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Archive for December, 2009

Porphyroblasts

A porphyroblast is a texture in many metamorphic rocks that have certain minerals that are larger than the rest of the rock matrix in grain size. The large grains are known as porphyroblasts and the rock has a porphyroblastic texture. Many porphyroblasts form in pelitic rocks. Some of these minerals are garnet, staurolite, cordierite, kyanite, sillimanite, and andalusite. In general the higher the metamorphic grade the larger the porphyroblasts can become. Different minerals form at different temperatures and pressures so rocks such as quartzite and marble that only contain one mineral do not form prophyroblasts.

Garnet porphyroblasts in a pelitic rock.

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Volcanic Necks

Volcanic necks form from feeder pipes that were once inside volcanoes, but once the volcano goes extinct and is in an erosional environment, the volcano weathers away but the neck remains. They are intrusive bodies, but near the surface the morphology of the necks can be cylindrical. At depth necks can become elliptical or even clover shaped. At great depths they can merge into several feeder dikes.

Devil’s Tower, Wyoming

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Flood Basalts

Flood basalts usually erupt in zones of crustal extension, this typically occurs along mid ocean ridges but can form on continental crust. Large flood basalts are called Large Ingneous Provinces (LIP). Flood basalts can be as large as millions of square kilometers in area. The flood basalts  form flat layers that mark each single eruptive event. Some ancient flood basalts are the Deccan traps in India that are from the Creataceous and they are believed to have some significance of the K-T extinction event. The Deccan traps are about 500,000 square kilometers in area and about 600 meters thick. The Miocene Columbia River basalts in the state of Washington are about 200,000 kilometers squared and about 1500 meters thick. Another large flood basalt that could have had some affect during the Permian extinction are the Siberian traps in Siberia. They range from 1,000,000 to 4,000,000 cubic kilometers in volumes. the flood basalts are usually dominated by randomly oriented plagioclase, pyroxene, and olivine grains.

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The study of progressive metamorphism began in the early 1900’s in the Scottish Highlands. George Barrow was mapping the geology of the area and he realized that there were index minerals that characterized each zone. If he was in the lower grade metamorphic zones ceratin minerals were no longer present. The zones that were closest to the principle stress had the highest grade of metamorphic minerals and the further away from there shows lower metamorphic grade. each mineral zone is separated by a boundary called an isograd. Barrow mapped each zone in the order of going from low grade to high grade starting with the chlorite zone –>biotite –>garnet –> kyanite –>sillimanite. These are now referred to as Barrow’s Zones.

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metamorphic facies are a group of minerals that form at certain pressure and temperature conditions. For example if the minerals in the rock formed at pressures greater than 1 GPa and 500 degrees C, then this mineral assemblage will fall into the eclogite facies. mineral assemblages that undergo high presures but lower temperatures such as at convergent plate margins are known as the bluescist facies. At even lower temperatures is the greeenschist facies. Some of the minerals that fall into the eclogite facies are garnet and orthopyroxene. Some of the minerals in blueschist facies can be kyanite and gluacophane, and some minerals in the greenschist facies can be chlorite, epidote, and actinolite.

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UPJ geology students….We need to raise money to get our asses to Iceland next summer. So start raising some money….even if we have to panhandle.

The volcanic rocks of the southern Laxardalsfjoll range in the northern Snaefellsnes rift are predominantly Tertiary tholeiitic basalt flows. Major and trace element data for these basalts, when compared to typical primary N-MORB, indicate they are evolved.

     

The plagioclase phyric andesitic unit (Strjugsskard andesite) contains up to 35% euhedral plagioclase phenocrysts ranging up to several centimeters in diameter. The groundmass is generally dark gray to purple near the bottom of the flow, becoming more red toward the top.

See figure 2 in text.

http://keckgeology.org/files/pdf/symvol/18th/iceland/schuyler.pdf

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This isn’t the creme de la creme of geology papers in my opinion, but i enjoyed it regardless.

Lava flows and sills of basaltic habit make up the bulk of St. Paul island. Some individual flows seem to merge into each other, but in many places they can be distinguished and thicknesses from 10 to 7 yards were measured; still thicker flows probably exists. Peridotite is probably the most interesting rock in St. George island. It is overlain by sills and lava flows.  Aplite is intrusive into the peridotite and composite in grained granular central portion.

Here is the link.  http://response.restoration.noaa.gov/book_shelf/535_prib5.pdf

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