Igneous Rocks

Rock Cycle

Types of rocks

Area of exposure on surface and volume fraction

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 Cooling of magma results in the systematic arrangement of ions into orderly patterns The silicate minerals resulting from crystallization form in a predictable order

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) –Example (basalt) Phaneritic (coarse-grained) texture –Slow cooling –Crystals can be identified without a microscope –Example (granite)

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 –Example (Andesite) Glassy texture –Very rapid cooling of molten rock –Example (obsidian)

Porphyritic texture

Glassy texture

Igneous Compositions Igneous rocks are composed primarily of silicate minerals Dark (or ferromagnesian) silicates –Olivine –Pyroxene –Amphibole –Biotite mica

Igneous Compositions Igneous rocks are composed primarily of silicate minerals Light (or nonferromagnesian) silicates –Quartz –Muscovite mica –Feldspars

Igneous compositions Granitic versus basaltic compositions Granitic composition –Composed of light-colored silicates –Designated as being felsic (feldspar and silica) in composition –Contains high amounts of silica (SiO 2 ) –Major constituents of continental crust

Igneous compositions Granitic versus basaltic compositions Basaltic composition –Composed of dark silicates and calcium-rich feldspar –Designated as being mafic (magnesium and ferrum, for iron) in composition –More dense than granitic rocks –Comprise the ocean floor as well as many volcanic islands

Igneous compositions Silica content influences a magma’s behavior Granitic magma –High silica content –Extremely viscous –Liquid exists at temperatures as low as 700 o C

Igneous compositions Silica content influences a magma’s behavior Basaltic magma –Much lower silica content –Fluid-like behavior –Crystallizes at higher temperatures

Feldspar Amphibole Pyroxene Olivine

Igneous compositions Naming igneous rocks – granitic (felsic) rocks Granite –Phaneritic –Over 25 percent quartz, about 65 percent or more feldspar –May exhibit a porphyritic texture –Very abundant as it is often associated with mountain building –The term granite covers a wide range of mineral compositions

Granite

Igneous compositions Naming igneous rocks – granitic (felsic) rocks Rhyolite –Extrusive equivalent of granite –May contain glass fragments and vesicles –Aphanitic texture –Less common and less voluminous than granite

Rhyolite

Igneous compositions Naming igneous rocks – granitic (felsic) rocks Obsidian –Dark colored –Glassy texture Pumice –Volcanic –Glassy texture –Frothy appearance with numerous voids

Obsidian

Pumice

Igneous compositions Naming igneous rocks – intermediate rocks Andesite –Volcanic origin –Aphanitic texture –Often resembles rhyolite

Andesite

Igneous compositions Naming igneous rocks – intermediate rocks Diorite –Plutonic equivalent of andesite –Coarse grained –Intrusive –Composed mainly of intermediate feldspar and amphibole

Diorite

Igneous compositions Naming igneous rocks – basaltic (mafic) rocks Basalt –Volcanic origin –Aphanitic texture –Composed mainly of pyroxene and calcium- rich plagioclase feldspar –Most common extrusive igneous rock

Basalt (Scoria)

Igneous compositions Naming igneous rocks – basaltic (mafic) rocks Gabbro –Intrusive equivalent of basalt –Phaneritic texture consisting of pyroxene and calcium-rich plagioclase –Makes up a significant percentage of the oceanic crust

Igneous compositions Naming igneous rocks – pyroclastic rocks Composed of fragments ejected during a volcanic eruption Varieties –Tuff – ash-sized fragments –Volcanic breccia – particles larger than ash

Ash and pumice layers