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Mantle plumes that form hotspots are thought to be relatively stationary whereas the overlying tectonic plates typically are not. Thus, as a plate moves over the location of a plume eruption, it carries successively older volcanoes with it.
The volcanic field is made of flood basalts that cover an area of 9,650 square miles (25,000 square km). The flows extend about 60 miles (100 km) north and south away from the arch. The youngest lava range in age from 750,000 to 900,000 years old. The oldest flows are about 2.7 million years old