Pyroclastic flow and lava aren't the only hazards created by volcanic eruptions. Other dangers are lahars -- mixtures of rock fragments and water that flood down volcanoes (mudflows are one type) -- landslides, gas emissions, and ash clouds. Ash clouds are a particular problem for aircraft. They can cause engine failure, damage electrical systems, scratch the outer surface of a plane, and contaminate its interior.

The effects of a volcanic eruption can also be felt over the long term. Eruptions releasing high concentrations of sulfur-rich gas -- like the eruptions of the Philippines' Mount Pinatubo, in 1991, and Mexico's El Chichón in 1982 -- can alter global climate. The sulfur mixes with water vapor in the atmosphere to form clouds of sulfuric acid. The acid droplets both absorb incoming solar radiation and bounce it back into space. The result: lower temperatures. In the year after the eruption of Pinatubo, for example, global temperatures dipped by nearly a degree.

Of course, volcanoes aren't always associated with plate boundaries. Volcano chains like the Hawaiian islands are formed by plumes of hot mantle material that rise up from the mantle and intrude on weak parts of the crust within the interior of a plate. The plumes are called "hot spots." 

The composition of volcanoes that form from hot spots 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. (Some basaltic lava flows -- in particular, flows of a ropy, smooth-skinned type of lava called "pahoehoe" -- can move downhill at speeds of over six miles per hour; the motion of a viscous lava, in contrast, is often imperceptible.) 

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.