Plate Tectonics & Earth's Interior

Earth's layered interior

From outside in: a thin rocky crust, a thick mantle (mostly solid rock that can flow very slowly), a liquid outer core, and a solid inner core (both core layers are iron and nickel). It gets hotter and denser with depth. The rigid outer shell — crust plus the top of the mantle — is broken into moving tectonic plates.

What moves the plates

Heat from the core sets up convection currents in the mantle: hot rock rises, spreads sideways, cools, and sinks in a continuous loop. These slow currents drag the plates along — the same physics as warm air circulating in a room, just at the speed your fingernails grow.

Three types of plate boundary

• Divergent — plates pull apart. Magma rises to fill the gap, creating new crust. At sea this builds mid-ocean ridges and drives sea-floor spreading; on land it forms rift valleys.
• Convergent — plates collide. Denser oceanic crust subducts beneath the other plate, forming deep trenches, volcanoes, and mountain belts. Continent-continent collisions push up ranges like the Himalayas.
• Transform — plates slide horizontally past each other. No crust is made or destroyed, but the grinding causes earthquakes (the San Andreas Fault is the classic example).

Evidence the continents move

• The coastlines of South America and Africa fit together like puzzle pieces.
• Matching rock belts and fossils line up across now-separated continents.
• Ocean crust gets older with distance from a mid-ocean ridge, and the magnetic stripes on the sea floor are symmetric on both sides — exactly what sea-floor spreading predicts.