A. IGNEOUS Rocks formed by cooling and hardening of hot molten rock called magma (within crust or at its surface).

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Gilbert Byrd
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1 EARTH SCIENCE 11 CHAPTER 5 NOTES KEY How Earth's Rocks Were Formed Early geologists believed that the physical features of the Earth were formed by sudden spectacular events called CATASTROPHES. Modern geology (the study of the Earth s interior) really began in 1795 with James Hutton s theory of UNIFORMITARIANISM. He said, The PRESENT is the key to the PAST. This meant that: geological processes now operating on Earth were also active in the past present physical features were formed by these geological processes Evidence of these changes is recorded in the ROCKS. To understand them, we must first learn about rock types. Three Families of Rocks A. IGNEOUS Rocks formed by cooling and hardening of hot molten rock called magma (within crust or at its surface). B. SEDIMENTARY Rocks formed by hardening of layers of sediments (rock fragments, plant and animal remains, or chemicals). C. METAMORPHIC Rocks formed when rocks that already exist are changed into new forms of rock (due to heat, pressure, chemicals and water). ***** Refer to the BLUE summary charts for an overview of this information. ***** A. Igneous Rocks When magma reaches the surface it cools quickly forming extrusive igneous rock or volcanic rock. Magma that reached earth s surface is called LAVA. Rocks formed by magma underneath earth s crust cool slowly and form INTRUSIVE igneous rock or PLUTONIC rock. These rocks generally have a more crystalline structure. Types of Magma 1. higher silica type relatively thick and slow flowing, acidic gives lighter colored minerals like quartz and orthoclase feldspar, and lighter colored rocks like granite. Called FELSIC or granitic. 2. lower silica type (more Ca, Fe, and Mg) hotter and more fluid gives darker minerals (ferromagnesian) like hornblende, augite, and black mica and dark rocks like basalt. Called MAFIC or basaltic.

2 Texture The size of crystals depends on how fast the magma cooled. SLOW cooling produces larger and more regular crystals. Rapid cooling produces a glass with no crystals. GRANITE is an intrusive rock with large mineral grains of uniform size granular or coarse grained. OBSIDIAN is an extrusive rock that has no crystalline structure. Basalt is an extrusive igneous rock with a fine grain. Porphyritic Texture Large crystals surrounded by a GROUNDMASS that is fine-grained or glassy is called a PORPHYRY. Families of Igneous Rocks I. GRANITE high silica, mostly orthoclase, feldspar and quartz generally light in color. a) Granite coarse grain b) Rhyolite fine grain c) Obsidian and pumice glassy II. DIORITE between granite and gabbro in mineral composition a) Grandiorite coarse grain b) Dunite - nearly all olivine III. GABBRO low silica magmas, mostly plagioclase feldspar and augite generally very dark and dense. a) Gabbro coarse grain b) Basalt fine grain B. Sedimentary Rocks Three main sources for the sediments that form sedimentary rocks are: 1. CLASTIC formed from fragments of other rocks such as clay, sand, or gravel. 2. CHEMICAL formed from mineral grains that precipitate out of solution by evaporation or chemical action. 3. ORGANIC- formed from remains of plant and animals. Page 2

Clastic FRAGMENTS range in size from large too small as: pebbles, gravels, sand, silt, and clay. fragments are carried by winds, waves, rivers, and glaciers and form in great layers of SEDIMENT.

when cement minerals precipitate into pore spaces between sand, gravel, or pebbles, they bind the fragments together, turning loose SEDIMENTS into firm cemented rock.

