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Geol 1002 Week Three

by: Gwendolyn Cochran

Geol 1002 Week Three GEOL 1002-10

Gwendolyn Cochran

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About this Document

These notes cover what makes a rock. These are probably the most important notes I have taken in this class so far as they detail how geologists differentiate different types of rocks and what the ...
Historical Geology
Catherine A. Forster
Class Notes
Historical Geology
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This 5 page Class Notes was uploaded by Gwendolyn Cochran on Thursday January 28, 2016. The Class Notes belongs to GEOL 1002-10 at George Washington University taught by Catherine A. Forster in Spring 2016. Since its upload, it has received 30 views. For similar materials see Historical Geology in Geology at George Washington University.


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Date Created: 01/28/16
Week Three  What makes a rock:  1. one or more minerals  2. each mineral has a unique chemical formula and a unique chemical  formula ­ leading to a unique set of physical properties (hardness, color,  shape) because of the elements that make up minerals  Most common elements  ● O oxygen  ● Si silicon  ● Al aluminum  ● Fe iron  ● Ca calcium  ● Na sodium ● K potassium   ● Mg magnesium  Most common minerals ● SiO2 quartz (for example silica tetrahedron) The basic unit of any rock is the unit cell­ the first compound mineral that  the rest of the crystal will form around. The crystal will grow by adding new  units in an extremely orderly fashion. Minerals will continue to add over  time causing the rock to grow larger in the very specific shape determined  by the unit cell. A crystal lattice forms on the inside of rock, determining the  external shape of the crystal.   By looking at the way a crystal grows, rocks can be divided into 7 different  crystal classes based on the configuration of the crystal. But there are just  two important mineral classes based on composition: Silicates (based in  silicone­ quartz, feldspars, micas, clays) and Non silicates (do not contain  silicone­ limestone, evaporites {salt}, and ironstones) Properties specific to each mineral  ● chemical formula  ● crystal structure  ● hardness  ● density and specific gravity ● color  ● cleavage (how the mineral breaks) ● melting point Minerals are compelled to form in three different situations 1. Solidification from a “melt” (melt= molten rock) a. This is how igneous rocks form­ minerals crystallizing as they cool down  from lava b.  Determinants of what forms from cooling lava i. temperature  ii. availability of elements c. Bowen's reaction series  High  Olivine (1200 Ca­ feldspars  T C) Pryovenes Ca.. Na Amphiboles Ca­Na feldspars biotite mica Na feldspars Potassium  feldspars  Muscovite Cool  Quartz (600  T C) i. By looking at bowens, you can tell which minerals will be associated  together, as only rocks close by on the table will form near or around each  other (ie. olivine and quartz will never be together) ii. This order is very well defined and non random  2. Precipitation from saturated water a. This is how sedimentary rocks form  i.  limestones  ii. evaporites (salts)  3. Solid state diffusion ­ metamorphism a. when you apply heat and pressure to an existing rock­ Recrystallization  without melting b. This is how metamorphic rocks are formed   Importantly… elements deplete as you cool from hot to cold, so there will  be less of certain elements to help form minerals and rocks. Similarly the  different minerals that go together will be side by side as the rock cools (ie.  Amphiboles and feldspars would be by each other) Igneous rocks  1. Melts cool at varying rates, thus the size of the crystals (which take time to  form) determine the texture of the rock. Cooling quickly will result in very  fine textures, so different rock despite identical minerals.  2. TEXTU Coarse Fine RE high  gabbro basalt temp mid  diorite andesite temp s low  granat rhyolite  temp e Sedimentary rocks 1. precipitation from oversaturated water (rock salt and carbonates) 2. Formed from the detritus of older rocks (clay and sand) 3. They only form at the surface of the earth at low temp and pressure  Metamorphic rocks  1. undergo solid state diffusion­ their original minerals are altered without  melting or recomposition 2. any type of rock can become metamorphic 3. Thermal history­ we can tell how much pressure was applied through  varying metamorphic grades 4. Foliation: occurs as a response to pressure. Minerals respond to the  pressure exerted on them by aligning themselves perpendicular to pressure a. Slate­ foliation can only be seen from a microscope  b. Schists­ fine grains of minerals, lots of mica 5. Nonfoliated rock­ random makeup, you cannot see any response to  pressure 6. Temp (what minerals are by eacho ther) pressure (foliation vs non foliation)


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