The pressure & friction from deep in the earth is what is producing all that heat in those cracks. And the expansion of that water is forced up and out of the earth wherever there's an opening. And yes, the ground underneath is definitely shifting along with that lakebed. The smell of sulfur around those plumes was 'otherworldly', as I recall.
Yeah I loved my 5 years living near Yellowstone. Got to hike and visit everything worth seeing. One of my favorite areas in the world. It will be fun to see it erupt in December.