Columbia River Basalt Group - Related to Cascade Volcanoes?

Massive lava flows covered much of the inland Northwest 17-6 million years ago. The Ice Age Floods Missoula Floods later cut deep coulees and channels into the basalt flows.
The basalt surged to the surface through fissures - like cracks in the bottom of a row boat. The stack of basalt layers is more than 2 miles thick at some locations. More than 300 lava flows erupted.