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ROCKS What is a Rock? • A group of minerals bound together • Naturally occurring solid Can consist of mostly one mineral or Several different minerals in varying quantities Rocks are classified according to the processes by which they are formed 3 Major Rock Groups 1. Metamorphic – rocks formed when an existing rock is transformed by heat, pressure, or chemical reactions. • Rock pulled beneath Earth’s surface; movements in crust • Rock is heated and undergoes changes • Metamorphic change happens in solid state ; w/o melting • Pressure changes the way the particles line up 2. Igneous – rocks formed when hot, molten rock, or magma, cools and hardens either inside Earth or at the surface • Rock pulled deeper under the surface, melt at 600 -1300˚ C • The molten rock called Magma rises, cools, and solidifies 3. Sedimentary – rocks formed over time as sediment is compacted and cemented together • Weathering • Sediment • Erosion • Deposition • Compaction • Cementation http://www.youtube.com/watch?v=BpXR-z47G8o http://www.youtube.com/watch?v=pg_jKJFbA 2A Igneous Rock 2 Types I. Extrusive Igneous Rock a. Magma explodes out of the Earth, becomes Lava b. Lava cools quickly, small crystals c. Volcanic rocks II. Intrusive Igneous Rock a. Magma cools and hardens underground as it is pushed toward surface b. Slow cooling, large crystals c. Plutonic rocks d. Intrusions – masses of rock that harden with the Earth How Does Magma Form? 1. Mantle Rock Rising – less pressure allows for melting 2. Addition of Volatiles – Chemicals that exist in gasous form and mix with hot rock to break bonds 3. Heat Transfer – rising rock brings heat to melt crustal rock Pyroclastic Debris: pyro – fire Clast – broken •Fine particles of glass or volcanic ash that form as a spray of lava erupts into air and quickly cools •Or large bombs erupted as liquid and cooled Identifying Igneous Rocks a) Grain size i. Coarse – large enough to identify ii. Medium – can see grains but can’t identify iii. Fine – too small to be seen iv. Glassy – no grains at all b) Texture- arrangement of grains i. Crystalline – interlocking grains ii. Fragmental – Chunks or large pieces cemented together iii. Glassy – shiny, no grains iv. Porous – sponge - like c) Composition – proportions of chemicals/minerals that make up the rock i. Felsic – rich in silica (silicon & oxygen), light colored ii. Mafic – poor in silica, dark colored iii. Ultramafic – very low silica iv. Intermediate – in between