span style="font-family: "georgia" , "times new roman" , serif;">NOVEMBER</span></span></h4>
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November birthdays is Topaz symbolizes love and affection, and is said to bestow strength and intelligence on the person who wears it, according to the Old Farmer’s Almanac, and a gift of blue topaz is said to be a promise of love and fidelity.<br />
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<b><span style="font-family: "georgia" , "times new roman" , serif;">Geological Information:</span></b><br />
Topaz is a silicate mineral of aluminium and fluorine with the chemical formula <span style="font-family: "times" , "times new roman" , serif;">Al<sub>2</sub>SiO<sub>4</sub>(F,OH)<sub>2</sub></span>. Topaz crystallizes in the orthorhombic system, and its crystals are mostly prismatic terminated by pyramidal and other faces. It is one of the hardest naturally occurring minerals (Mohs hardness of 8) and is the hardest of any silicate mineral. <br />
<h4 style="text-align: left;">
<span style="color: blue;"><span style="font-family: "georgia" , "times new roman" , serif;">DECEMBER</span></span></h4>
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December birthdays are; Zircon, and Turquoise. Each of these gemstones carries a unique blue tone making it a perfect birthstone for Minnesota’s frigid December winters. Zircon can be found in a variety of colors, but blue is the overwhelming favorite.<br />
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<b><span style="font-family: "georgia" , "times new roman" , serif;">Geological Information:</span></b><br />
<b>Zircon </b>is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is <span style="font-family: "times" , "times new roman" , serif;">ZrSiO<sub>4</sub></span>. The crystal structure of zircon is tetragonal crystal system. The natural color of zircon varies between colorless, yellow-golden, red, brown, blue, and green. Colorless specimens that show gem quality are a popular substitute for diamond and are also known as "Matura diamond".<br />
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<b>Turquoise </b>is an opaque, blue-to-green mineral that is a hydrated phosphate of copper and aluminium, with the chemical formula <span style="font-family: "times" , "times new roman" , serif;">CuAl<sub>6</sub>(PO<sub>4</sub>)<sub>4</sub>(OH)<sub>8</sub>·4H<sub>2</sub>O</span>. It is rare and valuable in finer grades and has been prized as a gem and ornamental stone for thousands of years owing to its unique hue. In recent times, turquoise, like most other opaque gems, has been devalued by the introduction of treatments, imitations, and synthetics onto the market.<br />
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<br />
<span style="font-family: Georgia, "Times New Roman", serif;"><a href="https://www.geologyin.com/2015/10/birthstones-meaning-significance-by.html" target="_blank">The first part: Birthstones’ Meaning & Significance by Month </a></span></div>

Find Your Birthstones And Get Interesting Facts About Them II

The first part: Birthstones’ Meaning & Significance by Month JULY Rubies, the birthstone of July, are considered the king of ...

How long is the interval between the trigger for a volcanic eruption and the eruption itself?

earth, volcano 8:56 PM

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<br />How long is the interval between the trigger for a volcanic eruption and the eruption itself? A new study by LMU volcanologists indicates that compositional variations in erupted Magmas can answer this question. <br /> <br />In the lithosphere beneath an active volcano the magma is never at rest. When melts with different chemical compositions come into contact, the result is often an explosive mixture: the molten rock begins to bubble, and an eruption becomes inevitable. A research team led by Professor Donald Dingwell, Director of the Department of Earth and Environmental Sciences at LMU, and Professor Diego Perugini of the University of Perugia (Italy) has now developed a model which, for the first time, allows one to estimate the time that has elapsed between the initial mixing event and the subsequent eruption. The technique has obvious application to ongoing efforts to enhance the forecasting of volcanic eruptions, as the authors demonstrate in their paper, which appears in "Nature Scientific Reports". <br /><br />The 'foaming' phenomenon is due to the oversaturation of the magma mixture with gases that are released as a result of the mixing process itself. These gases in turn provide the buoyancy that causes the magma to rise. "We know that, very often, the time that elapses from the first encounter between compositionally distinct melts and the ensuing eruption is insufficient to allow complete homogenization of the mixture to occur," says Dingwell. "So the material that is ultimately extruded is still quite heterogeneous. Here, we use this heterogeneity to model the temporal pattern of the mixing process." <br /><br /><b>Simulating eruptions in the laboratory </b><br /><br />With its unique combination of instrumentation, Dingwell's laboratory is ideally equipped to experimentally simulate the conditions of high temperature and high pressure that prevail in the interior of an active volcano. In their latest experiments they used two kinds of samples of erupted material collected in the Campi Flegrei near Naples, one of the most volcanically active areas in Europe, and compared their chemical compositions and mixing ratios. Their results showed that different chemical fractions of the component melts mixed with each other at different rates: Water, for instance, becomes uniformly distributed in mixtures very rapidly, but other substances take much longer to mix thoroughly. "On the basis of the variations in mixing ratios of particular components of the rock between the two samples of erupted material, we can estimate the duration of the period between the initiation of the mixing process and the eruption", Dingwell explains. <br /><br /><b>Little advance warning </b><br /><br />The results indicates that, in the Campi Flegrei, the interval between the onset of magma mixing and eruption is less than an hour – astonishingly short by comparison with previous estimates of several days. "The more studies of this kind are carried out on other volcanic systems, the sooner we should be in a position to estimate the duration of the run-up phase prior to an eruption using the variance of concentration ratios as a volcanic chronometer," says Dingwell. <br /><br />The new method can also contribute to improving the monitoring of active volcanos: Once it is known from studies of erupted material how much time elapsed between initial magma mixing and a given eruption, one can retrospectively identify those events in the monitoring record that represent true precursors of the eruption. "In future, this approach could make it easier to filter the really crucial signals out of the plethora of information picked up by the monitoring system," says Dingwell. "So our model could help us to provide a realistic estimate of the lead time prior to an eruption – a parameter which is essential for the design of appropriate and practical measures to protect the public."<br />
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<u><i><span style="font-size: x-small;"><span style="font-family: inherit;"> Provided by: Ludwig Maximilian University of Munich</span></span></i></u></div>

