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Convection Currents:

Currents in the Earth's System

Convection currents occur within:

• the geosphere – plate tectonics

• the atmosphere - wind

• the hydrosphere - ocean currents

Focus Question: What is the source of energy for convection currents in the geosphere?

Convection currents in the magma drive plate tectonics.

Heat generated from the radioactive decay of elements deep in the interior of the Earth creates magma (molten rock) in the aesthenosphere.

The aesthenosphere (70 ~ 250 km) is part of the mantle, the middle sphere of the Earth that extends to 2900 km. It contrasts with the more rigid lithosphere, the outer shell of the Earth (0 ~ 70 km) that contains the continental crust (made up of less dense granitic rocks) and the oceanic crust (more dense basaltic rocks) that are broken up into more than a dozen rigid plates. For more info, see: Why do the plates move?

Large convection currents in the aesthenosphere transfer heat to the surface, where plumes of less dense magma break apart the plates at the spreading centers, creating divergent plate boundaries.

As the plates move away from the spreading centers, they cool, and the higher density basalt rocks that make up ocean crust get consumed at the ocean trenches/subduction zones. The crust is recycled back into the aesthenosphere.

Subduction of Plates

Because ocean plates are denser than continental plates, when these two types of plates converge, the ocean plates are subducted beneath the continental plates. Subduction zones and trenches are convergent margins. The collision of plates is often accompanied by earthquakes and volcanoes.

Focus Question: Where is the source of heat in the atmosphere-hydrosphere system?

The source of heat is from the sun, above.

The teachers were asked to sketch the variation in the distribution of heat from the equator to the poles, noting the difference in the angle of incidence with latitude and how this would affect heating.

This led to discussions about the multiple currents/cells that are driven by unequal heating driving currents both vertically (creating high and low pressure systems by descending and ascending air masses) and horizontally.

Focus Question: If the hydrosphere were a closed system with only an external source of heat from the sun, what simple temperature patterns would you expect to see in the ocean basins?

One would expect to see warmer temperatures at the equator and cooler temperatures at the poles leading to two large convection cells from the equator to the poles, one in each hemisphere.

Teachers map their hypotheses:

Focus Question: How well does this simple basin model illustrate the real convection cells in the ocean-atmosphere system?

The teachers pondered this and other questions to be addressed further during Session #2 on November 2, 2002.

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