Week 2 - Rocks (2).pdf

Minerals and Rocks Earth Science Rocks Definition of Rocks Rocks are aggregates of minerals that have different melting points based on their composition and water content. Melting of rocks typically occurs in stages. Some are formed through accumulation of rock or mineral fragments on s...

Minerals and Rocks Earth Science Rocks Definition of Rocks Rocks are aggregates of minerals that have different melting points based on their composition and water content. Melting of rocks typically occurs in stages. Some are formed through accumulation of rock or mineral fragments on surface, precipitation, etc. Some changes due to variations of temperature, pressure, and fluid activity. Igneous and Sedimentary Rocks Igneous Rocks - Formation Igneous Rocks are formed by melting, cooling, and crystallization of other rocks. igneous rocks form as magma cools and crystallizes. Igneous rocks form as a result of volcanic activity, hot spots, and melting that occurs in the mantle. 2 Types of Igneous Rock Extrusive igneous rocks form when magma reaches the Earth's surface a volcano and cools quickly. Most extrusive (volcanic) rocks have small crystals. Examples include basalt, rhyolite, andesite, and obsidian. Intrusive, or plutonic, igneous rocks form when magma cools slowly below the Earth's surface. Most intrusive rocks have large, well-formed crystals. Examples include granite, gabbro, diorite and dunite. EXTRUSIVE IGNEOUS ROCKS INTRUSIVE IGNEOUS ROCKS TEXTURE Crystal size is used to classify igneous rocks. Crystals form as the rock cools, and the crystal size can tell us a lot about its cooling history: The larger the crystals, the slower it cooled. TEXTURE Glassy igneous rocks have no crystal structure, and probably formed by very rapid cooling (such as on the surface of a lava, or when a lava enters the water.) Aphanitic rocks have no visible crystals, and probably formed by fast cooling above ground. TEXTURE Phaneritic rocks have visible crystals, and probably formed by slow cooling below ground. Porphyritic rocks have both visible and nonvisible crystals, and probably formed by two different cooling events. TEXTURE Vesicular voids created by rapid cooling which causes air bubbles to be trapped inside. TEXTURE Dark igneous rocks are formed from basaltic or mafic magma. (Mafic because it contains a lot of magnesium and iron). The magma that forms these rocks is usually very hot (around 1000°C) (about the same viscosity as ketchup.) Light colored igneous rocks are formed from silicic (high silica content) or felsic magmas. The magmas that form these rocks is usually more cool, (lower than 850°C), and more viscous (about the viscosity of peanut butter.) IGNEOUS ROCK COMPOSITION Igneous rocks are also classified according to silica content and relative amounts of K, Na, Fe, Mg and Ca. They can be classified as felsic, intermediate, mafic and ultramafic, practically based on presence of light and dark colored minerals. IGNEOUS ROCK COMPOSITION Course-grained(intrusive) Fine-grained(extrusive) Granitic (felsic) - a coarse-grained igneous rock composed chiefly of orthoclase and albite feldspars and quartz Basaltic (Mafic) Rocks that contain substantial dark silicate minerals and calcium-rich plagioclase feldspar. IGNEOUS ROCK COMPOSITION Course-grained(intrusive) Fine-grained(extrusive) Andesitic (Intermediate) Rocks,- are rocks with a composition between granitic and basaltic rocks Ultramafic Rocks- igneous rocks composed almost entirely of dark silicate minerals Peridotites Komatiites IGNEOUS ROCKS COMPOSITION felsic: granitic: >65% silica, generally light-colored intermediate: andesitic: 55-65% silica, generally medium colored (medium gray) - mafic: basaltic: 45-55% silica, usually dark colored - ultramafic: 50% clasts diameter >2mm made up of primarily rock fragments Breccia ARANACEOUS ROCKS ARANACEOUS ROCKS: Sediment Group: Sand -a loose granular substance, typically pale yellowish brown, resulting from the erosion of siliceous and other rocks and forming a major constituent of beaches, riverbeds, the seabed, and deserts. Sediment Name: Very Course, Course, Medium, Fine, Very fine Sand Arenaceous Rocks: (arenites) >50% sediments diameter between 0.063-2mm can contain high quartz % ARGILLACEOUS ROCKS ARGILLACEOUS ROCKS Sediment Name: Silt, Clay Argillaceous Rocks: (argillites) >50% sediments diameter