The plume of partly molten rock that sits under Yellowstone Park may be even bigger than anyone thought.

University of Utah geophysicists have made the first large-scale image of the electrical conductivity of the plume, which feeds the Yellowstone supervolcano. The supervolcano is an ancient structure that covers a sizeable portion of the park from which the last big lava flows were 70,000 years ago. The plume of hot magma that sits underneath it is similar to the one underneath Hawaii that erupts molten rock on a regular basis.

Earlier images were made using earthquake waves. Dozens of small earthquakes happen every year in the park and the surrounding area, which is riddled with fault lines.

By measuring the changes over time of the Earth's magnetic field in dozens of locations around the park, the researchers could indirectly measure the electrical conductivity of the magma material underground.

Michael Zhdanov, the principal author of the new study, said that while you can't tell what the material is exactly, there are certain things it is more likely to be. Generally molten silicate rocks and hot, briny water mixed in partly molten rock conduct electricity better. His team found that the region of high conductivity extends 400 miles in a generally east-west direction, and at least 200 miles deep.

Previous images, from a 2009 study, found the plume was about 150 miles across in a west-northwest direction and extended about 410 miles under the area marked by the border between Idaho and Montana. That study measured the waves generated by small earthquakes in the park. The wave method tends to highlight materials such as molten or partly molten rock that slow seismic waves, so it will show which parts of it are denser or colder.

Zhdanov noted one reason his images see a larger plume is that they are looking at different things. It's like an ultrasound and MRI in the human body, he said. They are different imaging technologies.

One thing Zhdanov and his team noticed was that the electrically conductive part of the plume seems to extend downward at a gentler angle, at about 40 degrees as opposed to 60 degrees that the previous study measured. Given the larger size of the plume that his method measured, it could mean a more electrically conductive region is enveloping the one measured by other methods. Hopefully this will tell us more about the physics of these things, he said.

The method Zhdanov used is not new - it is often employed in the oil and gas industry to find areas that might be promising to drill in. But this is the first time it has been used over such a wide area.

The area around Hawaii could also be studied this way, Zhdanov said, but it would be more expensive to do as the measuring stations would have to be on the sea floor to cover a large enough area. But the technology exists, he said.

The work he did does not say anything about whether the Yellowstone supervolcano will erupt again, or when it is likely to happen. Zhdanov noted that while the Yellowstone region isn't as active as Hawaii, eruptions are more violent when they do happen.

The study will be published in Geophysical Research Letters.