If you were to plot the locations of the world’s active volcanoes on a world map, you’d soon realize that these smoking mountains aren’t randomly scattered about the globe. You can see such a map here, in fact.
The patterns paint a striking image. The Pacific Ocean is almost surrounded by a dense line of volcanoes in the Americas, Asia and off Australia. That line is so well-known it’s been given a name: the “ring of fire.” There are other strings in the Mediterranean Sea (all of Italy, in fact) and near the Red Sea. Another line of volcanoes cuts across the horn of Africa and, if you squint just right, you can pick a sort of dotted line down the middle of the Atlantic Ocean that passes through Iceland.
What might seem strange, however, are a few isolated volcano clusters and even single mountains. Think of Yellowstone National Park in the middle of North America, the Hawaiian Islands 3,000 miles from anywhere in the Pacific, and Mt. Erebus all by its lonesome on Antarctica’s Ross Island. There are volcanoes on six continents that have erupted within human history, and at least one in Australia that has erupted within the past 5,000 years.
Geologists blame all volcanic activity on plate tectonics
This is the jostling of thin “plates” of rocky crust that form the surface of the earth. All these plates are in constant motion (slow but sure). Most of the Earth’s volcanic activity takes place where two plates meet each other at a boundary.
Take the Pacific Ocean, which has a tectonic plate all to itself. Asia and both Americas are moving toward the Pacific plate, which is sliding under the continents at their edges. Where that happens, the buried oceanic crust melts deep in the earth. Magma, or melted rock, rises to the surface to create volcanoes. The volcanic activity in the Philippines, Japan, the Aleutian and Cascade chains of North America and the Andes in South America is all caused by the Pacific plate sliding under the continents. The same sort of activity takes place in Italy, where the Mediterranean (stuck to the edge of Africa) is slipping under Europe. Geologists call oceanic crust sliding under a continent “subduction.”
There are many other lines of volcanoes related to subduction. These include the West Indies in the Caribbean Sea, strings of volcanoes in Melanesia and the Papua-New Guinea region and much of Indonesia. These strings of volcanoes are mostly located where small slivers of oceanic crust collide with other oceanic crust.
Another group of volcanoes is created in places where two plates are pulling in opposite directions. The classic example of this “rift volcanism” runs up the middle of the Atlantic Ocean, where Europe and North America are moving in opposite directions. Similar rifting takes place in Africa (the East African Rift and the Red Sea) and western North America (the Rio Grande Valley). Other, similar volcanoes can be found in the Niger River Valley of sub-Saharan Africa and on the island of Madagascar.
Together, subduction-related and rift-related volcanoes account for more than 90 percent of Earth’s volcanoes, though the majority (85 percent) are found in subduction zones. The remainder of the volcanoes are found within their respective plates instead of near the edges. The best-known examples are the Hawaiian Islands and the Yellowstone area, though Antarctica’s Mt. Erebus is in a similar setting.
These intra-plate volcanoes are believed to be hot spots that sit above places where the Earth’s middle layer, the mantle, is rising, carrying with it extra heat from the molten core. Not only do these mantle plumes cause volcanism, but they are also believed to be the forces that move the plates around the surface.
Some 550 different volcanoes have erupted within recorded history, but many more have erupted within the past 10,000 years or so. Chances are very good that, at this very minute, a dozen or more volcanoes are erupting somewhere in the world. That’s a lot of hot rock!
More information: The Smithsonian Institution Global Volcanism Project
