What caused the hot springs in Yellowstone Park?

Ages ago, Yellowstone National Park was a region of seething volcanic activity. Rivers of molten lava oozed up from below and spread over the surface. The top layer of the molten lava cooled in the air and became solid rock. This outer crust prevented heat from escaping from the deeper layers of lava. These layers, sealed below the surface, are still hot.

Ground water seeps down through cracks and crevices. Here and there it gathers in a pocket of hot rock which acts like a kettle. The water becomes warn. and bubbles up to the surface as a hot spring. There are thousands of these hot springs in the park, some warm, some hot, some seething with steam.