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Mantle Convection

Solid, but convecting?

The cool, outer thermal boundary layer of the earth is stronger because it is cool, and it is called the lithosphere. On the other hand, the deeper mantle (below about 100 km depth) is hot enough that it slowly deforms. Over millions of years, this slow deformation is equivalent to fluid behaviour, even though the mantle rocks are solid.

An odd kind of convection – sinking plates

Although the lithosphere is strong, evidently it is also brittle and is broken into about a dozen large pieces: the plates. Because they are cooler and denser than the deeper mantle, the plates tend to sink. Once part of a plate has begun to sink, the plate acts like a continuous conveyor belt: new plate forms at mid-ocean ridges, where plates are separating, and the new part of the plate slowly drifts away from the ridge, thickening as it goes. Eventually this section of plate reaches a subduction zone, where it sinks under its own weight. The whole process of forming, thickening and sinking of a plate is called plate dynamics.

Another kind of convection – rising plumes

Under the mantle lies the liquid iron core, extending about half way up from the center of the earth. Heat leaking from the core evidently heats the bottom of the mantle, forming a hot thermal boundary layer. Material in this boundary layer rises because it is buoyant, forming mantle plumes. Mantle plumes comprise a second mode of mantle convection.

Notes: Last modified: Wed, 19 Dec 2001