Igneous Compositions - Dynamic Earth - Lecture Slides, Slides of Earth Sciences

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Igneous Rocks

Igneous compositions

 Igneous rocks are composed primarily of

silicate minerals

• Dark (or ferromagnesian) silicates
  • Olivine, pyroxene, amphibole, and biotite mica
• Light (or nonferromagnesian) silicates
  • Quartz, muscovite mica, and feldspars

Igneous compositions

 Granitic versus basaltic compositions

• Granitic composition
  • Light-colored silicates
  • Termed felsic ( fe ldspar and si lica) in composition
  • High amounts of silica (SiO 2 )
  • Major constituent of continental crust

Igneous compositions

 Granitic versus basaltic compositions

• Basaltic composition
  • Dark silicates and calcium-rich feldspar
  • Termed mafic ( ma gnesium and fe rrum, for iron) in composition
  • Higher dense than granitic rocks
  • Comprise the ocean floor and many volcanic islands

Igneous Rock Classification

Igneous compositions

 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

Granite

Igneous compositions

 Naming igneous rocks – granitic rocks

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

Igneous compositions

 Naming igneous rocks – granitic rocks

• Obsidian
  • Dark colored
  • Glassy texture
• Pumice
  • Volcanic
  • Glassy texture
  • Frothy appearance with numerous voids

Pumice and obsidian

Igneous compositions

 Naming igneous rocks – intermediate

rocks

• Andesite
  • Volcanic origin
  • Aphanitic texture
• Diorite
  • Plutonic equivalent of andesite
  • Coarse grained

Diorite

Figure 4.

Igneous compositions

 Naming igneous rocks – basaltic rocks

• Basalt
  • Volcanic origin
  • Aphanitic texture
  • Composed mainly of pyroxene and calcium-rich plagioclase feldspar
  • Most common extrusive igneous rock