Facts of the Matter


Scientist lifts some
volcanic misconception

Following the news about the eruption of Mount Etna last week I fielded several questions about volcanoes and their raison d'etre. It is apparent that there is some confusion and misconception about volcanoes and where lava comes from. Adding to the confusion is a movie coming out later this month that is based on the false premise that lava comes from the core of the earth.

It's true that the core of the earth is hot, very hot, and that the outer part of it is molten, but there is nearly 2,000 miles of solid rock above that. Lava doesn't come from that deep. The temperature at the center of the earth is thought to be around 12,000 degrees Fahrenheit, which nearly as hot as the surface of the sun. At that temperature rock would be vaporized if it weren't for the confining pressure of several thousand miles of rock above it.

Temperature increases downward in the earth quickly and it is hot enough to melt solid rock only a few tens of miles beneath the surface. The story of the origin and eruption of molten rock that we call magma is more than a story of temperature alone.

Although the temperature is high enough to melt solid rock into magma at rather shallow depth in the earth, melting doesn't happen everywhere and not just anywhere. The confining pressure of overlying miles of rock prevents the rock at depth from melting under most circumstances. There are notable exceptions, and it is the exceptions that produce volcanoes.

Most oceanic volcanoes are part of a system of underwater volcanic mountains that winds around the earth in the ocean basins like the seams on a baseball. Magma of these mid-ocean ridges arises from the hot mantle below. At the ridges new oceanic crust is created as lava erupts underwater through rifts and cracks, replacing oceanic crust being reabsorbed at the trenches.

Magma that supplies volcanoes of island chains like our Hawaiian Islands originates several hundred miles beneath the surface above a region of high temperature aptly named a "hot spot." There 100 or so hot spots but the reason why they are where they are is not known.

Above a hot spot a rising plume of hot rock becomes increasingly less viscous as it encounters lower and lower confining pressures. As it rises chemical reactions within the magma change its composition. By the time it reaches the surface and punches a hole in the plate to erupt as lava the magma is quite fluid and erupts rather gently, for a volcano, over time forming a seamount or an island.

In the case of island chains such as the Hawaiian Islands, the plate moves over the plume leaving a trail of volcanic seamounts and islands. Hawaii's volcanoes get progressively older towards the northwest, the direction of motion of the Pacific plate. The plate movement is slow enough to allow the growth of tall, broad shield volcanoes. Currently the eastern part of the island of Hawaii is over the hot spot as it feeds Mauna Loa, Kilauea, and the growing volcanic mound of the still submerged Loihi.

Hawaiian Volcano Observatory

We could all be a little smarter, no? Richard Brill picks up
where your high school science teacher left off. He is a professor of science
at Honolulu Community College, where he teaches earth and physical
science and investigates life and the universe.
He can be contacted by e-mail at

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