Assignments

• Chapter 3-6 in Smith and Punn
• Earth Revealed: movie. Courtesy of Anneberg Media, URL <http://www.learner.org/resources/series78.html>.  Requires Windows media Player. Sign in and go view #14 Igneous Rocks, #17 Sedimentary Rocks, and 18 Metamorphic Rocks.
• Go to the Rock Cycle by R.N. Abbott, Jr., Department of Geology, Appalachian State University - and run through problems 1 and 2.
• Quiz 4 (quicktime movie)

Terms: igneous, sedimentary, metamorphic, intrusive, extrusive, magma, lava, lithification, cementation, recrystallization, glass, detrital, clastic, silicic, mafic, phanertic, aphanitic, weathering, erosion, sedimentation, texture, composition

Rocks

Rocks compose the solid earth. Minerals make up most rocks. However, some are composed of organic material, volcanic glass, and variety of inorganic amorphous compounds. The three categories of rocks described below are defined by the processes creating them and their environment of formation.Rock Categories

Igneous rocks are formed within or on the earth from the solidification of molten rock (magma or lava). Processes: melting and solidification. The two categories of igneous rocks determined by where they cooled are:

• Intrusive: formed beneath the earth's surface
  • These rocks are typically crystalline (composed of minerals)
  • Examples: granite, diorite, gabbro
• Extrusive: formed on the earth's surface
  • These rocks are either glassy or finely crystalline (aphanitic)
  • Examples: felsite, obsidan, basalt, pumice,

Igneous rocks can also be texturally classified as crystalline or glassy. Compositionally igneous rocks are either silicic, intermediate, mafic or ultramafic according to their silica content.

Classes of sedimentary rocks based on composition:

• Detrital rocks: formed from the accumulation and cementation of rock and mineral fragments (clasts)
  • Examples: shale, sandstone, conglomerate
• Chemical and Biochemical rocks: formed from the precipitation of mineral material from water
  • Examples: rock salt, coral reef, banded iron, coquina
• Organic rocks: Derived from the accumulation and lithification of organic matter.
  • Example: coal

Metamorphic rocks are formed by the recrystallization of preexisting rocks in response to increasing pressure and/or temperature. In contrast to igneous processes metamorphism does not entail melting.  It also does not require the disintegration of the rock.

• Processes: solid state transformation (recrystallization) of an unstable mineral into a more stable mineral.

Basic classes of metamorphic rocks (based on agent of metamorphism):

• Dynamothermal (regional): Metamorphosed by an increase in heat and pressure
  • Examples: slate, phyllite, schist, gneiss, marble
• Thermal (contact): Metamorphosed by an increased in heat associated with the cooling of a nearby pluton.
  • Examples: hornfels

Classification of rocks

The classification of rocks within each category is based on the texture and composition.

• Texture: The size, shape and arrangement of the various components composing the rock.
• Composition: The minerals and/or chemical compounds composing the rock.

Rock cycle Concepts:

1. Any rock can be produced from any other preexisting rock as a result of:
   • Sedimentary processes: Weathering, erosion, deposition and lithification
   • Igneous processes: Melting and solidification, or
   • Metamorphic processes: Heating (without melting) and increasing pressure
2. Also, all rocks are recycled albeit at different rates and by different processes. For example, igneous rocks of the ocean floor are created by seafloor spreading and destroyed by subduction, a process that typically takes less than 200 my. These rocks are rarely exposed and placed into the sedimentary environment. However, rocks in continental interiors are most commonly destroyed by weathering and recycled as sedimentary rocks in inland basins or along continental margins. It may take a billion years or more before weathering exposes shield rocks for recycling. Rocks along active margins, where sedimentary, igneous and metamorphic processes are driven by plate motion, may be transform into other rocks in only a few tens of million years.

Two common versions of the Rock Cycle.  The rock cycle for oceanic crust is typically less complex than for Continental Lithosphere--unless it becomes accreted or obduction to the continental lithosphere. (See rock cycle Wikipedia) Image Source : USGS 3D Parks

Energy driving rock cycle

1. Volcanism, deformation and uplift are driven by the earth's geothermal energy and gravity
2. Weathering, erosion and other sedimentary processes are driven by solar energy and gravity

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Plate tectonics and the rock cycle Plate tectonics, driven by geothermal energy and gravity, is a major process behind the rock cycle!

Recycling of oceanic crust: Oceanic crust is created by sea-floor spreading and destroyed through subduction. The igneous crust of an ocean basin is typically recycled in less than 200 million years.

Recycling of continental crust: Recycling of continental crust is accomplished through any number of the following stages: weathering, erosion sedimentation, deformation, metamorphism, and melting.  The fastest rates of recycling occur along the continental margins where there is the greatest tectonic activity.

The hydrologic cycle, which requires water and solar energy, plays and important role in weathering, erosion and the formation of sedimentary rocks. 

Additional Online Resources

• The Dynamic Earth - The story of Plate Tectonics, Kios and Tilling, United States Geological Survey <http://pubs.usgs.gov/gip/dynamic/dynamic.html>
• Lynn Fichter's from Geology Web Sites, James Mason University
• Rock Picture Gallery form About.com

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