The Rock Cycle & Igneous Rocks November 11, 2008 Copy of this lecture will be found at: http://www.owlnet.rice.edu/~esci101 With Some Graphics from Press et al., Understanding Earth, 4th Ed. (Copyright © 2004 by W. H. Freeman & Company)‏

The Rock Cycle Melting & Intrusion Solidification of melt Mountain Building Uplift & Exposure Weathering Erosion & Transport Deposition & Burial Metamorphism Fig 4.9

Genetic Classification of Igneous Rocks Intrusive: crystallized from slowly cooling magma intruded within the Earth’s crust; e.g. granite, gabbro. Granite Gabbro

Course-grained, interlocking Rock Texture Intrusive: Course-grained, interlocking Fig. 5.1

Genetic Classification of Igneous Rocks Extrusive: crystallized from rapidly cooling magma extruded on the surface of the Earth as lava, … Rhyolite Basalt

Rock Texture Intrusive: Course-grained, interlocking Extrusive: Fine-grained, discrete crystals, often glassy Fig. 5.1

Genetic Classification of Igneous Rocks Extrusive: … or erupted as pyroclastic material, i.e., fragmented pieces of magma ejected and cooled in the air. Ash Pumice Scoria

Formation of Igneous Rocks Pyroclasts Extrusive Intrusive Porphyry: partially crystalline Fig. 5.3

Process of Intrusion Intrusive rocks fracture and cross-cut the “country rock”. Igneous rocks cool quickly near the intrusive contacts, and cause contact metamorphism. Fig. 5.2

Composition and Classification of Igneous Rocks Chemistry: e.g. % SiO2 Mineralogy: e.g. Felsic - high silica Intermediate - intermediate silica Mafic - low silica Ultramafic - very low silica (continental crust)‏ (oceanic crust)‏ (mantle)‏

Felsic Igneous Rocks: Igneous rocks rich in minerals high in silica and low in iron and magnesium. They include: Granite Rhyolite

Mafic Igneous Rocks: Igneous rocks rich in minerals low in silica and high in iron and magnesium. They include: Gabbro Basalt

Common Minerals

Intermediate Igneous Rocks: Igneous rocks intermediate in composition between felsic and mafic igneous rocks. They include: Granodiorite (Dacite)‏ Diorite (Andesite)‏

Ultramafic Igneous Rocks: Igneous rocks with very low silica content, consisting dominantly of mafic minerals. The most common ultramafic rock is: Peridotite (no extrusive equivalent)‏

Fig. 5.4

Compositional Classification Granite Compositional Classification Quartz Orthoclase Biotite Granite Plagioclase

Compositional Classification Granite Compositional Classification Granodiorite Quartz Granite Amphibole Plagioclase

Compositional Classification Granite Compositional Classification Granodiorite Diorite Granite Plagioclase Amphibole

Compositional Classification Granite Compositional Classification Granodiorite Diorite Granite Gabbro Plagioclase Pyroxene

Compositional Classification Granite Compositional Classification Granodiorite Diorite Granite Gabbro Pyroxene Olivine Peridotite

Felsic Intermediate Mafic Table. 5.2 Granite Granodiorite Diorite Gabbro Rhyolite Dacite Andesite Basalt Viscosity Melting Temperature

When rocks melt (or partially melt). How do magmas form? When rocks melt (or partially melt). When do rocks melt? When the temperature exceeds the melting point of the rock or some minerals within the rock.

Factors that Affect Melting of Minerals (and Rocks) Composition: Felsic minerals melt at lower temperatures than mafic minerals Pressure: Increased pressures raises melting points Water Content: Increased water content lowers melting points

The Formation of Magma Chambers Magma pools in magma chamber Magma rises Buoyant melt migrates through rock pores and fractures. Less dense magma Melt is less dense than solid. Low density minerals tend to melt first. Partial melting Some minerals melt before others. Results in mixture of melt and solid.

Covers 15,400 mi2 !! Composition Granite Monzonite Granodiorite Diorite Even Gneiss All in one “magma chamber”

e.g., granite, granodiorite, diorite Why do magmas have such different compositions - even when derived from a single “parent magma” e.g., granite, granodiorite, diorite

Magma Differentiation The process by which rocks of various compositions can arise from a uniform parent magma Occurs because different minerals crystallize at different temperatures (i.e., the opposite of partial melting) Tune in Wednesday for the exciting conclusion!