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Field Trip to Mars

Columbia Plateau Basalt

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Columbia Plateau basalt areaThe 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, schist, gneiss, quartzite 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.

Click for larger image of Drying Mud

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.

Click for larger image of basalt column

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.

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Bubbles made by lava in waterPillow 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 (center) formed as shown in Pillow Basalt 2.

Click for larger image of basalt column

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. 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. Hackly basalt is more resistant to erosion and is why scoured knobs remain after the columns have been removed.

Most of the basalt seems to have erupted in northwestern Oregon near what is now the Grande Ronde river drainage. These Grande Ronde basalts are estimated to be about 85% of the total basalt with earlier and later flows originating in nearby areas. No one knows what process started the basalt flows but theories range from continental plate movements (plate tectonics) to an asteroid impact where the Idaho, Oregon and Nevada borders meet.

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Geology Terms

Here are some basic terms used in the tour. Find more geology terms in the Glossary.

Here are some basic terms used in the tour. Find more geology terms in the Glossary.

Basalt
Volcanic rock caused by partial melting of the Earth's crust.
Bedrock
Solid layers of rock in the Earth's crust beneath soil.
Caliche
One or more layers of calcium carbonate deposited as water holding the carbonates in suspension evaporates. This is similar to hard water deposits on drying dishes.
Coulee
Long winding channel cut through lava formations.
Geomorphology
The study of the changes in landforms due to volcanoes,
earthquakes, weather, floods, etc.
Gneiss
Metamorphic rock with bands or or streaks.
Granite
Coarse-grained igneous rock usually without obvious bands or markings.
Loess
Fine dirt deposited by wind usually from arid or glaciated areas.
Miocene
The period of geologic time that began about 24 million years ago and ended approximately ten million years ago.
Pleistocene
The period of geologic time that began about two or three million years ago and ended approximately 8,000 years ago.
Pliocene
The period of geologic time that began about ten million years ago and ended approximately two or three million years ago.
Rain Shadow
A mountain or mountain range that blocks rain clouds just as an object might block sunlight to form a shadow. Areas in the shadow are more dry as a result.
Sediment
Collection of sand, silt, gravel and organic material that sinks to the bottom of a river, lake or ocean. Some or all of these materials may be present.
Volcano
A vent at the surface where magma, gas and steam erupt. Also, the landform constructed by volcanic material.
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