Tectonic plates

Imagine an egg with a cracked shell.

The pieces still cover the egg. Earth is a bit like that. Its hard outside is cracked into giant pieces. We call them plates.

The ground beneath you feels perfectly still. But the U.S. Geological Survey (USGS) explains that the layer of Earth we live on is broken into a dozen or so huge, rigid slabs, called tectonic plates, and they are all moving.

The plates fit together like the pieces of a cracked eggshell. Some carry whole continents, some carry ocean floor, and many carry both. They move very slowly: NASA says the North American Plate moves at roughly the speed your fingernails grow. Over millions of years, that slow drift has opened oceans and built mountains.

Plate tectonics is the theory that Earth's outer layer is divided into rigid plates moving relative to one another. The USGS counts a dozen or so; the British Geological Survey (BGS) describes about 15 major slabs, plus smaller ones, so totals depend on how small a piece is counted as a plate.

Plates carry both continents and ocean basins. Their motion is slow, typically a few centimeters per year (BGS), and NASA compares the North American Plate's westward drift over the Pacific basin to the rate at which fingernails grow. Yet most of the action happens along narrow zones where plates meet: the USGS notes that most movement and earthquake activity is concentrated there. The idea that continents move began with Alfred Wegener's continental drift; the page on continental drift and Pangaea tells that story.

A plate is a giant slab of rock.

It is the crust plus the stiff top of the rock below. The plates move on softer, hotter rock underneath.

A plate is not just the crust. It is the crust plus the stiff, cooler top part of the mantle, welded together into one rigid layer called the lithosphere. The USGS says the lithosphere is usually at least 80 kilometers thick.

Under the plates is a hotter, softer layer called the asthenosphere. Its rock is not liquid, but over a very long time it can flow slowly, like very thick putty. The plates move over it. Ocean floor is made of heavier rock; continents are made of lighter rock that rides high.

Each plate is a piece of lithosphere: crust bonded to the cooler, rigid uppermost mantle, averaging at least 80 km thick and thinnest under oceans and in volcanically active continental areas (USGS). Beneath it, scientists believe, lies the asthenosphere, a relatively narrow zone of hot, semi-solid mantle that can soften and flow over geological time, on which the lithosphere moves.

What drives the motion is still studied. BGS lists three proposed forces: mantle convection carrying plates like a conveyor belt, ridge push as young, warm, elevated lithosphere slides away from mid-ocean ridges, and slab pull, as old, cold, dense lithosphere sinks at subduction zones and drags the rest of the plate. BGS says research shows slab pull is the main driver for most plates, because plates with more of their edges subducting move fastest, while recent work also gives ridge push a role.

Some plates move away from each other.

Hot rock comes up in the gap. It cools and makes new ocean floor. Iceland sits right on a crack like this.

Where two plates move apart, hot magma rises from the mantle, cools and makes brand-new crust. This is a divergent boundary. Most are hidden under the sea as long underwater mountain chains called mid-ocean ridges.

The Mid-Atlantic Ridge runs from the Arctic Ocean to beyond the southern tip of Africa. It spreads by about 2.5 centimeters a year. That sounds tiny, but over the past 100 to 200 million years it has turned a narrow strip of water into the whole Atlantic Ocean. In Iceland the ridge rises above the sea, and you can see the land cracking apart.

At divergent (constructive) boundaries, plates separate and new crust forms from magma rising from the mantle; the USGS pictures two conveyor belts carrying new oceanic crust away from the ridge crest. Normal faulting dominates, and earthquakes there tend to be frequent and small (BGS).

The Mid-Atlantic Ridge spreads at an average of about 2.5 cm/yr, or 25 km per million years, and seafloor spreading over the past 100 to 200 million years has grown the Atlantic from a tiny inlet into an ocean. Iceland straddles the ridge between the North American and Eurasian Plates; near Krafla volcano, rifting episodes from 1975 to 1984 opened the ground by a total of about 7 m (USGS). On land, the East African Rift may be an ocean in the making: the USGS notes that spreading has already separated Arabia from Africa, forming the Red Sea.

Some plates push into each other.

One plate can slide under the other. Or both crumple up into tall mountains. The Himalayas and the Andes were made this way.

