With several recent eruptions making the news, I'd like to do a series about volcanoes. Japan is an interesting place to start, not only because I have some nice pictures from there, but also because it is home to over 100 active volcanoes, 10% of the world's total. In fact, the islands of Japan were formed by volcanoes, similarly to those in Hawaii. The picture above shows the city of Kagoshima with the active volcano Sakurajima looming in the background, but more on that volcano later.
First, let's discuss what is meant by an "active" volcano. The term is a bit misleading in that it sounds like the volcano is currently spewing ash and lava or is otherwise showing activity in some way. While currently erupting volcanoes, such as Sakurajima, are certainly classed as active, any volcano that has been active within recorded history (or sometimes longer) can be considered "active". A volcano that was only active earlier than that but is expected to be active again is called dormant, while a volcano that's not expected to erupt again is called extinct. Unfortunately, the definition details of these terms vary depending on who you ask, but the general idea holds for all.
So, why does Japan have so many active volcanoes? Well, it's in an ideal location for volcano formation (and also earthquakes) - right above a subduction zone (or rather a few). Recall that plate tectonics describes how the Earth's crust, the thin, top layer of hard rock, is broken into pieces (plates) that drift around on the more fluid-like (but still solid!) parts of the inner Earth. A subduction zone is an area where one of these tectonic plates is being pulled or pushed into the Earth beneath another plate that is more buoyant. Think of two pieces of ice floating on a lake. If one is more dense, it won't float as easily allowing the lighter ice piece to move over it.
Since plates made of oceanic crust are denser than those with continental crust, they are the ones that subduct. The oceanic plates pull some water along down into the Earth, either within the rocks themselves or in the upper sediment layers. As the plate gets deeper, the temperature and pressure increase and the water is released into the surrounding "rock". Water lows the melting temperature of rock, allowing some of it to melt and become less dense. Buoyancy causes it rise to shallower parts of the Earth where it pools and can eventually erupt.
Over time, the eruptions deposit more and more material on top of the crust, building up a volcano. If the subduction zone is below water, the volcano won't be seen until it grows large enough to appear above the water. Therefore, ocean island volcanoes are much larger than they seem since most of their bulk is hidden from the surface. Because this process happens along the subduction zone, not just at one location, you can end up with a volcanic arc, like Japan. Some volcanoes are also close enough that they eventually begin to grow on top of one another. The result is larger islands made of multiple volcanoes - also seen in the main islands of Japan.
Recently, you may have seen the news of the new island that formed off the coast of Japan. This is a great example of these processes in action. Of course, this isn't the only way to form volcanoes or volcanic arcs. We'll get to the others in later parts of this series.
If you're interested in current volcano eruptions and warnings in Japan, you can check the Japan Meteorological Agency website.
VHP Photo Glossary: Volcano by USGS (United States Geological Survey)
Subduction Zone Volcanism by San Diego State University
Dramatic Video Shows Volcano Making New Island Off Japan by Brian Clark Howard
Volcanic Island Eats Another by Brad Lendon
Resources from Volcano Discovery:
Volcanoes of Japan (118 volcanoes)
What's erupting? List & map of currently active volcanoes