“With such wisdom has nature ordered things in the economy of this world, that the destruction of one continent is not brought about without the renovation of the earth in the production of another.” — James Hutton, Theory of the Earth, with Proofs and Illustrations, Vol. 1, 1795.
Hutton’s quote predates plate tectonics theory by about one-and-a-half centuries, but it seems as if he was talking about divergent and convergent plate boundaries. The next step in understanding the development of plate tectonics theory is to learn what it is that moves around on Earth’s surface. It’s not really a continent; it’s a plate. What is a plate?
What is a Plate?
What portion of Earth makes up the “plates” in plate tectonics? Again, the answer came about in part due to war. In this case, the Cold War.
During the 1950s and early 1960s, scientists set up seismograph networks to see if enemy nations were testing atomic bombs. These seismographs also recorded all of the earthquakes around the planet. The seismic records were used to locate an earthquake’s epicenter, the point on Earth’s surface directly above the place where the earthquake occurs.
Why is this relevant? It turns out that earthquake epicenters outline the plates. This is because earthquakes occur everywhere plates come into contact with each other.
[Figure 1]
Earthquakes outline the plates.
The lithosphere is divided into a dozen major and several minor plates (Figureabove). A single plate can be made of all oceanic lithosphere or all continental lithosphere, but nearly all plates are made of a combination of both.
The movement of the plates over Earth's surface is termed plate tectonics. Plates move at a rate of a few centimeters a year, about the same rate fingernails grow.
How Plates Move
If seafloor spreading drives the plates, what drives seafloor spreading?
This goes back to Arthur Holmes’ idea of mantle convection. Picture two convection cells side by side in the mantle, similar to the illustration in Figurebelow.
Hot mantle from the two adjacent cellsrises at the ridge axis, creating new ocean crust.
The top limb of the convection cell moves horizontally away from the ridge crest, as does the new seafloor.
The outer limbs of the convectioncells plunge down into the deeper mantle, dragging oceanic crust as well. This takes place at the deep sea trenches.
The material sinks to the core and moves horizontally.
The material heats up and reaches the zone where it rises again.
[Figure 2]
Mantle convection drives plate tectonics. Hot material rises at mid-ocean ridges and sinks at deep sea trenches, which keeps the plates moving along the Earth’s surface.
Plate Boundaries
Plate boundaries are the edges where two plates meet. How can two plates move relative to each other? Most geologic activities, including volcanoes, earthquakes, and mountain building, take place at plate boundaries. The features found at these plate boundaries are the mid-ocean ridges, trenches, and large transform faults (Figure below).
Divergent plate boundaries: the two plates move away from each other.
Convergent plate boundaries: the two plates move towards each other.
Transform plate boundaries: the two plates slip past each other.
[Figure 3]
The lithospheric plates and their names. The arrows show whether the plates are moving apart, moving together, or sliding past each other.
The type of plate boundary and the type of crust found on each side of the boundary determines what sort of geologic activity will be found there. We can visit each of these types of plate boundaries on land or at sea.
Summary
The plate in plate tectonics is a large chunk of lithosphere that can carry continental crust, oceanic crust, or some of each.
Plates can be identified by the locations of earthquake epicenters. At the boundaries of plates are mid-ocean ridges, trenches, and large faults.
Plates move by seafloor spreading, which is driven by mantle convection.
Plates meet at plate boundaries. The three types are divergent, convergent, and transform.
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