Rocks Rock- A group of minerals, glass, mineroid bound together in some way. All rocks fit into one of three categories: Igneous- formed by the cooling and hardening of hot molten rock Sedimentary- formed by the compaction and cementing of layers of sediments. The sediments may be rock fragments, plant and animal material or chemicals. Metamorphic- formed when rocks that already exist are changed by heat and pressure into new types of rocks. Igneous rocks Terms Magma- molten material beneath the surface of the earth Lava- molten material on the surface of the earth Crystallization- cooling and solidification of rocks into an orderly pattern. All material does not solidify at the same rate There are two main types of igneous rocks, intrusive and extrusive Intrusive (plutonic)- rocks that form underground from cooled magma. They are only seen at the surface when rocks that cover them are worn away. Extrusive (volcanic) - formed from the rapid cooling of lava or melted rock on the earth's surface Texture- term used to describe the size, shape and arrangement of the crystalline grains Crystal size is the most important factor affecting texture. Crystal size is determined by the speed of hardening (rate of cooling) Slower cooling = larger crystals Faster cooling = smaller crystals Immediate cooling= glass The longer the magma stays liquid, the larger the crystals can get. The amount of dissolved gas helps determine the speed of hardening. High gas, crystal grow faster. Intrusive rocks form when magma cools and hardens slowly deep underground allows for large well developed crystals. Igneous rocks with large minerals grains have a coarse -grained texture ex. granite ( the crust of the continent is made of granite)
Some intrusive rocks form from magma that cools slowly at first and then more rapidly as it reaches the surface. This allows a rock to have two distinct textures (large crystals surrounded by a fine grained mass of rock. This is called porphyry or porphyritic rocks Extrusive rocks- form when lava cools rapidly at the surface. Rapid loss of heat to air or water does not allow for large crystals to grow. most extrusive rocks are fine grained (can not see grains with the naked eye) ex. basalt some cool so fast that no crystals can form (immediate cooling) ex. obsidian others cools so fast that gasses escaping become trapped and form bubbles. ex pumice vesicular- full of holes from escaping gas Composition of Igneous rocks determined by the texture and mineral composition of the magma determines the igneous rocks family Granite family called felsic high in silica, SiO 2 light coloring main mineral components= orthoclase feldspar and quartz formed during mountain building intrusive magmas are thick and slow moving (high viscosity) viscosity- resistance to flow Basalt Family called mafic low silica, rich in iron dark colored main mineral component = biotite, hornblende, augite, olivine extrusive magmas are thinner, hotter and less viscous Diorite Family little or no quartz medium colored rocks main mineral composition = plagioclase feldspar, hornblende, and micas
Sedimentary Rock Notes Sedimentary rocks form when sediments harden into rocks. They are classified according to the kind and size of sediments that form them. Sedimentary rocks form under two main processes- compaction and cementation Compaction- the weight of overlying sediments causes pressure, pushing the fragments together and squeezing out air and water. Cementation- dissolved material carried by water is left between fragments and provide a cement to hold the fragments together. Cement may be silica (SiO2, ) calcite (CaCO3) or iron oxide ( FeO) There are three main types of sedimentary rocks: Clastic, Chemical and Organic. Clastic come from weathering of rocks that already exist formed from fragments of their rocks Range in grain size from: Gravel size 2mm-64mm ex. conglomerate Sand size- 1/16-2 mm ex. sandstone Mud Size- 1/256-1/16, silt size less than 1/256 mm ex. shale Running water collects most of these particles and causes them to be smoothed down and rounded. The farther a particle travels the more rounded it becomes Sediments get deposited when water slows down. 1st to drop out are larger pebbles 2nd smaller sands 3rd silts, and clays Conglomerate- coarsest of clastic rocks. quartz very common, usually a mixture of pebbles, gravel and sand size material Sandstone- Made mostly of grains of quartz may be cemented with silica or calcite may be 30% air space air spaces mean it is porous ( filled with small holes) permeable- allows water to flow through why do we care if the rocks are permeable? Shale- made of clay spaces between clay particles are so tiny that water can not pass through the rock (impermeable)
Chemical- form from minerals that were once dissolved in water some form from dissolved material that settles out due to temp. change. some form when water evaporates and leaves behind the minerals called evaporates ex. rock gypsum and rock salt Organic formed from the remains of living things coal and limestone are examples presence of calcite allows for easy ID often fossils are present Sedimentary Rock features Sedimentary rocks have a number of easily identifiable features: stratification (layering), ripple marks, mud cracks, fossils and concretions. Give indications of paleoclimate. Stratification- arrangement in visible layers, oldest layer in on the bottom most are in horizontal layers some show cross-bedding: sediments deposited by wind show changes in direction Ripple marks indicate wave action Mud cracks- result when muddy deposits dry and shrink Concretions- sedimentary rocks that contain lumps or nodules with a different composition from the main rock body. Inside filled with liquid that evaporates leaves behind crystals. Fossils- remains, impressions or any evidence or plant or animal pressed into rock Metamorphic Rock Notes Metamorphism- changing one type of rock to another by heat, pressure, and chemical processes. All metamorphic rocks from existing igneous, sedimentary or metamorphic rocks. Most form deep within the surface of the earth. Regional metamorphism- Large areas of rocks are under intense heat and pressure. Most metamorphic rocks are formed this way. movement of tectonic plates against each other creates tremendous heat and pressure Contact metamorphism- hot magma pushes through existing rocks only rocks near or touching the magma are metamorphosed
Classification Metamorphic rocks have either a foliated (visible parallel bands) or unfoliated structure(without visible bands) Foliated rocks Extreme pressure may flatten the mineral crystals in the original rock and push them into parallel bands slate, schist and gneiss are common metamorphic rocks Slate- easily splits into flat sheets schist- named for principal mineral gneiss- has bands of light and dark minerals Unfoliated do not have bands of crystals ex. quartzite