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Igneous Rocks: Composition, Texture, and Formation, Study notes of Earth Sciences

Jaypee University of Engineering & TechnologyEarth Sciences

An in-depth exploration of igneous rocks, their general characteristics, textures, and compositions. It covers the differences between granitic, basaltic, and intermediate compositions, as well as their volcanic and plutonic equivalents. The document also discusses the origins of magma, the role of pressure and volatiles in melting, and the process of magmatic differentiation and assimilation.

Typology: Study notes

2011/2012

Uploaded on 11/14/2012

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Chapter 3
Igneous Rocks
General Characteristics of magma
Consists of 3 components
Liquid portion = melt
Solids if any are silicate minerals
Volatiles= dissolved gases in the melt including water vapor, carbon
dioxide and sulfur dioxide
Pyroclastic texture
Fragmental appearance produced by violent volcanic eruption
Granitic versus basaltic compositions
Granitic composition
Light colored silicate
Term felsic (feldspar and silica) in composition
High amounts of silica
Basaltic composition
Dark silicates and calcium-high feldspar
Termed mafic (magnesium and ferrum for iron) in composition
Comprised the ocean floor and many volcanic islands
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
Silica content as an indicator of composition
Exhibits a considerable range in the crust
45% to 70%
Silica content influences magma behavior
Granitic magmas= high silica content and viscous
Basaltic magmas= much lower silica content and more fluid like behavior
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Chapter 3

Igneous Rocks

General Characteristics of magma

Consists of 3 components Liquid portion = melt Solids if any are silicate minerals Volatiles= dissolved gases in the melt including water vapor, carbon dioxide and sulfur dioxide

Pyroclastic texture

Fragmental appearance produced by violent volcanic eruption

Granitic versus basaltic compositions

Granitic composition Light colored silicate Term felsic (feldspar and silica) in composition High amounts of silica Basaltic composition

Dark silicates and calcium-high feldspar Termed mafic (magnesium and ferrum for iron) in composition Comprised the ocean floor and many volcanic islands

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

Silica content as an indicator of composition

Exhibits a considerable range in the crust 45% to 70% Silica content influences magma behavior

Granitic magmas= high silica content and viscous

Basaltic magmas= much lower silica content and more fluid like behavior

Naming igneous rocks-granitic 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

Rhyolite

Extrusive equivalent of granite May contain glass fragments and vesicles Aphanitic texture Less common and less voluminous than granite

Obsidian

Dark colored Glassy texture

Pumice

Volcanic Glassy texture Frothy appearance with numerous voids

Intermediate Andesite Volcanic origins Aphanitic texture

Diorite

Plutonic equivalent of andesite Coarse grained

Basalt

Volcanic origin Aphanitic texture Composed mainly of pyroxene and calcium righ plagioclase feldspar Most common extrusive igneous rock

Gabbro (intrusive)

Intrusive equivalent to basalt