In simplified form, the earth is in three layers, the super hot internal core, the mantle, and the crust. That super heated internal area forms mantle plumes where the magma comes closer to the surface. And then it escapes through fractures in the crust or as explosions when the pressure builds significantly. Once it reaches the crust, the magma is termed lava.
While the core and mantle stay put, the crust moves slowly (and by that I mean geologically slowly, an imperceptible creep except to the trained eyes of geologists), over hot spots. At this crust has moved, due to plate tectonics, a line of ancient volcanic activity traces a line across current northern Nevada, southern Idaho and completes, for the time being, under Yellowstone. Craters of the Moon is one of the places the lava erupted and oozed through fractures in the Great Rift running 13 miles through what is now the park. Different events and cooling have left behind cinder cones, spatter cones volcanic bombs, lava tubes and caves. In places the lava is what is called a'a, a Hawaiian term, which is usually very rough on the surface and fragmented. Other areas are dominated by Pahoehoe, a more liquid form that cools in flowing waves. Within those two basic types are the manifestations of three kinds of rock, and multiple variations within those, including pumice, which is filled with air, and floats in water, or obsidian which is also called black glass and is sharp enough to make long lasting knives and surgical instrument.
But enough of my primitive descriptions. Here are some photos of the landscape at Craters of the Moon and the hearty plants and animals which are reclaiming this land.
While I traveled off the main roads onto dirt roads to get the incredible storm clouds over this agricultural valley between Craters of the Moon and Arco, ID, I also had a flat tire on the car. Joy. But as I dealt with it, a nice young couple stopped and the young man put the donut on. I was able to get a permanent fix in Arco the next morning.
I am posting this from Idaho Falls where I am parked at the Elks Lodge and doing final chores before the last leg into the park and another summer driving one of the historic yellow roll-top busses on photo trips.