0 shallow · 0 intermediate · 0 deep
The block is 1,400 km wide and 400 km deep. Heights and the crust are drawn much thicker than real so you can see them; depths below the crust are to scale.
250 million years ago almost all the land was joined in one supercontinent, Pangaea. Press play to watch it break apart, or drag the slider. The continents move with a real plate reconstruction (Müller et al., 2019). Turn on Wegener’s evidence: fossils of the same animals and plants, and matching mountain belts, line up when the continents are put back together.
Fossil dots are approximate find areas. Positions are relative to the deep mantle; small islands are left out.
Earth’s magnetic field flips at irregular times, on average every few hundred thousand years (the last flip was about 780,000 years ago). Lava erupting at a mid-ocean ridge records the field as it cools, then moves away as new lava takes its place. So the sea floor carries a striped record, the same on both sides of the ridge. Press play to make 6 million years of sea floor using the real record of reversals, then tap the sea floor to drill a sample.
Reversal dates: Geologic Time Scale 2012. Faster spreading makes wider stripes. The Mid-Atlantic Ridge spreads at about 1–2.5 cm a year on each side; the East Pacific Rise up to about 7–8.
Every earthquake of magnitude 5 or more in 2018 (1,802 of them), colour-coded by depth, with the world’s plate boundaries and volcanoes. Most of them line up along the boundaries: in 2018, 82% were within 200 km of a plate boundary. Tap the map to find out about a boundary, an earthquake or a volcano.
The — earthquakes of 2018 near the Tonga Trench, plotted by depth. Near the trench they are shallow; farther west they get deeper and deeper, down to about 670 km. They outline the Pacific Plate sinking into the mantle, just like the slab in the 3D lab.
Data: earthquakes USGS (2018, magnitude 5+; 1965–2016, magnitude 7+); plate boundaries Bird (2003) PB2002; volcanoes Smithsonian Global Volcanism Program (Holocene volcanoes; darker triangles erupted in 1964 or later).
Heat escaping from inside the Earth keeps the mantle slowly moving, and gravity pulls on the plates. Choose a force to see it.
The cross-section you label in a test: a mid-ocean ridge where plates move apart, and a trench where an ocean plate sinks under a continent, with the volcano, magma, lithosphere, asthenosphere and convection currents. Switch between the labeled diagram, a blank one to test your memory, and a quiz where you place each label yourself. You can also download both versions or print a label worksheet.
Tap a word, then tap the numbered box it belongs to (on the diagram or in the list below). Tap a filled box to take the word back. On a phone, swipe the diagram sideways to see every number.
Ten questions about plates, boundaries and the evidence.
Tap a card, then tap the boundary it belongs to. Check when all twelve are placed.
There are seven big plates and many smaller ones. A plate can carry a continent, ocean floor or both. They move 2–10 cm a year, about as fast as your fingernails grow, which is enough to open a whole ocean in a hundred million years.
At a divergent boundary plates move apart and magma fills the gap, making new crust: a mid-ocean ridge under the sea, or a rift valley on land. At a convergent boundary they move together: a denser ocean plate sinks under another plate at a trench (subduction), making volcanoes and deep earthquakes, or two continents crumple into fold mountains. At a transform boundary they slide past each other along a fault.
In 1912 Alfred Wegener argued that the continents had drifted apart from one supercontinent, using the fit of the coastlines, matching fossils and rocks, and traces of ancient glaciers. Most scientists rejected it because he could not explain what moved them. Mapping of the sea floor after the Second World War showed that mid-ocean ridges, with a rift valley along their crest, run right around the globe; in the early 1960s Harry Hess and Robert Dietz proposed sea-floor spreading, and in 1963 Fred Vine and Drummond Matthews (and, independently, Lawrence Morley) showed that the magnetic stripes prove it. By the late 1960s these ideas had become the theory of plate tectonics.
Run the ocean–continent boundary and watch where the earthquakes appear: shallow near the trench, deeper inland. Then find the same pattern under Tonga in the real data. Play the drift map from Pangaea and stop at 66 million years. Make sea floor at 1 cm a year and at 8 cm a year and compare the stripes.
