Researchers at the California Institute of Technology identified the new sites in a study of 900 volcanoes in South America published in the journal Nature. The data was collected from 1992-2000 by a pair of satellites operated by the European Space Agency.

The four volcanoes were thought to be dormant, and none appears to be in danger of imminent eruption. Researchers said the discovery provides nearby communities with warnings of possible lava flows, landslides and other hazards. It also demonstrates how orbiting radar instruments can detect ground swelling and other subtle signals of volcanic activity.

Of the four locations, scientists said the Hualca Hualca volcano in southern Peru is noteworthy because it is swelling near a town of 20,000. It is part of the same volcano cluster as the Sabancaya volcano, which erupted in 1990.

A second volcano, Uturuncu in Bolivia, is bulging about one inch per year, the study shows, while the Robledo caldera in Argentina is deflating for unknown reasons. The fourth volcano lies on the border between Chile and Argentina.

Researchers said the satellites bounce a radar signal off the volcano's surface and measure the time it takes the signal to return. On a later orbit the satellite repeats the measurement. If the results are different, it means the volcano is changing as a result of magma flowing below.