According to the plate tectonics theory, the Earth's surface consists of about a dozen rigid slabs or plates, each averaging at least 50 miles thick. These plates move relative to one another at average speeds of a few inches per year -- about as fast as human fingernails grow.

Scientists recognize three common types of boundaries between these moving plates:

1. Divergent or spreading

Adjacent plates pull apart, such as at the Mid-Atlantic Ridge, which separates the North and South American Plates from the Eurasian and African Plates.

This pulling apart causes "sea-floor spreading" as new material is added to the oceanic plates.

2. Convergent

Plates moving in opposite directions meet and one is dragged down (or subducted) beneath the other.

Convergent plate boundaries are also called subduction zones and are typified by the Aleutian Trench, where the Pacific Plate is being subducted under the North American Plate.

3. Transform fault

One plate slides horizontally past another.

The best known example is the earthquake-prone San Andreas fault zone of California, which marks the boundary between the Pacific and North American Plates.

From: Tilling, Heliker, and Wright, 1987, Eruptions of Hawaiian Volcanoes: Past, Present, and Future: Department of the Interior/U.S.Geological Survey Publication