Virtual lab · Plate tectonics

Plate Tectonics Explorer

Pick a plate boundary and run time forward. Watch plates pull apart, collide or grind past each other, and see where trenches, mountains, volcanoes and earthquakes appear.

Printable activity

Boundary

Plates

All three plate pairs can meet at a convergent boundary.

Ocean–continent subduction Convergent boundary, ocean–continent

Time: 10 million years

Speed: 5 cm per year

5 cm per year for 10 million years: the plates move 500 km.

Show labels Show earthquakes Show volcanoes

Seen from above

Evidence

Plates move: Together. Dense oceanic plate sinks (subducts) under lighter continental plate. Water from the sinking slab helps the mantle above it melt.
Landforms: A deep ocean trench offshore
A chain of volcanoes (a volcanic arc) on the continent
Mountains that grow as the plates squeeze together
Earthquakes: Shallow near the trench, then deeper and deeper along the sinking slab, down to about 700 km.
Volcanoes: Yes: explosive volcanoes in a line roughly 100–300 km inland from the trench.
Real examples: The Andes, where the Nazca Plate sinks under South America
The Cascades, where the Juan de Fuca Plate sinks under North America

The three types of plate boundaries

Earth’s rigid outer shell, the lithosphere, is broken into seven or eight large plates and many smaller ones. They move a few centimeters a year, about as fast as your fingernails grow. Most earthquakes and volcanoes happen where plates meet, at their boundaries. There are three kinds, named for how the plates move.

1. Divergent boundaries: plates move apart

Where plates pull apart, hot mantle rock rises into the gap. As it rises the pressure on it drops, so some of it melts. The magma cools into new crust. Under the ocean this builds a mid-ocean ridge, a mountain chain longer than any on land, with new seafloor moving away on both sides. When a continent pulls apart, it first sags into a rift valley; if rifting goes on long enough, the sea floods in and a new ocean is born.

Mid-ocean ridge (divergent boundary, ocean–ocean). The Mid-Atlantic Ridge, which spreads about 2–5 cm per year.

Continental rift (divergent boundary, continent–continent). The East African Rift, opening less than 1 cm per year.

2. Convergent boundaries: plates move together

Where plates push together, what happens depends on the crust they carry. Ocean crust is thinner and denser than continental crust, so an ocean plate sinks, or subducts, under a continent or under a younger ocean plate. That makes a deep trench, and water squeezed out of the sinking slab helps the rock above it melt, feeding a line of volcanoes. When two continents meet, neither sinks far: the crust crumples and stacks up into the highest mountains on Earth.

Ocean–continent subduction (convergent boundary, ocean–continent). The Andes, where the Nazca Plate sinks under South America.

Ocean–ocean subduction (convergent boundary, ocean–ocean). The Mariana Trench and the Mariana Islands.

Continental collision (convergent boundary, continent–continent). The Himalaya, where India pushes into Asia.

3. Transform boundaries: plates slide past each other

Where plates slide past each other sideways, crust is neither made nor destroyed. The rocks lock together, bend, and then slip suddenly in an earthquake. There are no volcanoes, because nothing rises or sinks. Most transform faults are on the seafloor, cutting mid-ocean ridges into segments; a few, like the San Andreas Fault, cross continents.

Continental transform fault (transform boundary, continent–continent). The San Andreas Fault in California, between the Pacific Plate and the North American Plate.

Oceanic transform fault (transform boundary, ocean–ocean). The transform faults that cut the Mid-Atlantic Ridge into segments.

Plate boundaries compared

Boundary	Plates move	Landforms	Deepest earthquakes	Volcanoes	Example
Mid-ocean ridgeDivergent boundary, ocean–ocean	Apart	A mid-ocean ridge: a long underwater mountain chain A rift valley along the ridge crest New seafloor that gets older the farther it is from the ridge	Shallow	Yes	The Mid-Atlantic Ridge, which spreads about 2–5 cm per year
Continental riftDivergent boundary, continent–continent	Apart	A rift valley with steep walls of faulted rock Long, deep lakes on the valley floor Volcanoes along and near the rift	Shallow	Yes	The East African Rift, opening less than 1 cm per year
Ocean–continent subductionConvergent boundary, ocean–continent	Together	A deep ocean trench offshore A chain of volcanoes (a volcanic arc) on the continent Mountains that grow as the plates squeeze together	Deep	Yes	The Andes, where the Nazca Plate sinks under South America
Ocean–ocean subductionConvergent boundary, ocean–ocean	Together	A deep ocean trench, the deepest places on Earth A curved chain of volcanic islands (an island arc)	Deep	Yes	The Mariana Trench and the Mariana Islands
Continental collisionConvergent boundary, continent–continent	Together	A high, wide mountain range Folded and faulted rock layers A thick root of crust under the mountains	Intermediate	No	The Himalaya, where India pushes into Asia
Oceanic transform faultTransform boundary, ocean–ocean	Past each other	A fault that offsets the ridge into separate segments Long scars on the seafloor (fracture zones)	Shallow	No	The transform faults that cut the Mid-Atlantic Ridge into segments
Continental transform faultTransform boundary, continent–continent	Past each other	A long, straight fault line with a valley along it Streams, roads and rock layers offset where they cross the fault	Shallow	No	The San Andreas Fault in California, between the Pacific Plate and the North American Plate

Earthquake depths: shallow is less than 70 km, intermediate 70–300 km and deep 300–700 km.

Worked example: how far, and how old?

