Let's look at some of the important techniques of volcano watching and what we can learn by using them.

Geologic Structure First we look at the structure and location of a volcano to determine its type. This is important because the main types of volcanoes (Cascade, Island Arc, and Hot Spot) have characteristic types of eruptions: some explosive, some effusive, and some do both. We look at the size of the volcano and its neighbors. We look for thick ice packs or glaciers on its sides or summit to see if mudflows are possible. We look at how heavily eroded it is to determine if it is still active and if sections are likely to break off.

As an example, here is a SIR-C radar image of Mount Pinatubo in the Philippines showing the central cone and summit crater. Rough ash flows (red) are high on the volcano's sides, and smooth (dark) mudflows extend down the surrounding drainage valleys. This is an island arc volcano, which has many similarities to Cascade-type volcanoes like Mount Rainier and Mount Hood. Photo: Courtesy Jet Propulsion Laboratory. Copyright © California Institute of Technology, Pasadena, CA. All rights reserved. Based on government-sponsored research under contract NAS7-1260.

Past Eruptive History Now we look at volcanic deposits around the volcano to determine their age, type (lava flows, mudflows, ash flows), size, and distance from the volcano. This data will help to determine if the volcano is active. We look for patterns of activity. Does the volcano erupt or produce mudflows at regular intervals? Does it have a consistent sequence of events, such as a number of effusive eruptions followed by an explosive one or several small eruptions followed by a large one? Combining the sizes and ages of different types of deposits will help us evaluate the risk of a particular volcanic hazard at a particular volcano.

The diagram in this article at the Cascades Volcano Observatory gives a simple example of eruptive histories of the Cascade volcanoes.

Past history can only help us in a limited way, however, because experience with many volcanoes has shown that no two eruptions of a single volcano are exactly alike and no two volcanoes produce exactly the same sequence of eruptions. Since each volcano is unique, predictions concerning the next eruption of any given volcano always have an element of uncertainty. This is why any volcano showing current signs of activity needs to be monitored in "real time."

Seismic Activity The first evidence of an impending eruption is usually a series of seismic events or earthquakes. In order for an eruption to occur, lava must rise from its formation zone deep underground to near the surface. The lava must literally push overlying rocks aside to rise through them. The rocks are brittle and break as they are bent and twisted, releasing seismic energy that we record as earthquakes. By placing a number of seismic recorders around a volcano, the movement of the new mass of rising lava can be followed.

To minimize human risk on active volcanoes, these recorders are linked to a volcano observatory by radio.

Here is a short animation (MPEG or Quicktime) simulating the occurrence of earthquakes during the formation of Volcano "X." At great depths, in the asthenosphere, huge globules (called diapirs) of magma rise through the soft rock without causing any earthquakes. As the globule reaches the bottom of the lithosphere, it flattens out and pushes upward against the rigid barrier, fracturing the brittle rock. The magma can then continue to ascend by forcing a crack through the lithosphere to the surface. Many small to medium earthquakes occur around the crack tip and in the adjacent rock. As the magma rises, the earthquake activity follows. Near the surface, the magma may sometimes force its way through pre-existing weaknesses between rock layers. Earthquakes will occur wherever the rock is deformed enough to break. Clusters of small earthquakes called swarms also occur when magma flows quickly through narrow sections of magma-filled cracks. Both types of quakes are used to follow magma movement underground. Eventually, the tip of the rising crack reaches the surface, and a new volcano is born.

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