4 Plate Tectonics
Learn how tectonic plates move, interact at their boundaries, create and recycle seafloor, and carry continents across Earth.
Plates and their boundaries
Earth’s outer shell is divided into large, slowly moving pieces called tectonic plates. Each plate is a slab of : rigid crust together with the strong uppermost mantle. Plates may carry continental land, ocean floor, or both, so a continent is not itself a separate plate. The plates move over the hotter, weaker , which can flow slowly over geologic time.
Plate boundaries are the zones where neighboring plates meet. They are classified by the direction of plate motion, though some boundaries form broad, complex zones rather than a single narrow line.
Takeaway: Plates are pieces of rigid , and their interactions shape Earth’s surface.
How plates interact
At a , plates move apart. Magma rises into the opening and cools to form new crust, especially along mid-ocean ridges.
At a , plates move toward one another. Dense oceanic can sink beneath another plate through . When two continents converge, their buoyant crust resists sinking; instead, it is compressed and uplifted into mountains.
At a , plates slide horizontally past one another. Crust is neither created nor consumed there. The San Andreas Fault is a well-known example.
Takeaway: Whether plates separate, collide, or slide past one another determines the kinds of geological changes that occur at a boundary.
Forces that move plates
Earth’s internal heat sustains slow circulation in the mantle, helping make long-term plate motion possible. Plate motion is not simply a passive ride on one conveyor belt: gravity acting on the plates themselves is especially important.
: Oceanic stands higher near a mid-ocean ridge than farther away. As it cools and becomes denser, gravity helps it slide away from the elevated ridge.
: Old, cold oceanic is dense. When it sinks into the mantle at a zone, its descending weight pulls the rest of the plate behind it.
Mantle flow and forces at plate boundaries also affect motion. The balance of forces varies among plates, but and are key drivers.
Takeaway: Plate motion results from several interacting processes, including gravity-driven forces and mantle flow.
and moving continents
At a mid-ocean ridge, plates separate and mantle-derived magma rises into the gap. As the magma cools, it forms new oceanic crust. The continuing addition of rock moves older seafloor away from the ridge; this process is called . Older oceanic is eventually recycled into the mantle at many zones.
Evidence for spreading is preserved in the seafloor:
Rocks are youngest near a ridge and generally become older farther away.
Basalt formed at a ridge records the direction of Earth’s magnetic field as it cools. Because the field has reversed many times, alternating magnetic bands appear on the ocean floor.
The magnetic bands roughly mirror one another on either side of the ridge.
Together, the pattern of rock ages and magnetic bands supports the explanation that new crust forms at ridges and moves outward as the seafloor spreads.
Continents travel because they are embedded in moving plates; they do not plow independently through the ocean floor. In 1912, Alfred Wegener argued that continents had once been joined and later drifted apart. His proposal, , drew support from the way some continental margins fit together and from similarities in rocks, fossils, and ancient climates across continents now separated by oceans. Wegener could not provide a convincing mechanism for the movement, so the idea was not widely accepted at first. Evidence for and the recycling of oceanic crust helped explain how continents move: they travel with their plates.
Over geologic time, plate motion has assembled and broken apart supercontinents. The Atlantic Ocean, for example, has widened as carried the Americas away from Europe and Africa.
Takeaway: New oceanic crust forms at ridges and older crust is recycled at zones, while continents move as parts of plates.