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Geology and History, Natural and Human

First, the earth

According to the theory of plate tectonics, the earth consists of:

A rigid, rocky outer shell, the lithosphere (“rocky zone”)

Beneath the lithosphere, a hot, semifluid layer, the asthenosphere (“weak zone”)

A core that doesn’t play a part in this oversimplified discussion.

The lithosphere is broken into plates that move with respect to each other. Hot, fluid material, possibly from the asthenosphere or melted by contact with the asthenosphere, penetrates up through the lithosphere at three kinds of places:

Mid-oceanic ridges, where plates spread apart

Subduction zones, where plates collide and one dives under the other (subducts)

Hot spots, where a plume of molten material appears in the middle of a plate.

Next, the land

It’s believed that the Hawaiian Islands exist where the Pacific Plate, on which they ride, is moving northwest across a hot spot. An undersea volcano is built where the plate is over the hot spot. If the volcano gets big enough, it breaks the ocean’s surface to become an island. Eventually, the plate’s movement carries the island far enough away from the hot spot that volcanism ceases on that island. Erosion, which began the moment the new island appeared above the sea, tears the land down.