Plate speeds are tiny in a year but huge over geologic time. One centimeter a year for a million years is 1,000,000 cm, which is 10 km. So:

distance (km) = speed (cm per year) × time (million years) × 10

How far? Suppose a ridge spreads 2.5 cm per year (the Mid-Atlantic Ridge spreads about 2–5 cm per year). That is 25 km per million years, so in 10 million years the plates move 25 × 10 = 250 km apart. At that speed, two markers on opposite sides of the ridge end up 2.5 m farther apart every century.

How old? At a ridge, each plate carries half the new seafloor away, so each moves at half the spreading rate: 2.5 ÷ 2 = 1.25 cm per year, or 12.5 km per million years.

seafloor age (million years) = distance from the ridge (km) ÷ (half the spreading rate × 10)

Seafloor 500 km from the ridge is 500 ÷ 12.5 = 40 million years old. The farther from the ridge, the older the seafloor, on both sides. Measuring seafloor ages like this was some of the strongest evidence that plates move.

Try your own numbers in the explorer’s calculator, or press Use the explorer’s speed and time to check the slider.

Going deeper

Earthquakes trace the sinking slab

Plot the depths of earthquakes near a trench and they form a sloping band that dips under the volcanoes, down to about 700 km. This band, called a Wadati–Benioff zone, shows exactly where the cold slab is sinking through the mantle. The explorer colors each quake by its depth class so you can see the band form as the slab grows.

What moves the plates?

Heat escaping from Earth’s interior keeps the mantle slowly churning. The biggest single force is probably slab pull: an old, cold ocean plate is denser than the mantle beneath it, so where it sinks at a trench it drags the rest of the plate behind it. Plates also slide gently downhill away from the high, hot ridges. That is one reason plates attached to long subduction zones, around the Pacific, tend to move fastest.

Why ocean floor is young

New seafloor is made at ridges and destroyed at trenches, so it never gets very old: almost all of it is younger than about 200 million years. Continental crust is too light to sink, so it survives far longer, and some continental rocks are billions of years old.

Common misconceptions

“The plates float on liquid magma.” They don’t. The mantle under the plates is almost all solid rock. Magma forms only in small pockets where rock partly melts, mostly at ridges, rifts and above sinking slabs.
“The mantle is a sea of lava.” The mantle is solid, but it is so hot and under so much pressure that, over millions of years, it can flow very slowly, a few centimeters a year, like extremely stiff putty.
“Every plate boundary has volcanoes.” Transform boundaries have none, and continental collisions have few or none.
“Plates move suddenly.” Plates creep a few centimeters a year. Rock along a locked fault stores up that motion and releases it in a sudden slip: an earthquake.
“Continents sink at subduction zones.” Only ocean plates sink far. Continental crust is too thick and light, which is why colliding continents pile up into mountains instead.

What this model simplifies

The cross-sections are diagrams, not maps of real places. Depths below the surface follow the scale on the left, but mountains, trenches, ridges and the crust are drawn many times taller than they really are, or they would be too thin to see. Each boundary starts from a simple shape and grows at one steady speed; real boundaries change speed and direction, slabs sink at different angles, and many boundaries mix types. The earthquake dots show the pattern of depths, not real recorded earthquakes. Speeds and depths are rounded classroom values.

For the classroom

Plate boundaries activity

A one-page printable worksheet for grades 6–10, about 15 minutes: students compare six boundaries in the explorer, record landforms, earthquake depths and volcanoes, then answer six questions, including plate-speed and seafloor-age calculations.

Open the activity For teachers

Share the explorer with your class in one click. Students need no accounts or logins.

Good questions

Plate tectonics questions

What are the three types of plate boundaries?

At divergent boundaries plates move apart, as along the Mid-Atlantic Ridge and the East African Rift. At convergent boundaries they move together: one plate sinks under another, as under the Andes and Japan, or two continents collide, as in the Himalaya. At transform boundaries they slide past each other, as along the San Andreas Fault.

How fast do tectonic plates move?

Most plates move between about 1 and 10 centimeters a year, roughly as fast as fingernails grow. The Mid-Atlantic Ridge spreads about 2–5 cm per year. That sounds slow, but 1 cm per year adds up to 10 km every million years, which is enough to open an ocean over tens of millions of years.

Why do some earthquakes happen hundreds of kilometers deep?

Below a few tens of kilometers, rock is usually too hot to break suddenly; it bends and flows instead. At subduction zones, though, a cold slab of ocean plate sinks into the mantle, and rock inside it can still break. Its earthquakes get deeper and deeper away from the trench, down to about 700 km. Divergent and transform boundaries have only shallow earthquakes.

Do tectonic plates float on liquid magma?

No. The mantle under the plates is almost all solid rock. It is so hot that over millions of years it can flow very slowly, a few centimeters a year, and the plates move along on top of it. Magma forms only in small places where rock partly melts, such as under mid-ocean ridges and above sinking slabs.

Why are there no volcanoes at transform boundaries?

Magma forms when hot rock rises and the pressure on it drops, as at ridges and rifts, or when water from a sinking slab lowers the melting point of the mantle above it. At a transform boundary the plates only slide sideways past each other, so neither happens.

Can I use the Plate Tectonics Explorer in class?

Yes. It is free with no sign-up and runs on school Chromebooks. Use the printable activity to compare the boundaries, then let students try Challenge mode, eight questions that ask them to identify a boundary from a cross-section or from evidence. Teachers can find the answer key on the For teachers page.