Igneous Rocks Text ref. Ch5 (pg. 98)

Contents What are igneous rocks? Classifying igneous rocks.

What are igneous rocks? Text ref. ch.5.1 (pg 99-106)

Objectives Compare and contrast intrusive and extrusive igneous rocks Describe the composition of magma Discuss the factors that effect how rocks melt and crystallize

Igneous Rocks Formed from the crystallization of magma Lava is magma the flows out onto the Earth’s surface. Extrusive igneous rocks are fine grained and cool quickly on Earth’s surface (magma) Intrusive igneous rocks are coarse grained and cool slowly beneath the Earth’s surface. (lava flows)

Composition of Magma Magma is often a slushy mix of molten rock, gases, and mineral crystals. Magma is classified on its SiO2 content Group SiO2 content Rhyolitic 70 percent Andesitic 60 percent Basaltic 50 percent

Factors that Affect Magma Formation Temperature Pressure Water content Mineral composition

Temperature Temperature gradient compares the internal temperature of the Earth with depth.

Pressure Pressure also increases with depth. In general, as pressure on a rock increases, so does its melting temperature. Ex. Rock sample may melt at 1000°C at Earth’s surface, but 1400°C at a depth of 100m.

Water content Also affects magma formation. As water content increases, melting decreases

Mineral content Magma containing minerals such as olivine, calcium feldspar, and pyroxene melt at higher temperatures In general oceanic crust tends to melt at higher temperatures than continental crust, due to its Fe content.

How Rocks Melt Rocks melt depending on their composition Partial Melting Fractional Crystallization

Partial Melting The process whereby some minerals melt at low temperatures while other minerals remain solid is called partial melting.

Fractional Crystallization The process wherein different minerals form at different temperatures is called fractional crystallization.

Fe-Mg minerals

Iron-rich minerals Left branch of Bowen’s reaction series represents iron-rich minerals. Undergo abrupt changes during fractional crystallization.

Layered Intrusions Minerals sometimes form in layers according to Bowen’s reaction series. Known a layered intrusion Important formations in mining: largest sources of platinum in U.S. found in layered intrusions in Montana

Classifying Igneous Rocks Text ref. Ch.5.2, 107-113

Objectives Classify different types and textures of igneous rocks. Recognize the effects of cooling rates on the grain sizes of igneous rocks. Describe some uses of igneous rocks.

Mineral Composition Felsic rocks such as granite are light-colored, have high silica contents, and contain quartz and the feldspars orthoclase and plagioclase.

Mineral Composition Intermediate rocks such as diorite contain moderate amounts of biotite, amphibole, and pyroxene

Mineral Composition Mafic rocks such as gabbro are dark-colored, have lower silica contents, and are rich in iron and magnesium.

Mineral Composition Ultramafic rocks are unusual, containing peridotite, dunite, have low silica contents and very high levels of iron and magnesium.

Grain Size Igneous rocks differ in the size of the grains that compose them Cooling rates will affect grain size Fast cooling rocks will form smaller grains Slow cooling rocks will form larger grains

Texture Distinct crystal shapes caused by fractional crystallization can be used to indentify igneous rocks Porphyritic texture is characterized by large crystals surrounded by smaller crystals of the same or other minerals

Igneous Rocks as Resources Used as building materials Granite

Ore Deposits Veins of gold, platinum, silver, and other precious metals can be found in igneous rock deposits.

Pegmatites are veins that contain extremely large-grained crystals Pegmatite containing Amethyst

Kimberlites Diamond is found naturally occurring ultramafic rocks known as kimberlites. Named after Kimberly, South Africa where they were first discovered Form 150-300 km; intense pressures required for kimberlites to form