AP® Environmental Science · Unit 4 · Topic 4.1
Plate Tectonics
Plate tectonics is the slow motion of Earth's rigid outer plates, driven by heat-powered convection in the mantle below, which builds new crust, drags continents apart, and reshapes the planet over millions of years.
Earth's interior is hot enough to flow slowly, not melt completely. In the mantle, material heated near the core rises, cools near the top, sinks again, and repeats. This convection drags the lithosphere, the rigid outer shell broken into plates, along for the ride. Plates move only a few centimeters a year, about as fast as fingernails grow, but over tens of millions of years that adds up to continents crossing oceans.
At mid-ocean ridges, convection currents pull plates apart and magma rises to fill the gap. It cools into new basalt seafloor, and the process repeats, pushing older crust away from the ridge on both sides. This seafloor spreading is why rock gets older the farther you travel from a ridge, and it is the mechanism that pries continents apart, which is exactly what split one landmass into South America and Africa.
A common mix-up is treating crust and lithosphere as the same layer. Crust is just the thin outermost rock, oceanic or continental. The lithosphere is that crust fused to the rigid uppermost mantle beneath it, and this combined slab, not the crust alone, is what makes up a tectonic plate. Plate tectonics sets up everything else in the unit: where plates collide or pull apart shapes mountain ranges, volcanic belts, earthquake zones, and even the ocean basins that steer currents and climate discussed later.
Key terms
- Mantle convection
- The circulation of hot mantle rock rising, cooling, and sinking, which supplies the force that moves plates.
- Lithosphere
- The rigid outer layer made of crust fused to the uppermost mantle, broken into the plates that move across Earth.
- Seafloor spreading
- The process of new oceanic crust forming at a mid-ocean ridge as plates pull apart and magma rises to fill the gap.
- Mid-ocean ridge
- An underwater mountain chain where two plates diverge and new crust is continuously created.
- Continental drift
- The historical observation that continents have shifted position over time, later explained by plate tectonics.
Watch it explained
- How it worksHow does convection in the mantle drag continent-sized plates across the planet?
- How it worksWalk me through seafloor spreading building brand new crust at a mid-ocean ridge.
- Common mistakeWhy isn't the crust the same thing as the lithosphere that plates ride on?
- Going furtherWhat happens to the world map 100 million years from now at current plate speeds?
Check yourself
End of topic.