The Earth's surface is not one shell but a set of plates, and the plates move a few centimeters a year over the soft asthenosphere beneath them.
Where plates meet, the boundary is divergent if they move apart, convergent if they come together, or transform if they slide past each other, and each kind builds different features: ridges, trenches, fold mountains, and transform faults.
At a mid-ocean ridge, mantle material rises and keeps forming new oceanic crust, and the direction of Earth's magnetic field at the time is locked into the rock as stripes laid out symmetrically on both sides of the ridge.
In the diagram, changing the plate speed shows how wide the magnetic stripes become and how far the age marks on the ocean floor move from the ridge.
The Earth's puzzle pieces
Mark earthquake zones, volcanoes, undersea mountain ranges, and deep trenches on a map one at a time and they look like unrelated dots. Divide the surface into plates, though, and ask whether the two plates at each boundary are moving apart, coming together, or sliding past each other, and the dots all line up along plate boundaries. Plates move only 1 to 15 cm a year, about as fast as fingernails grow, yet over millions of years that is enough to open an ocean or raise a mountain range. This chapter starts at a mid-ocean ridge, where that motion is easiest to see.
🧩 What is plate tectonics?
①The Earth's surface is not a single shell but is divided into about a dozen large plates and many smaller ones
②A plate is a rigid piece of lithosphere about 100 km thick, made of crust plus the top of the mantle, and it moves over the asthenosphere, a layer that is partly molten and can flow slowly
③Plates move a few centimeters a year, and where they meet, earthquakes and volcanoes occur and mountain ranges and trenches form
Oceanic plates growing at a ridge
It is easy to picture a plate as a slab of continent, but a plate is a rigid piece of lithosphere, crust plus the top of the mantle, and the plates under the oceans count too. So both sides of a mid-ocean ridge are oceanic plates, and no widening gap opens between them: magma rising under the ridge fills the space with new crust as fast as the plates separate. The evidence is that oceanic crust gets older away from the ridge, and the magnetic field direction frozen into the cooling rock forms stripes that mirror each other across it. Nor is crust made or lost at every boundary; where plates slide past each other at a transform boundary, almost none is.
5 cm/yr
seegongsik.com
Cross-section of a mid-ocean ridge (divergent boundary): seafloor spreading, mantle convection, and symmetric magnetic stripes on both sides
The top band shows the seafloor's magnetic stripes seen from above: dark strips cooled while the field pointed as it does today (normal polarity), light strips while it was reversed. Below is a cross-section through the ridge: magma rises beneath it, the gray oceanic plates move apart along the green arrows, and farther from the ridge the plate is thicker and the seafloor deeper. In the brown mantle, material rising under the ridge flows outward beneath the plates and then sinks. Raising the speed slider makes the plates travel farther in each polarity period, so the stripes widen and the 2- and 4-million-year marks move away from the ridge. Lowering it packs the stripes tighter, but the mirror-image pattern stays.
💡 What to look for in the diagram
①Magma rising under the ridge cools into new oceanic crust, and the plates move away on both sides of the ridge
②The farther from the ridge, the older the oceanic crust, and the plate cools, thickens, and sits deeper
③The magnetic stripes mirror each other across the ridge, and the faster the spreading, the wider the stripes
④Moving 1 cm a year for a million years covers 10 km, so at 5 cm a year a plate travels 50 km in a million years
Three kinds of plate boundary
Boundary classification
Types of plate boundary
🌋 What happens to crust at each boundary
①At divergent boundaries, new crust is created
②At convergent boundaries, a subducting oceanic plate sinks into the mantle and is destroyed, or colliding continents thicken the crust
③At transform boundaries, crust is neither created nor destroyed; the plates only scrape past each other
What moves the plates, and the evidence
What moves the plates
heat inside the Earth → mantle convection → plate motion
Along with mantle convection, the push of plates away from the ridge and the pull of a sinking plate's own weight at subduction zones also move the plates.
Plate speed
v ≈ 1 to 15 cm/yr
Satellite positioning (GPS) can measure it directly, and it is about as fast as fingernails grow.
🔬 Evidence for plate tectonics
①Oceanic crust is youngest at the ridge and gets older with distance from it
②Paleomagnetic stripes on both sides of a ridge mirror each other
③Seafloor sediment gets thicker with distance from the ridge
④The coastlines of South America and Africa fit together, and the same fossils and rock layers appear on both continents
⑤Earthquakes and volcanoes gather in belts along plate boundaries
Worked Examples and Exam Practice
Example 1
What is the boundary called where, as at the Mid-Atlantic Ridge, two plates move apart and new oceanic crust forms?
1
A boundary where two plates move apart is a divergent boundary.
2
Mantle material rises into the opening and melts into magma, and as the magma cools, new oceanic crust forms.
▸ a divergent boundary (a ridge)
In the diagram, the divergent boundary is where magma rises under the ridge and the oceanic plates on both sides move apart. New crust fills in as fast as the plates separate, so no gap opens.
Example 2
If one plate moves away from a ridge at 5 cm a year, about how far from the ridge does its rock end up after 2 million years?
1
Distance moved = speed × time.
2
5 cm/yr × 2,000,000 yr = 10,000,000 cm = 100 km.
▸ about 100 km
With the speed set to 5 cm/yr in the diagram, the 2-million-year mark sits 100 km from the ridge. The plate on the other side moves the same distance, so the two plates end up 200 km apart.
CSAT-style problem
Which statement about plate boundaries is correct?
①Old crust is destroyed at divergent boundaries
②Trenches and fold mountains form at convergent boundaries
③New crust is actively created at transform boundaries
④Mantle convection has nothing to do with plate motion
⑤Plates move several centimeters per second
▸ ② Trenches and fold mountains form at convergent boundaries
1
At convergent boundaries, plates collide or one sinks beneath the other, forming trenches and fold mountains, with frequent earthquakes and volcanic activity.
2
Divergent boundaries create new crust (①), transform boundaries neither create nor destroy crust (③), mantle convection is the main force moving the plates (④), and plates move a few centimeters a year (⑤).
Summary
Plate tectonics in brief
mantle convection → plate motion → earthquakes, volcanoes, and mountain building at the boundaries
Heat energy inside the Earth is the underlying cause of geological activity at the surface.
🎯 Exam Points
①Plate boundaries are divergent, convergent, or transform, and each kind has its own landforms and geological activity
②Divergent boundaries have ridges and rift valleys, convergent boundaries have trenches and fold mountains, and transform boundaries have transform faults
③The age of oceanic crust and the thickness of seafloor sediment increase with distance from the ridge
④Paleomagnetic stripes mirror each other across the ridge, and they are wider where spreading is faster
⑤Mantle convection is the main force moving the plates, which move about 1 to 15 cm a year