The composition of volcanoes that form from hot spots or ocean rift zones are often much different than subduction zone volcanoes. Typically, the magma is basaltic -- it has a lower quantity of silica -- and so it flows much more easily than andesitic magma. In these volcanoes, gases are released with relative ease; as they escape, they often propel incandescent blobs of lava hundreds of feet into the air, creating spectacular fountains. Hot-spot volcanoes like those in Hawaii often form a characteristic broad, flat shape, like that of a warrior's shield, and are known as shield volcanoes.

Hawaii's Mount Kilauea has essentially been continuously erupting since 1983, which has made it an ideal test site for a new system to predict volcanic eruptions. The system, first tested by researchers from Stanford University in January 1997, uses a network of receivers hooked into the satellite Global Positioning System. By looking at the position of the receivers, which can be determined to within a fraction of an inch, researchers can determine if the ground beneath the volcano is shifting or deforming, as it would if it were filling with magma. (Other tell-tale signs of impending eruption  -- such as particular changes in gas emissions and the frequency of earthquakes -- are currently being studied by researchers at other volcanoes.) In the test, researchers did see signs that the ground swelled, by as much as eight  inches, in the hours before an eruption on January 30. At the time, however, their system was not working in real-time, so they didn't see the signals until after the eruption. Soon, however, they hope to be able to actually predict eruptions.