Now, the subducting oceanic plate going beneath the Pacific Northwest was still there, but around 17 million years ago a hole in that plate opened up underneath eastern Oregon.

P. W. Lipman, H. J. Prostka, R. L. Christiansen, Cenozoic Volcanism and Plate-Tectonic Evolution of the Western United States. I. Early and Middle Cenozoic, Philosophical Transactions of the Royal ...

R. L. Christiansen, P. W. Lipman, Cenozoic Volcanism and Plate-Tectonic Evolution of the Western United States. II. Late Cenozoic, Philosophical Transactions of the Royal Society of London. Series A, ...