Plates and plate motions
Plates are large segments of the earth's lithosphere that move around relative to each other.
Evidence for plates includes:

• lines of earthquakes outline zones where the lithosphere (outer brittle layer) of the earth is breaking
• accurate measurements using GPS (global positioning satellite) technology demonstrates that the blocks outlined by the earthquakes are moving relative to each other

Plates move

• away from each other
• toward each other
• past each other

Some plate edges are also marked by rows of volcanoes (= volcanic arc).

BUT - Volcanoes occur only where plates move away from each other or toward each other, not where they move past each other. Why?

- Where plates move apart lithosphere thins, asthenosphere bows up. Hot asthenosphere mantle rises to lower depth (=lower pressure) and mantle melts

- Where plates move together, one plate goes beneath another. This brings crustal material (low melting point) to higher temperatures and it melts.

One other kind of setting for volcanoes - spot locations. These are thought to be due to local 'hot spots' "Plumes" of heat rising from deep in the earth and locally raising the temperature of the mantle to its melting point.

So melting occurs
- by uplift and decompression of hot mantle rocks
- by downward transport and heating of low melting point crustal rocks
- by local heating of mantle rocks due to rising heat plume

 This melting can be studied experimentally. These experiments permit drawing of phase diagrams that summarize the conditions of temperature and pressure of formation of these melts.

For a survey of plate tectonics visit the USGS website This Dynamic Earth

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