EARTHQUAKES may destroy the works of man. But seen in planetary perspective, they are part of a recycling system that helps keep Earth livable.
This system continuously renews most of the planet's crust. It regulates the atmosphere's content of heat-trapping carbon dioxide (CO2) over geological time. It's a system that geophysicists call plate tectonics.
Our planet resembles a cracked egg. Its crust is broken into about a dozen plates, most of which appear on the accompanying map. Where they jostle together or draw apart, volcanoes erupt and earthquakes rattle.
A vigorous thermal engine drives this incessant activity. Primordial heat flows from Earth's core. Decay of radioactive substances in the mantle - the layer between the core and outer crust - adds more heat. The heat flow sets up convection in the mantle that, like simmering water, has upwellings of hot buoyant material and down flows of colder, denser matter.
Tracking this motion on a human time scale is like watching fingernails grow. Both processes proceed at a rate of a few centimeters a year. But on a geological scale that measures time in millions of years, it's a vigorous boiling that completely renews the oceanic crust - which is most of Earth's crust - within a mere 200 million years. That's only a fraction of Earth's 4.5 billion-year age.
It works like this. New seabed material wells up where plates move apart, as they do along the Mid-Atlantic Ridge. (See map, above.) This material becomes part of the seabed plates that are moving away from the ridge in conveyor-belt fashion. Meanwhile, across the ocean where these moving plates meet the land, the seabed plates dive beneath the continent-bearing plates. A descending plate returns its material to the mantle, forming deep ocean trenches and creating volcanoes and high mountains on the adjacent land. Elsewhere, oceanic and continental plates slide past each other, as they do along the North American West coast.
Sea-floor spreading from ocean ridges, overriding of one plate by another, and grinding together of plates sliding past each other generate most of the world's earthquakes. Only a few happen within continental plates due to causes not well understood.
An oceanic plate that descends into the mantle also helps regulate climate. Most of the atmosphere's CO2 comes from the earth's interior through volcanoes. Biological and chemical processes on land and sea remove much of this gas. The carbon is converted to carbonate sea-floor rocks that reenter the mantle with the descending oceanic plates. Some of it later re-emerges through volcanoes as CO2.
Climatologists are concerned that human activity may increase atmospheric CO2 levels enough to cause global warming. But without the large-scale carbon recycling of plate tectonics, Earth probably would have a climate that would be either too hot or too cold to sustain life as we know it. It is in this sense that geologists consider the earthquake-generating action of plate tectonics to be a key process in keeping our planet livable.