How long is the interval between the trigger for a volcanic eruption and the eruption itself?

How long is the interval between the trigger for a volcanic eruption and the eruption itself? A new study by LMU volcanologists indicate...

Cooling Climate Caused Crocodilian Retreat

climate, earth, Earth History, ocean 9:52 PM

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Fluctuating sea levels and global 
cooling caused a significant decline in the number of crocodilian 
species over millions of years</h4>
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<dl class="dl-horizontal dl-custom"><dd id="abstract"><i><span style="font-size: small;"><span style="font-family: inherit;">Fluctuating sea levels and global cooling caused a 
significant decline in the number of crocodilian species over millions 
of years, according to new research. In the future, the researchers 
suggest that a warming world caused by global climate change may favour 
crocodylian diversification again.
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Crocodylians include present-day species of crocodiles, alligators, caimans and gavials and their extinct ancestors. Crocodylians first appeared in the Late Cretaceous period, approximately 85 million years ago, and the 250 million year fossil record of their extinct relatives reveals a diverse evolutionary history. <br /><br />Extinct crocodylians and their relatives came in all shapes and sizes, including giant land-based creatures such as Sarcosuchus, which reached around 12 metres in length and weighed up to eight metric tonnes. Crocodylians also roamed the ocean -- for example, thalattosuchians were equipped with flippers and shark-like tails to make them more agile in the sea. <br /><br />Many crocodylians survived the mass extinction that wiped out almost all of the dinosaurs 66 million years ago, but only 23 species survive today, six of which are classified by the International Union for Conservation of Nature as critically endangered and a further four classified as either endangered or vulnerable. <br /><br />In a new study published in Nature Communications, researchers from Imperial College London, the University of Oxford, the Smithsonian Institution and the University of Birmingham compiled a dataset of the entire known fossil record of crocodylians and their extinct relatives and analysed data about Earth's ancient climate. They wanted to explore how the group responded to past shifts in climate, to better understand how the reptiles may cope in the future. <br /><br />Crocodylians are ectotherms, meaning they rely on external heat sources from the environment such as the Sun. The researchers conclude that at higher latitudes in areas we now know as Europe and America, declining temperatures had a major impact on crocodylians and their relatives. <br /><br />At lower latitudes the decline of crocodylians was caused by areas on many continents becoming increasingly arid. For example, in Africa around ten million years ago, the Sahara desert was forming, replacing the vast lush wetlands in which crocodylians thrived. In South America, the rise of the Andes Mountains led to the loss of a proto-Amazonian mega wetland habitat that crocodylians lived in around five million years ago. <br /><br />Marine species of crocodylians were once widespread across the oceans. The team found that fluctuations in sea levels exerted the main control over the diversity of these creatures. For example, at times when the sea level was higher it created greater diversity because it increased the size of the continental shelf, providing the right conditions near the coast for them and their prey to thrive. <br /><br />Interestingly, the Cretaceous-Paleogene mass extinction event, which wiped out many other creatures on Earth nearly 66 million years ago including nearly all of the dinosaurs, had positive outcomes for the crocodylians and their extinct relatives. The team found that while several groups did go extinct, the surviving groups rapidly radiated out of their usual habitats to take advantage of territories that were now uninhabited. <br /><br />In the future, the team suggest that a warming world caused by global climate change may favour crocodylian diversification again, but human activity will continue to have a major impact on their habitats. <br /><br />Dr Philip Mannion, joint lead author from the Department of Earth Science and Engineering at Imperial College London, said: "Crocodylians are known by some as living fossils because they've been around since the time of the dinosaurs. Millions of years ago these creatures and their now extinct relatives thrived in a range of environments that ranged from the tropics, to northern latitudes and even deep in the ocean. However, all this changed because of changes in the climate, and crocodylians retreated to the warmer parts of the world. While they have a fearsome reputation, these creatures are vulnerable and looking back in time we've been able to determine what environmental factors had the greatest impact on them. This may help us to determine how they will cope with future changes." <br /><br />The next step for the researchers will be for them to look at similar patterns in other fossil groups with long histories, such as mammals and birds to determine how past climate influenced them. </div>
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<u><i><span style="font-size: x-small;"><span style="font-family: inherit;">Philip D. Mannion, Roger B. J. Benson, Matthew T. Carrano, Jonathan P. Tennant, Jack Judd, Richard J. Butler. <strong>Climate constrains the evolutionary history and biodiversity of crocodylians</strong>. <em>Nature Communications</em>, 2015; 6: 8438 DOI: <a href="http://dx.doi.org/10.1038/ncomms9438" rel="nofollow" target="_blank">10.1038/ncomms9438</a>
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Cooling Climate Caused Crocodilian Retreat

