GEOLOGY 101 Course Website: http://fouch101.asu.edu Today: Chapter 6 “Up from the Inferno: Magma and Igneous Rocks” Instructor: Professor Matt Fouch Email: fouch101@asu.edu Office: PSF-540 Phone: x5-9292 TA: Kara Krelove Email: kara.krelove@asu.edu Office: PSF-209 Course Website: http://fouch101.asu.edu

Igneous Rocks Devil’s Tower, Wyoming

Igneous Rocks: Why should we care? Igneous rocks make up the bulk of the Earth’s crust Earth’s mantle is essentially an enormous igneous rock Igneous rocks are economically important Many igneous rocks form striking landscape features

Igneous Rocks Formed from the cooling and consolidation of lava or magma plutonic (intrusive) — cooled below the surface volcanic (extrusive) — cooled on the surface

Magma Vs. Lava What is the difference? Magma – molten rock beneath Earth’s surface Lava – magma that has traveled to the surface

Fig. 6.02 W. W. Norton

What Makes a Magma? Usually a silicate melt (liquid) at high temperatures (650 to 1200°C). Mixture of all the elements that make up minerals plus volatile components: H2O, CO2, Cl, F, S These components form gases and will boil off when pressure is released.

Melting Rocks How do we melt rocks? 3 ways: Raise the temperature “heat-transfer melting” Lower the pressure “decompression melting” Add volatiles H2O, CO2, etc.

Temperature scale digression… Celsius (centigrade) scale Fahrenheit scale Water boils 100oC 212oF oC = (oF - 32) 5 9 Water freezes 0oC 32oC

Fig. 6.04a W. W. Norton

Melting temperature increases with increasing pressure (=depth) SOLID LIQUID Melting temperature curve

Fig. 6.04b W. W. Norton

Melting temperature increases with increasing pressure (=depth) Add water: decreases melting temperature Temp. Depth SOLID LIQUID Melting temperature curve A wet rock melts more easily

From Magma to Igneous Rock Cools Solidifies (freezes) Forms silicate minerals

Composition of Magmas Silicates are divided into groups based on relative amounts of silica (SiO2), and magnesium (Mg) and/or iron (Fe) 4 primary types: Silicic (70% SiO2; low Mg and Fe) Intermediate (55% SiO2; low Mg and Fe) Mafic (< 50% SiO2; high Mg and Fe) Ultramafic (< 40% SiO2; very high Mg and Fe)

Minerals in Mafic and Felsic Rocks Rock type Rich in Poor in Minerals in Rock MAFIC (DARK) Fe and/or Mg Si olivine pyroxene amphibole biotite Silicic (LIGHT) Si, K, Na, Ca Fe and/or Mg quartz muscovite feldspars

Crystallization Ideally, crystallization (freezing) is the opposite of melting. In fact, the process is more complicated than that because rocks are complex aggregates of many minerals with different melting (crystallization) points.

Simple crystallization Example: Quartz When melt reaches the crystallization temperature of a mineral, the mineral forms and undergoes no further changes with subsequent cooling.

Fractional crystallization The modification of magma by crystallization and removal of mineral phases. Because only certain elements will go into a given mineral, this will tend to change the composition of the remaining liquid.

Early Crystallization

Partial melting Opposite of fractional crystallization Last minerals to form will melt at lowest temperature Biggest changes will be for small degrees of melting

Fig. 6.05a W. W. Norton

Viscosity: An Important Factor in Magma Movement Viscosity = Resistance to flow i.e., honey vs. water Factors in igneous rocks: Composition: higher SiO2 -> higher viscosity lower volatiles -> higher viscosity Temperature: lower temperature -> higher viscosity

Tectonic Settings of Igneous Activity

Volcanic Island Arc, Indonesia

Oceanic Hot Spot Hawaii

Continental Volcanic Arc N. Cascades

Types of Igneous Structures

Sill Sill

Dike

Dike (Grand Tetons, Wyoming)

Batholith – Sierra Nevadas

Volcanic Necks

Volcanic Neck (central France)

Classification of Igneous Rocks We classify igneous rocks using two major properties: Texture Is the rock extrusive or intrusive? Chemical Composition Relative amounts of Si, O, Mg, Fe, K, Al, Na, Ca

Intrusive Granite (large crystals)

Extrusive Basalt (small crystals + glass)

Igneous Textures Glassy no minerals present Crystalline rocks made of mineral grains a) Coarse grained b) Fine grained c) Mixture of coarse and fine Vesicular with bubble holes Pyroclastic fragmental texture

Xl Size and Cooling Rate crystal size cooling rate Slow cooling larger crystals Fast cooling small or no crystals

Igneous Textures forms far below surface slow cooling intergrown fast cooling magma/lava forms at or near surface sometimes holes present can’t see individual crystals forms far below surface slow cooling intergrown crystals magma cooled slowly for a while before erupting minerals crystallized at different temperatures and/or rates very rapid cooling ions unable to unite in orderly crystalline structure Fine-grained Coarse-grained Mixture of coarse and fine Glassy

Intrusive/Extrusive Igneous Rocks Granite and rhyolite Chiefly composed of quartz and feldspar Same chemical composition – different cooling rates Melting point ~800 °C High viscosity High silica content ~70-75% SILICIC Granite Rhyolite

Intrusive/Extrusive Igneous Rocks Diorite and Andesite Chiefly composed of plagioclase Same chemical composition – different cooling rates Melting point ~1000 °C Medium viscosity Intermediate silica content (~60%) INTERMEDIATE Diorite Andesite

Intrusive/Extrusive Igneous Rocks Gabbro-Peridotite and Basalt >50% pyroxene and olivine Same chemical composition – different cooling rates Melting point ~1200 °C Low viscosity Low silica content ~45-50% MAFIC Gabbro Basalt