Plate Movement and Convection Help (page 2)

By — McGraw-Hill Professional
Updated on Aug 31, 2011


The circulation of material caused by heat is called convection . In the Earth system, convection is affected by gravitational forces within the planet as well as heat and radioactive recycling of elements in the molten core.

All tectonic processes within the Earth involve movement of solid or malleable matter. Convection in the mantle, driven by the thermal gradient between the core and lithosphere, takes place by deformation (creep) of the rocks and minerals that comprise the upper/lower mantle and the transition zone. Think of it like those square, hand-held puzzle games where one piece is left out and you can only slide one square into the open place at a time. In order to complete a number sequence or picture, you must keep sliding the squares around (one-at-a-time) until you are able to slide all of them around to their correct spots to complete the puzzle.

Mantle creep is like that. Because of imperfections in the crystalline structures of minerals and rocks, there are gaps. When pressure is applied, the atoms in the structure shift (creep), one atom at-a-time to a new position.

Plate tectonics, as seen in mountain building, earthquakes, and volcanoes, takes place by plastic (malleable) or brittle bending of the rocks and minerals that make up the oceanic and continental lithosphere. Temperature, pressure, and rate of deformation to a large extent define the nature of deformation for most minerals and rocks in the interior of the Earth. However, the chemical environment (presence or absence of water, oxygen, silica, and other elements) may also have a big impact. By understanding the mechanisms by which rocks and minerals move and change shape under extreme temperature and pressures, we will add to our understanding of the processes that shape our planet.

The steady movement of magma deep within the Earth depends on differences in temperature and differences in density within large “pockets” of molten matter. Depending on conditions, magma rises in the pockets of hotter temperatures and falls in pockets of cooler temperatures. Since the Earth’s center is still hot, this endless thermal activity keeps the tectonic process going.

On a smaller scale, convection happens in liquids or gases, like the swirling currents of a pot of boiling soup. In the depths of the Earth, convection moves flowing magma that is heated from below by the core and then pushed upward over time and cooled from above. This solid flow movement is much slower than the liquid flow we saw earlier. Remember the lava lamp?

Convection is the process of heat transfer that causes hot, less dense matter to rise and cool matter to sink.

Convection affects rocks of different densities as well. Lighter density lithospheric rock tends to ride along above sea level, while denser asthenospheric rock sinks below sea level. The hard, rigid lithosphere is an unyielding outer shell, while the softer, wax-like asthenosphere is moldable and fluid when pushed.

When hot matter is forced up and out, it cools and adds to the outer crustal rock. As more material moves up, the earlier matter is pushed out of the way. The pressure from underlying rock is removed as it comes to the surface and the material in the magma chamber, a “crystalline mush” heats as it gets closer to the surface. This activity expands the area between the plates by a few centimeters per year. After a while, this new surface rock comes to another plate that will not yield. When this happens, plates argue and new rock gets pushed back down by subduction to be melted over again. Subduction occurs between opposing plates (mountains and magma chamber are not to scale), while cooling, rising magma causes spreading at ocean ridges as is shown in Fig. 4-5.

Plate Tectonics


Fig. 4-5. Magma creates new land at ocean ridges.

Practice problem of this concept can be found at: Plate Tectonics Practice Test

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