Layers and Layers of Lava
The Columbia Plateau basalt covers the pink area in the map at left. It also covers part of eastern Oregon and northern Idaho. Most commonly referred to as a saucer shape the Plateau varies in thickness. The basalt near Pasco, in the center of the state, is up to 10,000 feet thick. Geologists have counted up to 300 layers or flows which cover the bedrock made of granite and other forms of cooled lava. (Image: WA State DNR)
The basalt formed some 10 to 30 to million years ago according to dating of plant fossil remains and covers older rolling hills of granite and other rock. A few of the granite and quartzite hills protrude above the basalt. These steptoes, named after Steptoe Butte in southeastern Washington, may even have been covered and then the basalt was stripped away by weathering and erosion. On top of the various basalt layers are deposits of river silts, sand, gravel and lakebeds from ancient lakes. In addition, there are deposits up to 200 feet deep of very fine sand particles (loess) blown in by glacial winds.
The material in basalt is from magma deep within the Earth's crust. Fully melted rock in magma that reaches the surface cools into granite. When the magma is from partially melted rock the lava becomes basalt. As the basalt flow cools the top and bottom layers separate into 5 to 7 sided sections similar to drying mud. See Section A of the drying mud image at right. Section A has been outlined in black to show a geometric shape about one foot across. Click on image for a detail view.
As the cooling continues the column is formed from the bottom up and the top down. Very thick flows have a bottom row of columns and a top row of columns separated by an irregular, rough layer, called hackly, where the two columns didn't match up as they cooled. These flows can be seen quite readily in the Banks Lake area of Lower Grand Coulee. Fast cooling flows solidify before forming columns and are more block-like. Fractures can be seen in these columns at left. The orange-red spots are from iron oxides (rust). Click on image for a detail view.
The seams where the columns meet are weak and a basalt layer may break along these columnar joints. Water seeps between the joints and during winter can freeze which forces the joint apart. The distinct talus at the base of basalt cliffs is a result of this process. Near the top of these columns at left are areas that look like a stack of plates as you can see in the larger image. (Click on image for a detail view). The pressure from magma pushing towards the surface can also pry apart the joints. When this happens the cooled lava within the basalt is called a dike. These weak joints are also the reason a column can be plucked away from the others during floods.
Here are some basic terms used in the tour. Find more geology terms in the Glossary.
- Volcanic rock caused by partial melting of the Earth's crust.
- 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.
- Long winding channel cut through lava formations. A term primarily used in the northwestern United States.
- 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.
- Molten earth material (rock) that comes out of volcanoes or cracks in the Earth's crust.
- Fine dirt deposited by wind usually from arid or glaciated areas.
- Molten rock beneath the earth's surface. Magma is called "lava" when it erupts from a volcano.
Kids' Cosmos… Expanding Minds Beyond the Limits of the Universe
P.O. Box 14077, Spokane, WA 99206-4077
© 2011 Kid's Cosmos
This tour created with the support of: