Why Are There So Many Volcanoes Here?

One of the first things you notice in Indonesia is the topography. The islands of this country are dominated by volcanoes. Here in this picture taken from the space shuttle you can see several of the volcanoes that dot the island of Java (look for the clouds; the clouds are related to the mountains, but we'll save that for another blog post). Visitors first see the volcanoes from the air as they approach Jakarta or Jogjakarta (see the picture below my blog posts). Here in Jogja we have an everyday reminder of the volcanic nature of these islands: Gunung Merapi (gunung means "mountain" in Indonesian; volcano is gunung api; "fire mountain"). You can see Gunung Merapi in the picture I've included. It constantly emits a trail of smoke. These volcanoes are responsible for the remarkably beautiful and unique scenery you'll see when you come to Indonesia. Ive included a picture of Bromo National Park in Eastern Java so you can see what I mean. There's also a major volcano near the spot where I am doing my PhD project on the island of Sumatra.

But the volcanoes have a dangerous side too, as you can well imagine. Frequent eruptions force many people to flee their homes, ash, gas and other debris expelled from volcanoes cause health problems, and from time to time people are killed by lava flows. In addition to the volcanoes, there are earthquakes, which are sometime accompanied by related events, such as the devastating tsunami of 2004. You may recall from my post about Borobudor that volcanic eruptions and earthquakes were at least partially responsible for the abandonment of the monument.

What causes these volcanoes and earthquakes? The answer lies in the very nature of the earth itself. The surface of the earth, or crust, is similar to the shell of an egg. It is very thin, and beneath it is liquid rock, or magma which is extremely hot because of the intense heat of the interior of the earth. However, the crust differs from the shell of an egg in that it is divided into a number of different pieces. Geographers, geologists, and geophysicist call these pieces plates. There are around 14 big plates and a number of smaller plates. I've included a map so you can see the different plates (1).


These plates don't just sit still; they move around, and when they do they bump into and slide against one another. As you can probably imagine, since the plates are so big and heavy this creates a lot of pressure and force which is expressed in the form of an earthquake. That's right, whenever there is an earthquake, it means the earth is adjusting its "shell". Now, look at the map of the plates. Can you find Indonesia on this map? What do you notice about it? That's right, it right at the meeting point of two plates. In fact, the Australian Plate is actually sliding beneath the Eurasian Plate. This isn't just a coincidence; Indonesia was actually created by this process. When one plate slides under another, the plate sliding beneath is actually melted by the intense heat under the surface of the earth. And whenever something melts or burns, there is always exhaust of some kind or another. This "exhaust" makes its way up to the surface of the earth and creates a volcano. In addition, some of the plate that is being forced below is actually scraped off and becomes part of the plate that remains on top. This entire process is called subduction, and the place where it happens is called a subduction zone. I've included a diagram so you can get an idea of how this works. As you can see, Indonesia is in the heart of a subduction zone.


Now, let's see if we can figure out how this affects the landscape. Below I've included a physical map of Indonesia. Geographers use physical maps (3) to show features of the landscape. On most physical maps elevation is shown in brown; the browner the area, the higher it is. At the same time, depth in the ocean is shown with shades of blue. The bluer the color, the deeper the ocean. See if you can identify some mountainous areas, flat areas, and deep areas in the ocean. Now think about the map of the plates, and look at the diagram of the subduction process once again. Can you see the physical evidence of the plates interacting with one another? You might notice a chain of mountains that run from the north of Sumatra all the way across the island. This is the product of subduction.

When you come to Indonesia you can see the evidence for yourself. It's truly breathtaking and gives you a new understanding of the dynamic nature of the planet. I'll write more later, but in the meantime you might want to think about the questions I've provided below.

1. What plate is Hawai'i on?

2. Look at the map of tectonic plates and identify some other places where plates meet. Now find a physical map of that location using the internet, classroom resources, or the library. What sorts of landforms do you find at these locations?

3. What types of things might a physical map be very useful for? Can you think of some other types of maps?