Ubehebe Crater, Death Valley
Ubehebe Crater is a half-mile wide hole in the ground, around 600-700 feet deep. It’s a bit unlike your stereotypical volcano, in that there are no lava flows or ash deposits. It is a big hole, but the walls of the crater are composed of orange and yellow lake sediment, and a nicely exposed fault (above). There are just little fragments of basaltic rock spread over several square miles, and a dozen or so craters. Ubehebe is the biggest.
Read more about Death Valley and its volcanic activity here.