Igneous Rocks

Extrusive vs. Intrusive realm

Extrusive: above surface (ex. lava) → cools on surface

Intrusive: below surface (ex. magma) → cools below surface

What is classifying igneous rocks based on?

• Texture

  • Fine or coarse (depending on how quickly magma cooled)

• Mineral content

  • Depends on the origin & chemical evolution of the magma

Reflects the chemical & cooling history of the rocks

Texture: extrusive vs. intrusive

Extrusive: can’t see individual minerals (cooled at surface)

Intrusive: can see each individual mineral (bad at moving heat below surface)

6 minerals that are >95% of igneous rocks

Mafics (Mg-Fe): olivine, pyroxene, hornblende (amphibole)

Aluminosilicates (Si-Al-O): feldspar (plagioclase & K-spar), quartz, micas

3-4 of these minerals are common in each type of igneous rock

Silicate tetrahedron structure

SiO₄⁴⁻

Silicate minerals

• All have silicate tetrahedra as part of structure

• Some have many metal ions attached to the tetrahedral

  • Others are made of 3-dimensional framework of tetrahedral

Broad classification of igneous rocks

Felsic - Si + O

Intermediate - between the 2

Mafic - Mg + Fe

Fine vs. Coarse grained

Mafic rocks

Gabbro:

• Intrusive

• Cooled slowly

• Coarse-grained

Basalt:

• Extrusive

• Cooled quickly

• Fine-grained

Same chemical & mineralogical composition as olivine, feldspar, & pyroxene

Intermediate rocks

Diorite:

• Intrusive

• Cooled slowly

• Coarse-grained

Andesite:

• Extrusive

• Cooled quickly

• Fine-grained

Same chemical & mineralogical composition as feldspar, hornblende, & mica

Felsic rocks

Granite:

• Intrusive

• Cooled slowly

• Coarse-grained

Rhyolite:

• Extrusive

• Cooled quickly

• Fine-grained

Same chemical & mineralogical composition as mica, feldspar, & quartz

Rock families (3)

Basalt-Gabbro

• Mafic rocks

• Make up the ocean crust (ex. Hawaiian islands, hotspots, basalt plateaus)

Andesite

• Intermediate rocks

• Volcanic island arcs, active continental margins like the Andes, subduction zones

Granite-Rhyolite

• Felsic rocks rich in quartz, feldspar, few mafic minerals

• Granite & granodiorite most common form of continental crust

• Occurs primarily on the continents

Textures of Igneous rocks

Phaneritic: intrusive & can see many minerals in a row

• Porphyritic: different mineral sizes in a rock with larger K-feldspar “phenocrysts”

Aphanitic: extrusive & can’t see minerals (came out of volcano & cooled really quickly)

• Porphyritic: larger amphibole phenocrysts

They have the same chemical/mineralogical composition but different cooling rates

Bowen’s Reaction Series

Describes the specific sequence in which minerals crystallize from cooling magma. Different minerals form at distinct temperatures, from high-temperature - iron-rich, mafic minerals - (e.g., olivine) to lower-temperature - silica-rich, felsic minerals - (e.g., quartz).

Partial Melting

Makes the melt silica-enriched because felsic minerals melt first

Always makes a more felsic magma than the rocks you start with

Different types of magma

• Partial melting of upper mantle (felsic) rocks make basaltic (mafic) magma

• Sedimentary rock + basaltic oceanic crust make andesitic (intermediate) magmas at subduction zones (ex. Andes)

• Melt of sedimentary, igneous, & metamorphic crustal rocks make granitic (felsic) magma - only found on the continents

Lava Viscosity

Mafic → Felsic with low → high viscosity

Shield Volcano

Low silica content, low viscosity, runny lava placid eruptions

Mafic volcanoes which are the biggest on earth due to the lava flowing down, forming a small hillC

Composite Volcano

High silica content, stiff, viscous lava, highly explosive

Felsic volcanoes

Violent eruptions from gas buildup due to stiff, viscous, high-silica magmas (rhyolite)

“Cone” shape is made from debris (“ash”: small rock pieces) that falls out near volcano vent