Illinois State Museum

heat drives the system graphic

Heat Drives the System

Convection currents of magma (molten rock) circulate through Earth's asthenosphere. The plates are carried on these convection currents. As they move, plate material is continually being created and destroyed along the margins.

Divergent Boundaries
Where magma squeezes up through cracks between the ocean floor plates, it creates new crust, pushing the plates apart.

Convergent Boundaries
At convergent boundaries, plates come together. There are two kinds of convergent boundaries: subduction zones and collision margins. Subduction zones occur where two plates come together, and one plate slides under the other. The subducted crust melts into magma in the asthenosphere and is recycled. Subduction can occur when two plates made of oceanic crust come together, or where continental crust and oceanic crust meet. Ocean crust is heavier than continental crust and is forced under it when the two meet. Earthquakes and volcanic activity can occur along subduction zones, as the crust melts underneath the overriding plate.

When the movement of the plates pushes continents together, one does not slide under the other. At collision margins, the continental crust buckles and crumples forming mountain ranges.

Continued on next page.