Although silicate minerals are most common, there are non-silicate mineral groups. Examples: native elements (gold,silver,copper); oxides; carbonates (inc. calcite, which is common in Fla); phospates (imp. resource in Fla)

Igneous Rocks (ch. 4)
• Form when magma (molten rocks and gases) cools and crystallizes.
• Lava: magma that flows out onto surface of earth.

On surface, magma cools quickly.
• Small crystals.
• Forms extrusive (volcanic) rocks.

• If cooling is extremely rapid, no crystals have a chance to grow.
• forms volcanic glass (obsidian  or pumice)

Intrusive rocks
Underground magma cools slowly
Large crystals can grow

 Textures
• Glassy
• fine grained
• coarse grained
• porphyritic (2-sizes)
 

Compositions: grouped by amount of silica.
 
 

	Coarse-grained	Fine-grained
Mafic	Gabbro	Basalt	Least Si, Most Fe,Mg, densest, darkest color
Intermediate	Diorite	Andesite
Felsic	Granite	Rhyolite	Most Si, Least Fe, Mg,  least dense, lightest color

Ultramafic rocks: peridotite (intrusive)
most dense
least silica
most Fe, Mg
makes up the Earth’s mantle

Where do magmas form?
Controlled by:
temperature
pressure (higher P raises melting point)
water (lowers melting point)
composition (minerals melt at different T)

Bowen’s reaction series, lists in order of melting T from high to low:
    olivine, pyroxene, amphibole, biotite mica, potassium feldspar, muscovite mica, quartz

partial melting leads to melt higher in Si than remaining rock
 

Igneous rocks and Plate Tectonics

divergent margins: mantle partially melts to form mafic magma.

Subduction zones:
partial melting of mafic and ultramafic rocks (with water) forms intermediate rocks
Partial melting of continental crust forms felsic magma

How magmas move
Move upward due to lower density
Flow slowed by:
 overlying rock
magma viscosity
greatest at low temperature and high Si

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Curious about igneous rocks? web pages have been linked by the textbook publisher.