Geysers & Hot Springs.

Yellowstone already shows up twice on this site — as the source of real-world obsidian and rhyolite — but purely for its rock, never for the hydrothermal activity that actually made it famous. Worth telling that story too, and it ties directly back to the Volcanism page.

Yellowstone sits above an unusually large, unusually shallow body of magma — a hotspot, the same general mechanism already explained on the Volcanism page, just with the plate above it (North America, in this case) drifting over it far more slowly than the Pacific Plate drifts over Hawaii's hotspot. That shallow magma doesn't need to erupt at the surface to have a dramatic effect: it's close enough to heat groundwater trapped in cracks and chambers in the rock above it to well past the normal boiling point, kept liquid only because it's under intense pressure from the rock and water sitting on top of it.

A hot spring forms where that superheated water can rise and circulate relatively freely, reaching the surface as a continuously flowing pool, often vividly colored by heat-loving microorganisms living at the water's edge. A geyser is what happens when the underground plumbing is more constricted — a narrow channel where the pressurized, superheated water can't circulate freely. Pressure builds until part of the trapped water flashes into steam all at once, violently pushing the water above it out through the surface opening in an eruption — after which the pressure resets, the chamber slowly refills and reheats, and the whole cycle begins again. That's why a geyser like Old Faithful erupts on a semi-predictable schedule, while a hot spring simply flows continuously.