Rocks (Igneous)

Rocks Naturally formed, consolidated material composed of the grains or one or more minerals Three Types of Rocks Igneous Sedimentary Metamorphic

Igneous Rocks Igneous rocks form from cooling magma or lava Magma: molten rock below the Earth’s surface Lava: magma on the Earth’s surface

Classifying Igneous Rock Igneous rocks can be classified by: Mineral composition Texture (crystal size)

Types of Igneous Rocks Intrusive Igneous Rocks: formed from magma cooling Cool inside the Earth Large crystals, coarse-grained (most crystals >1mm) Phaneritic Cools slowly Extrusive Igneous Rocks: formed from lava cooling Cool on the surface Small crystals, fine-grained (most crystals <1mm) Aphanitic Cools quickly Fine-grained igneous rock Coarse-grained igneous rock

Other Igneous Textures Porphyritic Texture Minerals form at different temperatures (1st slow, then fast) Large crystals are embedded in a matrix of smaller crystals Glassy Texture Very rapid cooling of lava Ex. Obsidian

Porphyritic Texture

Glassy Texture

Other Igneous Textures Pyroclastic Texture Composed of fragments ejected during a volcanic eruption Tuff: ash-sized fragments Volcanic breccia: particles larger than ash Vesicular Texture Volcanic rock having many cavities (vesicles) at the surface and inside

Pyroclastic Texture

Vesicular Texture

Igneous Compositions Igneous rocks are composed primarily of silicate (SiO2) minerals Dark (or ferromagnesian) silicates Olivine, pyroxene, amphibole, and biotite mica Light (or nonferromagnesian) silicates Quartz, muscovite mica, and feldspars

Igneous Compositions Felsic versus Mafic compositions Felsic composition Light-colored silicates Termed felsic (feldspar and silica) in composition High amounts of silica (SiO2) Major constituent of continental crust

Igneous Compositions Felsic versus Mafic compositions Dark silicates and calcium-rich feldspar Termed mafic (magnesium and ferrum, for iron) in composition Denser than granitic rocks Comprise the ocean floor and many volcanic islands

Igneous Compositions Other compositional groups Intermediate (or andesitic) composition Contains 25% or more dark silicate minerals Associated with explosive volcanic activity Ultramafic composition Rare composition that is high in magnesium and iron Composed entirely of ferromagnesian silicates

Rock Types Naming igneous rocks – Felsic rocks Granite Phaneritic Over 25% quartz, about 65% or more feldspar Very abundant - often associated with mountain building The term granite includes a wide range of mineral compositions

Granite

Rock Types Rhyolite Naming igneous rocks – Felsic rocks Extrusive equivalent of granite May contain glass fragments and vesicles Aphanitic texture Less common and less voluminous than granite

Rhyolite

Pumice is very glassy and sharp, with countless vesicles. Rock Types Naming igneous rocks – Felsic rocks Obsidian Dark colored Glassy texture Pumice Volcanic Frothy appearance with numerous voids Pumice is very glassy and sharp, with countless vesicles.

Rock Types Andesite Diorite Naming igneous rocks – intermediate rocks Volcanic origin Aphanitic/Porphyritic texture Diorite Plutonic equivalent of andesite Coarse grained- Phaneritic

Andesite

Diorite

Rock Types Basalt Naming igneous rocks – Mafic rocks Volcanic origin Aphanitic texture Composed mainly of pyroxene and calcium-rich plagioclase feldspar Most common extrusive igneous rock

Basalt

Rock Types Gabbro Naming igneous rocks – mafic rocks Intrusive equivalent of basalt Phaneritic texture consisting of pyroxene and calcium-rich plagioclase Significant % of the oceanic crust

Gabbro

Andesitic (Intermediate) COMPOSITION Felsic Andesitic (Intermediate) Mafic Ultramafic >66% Silica 54-66% Silica 45-54% Silica <45% Silica ORIGIN TEXTURE ROCK NAME Intrusive Phaneritic Granite Diorite Gabbro Peridotite (coarse-grained) Porphyritic   Extrusive Aphanitic Rhyolite Andesite Basalt Komatite (rare) (fine-grained) Glassy Obsidian Vesicular Pumice Scoria Pyroclastic Rhyolite Tuff Andesite Tuff Basalt Tuff