Layers of Earth and Plate Tectonics

Earth looks solid from where you stand, but it is built like a giant onion. Slice it open and you find four layers stacked from the center outwards — and the outer skin is broken into giant pieces that slide and grind against each other. Those pieces are called tectonic plates, and the way they move is what builds mountains, opens oceans, fires volcanoes, and rattles the ground in earthquakes.

The Four Layers of Earth

• Inner core — a ball of solid iron and nickel, about as hot as the surface of the Sun. Pressure is so huge it stays solid even at thousands of degrees.
• Outer core — liquid iron and nickel that swirls around the inner core. Its motion creates Earth's magnetic field, which makes a compass needle point north.
• Mantle — thick, hot, slow-moving rock between the core and the crust. It is mostly solid but flows like very stiff toffee over millions of years.
• Crust — the thin, brittle outer layer where we live. Continental crust is thick and old; oceanic crust is thinner, denser, and younger.

Tectonic Plates and Convection

The crust and the very top of the mantle together form a rigid shell called the lithosphere. That shell is cracked into about a dozen large pieces — tectonic plates. The plates float on the slow-flowing mantle below. Heat from the core makes hot rock rise, cool, and sink again in giant loops called convection currents. Those currents drag the plates along, the way boiling water in a pot drags floating leaves around the surface. Each plate moves only a few centimeters a year (about 1 in/year) — about as fast as your fingernails grow — but over millions of years that adds up to whole continents shifting.

Three Kinds of Plate Boundary

• Divergent — two plates pull apart. Magma rises into the gap and cools to form new crust. This is how mid-ocean ridges and new ocean floor are made.
• Convergent — two plates push together. If one is oceanic and one is continental, the heavier oceanic plate slides under the lighter continent (subduction) and forms a deep trench plus a chain of volcanoes. If two continents collide, neither sinks easily and the crust crumples up into mountain ranges like the Himalayas.
• Transform — two plates slide past each other sideways. They snag and jerk, releasing the stored energy as earthquakes. The San Andreas Fault in California is the classic example.

What Plate Tectonics Builds and Breaks

Almost every dramatic feature on Earth's surface comes from plate motion. Volcanoes line the edges of plates above subduction zones (the Pacific Ring of Fire) or above mid-ocean ridges. Earthquakes happen wherever plates jerk past, under, or away from each other. Mountain ranges grow where plates collide. Even the shapes of the continents are clues: South America and Africa fit together like puzzle pieces because they were once joined as the supercontinent Pangaea, then split apart as new ocean crust formed between them. This slow drifting of continents is called continental drift.