Without water, clay keeps accumulating. The result is mud. Lots of mud that's thick and sticky. And the gases underneath are what cause the mud to 'plop' and splatter. Thermal features are the product of the fact that Yellowstone is a very active, energetic volcanic plateau, and that was created because North America is moving over a hot spot, just as Hawaii moves over a hot spot over the Pacific Ocean.
Hot spots bring heat up from the earth's crust.
The crust melts, creating volcanic activity and stores heat close to the surface. . . Yellowstone is blessed with lots of precipitation, mainly from winter snow. As a result, cold water sinks and hot water rises here. Each year, on average, Yellowstone has 12-18 'felt' earthquakes. There are tens of thousands of earthquakes in Yellowstone every year, but most of them are 'micro,' not felt.