Fluctuating sea levels and global cooling caused a significant decline in the number of crocodilian species over millions of years...

Fluorite on Quartz

Minerals 9:23 PM

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<b>Fluorite | ‪#‎Geology</b><br />
Formula: CaF<sub>2</sub><br />
System: Isometric<br />
Hardness: 4<br />
Colour: various Purple, lilac, ...<br />
Lustre: Vitreous, Dull <br />
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From De'an Mine, Wushan, Jiangxi Province, China<br />
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Fluorite on Quartz

How fossil corals can shed light on Earth's past climate

climate, Earth History 9:23 PM

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<i> Researchers have used radiocarbon measured in deep-sea fossil corals to shed light on carbon dioxide levels during Earth's last deglaciation. Fossil corals have the unique advantage that they can be precisely dated by radiometric uranium-series dating, giving an age scale that can be directly compared to the ice core records.</i></div>
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<span style="font-family: inherit;"><span style="font-size: small;">Around 18,000-11,000 years ago, Earth's climate system experienced a dramatic shift: a period known to paleoclimate scientists as the last deglaciation. During this period, atmospheric CO<sub style="border-radius: 0px ! important; bottom: -0.25em; box-sizing: border-box; line-height: 0; position: relative; vertical-align: baseline;">2</sub><span class="Apple-converted-space"> </span>concentration increased by ~80 parts per million (ppm), accompanied by sea level rise of almost 120 metres due to ice sheet melting and global warming.</span></span></div>
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<span style="font-family: inherit;"><span style="font-size: small;">Recent high-resolution ice core CO<sub style="border-radius: 0px ! important; bottom: -0.25em; box-sizing: border-box; line-height: 0; position: relative; vertical-align: baseline;">2</sub><span class="Apple-converted-space"> </span>records have revealed that there were three abrupt centennial-scale atmospheric CO<sub style="border-radius: 0px ! important; bottom: -0.25em; box-sizing: border-box; line-height: 0; position: relative; vertical-align: baseline;">2</sub><span class="Apple-converted-space"> </span>increases of ~10 ppm superimposed on the more gradual millennial-scale deglacial CO<sub style="border-radius: 0px ! important; bottom: -0.25em; box-sizing: border-box; line-height: 0; position: relative; vertical-align: baseline;">2</sub><span class="Apple-converted-space"> </span>rise. The second and third of these events also coincided with abrupt warming of the high latitude North Atlantic region.</span></span></div>
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<span style="font-family: inherit;"><span style="font-size: small;">The rate of Atlantic Meridional Overturning Circulation -- that is, the deep water formation in the high latitudes and associated upwelling -- is closely related to the temperature of the North Atlantic region and thus might also be related to these CO<sub style="border-radius: 0px ! important; bottom: -0.25em; box-sizing: border-box; line-height: 0; position: relative; vertical-align: baseline;">2</sub><span class="Apple-converted-space"> </span>releasing events. However it has been remarkably hard to find marine archives that can show how deep oceans behave on rapid timescales.</span></span></div>
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<span style="font-family: inherit;"><span style="font-size: small;">Researchers from the University of Bristol, University of St Andrews and University of California Irvine tackled this problem using radiocarbon measured in deep-sea fossil corals. The corals were recovered by scientific research expeditions to the Equatorial Atlantic and Southern Ocean, funded by the European Research Council and the US National Science Foundation.</span></span></div>
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<span style="font-family: inherit;"><span style="font-size: small;">Fossil corals have the unique advantage that they can be precisely dated by radiometric uranium-series dating, giving an age scale that can be directly compared to the ice core records. Radiocarbon is introduced into the ocean at the surface and penetrates to deeper layers through deep water formation. During this process radiocarbon decays away, so that deep-sea radiocarbon -- and, therefore, the reconstructed fossil coral radiocarbon -- can provide information on the past strength of deep ocean circulation.</span></span></div>
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<span style="font-family: inherit;"><span style="font-size: small;">The measurements revealed two massive transient events where the water becomes homogenized and enriched in radiocarbon in the mid-depth equatorial Atlantic and the Drake Passage, in phase with the second two abrupt increases of the atmosphere CO<sub style="border-radius: 0px ! important; bottom: -0.25em; box-sizing: border-box; line-height: 0; position: relative; vertical-align: baseline;">2</sub><span class="Apple-converted-space"> </span>concentration during the last deglaciation.</span></span></div>
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<span style="font-family: inherit;"><span style="font-size: small;">Lead author, Dr Tianyu Chen of Bristol's School of Earth Sciences said: "Our radiocarbon data are consistent with two transient and enhanced deep Atlantic overturning events which flushed out respired carbon in the deep water, causing a rapid rise of atmosphere CO<sub style="border-radius: 0px ! important; bottom: -0.25em; box-sizing: border-box; line-height: 0; position: relative; vertical-align: baseline;">2</sub><span class="Apple-converted-space"> </span>concentration and abrupt warming of the high latitude North Atlantic."</span></span></div>
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<i><u><span style="font-size: x-small;"><span style="font-family: inherit;">The above post is reprinted from<span class="Apple-converted-space"> </span><a href="http://www.bristol.ac.uk/news/2015/september/fossil-corals-and-past-climate.html" rel="nofollow" style="background-color: transparent; border-radius: 0px ! important; box-sizing: border-box; color: #4c7a9f; text-decoration: none;" target="_blank">materials</a><span class="Apple-converted-space"> </span>provided by<span class="Apple-converted-space"> </span><a href="http://www.bristol.ac.uk/" rel="nofollow" style="background-color: transparent; border-radius: 0px ! important; box-sizing: border-box; color: #4c7a9f; text-decoration: none;" target="_blank"><strong style="border-radius: 0px ! important; box-sizing: border-box; font-weight: bold;">University of Bristol</strong></a>.<span class="Apple-converted-space"> </span><em style="border-radius: 0px ! important; box-sizing: border-box;">Note: Materials may be edited for content and length.</em></span></span></u></i></div>
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How fossil corals can shed light on Earth's past climate

