Photos of Pacific Coast, Cascades, Columbia Plateau
Geology of the Pacific Northwest

Basics -- The Rock Cycle

Introduction

The idea that rocks are unchanging is reflected in some common expressions ("solid as the Rock of Gibraltar," "rock solid") but rocks actually do change. Geological materials are constantly undergoing transformation from one type of rock to another. The rock cycle describes the pattern of change.

What is the rock cycle?

The rock cycle is an idea that was first stated on a scientific basis, with supporting evidence, by James Hutton, a Scotsman, in the late 1700s. He identified the starting point of new rocks of the crust as solidification of magma, forming igneous rock. This is followed by weathering and erosion of the rock.

The eroded material is deposited as layers of sediment. The sediment is buried beneath more sediment, which subjects it to greater heat and higher pressure as it is buried deeper within the earth. Eventually the sediment is compressed and cemented together to form sedimentary rock.

Upon deeper burial and increased heat and pressure within the earth, the rocks become recrystallized into metamorphic rocks. Even deeper burial and increased heat eventually melts some of the metamorphic rock, forming new magma and starting the cycle over.

Rock Cycle Diagram

diagram of the rock cycle

Why does the rock cycle work the way it does?

Part of understanding the rock cycle is realizing that the greater the depth within the earth, the higher the temperature and the greater the pressure. The earth is still producing internal heat by the breakdown of radioactive elements. Heat rises from the earth's interior. The pressure at a given point inside the earth comes from the weight of all the rocks above that point. This is called the lithostatic pressure. The deeper inside the earth, the more rocks there are above that point weighing things down, so the greater the lithostatic pressure.

Another part of understanding the rock cycle is visualizing it as a huge cycle that happens all the time, simultaneously. In the Cascade Range right now new igneous rocks are forming from magma that is intruding the crust below the earth's surface, and from new volcanic eruptions every century. Deep within the crust of the Cascade Range, rock is undergoing recrystallization and being metamorphosed into new rock, in response to all the heat and stress there. At the same time, the mountains are undergoing erosion and being turned into new sediments at the earth's surface.

A subtlety to understanding the rock cycle is the realization that not all rocks go all the way around the cycle. For example, some intrusive igneous rocks go straight from being igneous rock to becoming metamorphic rock, skipping the sedimentary part of the cycle by staying buried within the earth's crust and being subjected to changes in heat and pressure. Many metamorphic and sedimentary rocks are uplifted directly from the sedimentary rock or metamorphic rock stage and exposed to weathering and erosion, contributing to a new batch of sediments and sedimentary rocks.

These shortcuts that rocks can take from one part of the rock cycle to another show up in the rock cycle diagram as arrows across the center of the circle.

Web links

To read more about the rock cycle and the categories of rock types, check out the PhysicalGeography.net Web page about it at: http://www.physicalgeography.net/fundamentals/10a.html.

For an alternative way of viewing the rock cycle, see the UWired Web page on the rock cycle at http://www.washington.edu/uwired/outreach/teched/projects/web/rockteam/WebSite/rockcycle.htm.htm.

Glossary terms that appear on this page: magma; igneous rock; sedimentary rock; metamorphic rock


Geology of the Pacific Northwest
Basics--The Rock Cycle
© 2001 Ralph L. Dawes, Ph.D. and Cheryl D. Dawes
updated: 7/14/13