What this lesson teaches
I can explain what forms when plates move apart, collide or slide past each other.
Syllabus K229, Topic 3.1, Key Question 4. What happens at plate boundaries when tectonic plates move?
Make a guess
Two continental plates push into each other. What is most likely to form?
- Nothing, because continents cannot move.
- A deep trench with a line of volcanoes.
- Fold mountains, with earthquakes.
- A mid-ocean ridge with new crust.
Show the answer
Fold mountains, with earthquakes.
Continental crust is too light to sink easily. When two continents collide, the crust crumples and thickens, as India pushing into Eurasia built the Himalayas.
How plates move, and what type they are, decides whether a boundary makes new crust, subduction zones, mountains or faults.
A plate boundary is where plates meet. At divergent boundaries, plates move apart. This forms ocean ridges, rift valleys on land, undersea and island volcanoes, and earthquakes. At convergent boundaries, plates move towards each other. At transform boundaries, plates slide past each other.
When plates converge, the plate type matters. Dense ocean plate sinks under the other plate, forming a deep trench and volcanoes. When two continents collide, neither sinks easily. The crust crumples and thickens into fold mountains.
Sort these
Which type of plate boundary does each clue point to?
Groups: Divergent (moving apart), Convergent (moving together), Transform (sliding past).
- The Pacific and North American Plates slide past each other in California
- Frequent earthquakes but no line of volcanoes
- A rift valley opens up in East Africa
- India pushing into Eurasia built the Himalayas
- Volcanoes form above a sinking ocean plate in Indonesia
- New ocean crust forms along the Mid-Atlantic Ridge
- Ocean plate sinks into a deep trench near Japan
Show the answer
Divergent (moving apart)
- A rift valley opens up in East Africa Land splitting as plates move apart creates a rift valley at a divergent boundary.
- New ocean crust forms along the Mid-Atlantic Ridge Crust forms where plates pull apart and magma fills the gap, so this is divergent.
Convergent (moving together)
- India pushing into Eurasia built the Himalayas Two continents colliding and crumpling into fold mountains is convergence.
- Volcanoes form above a sinking ocean plate in Indonesia Subduction at a convergent boundary releases water that helps the mantle melt, feeding volcanoes.
- Ocean plate sinks into a deep trench near Japan Dense ocean plate sinking under another plate happens only where plates move together.
Transform (sliding past)
- The Pacific and North American Plates slide past each other in California Plates moving sideways past each other, as along the San Andreas Fault, form a transform boundary.
- Frequent earthquakes but no line of volcanoes Sliding plates strain rock and cause earthquakes, but they do not create magma.
Earthquakes happen at all three boundary types. Stress builds in the rock until it suddenly slips along a fault. Volcanoes are common at divergent boundaries and subduction zones. They are rare where continents collide or at transform boundaries.
Rates help you show scale. Plates move only a few centimetres a year, about as fast as fingernails grow. The Mid-Atlantic Ridge widens by about 2.5 cm a year. The Pacific Plate slides past the North American Plate at about 5 cm a year. Over millions of years these small movements build whole mountain ranges and oceans. In an answer, name a boundary, its two plates, the movement and one landform or event with a figure.
- Divergent
- Plates move apart: ridges, rifts, volcanoes.
- Convergent
- Plates move together: trenches, volcanoes or fold mountains.
- Transform
- Plates slide past: faults and earthquakes.
Step by step
Read the movement
Apart, together or sliding past?
Identify plate types
Ocean plate sinks; continents crumple.
Link to landforms
Ridges, trenches, volcanoes, fold mountains or faults.
Real examples to use
Divergent, ocean: Mid-Atlantic Ridge
The North American and Eurasian Plates move apart at about 2.5 cm a year (USGS). New ocean crust forms along the ridge, which runs through Iceland.
Divergent, continent: East African Rift
The African Plate is splitting into the Nubian and Somali Plates, by only a few millimetres a year. The crust stretches and drops into a rift valley, with faults and volcanoes.
Ocean meets continent: Andes
The ocean Nazca Plate sinks under the South American Plate at the Peru-Chile Trench. This built the volcanic Andes. Here, on 22 May 1960, the Valdivia earthquake in Chile reached magnitude 9.5, the largest ever recorded (USGS).
Ocean meets ocean: Mariana Trench and Islands
The Pacific Plate sinks under the Philippine Sea Plate. The Challenger Deep at the trench's southern end is nearly 11,000 m deep (USGS). Volcanoes on the overriding plate form the Mariana Islands, an island arc.
Continent meets continent: Himalayas
India collided with Asia about 40 to 50 million years ago. The crust crumpled into the Himalayas, which still rise by more than 1 cm a year (USGS). There is no chain of volcanoes.
Transform: San Andreas Fault
In California, the Pacific Plate has slid past the North American Plate for about 10 million years, at an average of about 5 cm a year (USGS). The fault makes earthquakes but no volcanoes.
Worked example: Andes and Himalayas: both converge, differently
At the Andes, the ocean Nazca Plate sinks under the South American Plate, forming a trench and volcanoes. At the Himalayas, India's continent crashes into Eurasia. The crust crumples into huge mountains, with no chain of volcanoes.
Watch out for this
All convergent boundaries make volcanoes.
Check the plate types. When continents collide, they make fold mountains and earthquakes, but rarely volcanoes.
Check your understanding
Two continental plates move towards each other. What is most likely?
- A trench and a volcanic island arc form.
- A ridge forms as the crust is squeezed up.
- The crust crumples into high fold mountains.
Show the answer
The crust crumples into high fold mountains.
Yes. Continents do not sink easily, so they crumple.
Check your understanding
The Himalayas formed where India pushes into Eurasia. What type of plate boundary is this?
- A convergent boundary of two continents.
- A transform boundary, where plates slide past.
- A divergent boundary between two plates.
Show the answer
A convergent boundary of two continents.
Continental crust crumples upwards into mountains.