The Rock Cycle

The History of Catastrophism and Uniformitarianism

  • Neptunists vs. Plutonists: On the origin of basalt
  • Hutton "No vestige of a beginning, no prospect of an end"
  • Cuvier and Catastrophism
  • Lyell's Principles of Geology
  • Darwin's Gradualism based in part on Lyell
  • Actualism, uniformitarianism in disguise
  • The current debate: gradualism vs. punctuated equilibrium

The Rock Cycle

Exogenetic Processes Occur at or near the surface of Earth under conditions of Standard Temperature and Pressure (STP) which is generally about 25° C and one bar. They are driven essentially by forces generated by the Sun-Earth System and by the pull of gravity.

  • Weathering: Chemical vs. Physical breakdown of rocks and minerals
  • Erosion: the removal of weathered rock
  • Transport: the movement of weathered particles and dissolved ions
  • Deposition: Sedimentation of transported particles

Endogenetic Processes produce chemical changes and tectonic uplift. They are driven by heat derived from radioisotope decay and pressure (gravity).

diagensis: the process by which loose, unconsolidated sediments are formed into hard,lithified rock

takes place far below Earth's surface under conditions of elevated temperatures and pressures. This condition of increasing T and P, causes changes in mineral structure and composition. Rocks altered in this way are said to be metamorphosed and are referred to as metamorphic rocks. Metamorphic rocks are always formed from prior rocks, whether igneous, sedimentary, or even metamorphic rocks.
Magma is silicate melt, when rock materials are heated to high enough temperatures, they may completely melt to form molten magma. Lava, flowing at Earth's surface, is essentially the surficial expression of molten magma.

Rock Types
Sedimentary: Sandstone, shale, limestone
Igneous: basalt and granite
Metamorphic: eg. slate, gneiss

Earth Systems Science

Modeling Earth Systems
  • Sources--> resevoirs -->sinks
  • fluxes, rate of mass exchange
  • Residence time is the average length of time that an element or compound spends in a resevoir. Residence time is very important to help give you a sense of how one component in a system can affect another. For example, The atmosphere contains 3.8 X 1019 moles of O2 (the resevoir) which is supplied at a flux of 1016 moles per year. Dividing the resevoir amount by the rate of O2 input yields a residence time ~4,000 years. This is the time it would take for Oxygen in the atmosphere (on average) to respond to changes in photosynthesis on Earth.

Chemical Differentiation of Earth over time
  • Silicates are recycling--> spreading ridges (source) and trenches (sinks)
  • Sediments are recycling--> The Ronov Curve
  • The formation of granitic crust from ultramafic and mafic origins
  • Changes in volatile composition, esp, shift from neutral atmosphere to oxidizing atmosphere

Plate Tectonics

Alfred Wegener and Continental Drift
Vine and Mathews and the magnetic anomolies in the Atlantic
Descriptive Plate Tectonics
  • plates are uppermost rigid (cool) mantle fused to the crust and ridge on a partially molten zone called the Asthenosphere.
  • Plate boundaries: subduction zones, transform faults and Mid-ocean rifts and ridges
The Wilson Cycle
On an average of about 500 Myr, the continents amass into a large supercontinent which, subsequently splits apart and separate continents are again set adrift. The last supercontinent was extant about 250 Ma and was called Pangea. The supercontinent prior to Pangea was called Rodinia. It existed around 1 Ga.

Web Resources

  • A Dictionary of Geologic Terms. “This glossary of geologic terms is based on the glossary in Earth: An Introduction to Geologic Change, by S. Judson and S.M. Richardson (Englewood Cliffs, NJ, Prentice Hall, 1995). Where possible, definitions conform generally, and in some cases specifically, to definitions given in Robert L Bates and Julia A Jackson (editors), Glossary of Geology, 3rd ed., American Geological Institute, Alexandria, Virginia, 1987. Photographs appearing on pages linked to this glossary from the server at Iowa State University are the property of Steven M. Richardson, unless otherwise acknowledged, and should not be copied without attribution.” (from the website).
  • Here is an interactive Rock Cyle Website with hypertext to the various parts of the diagram. (from Introduction to Petrology at The University of British Columbia.)
  • An Earth Systems Science course at the University of Arkansas by Stephan K. Boss.
  • An excellent site on Plate Tectonics from A Teacher's Guide to the Geology of Hawaii Volcanoes National Park by the Hawaii Natural History Association.

Update 15 August 2002