Seething Below the Surface
Famous for its hot springs, geysers, and breathtaking natural beauty, Yellowstone National Park in Northwest Wyoming is one of the premier vacation spots in the United States. Yet most people don't know that lying beneath Yellowstone and causing all the wonderful geothermal activity is a mammoth reservoir of pressurized magma pushing up against the surface. In effect, Yellowstone sits on top of a giant super volcano.
We tend to think of volcanoes as giant cones, or mountains, such as Mt. St. Helens. Indeed, most volcanoes form when a column of magma from deep within the earth erupts onto the surface and forms a cone as it flows down the sides and cools.
Tick, Tick, Tick
But some of the largest and most powerful volcanoes occur when instead of erupting, rising magma creates a giant reservoir in the Earth's crust. Pressure builds until the chamber erupts with a force thousands of times greater than a normal volcano, spewing thousands of cubic yards of ash, dust, and sulfur dioxide into the atmosphere. The explosion creates a giant crater called a caldera.
One of the world's largest, the super volcano beneath Yellowstone last erupted over 600,000 years ago, covering a 3,000 square mile area with volcanic ash. In other words, ash blanketed over half the United States. Recent measurements indicate that over the past century the earth above the Yellowstone magma chamber has risen almost 19 inches--telling evidence of building pressure. Since the Yellowstone super volcano erupts approximately every 600,000 years, the next eruption may already be overdue.
"Yellowstone Supervolcano" (Solcomhouse)