Using tomographic images based on seismic waves, relatively narrow, deeply seated, active convective plumes have been detected under Yellowstone and several other hot spots.These plumes are much more focused than the upwelling observed with large-scale plate-tectonics circulation.This subsidence of the crust produced a large, slightly depressed lava plain now known as the Columbia Basin or Columbia River Plateau.The northwesterly advancing lava forced the ancient Columbia River into its present course.The Ginkgo basalt was examined over its 500 km (310 mi) flow path from a Ginkgo flow feeder dike near Kahlotus, Washington to the flow terminus in the Pacific Ocean at Yaquina Head, Oregon.The basalt had an upper melting temperature of 1095 ± 5 °C and a lower temperature to 1085 ± 5 °C; this indicates that the maximum temperature drop along the Ginkgo flow was 20 °C.Once that magma breaches the surface, it flows as lava and then solidifies into basalt.Prior to 17.5 million years ago, the Western Cascade Stratovolcanoes erupted with periodic regularity for over 20 million years, even as they do today.
The cooling/hydraulics analyses are supported by an independent indicator; if longer periods were required, external water from temporarily dammed rivers would intrude, resulting in both more dramatic cooling rates and increased volumes of pillow lava.
An abrupt transition to shield volcanic flooding took place in the mid-Miocene.
The flows can be divided into four major categories: The Steens Basalt, Grande Ronde Basalt, the Wanapum Basalt, and the Saddle Mountains Basalt.
The Latah Formation sediments of Washington and Idaho are interbedded with a number of the Columbia River Basalt Group flows, and outcrop across the region.
Absolute dates, subject to a statistical uncertainty, are determined through radiometric dating using isotope ratios such as (800,000 sq mi) in Russia.
Some time during a 10–15 million-year period, lava flow after lava flow poured out, eventually reaching a thickness of more than 1.8 km (5,900 ft).