During the middle to late Miocene epoch, one of the largest flood basalts ever to appear on the earth's surface engulfed about 163,700 km2 (63,000 mile²) of the Pacific Northwest, forming a large igneous province with an estimated volume of 174,300 km³. Eruptions were most vigorous from 17 to 14 million years ago, when over 99% of the basalt was released.
During the middle to late Miocene epoch, one of the largest lava flows ever seen on Earth's surface covered approximately 163,700 square kilometers (63,000 square miles) of the Pacific Northwest, forming a significant volcanic region with an estimated volume of 174,300 cubic kilometers. Eruptions were most intense from 17 to 14 million years ago, when over 99% of the lava was emitted.