Flood basalts are thought to typically occur when the head of a giant mushroom-shaped upwelling of hot rock rising from near the Earth's core, known as a mantle plume,reaches the surface. Now researchers suggest a new way for these massive eruptions to form - a breach in a massive slab of the Earth's crust.
Scientists generated computer models of how the complicated structure of the Earth's mantle layer under the western United States evolved over the past 40 million years. They based their work on data from the USArray, a mobile seismic networkof 400 sensor stations traveling across the United States.
The researchers suggest that about 17 million years ago, a giant chunk of rock known as the Farallon slab that was diving underneath the western United States began ripping apart. This led to massive outpourings of magma, the pattern and timing of which appear consistent with the Steens–Columbia River flood basalts.
"When the slab is first opened, there's a little tear, but because of the high pressure underneath, the material is able to force its way through the hole," said researcher Dave Stegman, a geophysicist at the University of California, San Diego. "It's like in the movies when a window breaks in an airplane that is at high altitude - since the cabin is at higher pressure, everything gets sucked out the window."