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 17–14 million years ago, when over 99% of the basalt was released.
During the middle to late Miocene epoch, a massive lava flow that covered about 163,700 square kilometers (63,000 square miles) of the Pacific Northwest formed a large igneous province with an estimated volume of 174,300 cubic kilometers. The eruptions were most intense between 17 and 14 million years ago, when over 99% of the lava was released.