Ask GeoMan...

Which country on earth has the most basalt?

-Landon, 3rd grade, Ogden, Iowa

Basalt is an igneous rock. Plate tectonics causes some areas of the crust to separate, and mafic magma from the upper mantle is allowed to move to the surface and erupt as a volcano. Because basalt is mafic, it is generally heavier than the granitic rocks that make up the continents. Because of this density difference, basalt is usually found on the seafloor. Therefore, since it's what makes the seafloor, MOST basalt is not part of ANY country.

However, most of the continents will have some basalt exposed at the surface. These areas of basalt are often a piece of the seafloor which has been folded and pushed up onto the land (and is now called an "ophiolite"). Other areas of basalt are related to what is called "continental rifting," where the land is being pulled apart and basaltic magma is allowed to escape to the surface. These types of lava flows are called "flood basalt" because they literally flood the land with lava.

The western United States has quite a bit of both types. Much of the Klamath Mountains of northern California and southern Oregon is broken-up pieces of 150 to 450 million year old seafloor crust (the 150 million year old Josephine Ophiolite is a good example), while most of eastern Washington and Oregon (the Columbia River Basalt), and northeastern California (the Modoc Plateau Basalt) are underlain by flood basalt.

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