May 16,1997

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

HVO Goes to the Movies!

A few evenings ago the staff of the Hawaiian Volcano Observatory took a field trip to Prince Kuhio Plaza to see the latest disaster film, VOLCANO! Generally, we on the Big Island take volcanic phenomena very seriously. The 1990 destruction of Kalapana by Kilauea's east rift eruption and the near-disaster in 1984 when flows from Mauna Loa threatened Hilo are both vivid reminders of the real-life hazards of living on a volcano. But, come on Hollywood! Lava gushing out of the Los Angeles subway system? How about a sequel featuring an eruption in Washington, D.C.?

The truth is, neither scenario is very likely. Centers of volcanic activity don't just pop up randomly across the globe. They are preferentially located at the margins of the Earth's "tectonic plates."

By the turn of the 20th Century, scientists had recognized that the Earth is composed of three main layers: the surface crust, and the mantle and core at progressively deeper levels. In the 1960's, scientists discovered that the crust and upper part of the mantle is actually broken up into a mosaic of rigid rock plates. There are a dozen or so of these plates, some up to a hundred miles thick and thousands of miles across.

The plates are resting on a partially molten, mobile layer within the Earth's mantle known as the "asthenosphere." The asthenosphere is slowly circulating, in much the same way that a pan of melted fudge churns when heated on a stove top. The overlying plates are dragged along by the asthenosphere in conveyor-belt fashion. The internal circulation patterns are complex; thus, the direction and rate of plate motion varies. Some plates collide head on (convergent plate boundary), some are pulled in opposite directions (divergent plate boundary), while still others scrape past one another horizontally (transform fault). It is at convergent and divergent plate boundaries that we expect volcanoes to appear.

At a convergent plate boundary, the denser of the two plates is thrust downward, or "subducted." The subducted plate dives into the hot mantle, where parts of it begin to melt. This melt, or "magma", separates and rises toward the surface where it erupts. The volcanoes of the Aleutian Islands (Alaska) and the Cascade Range (Washington, Oregon, Northern California) are of this type.

At divergent boundaries, the two plates are pulled apart, and magma from the asthenosphere wells up in the spreading gap forming between them. Active volcanoes dot the divergent boundary known as the Mid-Atlantic Ridge, which stretches across the sea floor from the Arctic to the southern tip of Africa.

In the third type of plate interaction, transform faulting, the two plates ride past each another like cars going in opposite directions on a two-lane highway. Earthquakes occur along transform faults, but not volcanoes. There is no melting of a subducted plate, or filling of a spreading gap with magma from the asthenosphere. Los Angeles sits near such a boundary the infamous San Andreas fault. So, Hollywood, take note! We expect Los Angeles to suffer earthquakes, but not volcanic eruptions.

But wait! What about Hawai'i? Hawai'i sits in the middle of the Pacific plate, more than 2000 miles from the nearest plate boundary. This relatively rare occurrence is known as "hot spot" volcanism. Scientists hypothesize that it is the result of a "thermal anomaly" deep in the Earth's mantle. There are only a few intra-plate hot spots world-wide. These have remained in fixed positions for periods of 50 million to 100 million years. And so, Los Angeles, there is really no need to worry...

Kilauea Eruption Status--May 16