Researchers have used radiocarbon measured in deep-sea fossil corals to shed light on carbon dioxide levels during Earth's la...

Meteorite Bashing Changed Earth's Chemistry

earth, Geochemistry, Meteorite 8:56 PM

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<tr><td class="tr-caption" style="text-align: center;"> This artist rendering handout by the CNRS on Wednesday shows "Collision of a rock body with the early Earth, causing pulverization of terrestrial crust" </td></tr>
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In its early life, Earth suffered a meteorite pummelling that lasted 100 million years and may have changed its chemical makeup forever, researchers said Wednesday. <br />
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The steady stream of Earth-shattering collisions shortly after the birth of our solar system, ripped up the planet's surface and altered the very composition of the rock we call home. <br />
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At the same time, those ancient meteorites called chondrites were adding to the Earth's bulk, which like all planets form through a process of accretion—where material is pulled in by gravity, according to the study published in the journal Nature Communications. <br />
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While the Earth lost chunks as the meteorites crashed into it, it nevertheless experienced a net gain in mass during this violent period. <br />
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The finding is the latest in a long scientific quest to resolve a puzzle over the makeup of meteorites—building blocks of early Earth—and the chemical signature of our present-day planet. <br />
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Embryonic Earth some 4.7 billion years ago, the new research suggests, did in fact have the same chemical mix as at least one type of chondrite (called enstatite). <br />
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But the extended bombardment changed the chemical signature of the planet itself. <br />
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"Repeated episodes of production and erosion of the terrestrial crust removed large quantities of silicon, leaving the relative excess of magnesium we observe today," the researchers concluded. <br />
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The crust is the outermost layer of Earth, a thin layer over the mantle, which itself covers the planet's outer and inner core. <br />
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If scientists could extract samples from its deep layers, they might have a better idea of what a proto-Earth was made of. <br />
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But they have no choice but to rely on other evidence—including meteorites. <br />
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For the study, Asmaa Boujibar and colleagues at France's National Centre for Scientific Research (CNRS) used lab experiments and modelling to test their theory. <br />
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They reproduced the conditions under which Earth's primitive crust accumulated, by melting chondrites under various pressures. <br />
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The results, the study concludes, explains the makeup of the molten rock that eventually cooled into Earth's crust.<br />
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<br />Journal reference: Nature Communications </div>

Meteorite Bashing Changed Earth's Chemistry

This artist rendering handout by the CNRS on Wednesday shows "Collision of a rock body with the early Earth, causing pulverization ...

