What are the different types of volcanoes?

There are pretty much 3 types of volcanoes, but as with all kinds of classifications you can find exceptions. The three types that I am thinking of are: 1) shield volcanoes (such as we have here in Hawai'i); 2) stratovolcanoes (such as Mt. St. Helens and Pinatubo); and 3) large rhyolite complexes (such as Yellowstone and Taupo). You might also want to add the mid-ocean ridges, flood basalts, and monogenetic fields to make that 6 types in all. "Volcanoes of the World" by Tom Simkin and Lee Seibert, lists 26 different types, but that's probably kind of extreme.

Here are some brief descriptions:

The Piton de la Fournaise is basaltic shield volcano on Reunion Island in the western Indian Ocean. It is one of the most active volcanoes on Earth. Piton de la Fournaise has erupted at least 153 times since 1640. The most recent eruption was in 1992. About 600,000 people live on the 950 square mile (2,510 square kilometers) island.

1. Shield volcanoes--the largest of all volcanoes on Earth (not counting flood basalt flows). The Hawaiian volcanoes are the most famous examples. These volcanoes are mostly made up of basalt, a type of lava that is very fluid when erupted. For this reason these volcanoes are not steep (you can't pile up a fluid that easily runs downhill). These volcanoes are only explosive if water somehow gets into the vent, otherwise they are characterized by low-explosivity fountaining that forms cinder cones and spatter cones at the vent, however, 95% of the volcano is lava rather than pyroclastic material. Shield volcanoes are the common product of hotspot volcanism but they can also be found along subduction-related volcanic arcs and out by themselves as well.

   Kanaga is one of the most southerly stratovolcanoes in the Aleutian arc. This photo of this nearly symmetric volcano is to the northwest from Adak Island. Photo by Howard Powers, U.S. Geological Survey, ca. 1950.

2. Stratovolcanoes--making up the largest percentage (~60%) of the Earth's volcanoes, these are characterized by eruptions of cooler and more viscous lavas than basalt. The usual lavas that erupt from stratovolcanoes are andesite, dacite, and occasionally rhyolite. These more viscous lavas allow gas pressures to build up to high levels (they are effective "plugs" in the plumbing), therefore these volcanoes often suffer explosive eruptions. They are usually about 50/50 lava and pyroclastic material, and the layering of these products gives them their other common name of composite volcanoes. Stratovolcanoes are commonly found along subduction-related volcanic arcs.

3. Large rhyolite caldera complexes--the most explosive of Earth's volcanoes. These are volcanoes that often don't even look like volcanoes. They are usually so explosive when they erupt that they end up collapsing in on themselves rather than building any tall structure. The collapsed depressions are called calderas, and they indicate that the magma chambers associated with the eruptions are huge. Fortunately we haven't had to live through one of these since 83 AD when Taupo erupted. Yellowstone is the most famous U.S. example of one of these. Their origin is still not well-understood. Many folks think that Yellowstone is associated with a hotspot, however, a hotspot association with most other rhyolite calderas doesn't work.

   View into the floor of a young maar volcano on the Ethiopia/Kenya border. The Mega volcanic field includes a number of maars that cut through ancient crystalline rock. Like many maars, this one has collapsed so that its floor is lower than the surrounding plain. If this area did not have a desert climate, the maar would probably contain a lake. Photo by Chuck Wood, 1972.

   Paricutin, 1946. Paricutin is probably Mexico's most famous volcano but it is a small one. Paricutin is but one of >1000 vents in the Michoacan-Guanajuato monogenetic field. Photograph by K. Segerstrom, U.S. Geological Survey.

4. Monogenetic fields. These also don't look like a "volcano", rather they are a collection of sometimes hundreds to thousands of separate vents and flows. These are the product of very low supply rates of magma. The supply rate is so slow and spread out that between the times of eruptions the plumbing doesn't stay hot so the next batch of magma doesn't have any preferred pathway to the surface and it makes its own path. A monogenetic field is kind of like taking a single volcano and spreading all its separate eruptions over a large area. There are a number of monogenetic fields in the American southwest, and there is a famous one in Mexico called the Michoacan-Guanajuato field.

   Cross-sectional view of lava flows of the Columbia River flood basalts, part of the lava plains of the western United States. Photo by Steve Mattox, July 1989.

5. Flood basalt provinces--another strange type of "volcano". Some parts of the world are covered by thousands of square kilometers of thick basalt lava flows--some flows are more than 50 meters thick, and individual flows extend for hundreds of kilometers. The old idea was that these flows went whooshing over the countryside at incredible velocities. The new idea is that these flows are emplaced more like pahoehoe flows--slow moving, with most of the great thickness being accomplished by injecting lava into the interior of an initially thin flow. The most famous U.S. example of a flood basalt province is the Columbia River Basalts, covering most of SE Washington State, and extending all the way to the Pacific and into Oregon. The Deccan Traps of northwest India are a much larger flood basalt province.

6. Mid-ocean ridge volcanism occurs at plate margins where oceanic plates are created. There is a system of mid-ocean ridges more than 70,000 km long that stretches through all the ocean basins--some folks consider this the largest volcano on Earth. Here, the plates are pulled apart by convection in the upper mantle, and basalt lava intrudes to the surface to fill in the space. Or, the basalt intrudes to the surface and pushes the plates apart. Or, better yet, it is a combination of these two processes. Either way, this is how the oceanic plates are created. A recent mid-ocean ridge eruption took place along the Gorda Rise--the mid-ocean ridge that separates the Juan de Fuca plate from the northern part of the Pacific plate.

   Scott Rowland