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What Is the Rock Cycle?

The stuff that rocks are made from has stayed the same, but the rocks themselves have not. Over time rocks are recycled into other rocks. Moving tectonic plates are the primary agents destroying and forming various types of rocks. The processed by which rocks are made and destroyed is called the rock cycle.

The cycle diagram below (click for larger size) shows the basic processes involved with the rock cycle.

Rock Cycle

This diagram shows both the primary and secondary processes of the rock cycle.
Credits: Original artwork by Windows to the Universe staff (Dennis Ward ).

  • Sedimentary rocks are formed via weathering and transport of existing rocks, and then deposition, cementation, and compaction into a sedimentary rock.
  • Igneous rocks are formed by cooling and crystallization of molten rock.
  • Metamorphic rocks are formed when heat or pressure are applied to other rocks.
In addition to the primary cycle (Igneous-Sedimentary-Metamorphic), this cycle diagram includes the secondary processes that show that any rock type can be reformed into any other rock type. For example, a sedimentary rock can be weathered and eventually the sediments are deposited to become another sedimentary rock. And an igneous rock can be melted and cooled to become another igneous rock.

These secondary processes are often omitted from textbook illustrations. Follow this link to perform a Google image search for "rock cycle diagram." Compare the various diagrams published by the USGS, numerous textbook publishers, and other sources. If you teach the rock cycle, hows does this compare to your textbook's treatment? Feel free to discuss your findings in this week's discussion forum.

Last modified June 21, 2007 by Dennis Ward.

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The Winter 2009 issue of The Earth Scientist, focuses on Earth System science, including articles on student inquiry, differentiated instruction, geomorphic concepts, the rock cycle, and much more!

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