The Rock Cycle The Rock Cycle illustrates the origin of igneous, sedimentary and metamorphic rocks Igneous rocks form as molten magma or lava cools and solidifies. Magma is completely or partly molten material. Magma that reaches the surface is known as lava. Once the magma has formed, it is less dense than the surrounding rocks and rises buoyantly towards the Earth's surface. Igneous rocks form at high temperatures; at the Earth s surface they are chemically unstable and will begin to disintegrate and decompose in a process known as weathering. The products of weathering are particles known as sediment (sand grains, gravel, etc.) and rock constituents dissolved in water. Most sediments are deposited in the ocean basins but they are also deposited in other environments such as lakes, streams, desert basins, etc. Sediment is loose, unconsolidated material and is transformed into sedimentary rock through a process known as lithification (cementation and compaction). Sedimentary rocks may be buried deep enough in the Earth that they are transformed by high temperatures and pressures into metamorphic rocks. The process to form metamorphic rocks is known as metamorphism. Metamorphism may be taken to the point where the rock melts, forming magma, and the whole process starts over. We considered the outer circle in this diagram of the rock cycle. Other paths are possible; for example, igneous rocks may be transformed directly into metamorphic rocks. Additionally, sedimentary and metamorphic rocks may be weathered to form sediments. 1
What Happens in Magma? Magma is generated from Heat Geothermal gradient Friction between lithospheric plates Pressure release Pressure gradient Decompression melting Water and other volatiles Wet melting points Subduction Igneous Rocks If the magma makes it to the Earth's surface, it may produce a spectacular volcanic eruption. Igneous rocks that solidify at the surface are classified as extrusive or volcanic. If the magma crystallized before it reaches the surface, it is classified as intrusive or plutonic. Crystallization of Magma Magma usually contains some suspended crystals and dissolved gases such as H 2 O and CO 2. As magma cools, the random motion of the ions begin to slow down and arrange themselves into orderly crystalline structures. Crystallization process by which magma cools to form crystals Types of Igneous Textures Depending upon the cooling rate and mode of formation, igneous rocks can possess a variety of textures. It is important to understand that formation history can be inferred from texture. For instance, slow plutonic cooling may result in a "coarse-grained" igneous rock. There are several common igneous textures: 1. fine-grained (aphanitic) 2. course-grained (phaneritic) 3. porphyritic 4. glassy 5. frothy Igneous Textures Three factors contribute to the textures of igneous rocks: 1. the cooling rate most important 2. amount of silica present 3. amount of dissolved gases Contrasting Textures Phaneritic Texture Granite slow cooling Aphanitic Texture Ryolite rapid cooling 2
Porphyritic Texture 2 cooling times Grand Canyon Pegmatite (at the bottom!) Slow cooling with many volatiles Some Volcanic Igneous Rock Textures Igneous Compositions Igneous rocks are mainly composed of silicate minerals. Mafic composition Rapid cooling high silica Rapid cooling high volatiles Felsic composition Ultramafic rocks are very rich in magnesium and iron. Although these rocks are rare at the Earth s surface, peridotite is the main constituent of deep interior portions of the Earth (upper mantle). You can see from this diagram that gabbro is the plutonic/intrusive analog to basalt. Gabbro forms by the slow cooling of a basaltic (mafic) melt deep in the Earth. 3
Basaltic Rocks Basalt is dark green/gray to black in color and is fine-grained (aphanitic) in texture. The fine-grained texture indicates that basalt is volcanic in origin. Basalt is the most common volcanic/extrusive igneous rock - the seafloor, many volcanic islands and large portions of the continent are composed of basalt. Mafic rocks like basalt tend to be more dense than felsic rocks. Basaltic Lava Basalt commonly has a vesicular texture - that is, it contains holes where gas bubbles escaped during the cooling and crystallization of the basalt. This variety of basalt is known as vesicular basalt; when it is coarsely vesicular, it may be called scoria. Owens Valley Basaltic Lava Flow Underwater formation of Pillow Basalt Miocene Pillow Basalts 4
Intermediate rocks fall between felsic (granitic) and mafic (basaltic) igneous rocks. Andesite is a common volcanic rock that occurs along the margins of continents. Mt. St. Helens Eruption Andesite Volcano Andesite Diorite Granite is a coarse-grained (phaneritic) felsic igneous rock. It forms by slow cooling at depth in the Earth and is exposed at the surface by erosion mountain building and erosion. Granitic Rocks Granite is a familiar rock that is composed of predominantly lightcolored silicate minerals such as quartz and feldspar. Geologists refer to the composition of granitic rocks as felsic - meaning that it contains a lot of feldspar and silica. Granite is a common building material and decorative material. 5
Sierra Nevada Batholith Granite in Outcrop Rhyolite is the volcanic/extrusive equivalent of granite; the texture of rhyolite is fine-grained (aphanitic). Rhyolite commonly contains volcanic glass and ash. Ryolite Volcanoes Mt. Lassen, California Mt. Shasta, California Norman Bowen (1887-1956) demonstrated the complicated chemical behavior of crystallizing magmas in the laboratory. He showed that as a magma cools, certain minerals crystallize first at high temperatures and that as the magma continues to cool the identity of the minerals that crystallize change. This is known as the Bowen s reaction series. Bowen s reaction series indicates that we can produce a magma of nearly any composition from an initial ultramafic or mafic (basalt) magma through magmatic differentiation. 6
Weathering of Rocks to Form Sediment Earth is a very dynamic environment: Volcanic & other mountain building processes elevate portions of the Earth's surface, while opposing forces move material from high elevation to lower elevation. Weathering The physical breakdown and chemical disintegration of rocks at the earth s surface Weathering Two Types: Mechanical Weathering - physical forces that break rock into smaller and smaller pieces without changing the rock's mineral composition (same minerals). Chemical Weathering - involves a chemical transformation of rock into one or more new compounds. Weathering is the response of rocks to a changing environment: Erosion -- The physical removal of material by mobile agents such as water, wind or ice. Mechanical Weathering Mechanical weathering increases the surface area available for chemical weathering. Physical processes lead to mechanical weathering: 1. Frost Wedging 2. Unloading 3. Biologic Activity Erosional agents (water, ice) also result in "breaking rocks." Jointed Rock, Sierra Nevada Exfoliation and Domes 7
Mechanical Weathering (cont.) 3. Biological Activity Mechanical weathering can be accomplished by organisms such as trees and burrowing animals. Plant roots grow into fractures, and as they grow larger, they wedge the rock apart. Mechanical and Chemical Weathering by Plants Chemical Weathering Chemical weathering is a chemical process that breaks down minerals. 1. Dissolution 2. Oxidation 3. Hydration Spheroidal Weathering a chemical weathering process Grand Canyon of Kawai - Oxidation What s Left? Unlike other silicate minerals, quartz is particularly resistant to chemical weathering - that is why it tends to accumulate on beaches and sand dunes since everything else has weathered away. 8