"I DON'T think visitors appreciate that they're standing directly on top of the largest, most dynamic magmatic system on the planet," says geologist Daniel Dzurisin. While the supervolcano that is Yellowstone National Park won't be erupting any time soon, he and his colleagues have uncovered a surprising source of volcanic activity beneath tourists' feet, which was probably the reason trails had to be closed in 2003.

The Yellowstone caldera formed 640,000 years ago in an explosion of magma more than 1000 times greater than the Mount St Helens eruption in 1980. While it is common knowledge that the caldera floor rises and falls, the source of the motion remains uncertain. According to a previous popular theory, the accumulation and release of fluid not far beneath the surface is driving the cycles, but Dzurisin, of the David A. Johnston Cascades Volcano Observatory in Vancouver, Washington, and his colleagues say a deeper source best explains their latest findings.

The team, led by Charles Wicks of the US Geological Survey in Menlo Park, California, used a series of satellite measurements to determine the fluctuations in elevation - up to 120 millimetres - over a seven-year period.

Although the floor of the caldera began subsiding in 1997, the researchers uncovered a new region of activity beneath the north rim of the caldera that continued to swell from 1995 until 2002. Models incorporating the measurements indicate that the source of the upward push was 10 to 16 kilometres beneath the surface in the basaltic magma layer, well below the level of the fluid suggested as the source of the motion (Nature, vol 440, p 72).

The picture is one of magma flow driving the undulation of the surface, flowing upward from beneath the caldera floor towards the northern rim and then down and out from beneath the rim. Seismic activity near the exit acts as a valve, suggests Wicks, blocking or releasing the magma outflow. This explains why the rim and floor can swell and sink at different times.

The idea also explains the rise in thermal activity in the rim area in 2003, Wicks says, when some trails had to be closed because of increased steam releases and a rise in surface temperatures. The swelling magma could have cracked the crust, creating new avenues for steam to escape to the surface, he says.