The Largest Black Diamond Ever Found

Diamonds, famous, Minerals 5:59 PM

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<a href="https://2.bp.blogspot.com/-oCUZ5u2frRU/WuIFVrVvecI/AAAAAAAAPvo/YRIlcc-QkjoFJ5Xa70N7kHy-qOfd0ZTnACLcBGAs/s1600/the-largest-black-diamond-ever-found%2B%25281%2529%2B%25281%2529.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img alt="The Largest Black Diamond Ever Found" border="0" data-original-height="921" data-original-width="1274" height="462" src="https://2.bp.blogspot.com/-oCUZ5u2frRU/WuIFVrVvecI/AAAAAAAAPvo/YRIlcc-QkjoFJ5Xa70N7kHy-qOfd0ZTnACLcBGAs/s640/the-largest-black-diamond-ever-found%2B%25281%2529%2B%25281%2529.jpg" title="" width="640" /></a></div>
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<span data-offset-key="cn0ql-0-0" data-reactid=".1c6.1:4.0.$right.0.0.0.0.1.0.0.$editor0.0.0.$cn0ql.0:$cn0ql-0-0"><span data-reactid=".1c6.1:4.0.$right.0.0.0.0.1.0.0.$editor0.0.0.$cn0ql.0:$cn0ql-0-0.0">Sergio is the largest carbonado and the largest rough diamond ever found. It weighs 3167 carats and was found in the State of Bahia in Brazil in 1895. Like other carbonados it is believed to be of meteoritic origin.</span></span><br />
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<span data-offset-key="cn0ql-0-0" data-reactid=".1c6.1:4.0.$right.0.0.0.0.1.0.0.$editor0.0.0.$cn0ql.0:$cn0ql-0-0"><span data-reactid=".1c6.1:4.0.$right.0.0.0.0.1.0.0.$editor0.0.0.$cn0ql.0:$cn0ql-0-0.0">Black diamonds are different from other coloured rocks because they do not get their shade from chemical impurities, such as nitrogen, hydrogen or boron. Rather, black diamonds owe their colour to numerous dark inclusions (mostly graphite), and their opaqueness is caused by a “polycrystalline” structure that inhibits the reflection of light.</span></span></div>

The Largest Black Diamond Ever Found

Sergio is the largest carbonado and the largest rough diamond ever found. It weighs 3167 carats and was found in the State of Bahia...

