What is Yellowstone's caldera?
Let’s start with some background information. The Earth is made of three layers: the core at the center, surrounded by the mantle, and then the crust.
Millions of years ago, a source of immense heat known as a hotspot formed in the Earth’s mantle below what today is Yellowstone. Roughly 600,000 years ago, the hotspot pushed a large plume of magma toward the Earth’s surface. This caused the crust to jut upward. Bob Smith, a seismologist at the University of Utah, described the phenomenon to Fox News like this.
"This crustal magma body is a little dimple that creates the uplift.It's like putting your finger under a rubber membrane and pushing it up and the sides expand," he said.
The pressure on the surface finally gave way when cracks formed around the plume’s edges. When the surface could no longer withstand the pressure there was a massive explosion of magma, emptying more than one hundred cubic miles below the surface of molten rock. With nothing beneath the surface to hold it up, the crust caved in.
A caldera is that volcanic depression that occurs when a magma reserve is emptied, the “caved in,” typically round in shape, section. The Yellowstone caldera is 35 miles wide and 50 miles long, although a recent study suggests the caldera is larger than previously thought.