| Boundary | Plates move | What happens | Landforms | Examples |
|---|---|---|---|---|
| Divergent (ocean) | apart | magma rises and makes new ocean crust | mid-ocean ridge, rift valley along its crest | Mid-Atlantic Ridge, East Pacific Rise |
| Divergent (continent) | apart | the continent stretches, thins and may split | rift valley, volcanoes, later a narrow sea | East African Rift, Red Sea |
| Convergent: ocean–continent | together | the ocean plate sinks (subduction) | trench, volcanic mountain range | Andes, Cascades |
| Convergent: ocean–ocean | together | the older, denser ocean plate sinks | deep trench, volcanic island arc | Mariana Islands, Japan, Aleutians |
| Convergent: continent–continent | together | neither sinks far; crust folds and thickens | high fold mountains, plateaus | Himalayas and Tibet, Alps |
| Transform | past each other | crust is neither made nor destroyed | long fault, offset streams and roads | San Andreas Fault, Alpine Fault |
| Evidence | What it shows |
|---|---|
| Fit of the continents | South America and Africa fit like puzzle pieces, best at the edges of their continental shelves. |
| Fossils | Mesosaurus, Cynognathus, Lystrosaurus and the plant Glossopteris are found on continents now separated by oceans they could not have crossed. |
| Rocks and mountains | Rock types and old mountain belts match across the Atlantic: the Appalachians continue in the Caledonian mountains of Greenland, Scotland and Norway. |
| Ancient climates | Scratches left by glaciers about 300 million years ago are found in South America, Africa, India and Australia; coal and fossil forests are found in Antarctica, which is now far too cold for trees. |
| Sea-floor ages and magnetic stripes | The ocean floor is youngest at ridges, older away from them, and nowhere older than about 180 million years; magnetic stripes are mirrored across ridges. |
| Earthquakes and volcanoes | They form narrow belts along the plate boundaries; deep earthquakes trace sinking plates. |
| GPS | Satellites measure the plates moving today, a few centimetres a year. |
| Value | |
|---|---|
| Plate speeds | about 2–10 cm a year (fastest spreading about 15 cm a year in total, on the East Pacific Rise) |
| Mid-ocean ridge system | about 65,000 km long, the longest mountain range on Earth |
| Deepest trench | Mariana Trench, Challenger Deep: about 10.9 km |
| Highest mountain | Mount Everest, 8,849 m, made by the India–Asia collision |
| Oldest ocean floor | about 180 million years (western Pacific); some continental rock is about 4 billion years old |
| Deepest earthquakes | about 700 km, inside sinking plates |
The theory that the Earth’s rigid outer shell (the lithosphere: the crust plus the top of the mantle) is broken into large plates that move slowly over the hot, soft asthenosphere. Their movements make earthquakes, volcanoes, mountains, trenches and new ocean floor, and over millions of years they move the continents.
Key takeaway: moving plates of lithosphere shape the Earth’s surface.Divergent (plates move apart: mid-ocean ridges and rift valleys), convergent (plates move together: trenches, volcanic arcs and fold mountains) and transform (plates slide past each other along faults such as the San Andreas Fault).
Key takeaway: apart, together, past.Usually 2–10 cm a year, about as fast as fingernails grow. GPS stations measure it directly. Over 100 million years, 5 cm a year adds up to 5,000 km, enough to open an ocean.
Key takeaway: a few centimetres a year.Heat from inside the Earth drives slow convection in the mantle. The main forces on the plates come from gravity: slab pull, where the cold, dense edge of a plate sinks at a trench and pulls the rest, and ridge push, where young hot lithosphere at a ridge slides away down a gentle slope. Slab pull is thought to be the largest.
Key takeaway: mostly slab pull, plus ridge push, driven by the mantle’s heat.Pangaea was a supercontinent that held almost all of Earth’s land. It was fully assembled by about 300 million years ago and began to break up about 200 million years ago, when the central Atlantic started to open. The pieces became today’s continents.
Key takeaway: one supercontinent, breaking up from about 200 million years ago.The fit of the continents; the same fossils (Mesosaurus, Glossopteris, Lystrosaurus) on continents now apart; matching rocks and mountain belts across oceans; traces of ancient glaciers in today’s tropics; young, mirrored, magnetically striped ocean floor; belts of earthquakes and volcanoes along plate edges; and GPS measurements today.
Key takeaway: fossils, rocks, fit, old climates, the sea floor, earthquakes and GPS.That is where plates grind, pull apart or sink. The rock bends until it breaks and slips, making earthquakes. Magma forms where plates pull apart (rock rises and partly melts) and above sinking plates (water from the plate lowers the melting point of the mantle). In 2018, 82% of magnitude 5+ earthquakes were within 200 km of a plate boundary.
Key takeaway: boundaries are where rock breaks and melts.A horseshoe of subduction zones around the Pacific Ocean, from New Zealand through Japan, Alaska and the Andes. Ocean plates sink there, so it has most of the world’s explosive volcanoes and many of its biggest earthquakes.
Key takeaway: the subduction zones around the Pacific.Lava records the direction of Earth’s magnetic field when it cools, and the field has reversed many times. If new crust forms at the ridge and moves away on both sides, the stripes must be mirror images across the ridge, with ages that match the known dates of reversals. That is exactly what ships found in the early 1960s.
Key takeaway: mirror-image stripes record reversals as the sea floor spreads.Hawaii sits over a hotspot: a plume of unusually hot mantle rising from deep below. The Pacific Plate moves over it, so it makes a chain of volcanoes that get older to the northwest. The bend in the Hawaiian–Emperor chain records a change in plate motion about 47 million years ago.
Key takeaway: a hotspot under a moving plate makes a chain of volcanoes.