04 Plate Tectonics
Learn how Earth’s moving plates are structured, what evidence supports their motion, and how new ocean floor forms and is recycled.
Earth’s layers and moving plates
Earth’s crust, mantle, and metallic core differ in composition and behavior. The core has a liquid outer part and a solid inner part. For understanding plate motion, the key distinction is between the rigid outer shell and the warmer mantle beneath it.
The consists of the crust and rigid uppermost mantle. It is broken into plates that carry both continents and ocean floor. Beneath it, the is mostly solid rock that can deform and flow slowly over long periods. It is not a global ocean of magma.
This difference in behavior allows stronger lithospheric plates to move over the more ductile mantle. Earth’s interior is understood through evidence such as earthquake waves, gravity, heat flow, and laboratory studies.
Takeaway: Plates are pieces of rigid , not just pieces of continental crust.
From continental drift to plate tectonics
In the early twentieth century, Alfred Wegener argued that continents now separated by oceans had once been joined. He proposed that they had formed part of the supercontinent and later drifted apart.
Several observations supported the idea:
The continental margins of South America and Africa fit together roughly, especially when their submerged margins are considered.
Matching fossils and rock formations occur on continents now separated by oceans. For example, fossils of the freshwater reptile Mesosaurus occur in both southern Africa and South America.
Ancient climate clues, such as glacial deposits in areas that are now warm, make sense when the continents are reassembled.
Wegener could not provide a convincing physical mechanism for moving continents, so his proposal was not initially accepted. Later discoveries about the ocean floor supplied the missing framework: continents move because they are embedded in moving plates, not because they plow through oceanic crust.
How the ocean floor supports the theory
explains how oceanic crust forms and moves. At mid-ocean ridges, magma rises, cools, and forms new oceanic crust. As new crust forms, older seafloor moves away from the ridge. At many deep ocean trenches, oceanic bends and sinks beneath another plate in a process called , returning material to the mantle.
The theory is supported by several independent patterns:
Magnetic stripes: Iron-bearing minerals in lava record the direction of Earth’s magnetic field as the lava cools. Because the field has reversed many times, the seafloor preserves alternating magnetic bands. Matching patterns on opposite sides of ridges support the idea that new crust forms there and moves outward.
Seafloor age: Oceanic crust is youngest near spreading ridges and generally becomes older farther away, consistent with crust formation at ridges and recycling at zones.
Earthquakes and volcanoes: Their global patterns outline many plate boundaries. Shallow earthquakes occur along spreading ridges and transform faults, while earthquakes can extend deep beneath zones.
Direct measurements: Satellite-based GPS tracks plate motion today. Plates move slowly, often at rates comparable to fingernail growth.
Matching geology: Fossils, rock units, mountain belts, and past-climate evidence can line up across oceans when continents are brought together.
Together, these lines of evidence show that plates move, that oceanic crust is created and destroyed, and that plate boundaries are active regions.
Forces that move plates
Plate motion involves Earth’s internal heat and gravity. Hot mantle material tends to rise and cooler material tends to sink, contributing to slow mantle circulation. However, plates are not simply passive rafts carried by a simple conveyor belt.
Two important gravitational forces are and . In , old, cold, dense oceanic sinks at a zone and pulls the rest of its plate behind it. In , newly formed stands higher at a mid-ocean ridge than older, cooler seafloor, and gravity helps move the plate toward lower elevations.
Mantle flow, , , and resistance along plate boundaries interact. No single simple force explains every plate’s motion.
Takeaway: Plate motion results from interacting processes involving heat and gravity, rather than one universal driving force.