Earth is not getting bigger, so if new crust is made in one place, old crust must be destroyed somewhere else. That happens at convergent boundaries, where plates move toward each other.

When heavy ocean floor meets a continent, the ocean plate sinks underneath. This is called subduction. It makes deep trenches in the sea floor, 8 to 10 kilometers deep, and lifts up mountains and volcanoes like the Andes. When two continents meet, neither one sinks, because continental rock is too light. Instead the crust crumples upward. India crashing slowly into Asia pushed up the Himalayas.

Because Earth's size has not changed significantly for at least 600 million years, crust must be destroyed about as fast as it is created, as Harry Hess reasoned (USGS). This happens at convergent (destructive) boundaries, dominated by reverse faulting. Where an oceanic plate meets a continent, as the Nazca Plate does under South America at the Peru-Chile trench, the dense oceanic slab is subducted, the overriding plate is lifted to form the Andes, and subduction zones produce most earthquakes above magnitude 6 and the deepest ones (BGS). Where two oceanic plates meet, one subducts beneath the other and volcanic island arcs such as the Marianas and Aleutians form.

Where two continents collide, neither subducts, since continental rock is relatively light; the crust buckles and is pushed up or sideways. India's collision with Asia, which the USGS dates to 50 million years ago, crumpled the Indian and Eurasian Plates, and slow convergence since then raised the Himalayas and the Tibetan Plateau, most of it in the past 10 million years.

Some plates slide past each other.

They rub side by side. Sometimes they get stuck, then slip. That makes the ground shake.

At a transform boundary, two plates slide past each other sideways. No crust is made and none is destroyed. The edges often lock together, then suddenly slip, which causes earthquakes.

Most transform boundaries are on the ocean floor, but one famous one is on land: the San Andreas fault zone in California. It is about 1,300 kilometers long. Along it, the Pacific Plate has been grinding past the North American Plate for about 10 million years, at an average of about 5 centimeters a year. The USGS points out that California will not fall into the sea: the plates slide sideways past each other, and the fault zone is only about 15 kilometers deep.

Transform (conservative) boundaries involve horizontal motion with no creation or loss of crust, dominated by strike-slip faulting; they typically produce large, shallow-focus earthquakes (BGS). The concept of transform faults came from Canadian geophysicist J. Tuzo Wilson, who proposed that they connect spreading centers or, less commonly, trenches. Most offset mid-ocean ridges into zig-zag margins on the sea floor.

The San Andreas fault zone is one of the few on land: about 1,300 km long and in places tens of kilometers wide, it connects the East Pacific Rise to the south with the Juan de Fuca and Explorer ridges to the north. The Pacific Plate has moved northwest past the North American Plate along it for 10 million years at about 5 cm/yr (USGS). The USGS adds that the fault zone extends only about 15 km deep, roughly a quarter of the thickness of the continental crust there, which is one reason the popular idea of California breaking off into the Pacific has no scientific basis.

Scientists can measure how fast plates move.

They use satellites in space. Some plates move about as fast as your fingernails grow.

How do we know the plates move, and how fast? Today scientists put GPS receivers on the ground and use satellites to measure their exact positions again and again. Year by year, points on different plates move farther apart or closer together.

The speeds are different for each place. The USGS says the ridge in the Arctic Ocean spreads at less than 2.5 centimeters a year, while the East Pacific Rise near Easter Island spreads at more than 15 centimeters a year. Scientists also read the rocks of the ocean floor, which hold a record of the plates' movement over millions of years.

Present-day motion is measured by space geodesy: very long baseline interferometry, satellite laser ranging and, most usefully, GPS, which fixes ground stations by timing signals from several satellites and tracks how the distances between them change (USGS). These data confirm that rates and directions averaged over a few years match those averaged over millions of years.

Long-term rates come from the ocean floor. Its rocks record reversals of Earth's magnetic field as stripes; knowing roughly how long each reversal lasted, scientists calculate average spreading rates. Rates vary widely, from less than 2.5 cm/yr on the Arctic Ridge to more than 15 cm/yr on the East Pacific Rise near Easter Island. Matching rock formations of known age across a boundary, and measuring how far they have been offset, gives another estimate, used on both the Mid-Atlantic Ridge and the San Andreas Fault.