US Dept. of the Interior - National Park Service
©National Park Service
At Yellowstone and some other volcanoes, some scientists theorize that the earth's crust fractures and cracks in a concentric or ring-fracture pattern. At some point these cracks reach the magma "reservoir," release the pressure, and the volcano explodes. The huge amount of material released causes the volcano to collapse into a huge crater - a caldera.

A minor earthquake rattled remote northeastern Yellowstone National Park early Tuesday.

The U.S. Geological Survey's National Earthquake Information Center in Golden, Colo., said the magnitude 4.1 quake struck the park at 5:59 a.m.

The quake was centered about 15 miles north of Yellowstone's east entrance. People reported feeling the quake as far away as Thermopolis in western Wyoming and Billings in southern Montana.

USGS geophysicist John Bellini said the quake was strong enough to wake people up but it was probably not strong enough to cause much, if any, damage.

Yellowstone is a hotspot of geological activity and very small earthquakes occur in the park regularly.

Proposed new stream monitoring, gas sampling, GPS, tilt, seismic and strain monitoring instruments to be added to the Yellowstone volcano and earthquake monitoring network. Thick black line is the boundary of the Yellowstone Caldera. Thin black lines are roads. Gray outline is park boundary. Red regions are thermal areas. Image by USGS