Photo: NGDC/NOAA.

Photo: NGDC/NOAA.

There are many different types of volcanic eruptions. Different types of eruptions tend to form different types of volcanoes. Most volcanic eruptions are "builders," adding thin layers of lava or ash to the sides of a volcano, slowly building the easily recognized cone-shaped volcanoes seen around the world. Some eruptions, however, are "destructors," knocking off pieces of a volcano's summit or even destroying the entire mountain. Generally, small-to-medium eruptions build volcanoes, and large explosive eruptions tend to destroy them. Let's look at simple animations of some of the different types of eruptions.

The first animation (in QuickTime™ or MPEG format) shows a "building" or constructive eruption of a typical strato-volcano like Mount Rainier or Fujiyama. The volcano itself consists of a pile of successive layers of ash (light brown) and lava (dark brown) resting on a preexisting surface. Before the surface eruption a new mass of lava has surged into the established vent system deep under the mountain (not shown). Inflation of the mountain is negligible because the amount of fresh lava is small. The eruption begins with the invasion of the fresh lava along the old eruption channel leading to the summit vent. (Successive eruptions do not always occur at the old summit. Often, the lava works its way through weak rock or fractures within the cone and bursts out on the side of the mountain as a "flank eruption"). In this case the first stage of the eruption begins with a mildly explosive fire fountain of fragmented liquid rock. The hot particles blast out of the vent, but since the gas content is low, the particles almost immediately begin to fall back under the influence of gravity. The particles coat the sides of the cone and build up a new layer of ash. In this case the layer is thick enough to flow slightly after landing, engulfing some of the neighboring vegetation. Next, there is a short break in activity, during which the ash layer cools.

The second stage of the eruption is effusive: A large mass of gas-free lava percolates to the summit vent, fills it, and flows down the sides of the cone. The resulting lava flows spread out beyond the original mountain and then cool to form a hard cap over the weaker ash layers. Summit vents are typically irregular in shape. For instance, one side is often higher than the other, in which case the lava flows down only one side of the volcano. Usually it takes several effusive eruptions to build a complete new lava layer around a volcano's cone. The illustrated eruption now ends with the cooling of the lava remaining in the vent.

The illustrated sequence of explosive activity followed by effusive is fairly typical. Rising lava usually has some dissolved gas. If the fresh lava slows or temporarily stops in the subsurface vent system, the dissolved gas tends to migrate to the top of the mass of lava. Thus, the first lava that comes out is usually more gas-charged and explosive than the lava expelled later during the same eruption. Of course, lava movements can be complicated, and lava compositions and gas contents do vary, so no two eruptions are alike.

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

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