Diane, I don't know exactly were you are in the Northwest but as you know large parts of Washington, Oregon and Idaho are covered with basalt. Before 15-17 million years ago the Cascade volcanoes were doing what they do now, exploding periodically and spewing out ash and lava. Suddenly they changed to shield volcanoes, like those in Hawaii, pouring out copious amounts of lava that in some places is greater than 6000 ft. thick, which cooled to become basalt.
Here's the neat part and it's called Hot Spot theory. Around the globe there are a number of these spots, mantle plumes which act like blowtorches punching holes in the overlying crust. Hawaii is over one. As the Pacific Plate moves northwest over the stationary plume new volcanoes form southeast of the older ones. The relative motion of the plate makes it seem that the plume southeast is moving but it's actually the crust moving above it. The Hawaiian chain can be traced to Midway Is. and beyond, eventually becoming old worn down sea mounts to the northwest. A new shield volcano is currently building below sea level southeast of the big island of Hawaii, which will extend the island chain in that direction.
Here's how that relates to your part of the world. As the North American Plate moved westward at a few centimeters a year over one of these hot spots the old strato-volcanoes of the Cascades were transformed into shield volcanoes and became gigantic lava flowing engines. When the hot spot moved (relatively speaking) eastward the Cascades reverted to their old, and present form, and the lava flows became increasingly younger in that direction. That Hot Spot is now located under Yellowstone National Park and that's why there is the potential for some really serious volcanic activity in the area.
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Very good! Thanks! I don't know a lot about geology, but I do know we have some unique stuff in the far north of California and southern Oregon. There are a lot of basalt formations and the area from about the Tule Lakes up through Chiloquin is one giant batholith. Bubble! There are hot springs and mineral springs all over this particular area - Little Bear Valley - and that's why the city of Ashland has become famous for its lithia water. For over 100 years people, including famous ones like Teddy Roosevelt and his shirttail cousin FDR, have been coming to the various springs around here. There are more, and better, back in them thar hills but so far only mountain goats and us Indians can get in there! It's all the volcanic activity. Mt Shasta is the main one but there is Crater Lake, a caldera, and Mt Mazuma, a collapsed caldera. Pilot Rock is a cinder cone and so named because small planes kept running into it. (The Indian name has been sanitized to Standing Stone but the real name is a certain part of male anatomy which the cinder cone resembles almost exactly!) The showpiece of the Rogue Valley is the two massive table rocks - Upper and Lower Table Rock - which are basalt formations. Mt Shasta was long considered dormant but ever since St Helens blew out, Mt Shasta has been listed as active. All the landscape around here has been formed by its eruptions. There are lumpy little hills all over the Shasta Valley to the north and northwest - that is an ancient slump. The mountain was once about three times as high as it is at present. Shastina, the smaller cone, is actually the top of the mountain. The last eruption was in the 1700s and she literally flipped her lid! It landed beside her - and legend has it, on top of a Shasta village. The area to the east, McCloud, is peculiar because it's made from floods caused by glaciers on the mountain melting during eruptions. Weed and Mt Shasta, towns toward the south, tended to get the gas and ashes. Sure a lot of ways to be killed by a volcano!