Arguably, the most unique volcano on Earth is the Yellowstone Hotspot. Most centers of volcanic activity on Earth occur along subduction zones or mid-ocean ridges. Volcanism also occurs at hotspots. Good examples include the Iceland Hotspot, the Hawaii Hotspot, and the Yellowstone Hotspot. The behavior of volcanoes at hotspots varies tremendously. Yellowstone tends to have tremendously powerful explosive eruptions. The last three major Yellowstone eruption have a noticeable periodicity of ~600,000 to 650,000 years. Yellowstone's last major ash eruption occurred 640,000 years ago.
The geology of the Yellowstone Hotspot is readily examined, as the entire area is preserved in a national park. Yellowstone National Park has the highest concentration of geothermal features anywhere in the world. The area is famous for its abundance of geysers, hot springs, fumaroles, etc. The bedrock at Yellowstone is almost exclusively ash beds & lava flows.