The 1980 eruption of Mount St. Helens is an excellent example of a destructive eruption. About the upper third of the beautiful pre-eruption cone (below, left) disappeared during the eruption (below, right). Where did it go? The material that was blasted upward so spectacularly and blown across eastern Washington and Idaho (about 1/4 cubic mile of material) was mostly made from the new gas-charged lava that had invaded the mountain's interior during the few months preceding the eruption. Nearly all of the rock missing from the pre-eruption cone went sideways down the hill instead of up into the air.

Photo: USGS EROS Data Center

Photo: Lyn Topinka

The main events of the Mount St. Helens' 1980 eruption are shown in a short animated movie (in Quicktime or MPEG format). The perspective shown is along a north-south slice through the mountain, the valley of the North Fork of the Toutle River and the Coldwater II campsite on the ridge to the north of the valley. David Johnston, a volcanologist who was killed in the eruption, was at the Coldwater II camp when the mountain finally exploded. The sequence begins with the initial invasion of a small amount of lava through the center of the cone causing the first small steam explosion near the summit on March 27, 1980. The next several frames show the invasion of a large body of lava into the upper heart of the cone during the next few months. The invading lava mass caused the north slope of the mountain to bulge upward and outward over 400 feet (greatly exaggerated in the movie), and generated fractures (magenta lines) through the mountain. Finally, the bulge and cracks grew so large that the whole mass became unstable. The main eruption on May 18, 1980, began when the part of the mountain above a huge crack finally broke loose and started to slide down the north slope of the mountain. As the top of the mountain slid off, the gas-charged lava inside was exposed and began exploding upward and outward. Meanwhile, the rock from the top of the mountain continued to accelerate downward, roaring across the upper valley of the North Fork Toutle River and over some of the nearby ridges as a giant landslide (thickness greatly exaggerated). The avalanche was deflected to the west by the ridge, but the superheated clouds of steam and ash blasted across the ridge and several miles beyond with scalding winds of hurricane force. The rest of the upper part of the cone collapsed as the mass of lava in the heart of the mountain was blasted away. The volcanic cloud deposited a thick layer of ash over the area. During the next few months much of the ash eroded away, and a new "dome" of pasty lava formed at the bottom of the empty mountain's shell.

As destructive as a Mount St. Helens-type eruption is to the original volcano, there are some eruptions in which the entire volcano is annihilated! This happened at Krakatoa (and several earlier eruptions such as Tambora and Santorini). Before the 1883 eruption Krakatoa was a majestic cone at least 7,000 feet high on an island 3 by 5 miles in size; after the eruption only a shallow lagoon surrounded by a ring of small islands remained. Where had the main island gone? Conventional wisdom said that it had been blown up into the sky. But Krakatoa was the first of this type of eruption to be studied in detail, and no old volcanic rocks were found in the ash blasted all over Indonesia. The old rocks were found underneath the new lagoon--the old volcano had not blown up, it had sunk! Geologic studies at similar eruption sites have shown the same thing: The center of the mountain in this type of eruption sinks straight down, leaving behind a large crater called a "caldera." Let's look at another short movie (in Quicktime or MPEG format) that illustrates how this type of volcano forms.

Initially, we find the dormant (resting) volcano sitting on the earth's surface but underlain by a huge underground lake of liquid rock called a magma chamber (magma is lava that is still underground). A "pipe," or conduit (not shown), connects the bottom of the magma chamber to the region where the liquid rock is formed. Occasionally, new masses of magma come up the conduit into the chamber. The new magma has dissolved gases that migrate to the top of the chamber. Just like putting air into a tire, adding new magma to the chamber increases the pressure inside and causes the old volcano on the surface to bulge upward and crack. Gas-charged magma creeps up the new cracks, causing the volcano to bulge even more. Eventually, the new magma opens a crack all the way to the surface, which becomes the exit vent for a new eruption. When that happens, the pressure in the magma at the top of the vent drops rapidly, causing the dissolved gas to form bubbles at an explosive rate. The magma in which the bubbles are forming is fragmented to ash and blown out the top of the vent. As that magma is blown out, the magma just below it in the vent suddenly has no rock on top of it, so it forms explosive bubbles and rockets out the vent as well, followed by the magma below it, and so on down the vent. The zone of explosive bubble formation and rocketing ash moves downward into the top of the magma chamber, allowing that magma to fragment and escape up the vent as well. As the upper part of the magma chamber empties, the overlying rock finds itself without visible (or any other) means of support, and does what comes naturally--it sinks. At the end of the eruption, most of the rock of the original cone has sunk down and left a big hole. Sometimes the hole fills with volcanic rocks from later, smaller volcanic eruptions (see the Valles Caldera); sometimes it fills with water forming a volcanic lake, like Crater Lake in Oregon (click here to see a stereo pair of Crater Lake). Click here for a topographic map. If at sea, the caldera fills with sea water, becoming a lagoon, like Krakatoa.

Magma chambers can get very large, as much as 100 miles across. Consequently, caldera-type eruptions can be very large. The largest known explosive volcanic eruptions are of this kind.

[ Types of Volcanoes ] [ Types of Lava ] [ Sizes of Eruptions ]
[ Volcano Eruption Animations: page 1 / page 2 ]
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