3 Clastic FRAGMENTS range in size from large too small as: pebbles, gravels, sand, silt, and clay. fragments are carried by winds, waves, rivers, and glaciers and form in great layers of SEDIMENT. ocean, lake, and ground water all contain DISSOLVED minerals; silica (from quartz), lime (from calcite), and iron (from iron containing minerals) may form natural CEMENTS. when cement minerals precipitate into pore spaces between sand, gravel, or pebbles, they bind the fragments together, turning loose SEDIMENTS into firm cemented rock. in fine sediments such as clay or silt, sometimes the pressure of the later deposit is enough to make the particles stick together, but usually cement is required. silica and lime give white or grey cement; iron cement is red, brown or rust-colored. water (or wind) carrying fragments deposit the fragments when the flow SLOWS down. Diagram of Sediment Deposits and the Rocks They Form (p. 57 Green, p.70 Brown) The heaviest/largest fragments are deposited FIRST. These are heavy pebbles and gravels. Next come the lighter sands and finally the silts and clays. Types of Clastic Sedimentary Rocks 1. CONGLOMERATES pebbles and gravels cemented together. Quartz is most common material. 2. SANDSTONE sands cemented together. Porous. 3. SHALE silts and clays cement together. Usually contains Kaolin. Smooth, soft, and easily broken. Note: Particles carried by water are smoothed by tumbling. The FARTHER they are carried, the smoother they become. Page 3

4 Types of Sedimentary Rocks of Chemical Origin 1. LIMESTONES tiny grains of calcite deposited from sea or lake water usually gray or tan, dense in appearance and smooth. 2. ROCK SALT made from halite in thick layers, formed by evaporation. 3. ROCK GYPSUM sedimentary layers in nearly pure veins of gypsum. Formed by evaporation. Types of Organic Sedimentary Rocks come from remains of plants and animals. limestone (from calcite and from lime forming plants and animals) can be identified by acid test. coal Stratification arrangements in visible layers of different materials in BEDDING PLANES. Fossils fossils are the hard part of DEAD plants and animals that remain and turn to rock. Ripple Marks formed by the action of WINDS, streams, waves or currents on sand becomes sandstone. Mud Cracks deposits of wet clay drying and contracting which may be later filled with another material. Geodes and Concretions. Distinguish between nodules, geodes and concretions. 1. A concretion is a hard, compact mass of mineral matter formed by precipitation from a WATER solution about a nucleus or center, such as a leaf, shell, bone, or fossil, in the pores of a sedimentary or fragmental volcanic rock. 2. A GEODE is a type of concretion with a hollow or partly hollow globular body. They are found in certain limestone beds. The cavity is partly filled by an inner lining of crystals. (AKA Thunder Eggs ) 3. A NODULE is a type of concretion that is a small, irregularly rounded knot, mass, or lump of a mineral, normally having a warty or knobby surface and no internal structure. (i.e. Manganese nodules are found on the floors of the world s oceans) Page 4

5 C. Metamorphic Rocks Metamorphic rocks are rocks formed from existing bedrock by action of heat, pressure, and chemicals, especially water. 1. Dynamic metamorphism occurs during mountain building movements when horizontal layers of sedimentary rocks are subjected to both high temperatures and pressures. 2. If it takes place over a large area, it is called REGIONAL metamorphism. 3. In general, pressure squeezes the grains closer together making them more dense and less porous. Heat and chemicals may rearrange ions or form new ions, making the rocks more crystalline or even changing minerals. 4. Sandstone changes to QUARTZITE. 5. Limestone changes to MARBLE. 6. Granite changes to GNEISS. 7. When under the pressure and temperature, shale becomes denser and more crystalline and new minerals like mica and hornblende are formed. Tremendous pressure squeezes the flakes of mica or needles of hornblende into parallel layers along which the new rocks split easily. This is called FOLIATION. Shale SLATE Phyllite SCHIST Gneiss 8. Gneiss has thick bands of light and dark coloured minerals. 9. Thermal Metamorphism When hot magma forces its way into overlaying rocks and heats surrounding rocks. The heated rocks change into other rocks. Also called CONTACT metamorphism. Hot magma enters intruded rock and reacts with minerals. Change is less dramatic and foliation is never produced. Hornfels from shale is by thermal metamorphism. The Rock Cycle Complete the diagram of the rock cycle on the following page. Page 5

6 Label the following: magma lava igneous rock (extrusive) igneous rock (intrusive) weathering transportation deposition erosion sediments sedimentary rocks metamorphic rocks uplift and exposure burial heat and pressure Page 6

7 Use this key to complete your pink Rock Cycle Quiz / Study Guide

Rocks Rock- A group of minerals, glass, mineroid bound together in some way.

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