Eruption in Progress

Molten rock forms a fountain
Credit: Jay Robinson / U. S. National Park System

Distinctive Patterns

If you plotted all of Earth's active volcanoes onto a map of the world, it wouldn't take much study to notice that volcanoes aren't scattered randomly across the globe. You don't have to do the work, though: scientists like those at the Smithsonian Institution[1] have already made the map of volcanoes; so take a look for yourself (or see the one below).

The patterns visible on a map of volcanoes are are quite striking. For instance, you'll probably notice that the Pacific Ocean is almost surrounded by a dense line of red dots, which show where volcanoes are  in North and South America, Asia, and near Australia. That pattern is so well-known it has its own name: the “Ring of Fire.”[2] There are other strings of dots in the Mediterranean Sea--down the spine of Italy's "boot"--and near the Red Sea. Another line of volcanoes slices off the horn of Africa and, if you squint a little you can pick a line of dots down the middle of the Atlantic Ocean, a line passing right through Iceland.

Of course, there are also isolated clusters of volcanoes and even single mountains. Consider Yellowstone National Park in the middle of North America or the Hawaiian Islands, which are 3000 kilometers from anywhere, out in the middle of the Pacific. There's even Mt. Erebus, all by itself on Antarctica’s Ross Island. Volcanoes on six of the seven continents have erupted within human history, and at least one in the seventh--Australia--has erupted within the past 5,000 years.

Here's that Map

Worldwide Distribution of Active Volcanoes
Credit: U. S. Geogical Survey

Why the Traffic Jams in Certain Places?

Geologists blame all that volcanic activity on plate tectonics, which is the jostling of thin “plates” of rocky crust that malke up the outermost few tens of miles of our planet. All these plates are constantly in motion: slow, but sure, motion[3]. Most of the earth’s volcanic activity takes place along the boundaries where two or more of these plates meet each other[1].

Take the Pacific Ocean, which has a whole plate to itself. Both Eurasia and North America are slowly creeping toward the Pacific plate, which can't just jump out of the way. As a result, the Pacific plate is sliding beneath the edges of those the continents. Anywhere that happens, the oceans crust gets buried deeper and deeper until it melts. That melted rock, which scientists call "magma," then bubbles up to the surface. When it gets there, it creates volcanoes. All the volcanic activity taking place in the Philippines Islands, Japan, the Aleutian Islands and Cascade Mountain chain of North America,[4] and the Andes in South America is ultimately caused by the Pacific plate slipping under the continents. The same sort of activity is taking place in Italy, where the Mediterranean, which is stuck to the edge of Africa, is slipping down below Europe.

Subduction 101

Cross-section of a Subduction Zone
Credit: Eround1 / Wikimedia Commons

When one plate slides under another, it's termed "subduction."[3] There are other lines of volcanoes on the map that can be traced to subduction. These include the West Indies in the Caribbean Sea, long strings of volcanoes in Melanesia and the Papua-New Guinea region, and many of the islands that make up Indonesia. These strings of volcanoes are mostly located where small slivers of oceanic crust collide with other oceanic crust.

The most Famous Volcano in America?

Where Plates Go in Opposite Directions

Other volcanoes occur along boundaries where the plates are moving away from each other. A classic example of this “rift volcanism” runs down the middle of the Atlantic Ocean[1], where Europe and North America are moving in opposite directions. Similar rift volcanoes can be found in Africa's Olduvai Gorge and the Red Sea, and western North America's Rio Grande Valley. Other, volcanoes from the same environment can be found in the Niger River valley of sub-Saharan Africa and on the island of Madagascar.

Spreading Centers

A Spreading Center off the U. S. East Coast.
Credit: Wikimedia Commons

Together, subduction- and rift-related volcanoes make up more than 90% of Earth’s volcanoes, of which the majority (85%) are found in subduction zones. The remaining volcanoes are often found within their respective plates instead of out at the edges. The best-known examples of mid-plate volcanoes are the Hawaiian Islands and Yellowstone, though Mt. Erebus is far from the edge of the Antarctic plate.

These volcanoes, called intraplate volcanoes, are believed to be hot spots sitting atop the places where the mantle, the Earth’s middle layer, rises carrying excess heat from the molten core. Not only does this rising heat cause volcanism, but the circulation caused in the mantle is believed to be the force that moves the plates around the surface.

About 550 different volcanoes have erupted within recorded history, but many more are known to have erupted within the past 10,000 years.[4] Chances are very good that, at this very minutes, at least a dozen volcanoes are erupting somewhere in the world. Add that’s a lot of molten rock!