geyser "silex spring" in a giant magma balloon under the surface.

Generally all geyser field sites are located near active volcanic areas
 The number of thermal features in Yellowstone is estimated at 10,000 and 200 to 250 geysers erupt in Yellowstone each year, making it the place with the highest concentration of active geysers in the world, thanks to its location in an ancient caldera. Many of these features build up sinter, geyserite, or travertine deposits around and within them.

yellowstone volcanic caldera is Like other calderas worldwide, the Yellowstone caldera landscape was created by the "roof collapse" on a subterranean chamber after molten rock — called magma — was ejected in massive prehistoric eruptions. It's almost as if there was a giant magma balloon under the surface that suddenly deflated. The deflation itself is the super-eruption, and the sunken land left behind is the caldera.
Also, as with many other calderas, there is still hot material not far underground at Yellowstone, which is why there are so many hot springs and geysers today.
As for what caused the land to inflate with magma and explode in the first place, it was a powerful "hot spot" welling up from deep in the Earth and melting rock closer to the surface into magma, says Smith.
But what caused the hot spot? And what can explain it today? Scientists are still learning answers to these questions.
"A hot spot is a long-lived point spot of magmatism," explains geologist Paul Ihinger of the University of Wisconsin at Eau Claire. Among the most famous places made by hot spots are the Hawaiian Islands, Iceland and, of course, Yellowstone.