The Diamond Cutting Process

Diamonds, gemstones, Mineralogy 9:54 PM

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<tr><td class="tr-caption" style="text-align: center;">World's Largest Pink Diamond Rough and Cut</td></tr>
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<b> </b><span style="font-size: small;"><span style="font-family: inherit;"><b>Diamond </b>cutting is the practice of changing a diamond from a rough stone into a faceted gem. Cutting diamond requires specialized knowledge, tools, equipment, and techniques because of its extreme difficulty.</span></span></div>
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<u><span style="font-family: "georgia" , "times new roman" , serif;"> The diamond cutting process includes these steps; planning, cleaving or sawing, bruting, polishing, and final inspection.</span></u></h4>
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<span style="color: blue;"><span style="font-family: "georgia" , "times new roman" , serif;"> Planning</span></span></h4>
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<tr><td style="text-align: center;"><a href="https://4.bp.blogspot.com/-fam5F94qhnk/WrqR_VGrjSI/AAAAAAAAPaw/v9OEYOErfu0JRohJa_pdtZCo5LJhY1qbgCLcBGAs/s1600/diamond-cutting-illustration%2B%25281%2529.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img alt="The Diamond Cutting Process" border="0" data-original-height="218" data-original-width="595" height="145" src="https://4.bp.blogspot.com/-fam5F94qhnk/WrqR_VGrjSI/AAAAAAAAPaw/v9OEYOErfu0JRohJa_pdtZCo5LJhY1qbgCLcBGAs/s400/diamond-cutting-illustration%2B%25281%2529.jpg" title="" width="400" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">(left to right) Rough Stone, Ideal Cut Diamond, Standard Cut Diamond</td></tr>
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<span style="font-size: small;"><span style="font-family: inherit;">Diamond manufacturers analyze diamond rough from an economic perspective, with two objectives steering decisions made about how a faceted diamond will be cut. The first objective is that of maximum return on investment for the piece of diamond rough. The second is how quickly the finished diamond can be sold. Scanning devices are used to get a 3-dimensional computer model of the rough stone. Also, inclusions are photographed and placed on the 3D model, which is then used to find an optimal way to cut the stone.</span></span><br />
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<span style="color: blue;"><span style="font-family: Georgia, "Times New Roman", serif;"> Maximizing value</span></span></h4>
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<span style="font-size: small;"><span style="font-family: inherit;">The process of maximizing the value of finished diamonds, from a rough diamond into a polished gemstone, is both an art and a science. The choice of cut is influenced by many factors. Market factors include the exponential increase in value of diamonds as weight increases, referred to as weight retention, and the popularity of certain shapes amongst consumers. Physical factors include the original shape of the rough stone, and location of the inclusions and flaws to be eliminated.</span></span></div>
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<span style="color: blue;"><span style="font-family: Georgia, "Times New Roman", serif;">Weight retention</span></span></h4>
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<span style="font-size: small;"><span style="font-family: inherit;"> The weight retention analysis studies the diamond rough to find the best combination of finished stones as it relates to per carat value. For instance, a 2.20 carat (440 mg) octahedron may produce (i) either two half carat (100 mg) diamonds whose combined value may be higher than that of (ii) a 0.80 carat (160 mg) diamond + 0.30 carat (60 mg) diamond that could be cut from the same rough diamond.<br />
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The round brilliant cut and square brilliant cuts are preferred when the crystal is an octahedron, as often two stones may be cut from one such crystal. Oddly shaped crystals, such as macles are more likely to be cut in a fancy cut—that is, a cut other than the round brilliant—which the particular crystal shape lends itself to.<br />
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Even with modern techniques, the cutting and polishing of a diamond crystal always results in a dramatic loss of weight, about 50%. Sometimes the cutters compromise and accept lesser proportions and symmetry in order to avoid inclusions or to preserve the weight. Since the per-carat price of a diamond shifts around key milestones (such as 1.00 carat), many one-carat (200 mg) diamonds are the result of compromising Cut quality for Carat weight.</span></span></div>
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<span style="color: blue;"><span style="font-family: Georgia, "Times New Roman", serif;"> Color retention</span></span></h4>
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<a href="https://4.bp.blogspot.com/-Kw7Bldqesv8/WrqUTA-D97I/AAAAAAAAPbE/fytgDE86iIEpSgW26FQB28jKYNJVFzmLACLcBGAs/s1600/Diamond%2BCuts%2B%25281%2529.jpg" imageanchor="1" style="clear: right; float: right; margin-bottom: 1em; margin-left: 1em;"><img alt=" Color retention" border="0" data-original-height="471" data-original-width="450" height="320" src="https://4.bp.blogspot.com/-Kw7Bldqesv8/WrqUTA-D97I/AAAAAAAAPbE/fytgDE86iIEpSgW26FQB28jKYNJVFzmLACLcBGAs/s320/Diamond%2BCuts%2B%25281%2529.jpg" title="" width="304" /></a></div>
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<span style="font-size: small;"><span style="font-family: inherit;">In colored diamonds, cutting can influence the color grade of the diamond, thereby raising its value. Certain cut shapes are used to intensify the color of the diamond. The radiant cut is an example of this type of cut.<br />
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Natural green color diamonds most often have merely a surface coloration caused by natural irradiation, which does not extend through the stone. For this reason green diamonds are cut with significant portions of the original rough diamond's surface (naturals) left on the finished gem. It is these naturals that provide the color to the diamond.</span></span></div>
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<span style="color: blue;"><span style="font-family: Georgia, "Times New Roman", serif;"> Turnaround Minimization</span></span></h4>
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<span style="font-size: small;"><span style="font-family: inherit;"> The other consideration of diamond planning is how quickly a diamond will sell. This consideration is often unique to the type of manufacturer. While a certain cutting plan may yield a better value, a different plan may yield diamonds that will sell sooner, providing an earlier return on the investment.</span></span></div>
