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UNIT 3 ROCKS AND THE ROCK CYCLE A Rock is composed of one or more minerals that are naturally occurring. There are three rock types a. Igneous-formed by cooling magma. b. Sedimentary-formed by the breakdown of other rocks. c. Metamorphic-formed when preexisting rocks are heated under pressure. • Some rocks are composed entirely of one mineral ex. Limestone (mineral) calcite. • Most rocks have more than one kind of mineral ex. Granite • Some rocks contain non-mineral matter. • Ex coal organic debris, and obsidian volcanic rocknot crystalline. rock minerals mineral Example: Granite & its constituent minerals: Quartz Amphibole (hornblende) Feldspar A. THE ROCK CYCLE 1. Igneous- formed by the crystallization of molten rock material called magma. 2. Sedimentary-formed from pre-existing rocks by weathering-chemical and physical breakup, erosion, and transport. 3. Metamorphic-formed by textural and compositional changes that occur when preexisting rocks are buried and subjected to increased temperatures and pressures. • The rock cycle connects the three rock groups to each other by processes. • The rock cycle is embedded within the hydrological and plate cycles. Weathering SEDIMENT Erosion Transport Deposition Volcanic IGNEOUS SEDIMENTARY Plutonic Increased P&T METAMORPHIC Crystallization Melting Uplift Burial MAGMA 7 • B. Igneous Rocks • Molten rock inside the Earth is called Magma. • Magma is buoyant, rises to the surface and sometimes breaks through as volcanic eruptions. • When magma reaches the surface it is called lava. • An igneous rock is formed when magma or lava cools and solidified. • Igneous rocks make up the bulk of the Earth’s crust. • Earth’s mantle is composed entirely of igneous rock. • Igneous rocks are important economically as building stones and as host rocks for a variety of mineral (ore) deposits. Volcanic activity is a geological hazard, igneous rock can tell us both the nature of past volcanic eruptions and the potential for future eruption hazards. Igneous rocks that form by magma at the surface are called volcanic or extrusive. Fragmented materials are called pyroclastic and consist of ash & cinders. In igneous rocks, texture is controlled by the cooling rate of the magma. Cooling Rate Crystal Size Slow cooling larger crystals Fast cooling small or no crystals Plutonic Igneous Rocks Igneous rocks that form deep below the surface are called plutonic (intrusive) igneous rocks. To see them, they must be uplifted to surface and the overlying rock eroded away. As a magma cools, atoms arrange themselves into orderly crystalline structures called minerals. This process is called: Crystallization Plutonic igneous rocks cool slowly at depth and are therefore coarser grained! Microscopic views of plutonic igneous rocks Subsurface intrusion called a dike All rocks are classified according to their texture and mineral composition. Texture involves the size, shape, and arrangement of the minerals making up a rock. Types of Igneous Textures Fine-grained Coarse-grained Porphyritic Glassy C. Types of igneous textures 1. Fine-grained-fast cooling magma/lava forms at or near surface sometimes gas holes present, hard to see individual crystals. 2. Coarse-grained-forms deep below the surface slowly cooling. The crystals are coarse and intergrown. 3. Porphyritic- magma cooled slowly for a while then erupted. Minerals crystallized at different temperatures and rates over a period of time, • 4. Glassy-rapid cooling at the surface. Cannot form orderly crystalline structures. Fig. 6.12 W. W. Norton Zooming in: Basalt Gabbro Zooming in Andesite Diorite Obsidian Volcanic Glass Pumice Volcanic “Froth” Basalt eruptions on land produce flows that travel great distances. Fig. 6.20a W. W. Norton Columbia River basalts Fig. 6.21a W. W. Norton Fig. 6.21b Stephen Marshak Fig. 6.02 W. W. Norton • D. Explosive Volcanic Eruptions. • Violently explosive volcanic eruptions produce rock fragments of all sizes, finely-fragmented ash, and molten bombs. They accumulate to form pyroclastic volcanic rocks. • Rock types from these volcano’s include: • a. Tuff-composed of ash, finely fragmented volcanic rock. • b. Scoria-red or black, frothy lave, denser then pumice. • c. Volcanic Breccia-coarse, angular rock fragments, usually in a matrix of fine to coarse ash. Fig. 6.08a W. W. Norton Types of Igneous Plutons Volcano Dikes Laccolith Sills p.140-141d Volcanic Pipe Original artwork by Gary Hincks Dikes Batholith Fig. 6.08b Stephen Marshak Fig. 6.10 W. W. Norton Fig. 6.11a Stephen Marshak Fig. 6.11de Paul Hoffmann Fig. 6.18 W. W. Norton