Plate movements and boundaries
AdvancedThe lithosphere is broken into seven major plates that creep along. Three boundary types — divergent (pulling apart), convergent (colliding) and transform (sliding past) — build rift valleys, fold mountains, trenches and more.
- Seven major plates, a few cm a year
- Boundaries: divergent, convergent, transform
- Divergent makes rifts and ridges; convergent makes folds and trenches
§Text Passage
1The crust is not one unbroken shell; like a jigsaw puzzle it is made of more than a dozen plates, drifting a few centimetres a year on the asthenosphere. This lesson covers the three types of plate boundaries — divergent, convergent and transform — and the landforms each one builds.
2The cause lies deep underground: convection currents in the mantle drag the plates along. Where plates pull apart, magma rises to build mid-ocean ridges and rift valleys. Where they collide, an ocean plate subducts to form trenches and volcanic island arcs, colliding continents crumple into mountain ranges, and plates scraping past each other create faults.
3World examples abound: the Mid-Atlantic Ridge is slowly pulling the Americas away from Europe and Africa; the Himalayas were born when India crashed into Asia and are still rising a few millimetres a year; Hong Kong has no active faults running through it, so its earthquake risk is far lower than Japan's or Taiwan's.
1Summary 重點摘要
The lithosphere is broken into seven major plates that creep along. Three boundary types — divergent (pulling apart), convergent (colliding) and transform (sliding past) — build rift valleys, fold mountains, trenches and more.
2Vocabulary cards Vocabulary
A large slab of the lithosphere, moving a few centimetres a year.
Two plates pull apart.
Two plates push into each other.
A boundary where two plates grind past each other, often making faults and quakes.
The denser oceanic plate dives beneath another plate.
Mountains buckled up by compression, e.g. the Himalayas.
The circulation of hot mantle material rising and cooler material sinking, dragging the plates.
A vast undersea mountain chain built where plates pull apart and magma rises.
3Step by step Steps
- The ground is a jigsawThe Earth's lithosphere is broken into plates fitted together like a jigsaw: the major ones include the Pacific, Eurasian, Indo-Australian, African, North and South American and Antarctic plates, all floating on the asthenosphere and creeping a few centimetres a year. The movement itself is hard to notice, but as the plates collide, pull apart and grind past one another, they are the root cause of earthquakes, volcanoes and mountain building.
- Three boundaries, three motionsPlate boundaries come in three kinds by their motion: divergent boundaries, where two plates pull apart; convergent boundaries, where they collide and one dives beneath the other; and transform boundaries, where they slide past each other, grinding out faults. Boundary zones are where the crust is most active, and the great majority of the world's earthquakes and volcanoes cluster there — so the plate boundaries on a map are at the same time the belts of quakes and volcanoes.
- Movement builds landformsPlate movement sculpts the land: at divergent boundaries, land is torn into rift valleys such as the East African Rift, and the sea floor opens along ridges such as the Mid-Atlantic Ridge; at convergent boundaries, squeezed rock folds up into fold mountains such as the Himalayas, subducting ocean floor carves trenches such as the Mariana Trench, and volcanoes rise; transform boundaries are dominated by faults and earthquakes. Memorise these pairings of boundary and landform.
- Cause, and Hong KongThe driving force is convection in the mantle: hot material rises, cooler material sinks, and like a conveyor belt the currents drag the plates along. Hong Kong has no active faults and sits on no plate boundary, so its earthquake risk is far lower than Japan's or Taiwan's. Understanding plate tectonics nevertheless explains the worldwide pattern of earthquake belts and volcanoes — why some regions suffer so much, and why everywhere values disaster preparedness.
- Convergence has three facesConvergent boundaries divide again into three cases. Ocean meets continent: the denser oceanic plate subducts into the mantle, building a trench and a coastal cordillera such as the Andes of South America. Ocean meets ocean: island arcs and deep trenches form, like the Mariana Trench. Continent meets continent: both plates are too buoyant to sink, so the rock layers crumple into fold mountains such as the Himalayas.
- How fast plates creepPlates move only about 2–10 cm a year — roughly the speed at which fingernails grow, imperceptible in a whole lifetime — yet over tens of millions of years it adds up enormously. The Indian plate pushed north at about 5 cm a year, squeezing the sedimentary layers between the two plates upward into the whole Himalayan range, which is still rising today. GPS satellites now measure plate motion to the millimetre, direct proof that the plates really are moving.
4Diagrams Diagrams
5Worked example Example
Example: how did the Himalayas form?
- The Indo-Australian plate kept pushing north
- It collided with the Eurasian plate — a convergent boundary
- Rock layers were squeezed and buckled into fold mountains, still rising today
- Similar case: the Alps were also squeezed up where plates converged
- Answer: The Himalayas are fold mountains built by converging plates. The Indo-Australian plate kept pushing north into the Eurasian plate; the sedimentary layers between were squeezed, buckled and heaved upward. The plates still creep a few centimetres a year, so the range is still rising — the Alps formed the same way. In exams, name the convergent boundary first, then describe the squeezing and buckling.
6Common mistakes Mistakes
7Quick check Quick check
Q1Name the three types of plate boundary.
Q2Which boundary type made the East African Rift?
Q3Why are earthquakes rare in Hong Kong?
Q4What happens when a continental plate collides with another continental plate? Give an example.
Q5Roughly how fast do plates move, and how is this measured today?
8Recap Recap
- Seven major plates, a few cm a year
- Boundaries: divergent, convergent, transform
- Divergent makes rifts and ridges; convergent makes folds and trenches
- Mantle convection is the engine that drives the plates
- The Indo-Australian plate crashing into Eurasia squeezed up the Himalayas