Nobody can cut the Earth in half, but by studying how earthquake waves speed up and bend as they pass through it, scientists worked out that the inside is made of four layers: the crust, the mantle, the outer core and the inner core.
The crust and the top of the mantle are broken into pieces called plates, and every year these plates move a few centimeters toward each other, away from each other or sideways past each other.
Where plates meet, mountain ranges, trenches and ridges form, and most earthquakes and volcanoes happen along these boundaries.
Press the three boundary buttons to compare how the landforms and events at a boundary change with the direction the plates move.
The Four Layers Inside the Earth
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Inside the Earth (the crust is drawn thicker than it really is so it can be seen)
💡 How the Four Layers Differ
①The crust is the thin outer layer, around 5 km under the oceans and 30–70 km under the continents; compared with the whole Earth it is thinner than the skin of an apple.
②The mantle is the largest layer by volume and reaches down to about 2,900 km. It is solid but flows very slowly and convects.
③The outer core is molten iron and nickel. The fact that S waves cannot pass through it is the evidence.
④The inner core is hotter than the outer core, but the pressure is so great that it is solid.
Three Ways Plates Move
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Three plate boundaries: moving together, moving apart, sliding past
🌍 What Happens at Each Boundary
①Convergent: two plates move toward each other and collide. Two continental plates push up fold mountains like the Himalayas; a heavy oceanic plate sinking under another forms a trench and volcanoes.
②Divergent: two plates move apart and magma rising into the gap becomes new crust, as at the Mid-Atlantic Ridge and the East African Rift.
③Transform: two plates slide sideways past each other, so crust is neither created nor destroyed, and earthquakes are frequent.
④Textbooks explain the force that moves the plates as mantle convection.
Where Earthquakes and Volcanoes Happen
Earthquakes and volcanoes
Earthquakes and volcanoes compared
Feature
Earthquake
Volcano
Cause
Rock breaks under stress built up by moving plates
P wave (longitudinal, fastest) → S wave (transverse) → surface waves (slowest, most damage)
P waves reach a station first, and the time gap between the P and S waves gives the distance to the focus.
🌋 Earthquakes, Volcanoes and Seismic Waves
①Earthquakes and volcanoes cluster in belts along plate boundaries, most of all in the Ring of Fire around the Pacific.
②P waves pass through solids and liquids, but S waves only through solids. A zone on the far side of the Earth that S waves never reach showed that the outer core is liquid.
③Speed order: P wave > S wave > surface waves.
Exam Key Points
Summary
mantle convection → plate motion → mountains, trenches, ridges, earthquakes and volcanoes at boundaries
The big changes on the surface come from energy inside the Earth moving the plates.
🎯 Exam Points
①From outside in: crust → mantle → outer core (liquid) → inner core (solid); the thickest layer is the mantle.
②The driving force of plate motion is explained as mantle convection.
③Convergent: mountains and trenches; divergent: ridges and rift valleys; transform: transform faults and earthquakes.
④Seismic wave speed: P > S > surface waves, and S waves cannot travel through liquid.
⑤Earthquakes and volcanoes gather along plate boundaries, as in the Ring of Fire.
Examples and Unit Test
Example 1
List the layers of the Earth from the outside inward. Which layer is liquid?
1
From the outside the Earth is made of crust → mantle → outer core → inner core.
2
The outer core, which S waves cannot cross, is liquid iron and nickel; the inner core is solid because of the enormous pressure.
Only one of the four layers is liquid, the outer core, and the S-wave shadow is the evidence.
Example 2
What landforms form where two plates push into each other, and what is this kind of boundary called?
1
A boundary where two plates move toward each other and collide is a convergent boundary.
2
Two continental plates push up fold mountains, an oceanic plate sinking under another forms a trench, and earthquakes and volcanoes are common.
▸ Mountains and trenches / convergent boundary
Know the direction the plates move and the landform follows: together gives mountains and trenches, apart gives ridges, sliding past gives transform faults.
Unit-test style
Which statement about tectonic activity is correct?
①The thickest layer of the Earth is the crust
②Mantle convection is the driving force that moves the plates
③The outer core is in a solid state
④At a divergent boundary, two plates collide
⑤Among seismic waves, the S wave arrives before the P wave
▸ ② Mantle convection is the driving force that moves the plates
1
Textbooks explain that plates are carried along by convection in the mantle, so ② is correct.
2
The thickest layer is the mantle (①), the outer core is liquid (③), plates move apart at a divergent boundary (④), and P waves arrive before S waves (⑤).