Our Boise Valley provides good examples of the lessons from Earth Sciences class: a river valley formed by erosion caused by the river itself and soil made from eroded rock. But to the east, the Snake River plain reveals a much different origin. Fringed by acres of lava, farms grow Idaho’s famous potatoes in soils that consist mostly of volcanic ash. Geologists believe that volcanic activity advanced steadily from the very southwest corner of Idaho (location 1 on the map), melting mountains in its path, all the way to Yellowstone (location 6). Geysers at Yellowstone National Park are evidence of the fury that convulsed that area in ancient times; the last eruption there was 1000 times larger than that of Mount St. Helens.
The progression of volcanic activity from the Owyhee country to Yellowstone reflects the movement of the crustal plate that bears southern Idaho over the earth’s mantle. The plate is carrying us to the southwest at a rate of two inches a year. A weak spot in the upper mantle has allowed hot molten magma to approach the earth’s surface. Scientists believe the topography of southern Idaho reveals the searing path of that hot spot during the last 13 million years.