Collaborative Research: Understanding the crustal link between the Columbia River flood basalts and lithospheric foundering.

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The Earth?s largest volcanic events are the rare flood-basalt eruptions that occur once every about 40 million years, on average. Some of these events appear to have caused major extinctions, and in net, the erupted lavas cover a significant portion of the Earth; they also represent the beginning of hotspots such as Yellowstone. The most recent and best-exposed flood basalt event erupted about 16 million years ago from eastern Oregon. This is the target of our study. The fundamental driving mechanism for flood basalt eruptions occurs below the lithospheric plate, and the event has a major effect on modifying the plate. Yet, we do not understand the fundamentals of these events. It our case, there is strong evidence that large pieces of the affected lithosphere foundered and sank into the asthenosphere, and that the distribution of the foundering lithosphere controlled the location and distribution of volcanism. Our study will use seismic imaging methods to image the structures created and modified by this event, both in the crust and in the underlying lithosphere and asthenosphere, with the specific goals of finding the primary magma chamber(s) and resolving their shape.We will deploy about 60 seismometers in a line extending north 150 km from the center of the main eruptive center in northeast Oregon into southwest Washington, out of the eruptive area. We will use seismic waves that propagate up from distant earthquakes that occur around the world. The two primary techniques to be employed involve (1) the conversion of P waves to S waves that occurs when the wave hits an interface (such as the boundaries of the now-frozen magma chamber(s) we are studying), and (2) the time advance (or delay) that occurs for each ray as it passes through the region of interest. These delays are used to make a tomographic image of the major volumes of Earth that have been created by the flood basalt event.