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<span style="color: blue;"><span style="font-family: Georgia, "Times New Roman", serif;"> Cleaving or sawing</span></span></h4>
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<span style="font-size: small;"><span style="font-family: inherit;"> Cleaving is the separation of a piece of diamond rough into separate pieces, to be finished as separate gems.<br />
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<b>Sawing </b>is the use of a diamond saw or laser to cut the diamond rough into separate pieces.</span></span></div>
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<span style="color: blue;"><span style="font-family: Georgia, "Times New Roman", serif;"> Bruting</span></span></h4>
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<span style="font-size: small;"><span style="font-family: inherit;"> Bruting is the process whereby two diamonds are set onto spinning axles turning in opposite directions, which are then set to grind against each other to shape each diamond into a round shape. This can also be known as girdling.</span></span></div>
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<span style="color: blue;"><span style="font-family: Georgia, "Times New Roman", serif;"> Polishing</span></span></h4>
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<span style="font-size: small;"><span style="font-family: inherit;"> Polishing is the name given to process whereby the facets are cut onto the diamond and final polishing is performed. The process takes the steps blocking, faceting, also called "brillianteering", and polishing.<br />
Final inspection<br />
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The final stage involves thoroughly cleaning the diamond in acids, and examining the diamond to see whether it meets the quality standards of the manufacturer.</span></span></div>
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<span style="color: blue;"><span style="font-family: Georgia, "Times New Roman", serif;"> Cutting process</span></span></h4>
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<span style="font-size: small;"><span style="font-family: inherit;"> It is possible only because the hardness of diamond varies widely according to the direction in which one is trying to cut or grind.</span></span></div>
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<tr><td style="text-align: center;"><a href="https://4.bp.blogspot.com/-_syi-AKN4Kc/WrqTXwTl5hI/AAAAAAAAPa8/SyVdGqY0awEQlxXXh3AGQN5fqGbDErlZQCLcBGAs/s1600/Cutting%2BDiamonds%2BIsdiamonds%2B%25281%2529.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img alt="Cutting Diamonds Isdiamonds" border="0" data-original-height="620" data-original-width="620" height="320" src="https://4.bp.blogspot.com/-_syi-AKN4Kc/WrqTXwTl5hI/AAAAAAAAPa8/SyVdGqY0awEQlxXXh3AGQN5fqGbDErlZQCLcBGAs/s320/Cutting%2BDiamonds%2BIsdiamonds%2B%25281%2529.jpg" title="" width="320" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Cutting Diamonds Isdiamonds</td></tr>
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<span style="font-family: Georgia, "Times New Roman", serif;"><br /></span><h4>
<span style="color: blue;"><span style="font-family: Georgia, "Times New Roman", serif;"> A simplified round brilliant cut process includes the following stages:</span></span></h4>
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<li><span style="font-size: small;"><span style="font-family: inherit;"> Sawing the rough stone.</span></span></li>
<li><span style="font-size: small;"><span style="font-family: inherit;"> Table setting where one facet is created. The table facet is then used to attach the stone into a dop (a lapidary tool holding gemstones for cutting or polishing).</span></span></li>
<li><span style="font-size: small;"><span style="font-family: inherit;"> Bruting the girdle.</span></span></li>
<li><span style="font-size: small;"><span style="font-family: inherit;"> Blocking four main pavilion facets.</span></span></li>
<li><span style="font-size: small;"><span style="font-family: inherit;"> Transferring to another dop in order to rotate the stone.</span></span></li>
<li><span style="font-size: small;"><span style="font-family: inherit;"> Blocking four main crown facets.</span></span></li>
<li><span style="font-size: small;"><span style="font-family: inherit;"> Cutting and polishing all pavilion facets.</span></span></li>
<li><span style="font-size: small;"><span style="font-family: inherit;"> Transferring to another dop.</span></span></li>
<li><span style="font-size: small;"><span style="font-family: inherit;"> Cutting and polishing all crown facets.</span></span></li>
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<span style="font-size: small;"><span style="font-family: inherit;"> <br />
This is just one, although a fairly common way of creating a round brilliant cut. The actual process also includes many more stages depending on the size and quality of the rough stone. For example, bigger stones are first scanned to get the three-dimensional shape, which is then used to find the optimal usage. The scanning may be repeated after each stage and bruting may be done in several steps, each bringing the girdle closer to the final shape.</span></span><br />
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</span></span><span style="font-size: small;"><span style="font-family: inherit;"><i> </i></span></span><span style="font-size: small;"><span style="font-family: inherit;"><i> </i></span></span><br />
<span style="color: blue;"><b><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: small;"><span style="font-family: inherit;">See also: </span></span></span></b></span><br />
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: small;"><span style="font-family: inherit;"><a href="http://www.geologyin.com/2018/03/garnet-group-colors-and-varieties-of.html" target="_blank">Garnet Group: The Colors and Varieties of Garnet</a></span></span></span><br />
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: small;"><span style="font-family: inherit;"><a href="http://www.geologyin.com/2017/12/how-does-trapiche-emerald-form.html" target="_blank">How Does a Trapiche Emerald Form?</a></span></span></span><br />
<span style="font-size: small;"><span style="font-family: inherit;"><span style="font-family: "georgia" , "times new roman" , serif;"><a href="http://www.geologyin.com/2017/06/what-is-petoskey-stone-and-where-can.html" target="_blank">What Is Petoskey Stone, and Where Can You Find It?</a></span><br />
</span></span><span style="font-size: small;"><span style="font-family: inherit;"><i> </i></span></span><span style="font-size: small;"><span style="font-family: inherit;"><i> </i></span></span></div>
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The Diamond Cutting Process

