Essentials_Ch03

Essentials_Ch03 - Characteristics of magma Igneous rocks...

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Unformatted text preview: Characteristics of magma Igneous rocks form as molten rock cools and solidifies General Characteristic of magma Parent material of igneous rocks Forms from partial melting of rocks inside the Earth Magma that reaches the surface is called lava Characteristics of magma General Characteristic of magma Rocks formed from lava at the surface are classified as extrusive, or volcanic rocks Rocks formed from magma that crystallizes at depth are termed intrusive , or plutonic rocks Characteristics of magma The nature of magma Consists of three components: A liquid portion, called melt , that is composed of mobile ions Solids , if any, are silicate minerals that have already crystallized from the melt Volatiles , which are gases dissolved in the melt, including water vapor (H 2 O), carbon dioxide (CO 2 ), and sulfur dioxide (SO 2 ) Characteristics of magma Crystallization of magma Texture in igneous rocks is determined by the size and arrangement of mineral grains Igneous rocks are typically classified by Texture Mineral composition Igneous textures Texture is used to describe the overall appearance of a rock based on the size, shape, and arrangement of interlocking minerals Factors affecting crystal size Rate of cooling Slow rate promotes the growth of fewer but larger crystals Igneous textures Factors affecting crystal size Rate of cooling Fast rate forms many small crystals Very fast rate forms glass Amount of silica (SiO 2 ) present Amount of dissolved gases Igneous textures Types of igneous textures Aphanitic (fine-grained) texture Rapid rate of cooling of lava or magma Microscopic crystals May contain vesicles (holes from gas bubbles) Phaneritic (coarse-grained) texture Slow cooling Crystals can be identified without a microscope Aphanitic texture Phaneritic texture Igneous textures Types of igneous textures Porphyritic texture Minerals form at different temperatures as well as differing rates Large crystals, called phenocrysts , are embedded in a matrix of smaller crystals, called the groundmass Glassy texture Very rapid cooling of molten rock Resulting rock is called obsidian Porphyritic texture An obsidian flow in Oregon Glassy texture Igneous textures Types of igneous textures Pyroclastic texture Various fragments ejected during a violent volcanic eruption Textures often appear to more similar to sedimentary rocks Pegmatitic texture Exceptionally coarse grained Form in late stages of crystallization of granitic magmas Mineralogy of common igneous rocks Igneous Compositions Igneous rocks are composed primarily of silicate minerals Dark (or ferromagnesian ) silicates Olivine Pyroxene Amphibole Biotite mica Igneous Compositions...
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This note was uploaded on 07/17/2008 for the course GS 100 taught by Professor Leavell during the Spring '06 term at Ohio State.

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Essentials_Ch03 - Characteristics of magma Igneous rocks...

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