Pillow Basalt

Back to Basalt Lava

Columbia Plateau Basalt

Geological History

Bubbles and Steam

Bubbles made by lava in water Pillow Basalt
Flows that encounter water cool rapidly into shapes called Pillow Basalt. Hot basalt and steam react to form palagonite, a soft yellowish rock full of clay as seen in Detail A at right. The area is about 18 inches across. The palagonite is mixed in with the sand and mud which surrounds the pillow basalt. Pillows can be as small as pebbles or as large as a few feet across.

The pillow basalt images below show some of the different forms the basalt can take as it enters water.

Click for larger image of Pillow Basalt 1

Click for larger image of Pillow Basalt 2

Click on images for a larger view.
The Pillow Basalt 1 image (left) shows forms that are about 3 feet long.
Football-sized pillows (right) formed as shown in Pillow Basalt 2.

The weathered basalt in the road cut (below top) looks very different than the pillow basalt of the next road cut (below bottom).

Weathered basalt in the road cut

Pillow basalt

Weathered basalt above and pillow basalt below.

Some 200 million years ago the western edge of the North American continent was about 60 miles west of where Spokane is now. Fossils of ocean plants and animals have been found in the now-dry eastern Washington area.

How did this lush seacoast become a semi-arid desert? The story continues on the Geological History page.

Geology Terms

Here are some basic terms used on this page. Find more geology terms in the Glossary.

Basalt: Volcanic rock caused by partial melting of the Earth's crust.
Channel: The deepest part of a river or bay.
Channeled Scabland: Area in Washington state where huge floods made channels in a large, deep basalt flow. Named by J Harlan Bretz during the 1920's in various publications. See also Glacial Lake Missoula.
Coulee: Long winding channel cut through lava formations. A term primarily used in the northwestern United States.
Erosion: Lifting and removal of rock, dirt, sand and the like caused by wind, water, or glacial ice.
Ice Age: A period in Earth's history when much of the continents are covered with ice sheets and glaciers.
Lava: Molten earth material (rock) that comes out of volcanoes or cracks in the Earth's crust.
Loess: Fine dirt deposited by wind usually from arid or glaciated areas.
Magma: Molten rock beneath the earth's surface. Magma is called "lava" when it erupts from a volcano.
Palagonite: Pale yellow, glassy material that forms when hot steam and other gases contact water during a lava flow. Sand and clay is usually mixed in as well.
Pillow Basalt: Basalt formed underwater or as a basalt flow contacts a river or lake.

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