World's Largest Pink Diamond Rough and Cut Diamond cutting is the practice of changing a diamond from a rough stone into a fac...

Rifts: Plate Divergence in Continental Setting

edu, Tectonic, Terminology 9:08 PM

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<br />Rifts: Plate divergence in continental setting<br /><br /><b>A rift </b>is a linear zone where the Earth's crust and lithosphere are being pulled apart and is an example of extensional tectonics.<br /><br />The early stages of continental break-up are accomplished by normal faulting and produce rift systems.<br /><br /><div class="Default">
<span style="font-size: small;"><span style="font-family: inherit;">Active rifts are long and narrow depressions on the earth crust; they are characterised by seismic activity down to depths of ca. 15 km, high heat flow, and topographically elevated shoulders due to flexural isostatic compensation of the lithosphere. Some of the best examples comprise the Red-Sea, the African Great Rift Valley and the Gulf of Aden that meet in a triple junction in the Afar region. Rifting produces long and linear crustal depressions that are sites of extensive sediment accumulation and whose characteristics are: </span></span></div>
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<span style="font-size: small;"><span style="font-family: inherit;">- A relatively narrow width (30-60 km), which is about the thickness of the rifted continental crust, irrespective of the length of the rift valley. Since active rift zones are initially elevated above the sea level, <b>syn-rift sediments </b>are often continental or lagoonal and strongly influenced by syn-rift volcanism in places. Huge down-dropped blocks are sites of continental basins, which are filled by thick (> 1 km) clastic debris derived from adjacent high-standing blocks and deposited in alluvial fans. These sediments impart the relatively flat topography of the rift valley although they are deposited within an irregular pattern of partial deposition centers. Major lakes and rivers are common in this central depression. </span></span></div>
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<span style="font-size: small;"><span style="font-family: inherit;">- Steep and subparallel flanks are 3–5 km higher than the rift floor; this outstanding landscape feature results from cumulated normal-fault scarps, with the faults stepping down towards the central lowland. Some of these faults are very long but most of them relay each other, occasionally in an <i>en échelon </i>manner. Shoulder uplift arises from the mechanical unloading of the footwall block. Isostasy calculations indicate that the total relief of a rift wall is about 2.25 times the uplift. The amplitude of the uplift as well as the width of the rift flank uplift is determined by the elastic thickness of the lithosphere Te. The width of the shoulders varies from 80 km for Te=15 km (Rhine Graben) to 200 km for Te =50km (Baikal). </span></span></div>
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<span style="font-size: small;"><span style="font-family: inherit;">- Rifts are not equally deep all along their length. Thinning of the continental crust usually occurs on a series of sub-planar to listric faults with a throw of several km. Rifts bounded by such serialized faults are cut into segments and comprise an assemblage of half-grabens of varying polarity, arranged end-to-end or side-by-side, and linked through cross-rift accommodation zones. </span></span><br />
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<span style="font-size: small;"><span style="font-family: inherit;">The dimension of the half-graben is determined primarily by the thickness of the seismogenic brittle crust, and is typically 25-100 km long and 20-50 km wide. Fault-bounded, long and narrow basins develop on the rift-parallel extensional blocks that rotate about a horizontal axis as the rift zone widens. </span></span><br />
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<span style="font-size: small;"><span style="font-family: inherit;">The drainage patterns determine sediment input into the half-grabens. Their sedimentary fill is asymmetric, with coarse terrigenous sediments in aprons and fans rimming the fault escarpment and finer-grained sediments depositing on the moderately dipping counter-slope. Polarity reversals in half-grabens, due to the development of new faults as old systems become inactive, may complicate sediment distributions. These descriptions imply that many rifts are asymmetric, with one margin being higher than the other and the sub-sediment floor being tilted. </span></span></div>
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Episodic pulses of extension create space for sediment accumulation at very fast rates. Stages of rapid mechanical subsidence are typically followed by periods of relative tectonic quiescence, when sediment supply fills the available space (called <b>accommodation</b>). Syn-sedimentary tectonics (block-tilting with sedimentary fanning, angular unconformities, stratigraphic gaps, etc) are very active.<br />
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Alluvial and fluvial early-rift sediments pass upward into shallow and deep lacustrine facies (conglomerates, sandstones, coal, fine-grained mud) where intermittent lakes occur along the axis of the basin. As the rift widens, its floor further subsides and eventually sinks at sea level. At this stage, the sea that covers the basin is very shallow and may dry up. Evaporites precipitate in this environment. With continued extension the basin broadens and deepens, thereby becoming a narrow ocean in which marine strata bury the evaporites.<br />
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<span style="font-size: small;"><span style="font-family: inherit;"><b>Normal faults</b> accommodating continental crustal pull-apart and accompanied by parallel dyke swarms and outpouring flows of both tholeiitic and alkaline basalts. Mounts Kenya and Kilimandjaro are big volcanoes that exemplify this magmatism. Partial melting of the granitic crust may produce rhyolitic magma. </span></span><br />
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<span style="font-size: small;"><span style="font-family: inherit;">The bimodal association of acid and basic volcanic rocks is characteristic of within-continent rift systems. In fact, magmatic rocks show a great range and also include extreme compositions that are rarely found outside rifts such as extremely sodic and potassic rocks, often rich in carbonate. The carbonatite complexes of East Africa are considered to be the subvolcanic equivalents of such unusual volcanoes, which erupt sodium carbonate lavas and ash (Oldoinyo Lengai, in Tanzania). </span></span></div>
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- Post-rift, thermal subsidence is initiated by cooling of the asthenospheric material that was upwelled to lithospheric levels during rifting. The <b>post-rift sediments</b> are mostly marine, pelagic and poor in terrigenous material. The thermal subsidence seizes wide zones beyond the rift zone itself, but the syn-rift rugged morphology of the basin may be still reflected by various types of sediments.<br />
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The relative elevations - pelagic swells are sediment-starved with typical condensed facies, sedimentary gaps, neptunian dikes etc. The basins are marked by hemipelagic and eupelagic sedimentation, discontinuously affected by resedimentation events bringing material from nearby elevations.</div>
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Rifts: Plate Divergence in Continental Setting

Rifts: Plate divergence in continental setting A rift is a linear zone where the Earth's crust and lithosphere are being pulled ...

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