Where Volcanoes Come From

The World’s Volcanoes

The Smithsonian Institution’s Global Volcanism Program lists 1,214 volcanoes that have erupted, or may have, in roughly the last 12,000 years. Put them on the globe and most line up along plate boundaries, above all around the Pacific.

The red ones stand where plates collide and one dives beneath the other — about seven in ten. The teal ones are where plates pull apart; the yellow ones sit in the middle of a plate; a few grey ones have no setting recorded.

Volcanoes: Smithsonian Institution, Global Volcanism Program

A Sinking Plate Carries Water

When an ocean plate dives under another, it carries wet rock down with it. About a hundred kilometers down, the water is squeezed out and seeps into the hot rock above.

Wet rock melts more easily, the way salted ice melts more easily. The melt is lighter than the rock around it, so it slowly rises and builds a line of volcanoes on the surface — like Japan, or the Andes of South America.

Cross-section, schematic · left: plates pulling apart; middle: a hot spot; right: a plate diving

Where Plates Pull Apart

At mid-ocean ridges, two plates pull away from each other and hot rock below rises to fill in. Deep down, rock is squeezed so hard it stays solid even when it is hot enough to melt. As it rises, the squeeze eases, and some of it melts.

Most of the Earth’s magma forms quietly like this, on the seafloor, making new ocean floor. Most of it erupts unseen, deep underwater, so it barely shows on the map. Iceland is where a ridge and a hot spot meet: so much magma that the ridge rises above the sea.

Hot Spots

Some volcanoes sit far from any plate boundary — Hawaii, for one. Scientists think a column of unusually hot rock rises slowly from deep in the mantle there: a hot spot.

The hot spot stays nearly still while the plate moves over it. Each volcano is carried away once it is built, and a new one grows over the hot spot, so they line up in a long chain.

A Chain That Tells Time

The farther from the hot spot, the older the volcano: West Maui about 1.3 million years, Kauai about 5.1 million, Midway about 27 million.

Divide distance by age and you get the speed of the Pacific plate: about 9 centimeters a year — more than twice as fast as your fingernails grow.

Volcano ages: Clague & Dalrymple (1989); Midway: Dalrymple et al. (1977)

A Cone Above Sinking Plates

Here is a real volcano, cut through its summit: Mount Fuji, at 3,776 meters the highest volcano in Japan, which stands above sinking plates.

Fuji is a stratovolcano, a steep cone of lava flows layered with the cinders and ash of explosive eruptions. In this elevation data, its slopes above 2,500 meters average about 30 degrees.

Japan · 35.4°N 138.7°E · cut through the summit · heights × 2.5

A Shield Over a Hot Spot

Beside it, at the same scale, is Mauna Loa, 4,169 meters high, which grew over the Hawaiian hot spot. It is only a little taller than Fuji, and far broader.

Mauna Loa is a shield volcano, built from thin flows of runny lava that pour from its summit and from long rifts on its flanks, traveling far before they stop. Most of its slopes are under 10 degrees.

Mount Fuji and Mauna Loa · one scale · heights × 2.5

Measured From Its Base

Above the waves, Mauna Loa rises a little more than 4 kilometers. Its flanks carry on beneath the sea for another 5, down to the ocean floor.

Its weight has pressed that floor down another 8 kilometers. Measured from its base, then, the summit stands about 17 kilometers high, almost twice the height of Everest above sea level. With a volume of at least 75,000 cubic kilometers, Mauna Loa is the largest active volcano on Earth.

Island of Hawaii · 19.5°N 155.6°W · 200 km across · heights × 2.5

Erupting Lately

On this list, 231 volcanoes have erupted since the year 2000. About four in five of them are where one plate dives beneath another.