The Why Files -- whyfiles.org

POSTED 3 NOVEMBER 2005
A moving story
Earthquakes prove that Earth is as shifty as a Las Vegas con man. Over the long haul, Earth's crust -- the top five to 40 kilometers -- is constantly being redefined as the huge tectonic plates float on the semi-solid rock of the upper mantle. ("Tectonic" is seismo-jargon for processes that deform the crust.)

The earth's crust is broken into seven large and many small moving tectonic plates. Earthquakes happen when these giant plates grind and lurch past each other. Photo: C. Prentice, USGS

The upper mantle, in turn, moves about through the force of convection -- the rise of lighter fluid inside a container. Just as hot mash rises from the bottom of a still, the hot, gooey rocks inside Earth move in a three-dimensional planetary ooze.

When a moonshiner heats a liquor-making still, the force of convection stirs the mash. Heat causes a similar movement in Earth's mantle. Photo: NPS

If you don't distill white lightning out in the backwoods, it may be more helpful to compare the fluid mantle to the stuff that oozes around inside a lava lamp. But the mantle is warmed by something a lot more powerful than a light bulb:

heat left from the hot gas and dust that formed Earth

gravitational energy

radioactive decay

To understand how these solid-as-rock plates move past each other, you need to know about plate boundaries, where adjacent plates rub against each other. The nature of the boundary determines if earthquakes will occur, and how strong they will be.

Divergent boundary: New crust is generated as two plates pull away from each other. Divergent boundaries appear mainly at spreading centers of the oceans, like the Mid-Atlantic Ridge, where hot rock, or magma, emerges, solidifies, and forms new ocean floor that starts to flow away from the spreading center. The movement is usually gradual and generally not a source of large earthquakes. Africa's Rift Valley is another divergent boundary, where the continent is gradually splitting apart.

Convergent boundary: Lithosphere (crust and the upper mantle beneath it) is destroyed when the denser ocean floor plate dives under a continental in a "subduction zone" (one piece of ocean floor may also subduct beneath another). Subduction zones usually cause volcanoes and earthquakes. Examples include the Sunda Fault, which caused 2004's gigantic tsunamis, and the Cascadia Subduction Zone, along the Pacific Northwest coast, which causes a string of volcanoes including Mt. Rainier and Mt. St. Helens.

Transform boundary: Crust is neither produced nor destroyed as the plates slide horizontally past each other. At the San Andreas Fault, for example, the Pacific Plate in moving north relative to the North American Plate. The San Andreas and related faults shake much of California.

The three types of plate boundary occur at different locations on Earth, and cause different types of earthquakes. Lithosphere = crust plus associated upper mantle; asthenosphere = more fluid part of mantle. Graphic: USGS

While the hot rock of the mantle can change shape like the gook in a lava lamp, the cold rock in a tectonic plate changes shape more like a rubber ball -- if it deforms, it tends to bounce back to the original shape. The result is that when plate boundaries at convergent and transform boundaries are stressed, they slip, grind, lurch or crunch past each other.

That's an earthquake.

One of Earth's tectonic plates "subducts" under an adjacent plate, in a subduction zone. An earthquake happens when stuck plates suddenly start sliding past each other. Graphic: USGS

The details of this slipping, grinding or crunching determine whether anybody outside the seismology business takes notice of the plate boundary. Boundaries can practice regular, gentle movement that relieves stress -- think yoga for the tectonic plates. Or plate boundaries can jam for decades or centuries as strain builds up. When that type of fault suddenly busts loose, people do notice, because the grinding, lurching motion can be a major earthquake.

Although earthquakes occur in many parts of the world, the largest concentration occurs in a band around the Pacific Ocean, where the Pacific plate rubs up against continental plates. This band largely parallels the volcanic "ring of fire," where subduction zones have formed hundreds of volcanoes.

The Ring of Fire around the Pacific Ocean is home to many volcanoes and a large number of earthquakes. Map: USGS

Slippin' and slidin'
Faults can move in several ways. Where the rock is weak, as it is along parts of the San Andreas Fault, the opposing sides may gradually creep against each other, forming weak earthquakes that do little or no damage.

But when a giant rock hits an immovable object -- when one tectonic plate rams against another -- and where there is no way to relieve the stress, a major earthquake can result.

What were the lessons of the great earthquake that caused the tsunamis of 2004?

There are