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Lecture 12

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Earth & Planetary Sciences
EPSC 201
Anthony Williams- Jones

EPSC201 - Lecture 12 Notes What is a rock? It is an assembly of minerals. It could be many minerals, or only one mineral. Com- monly, rocks have about 5-10 different minerals. Granite is a rock with a particular assembly of miner- als. At hardware stores, they sell different types of “granite” countertops, but they are not actually gran- ite. Granite is usually light, but depending on impuri- ties, it can take other colors. Potassium feldspar in granite gives a pink color. If the granite is white, the impurity could be albite. (al- bite = variety of feldspar). So the color allows us to identify types of minerals present in the granite. Glassy mineral is quartz. Biotite – a flaky mineral, formed from sheet silicate. The sheets make it easy to peel Amphibole – a dark mineral formed from chain silicate, that does NOT flake like biotite. 3 types of rocks These rocks are formed through the rock cycle. They are all related to each other. Igneous Rocks – igneous means fire. These rocks originate from hot molten material. Basalt is an example of igneous rock because it formed from lava flowing out of the earth. Granite is also an igneous rock. These rocks are formed from a liquid (magma). The process of forming a rock from a liquid is called crystallization. These crystals clump together to make rocks. Igneous rocks are further divided into two classes. Plutonic Rocks – magma that inject into the earth’s crust, but do NOT make it to the sur- face of the earth. So igneous rocks that form underground. The magma that almost makes it to the surface is called hyperbisil. The question is at what point does magma become hyperbisil, and the answer is arbitrary. Volcanic – fine grained igneous rocks caused by rapid cooling. Not enough time to form large crystals because the lava cools quickly. Intrusive – plutonic, magma stays below the earths surface Extrusive – volcanic, magma comes out of the earth on to the surface Sedimentary Rocks – once igneous rocks are on the surface, they are exposed to weathering. Chemical and physical weathering breaks down the rocks. Water breaks down rocks. As the rocks are broken down into smaller pieces, they form sediment. If the sediment is buried, it is turned into rock. Metamorphic Rocks – if the sedimentary rock is buried deep enough into the earth, or heated enough, it is changed into a metamorphic rock. This heating changes the mineral composition. Igneous rocks are at the beginning of the cycle. These rocks weather and compact into sedimen- tary rocks. If this sedimentary rock are heated and buried deep enough, they turn into metamor- phic rock. If the metamorphic rock is buried deep enough, it melts into magma, and will later form into igneous rocks. Hot lava is not very viscous material, so it flows like a river on the surface. As it cools, the lava solidifies and forms a crust. It turns into a dark black material instead of the orange hot lava. The surface of the lava cools quickly and forms a skin. As the liquid under the skin moves, the skin folds. This type of lava is called pahoehoe. This means ropy lava, and is a Hawaiian term. Hot lava = low viscosity (flows easily) Glass hairs form on the surface of the lava. The wind causes little ripples in the lava, and stretch- es it out, forming Paley’s Hairs. Aa lava – blocky lava, Hawaiian term. It moves very slowly, so lots of time to escape it. Pahoe- hoe lava flows about 15 km/h whereas aa lava flows meters per day. Even though the two magmas were basalt, they formed two completely different lavas. The aa was viscous, and the pahoehoe lava flowed quite easily. Lavas have internal structures. Little crystals are the starting point. Viscosity can be related to these small crystals. Viscosity – related to composition of lava Lava with only little silica tetrahedron will flow very quickly. Lava with long chains will flow much slower. A rock with lots of quartz and feldspar will have a high viscosity, because of the chain like structure of these minerals. A rock with olivine will have lots of tetrahedrons, and flow very quick- ly. Rhyolite – a igneous, volcanic rock made up of quartz, albite, potassium feldspar Plutonic (intrusive) rock – granite Andesite – an igneous volcanic (extrusive) rock – an intermediate between basalt and rhyolit
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