Inside Earth
No one has ever travelled to Earth’s centre. The deepest hole ever drilled, Russia’s Kola Superdeep Borehole, reached about 12 km, less than 0.2% of the way. Scientists learn about Earth’s interior by studying how earthquake waves travel through it.
Earth’s layers
| Layer | Thickness | State | Made of | Temperature |
|---|---|---|---|---|
| Crust | 5–70 km | Solid | Rock (granite and basalt) | Up to about 900 °C at its base |
| Mantle | about 2900 km | Solid but slowly flowing (plastic) | Hot rock rich in iron and magnesium | About 1000–3700 °C |
| Outer core | about 2200 km | Liquid | Iron and nickel | About 4000–5000 °C |
| Inner core | about 1200 km radius | Solid (from immense pressure) | Iron and nickel | About 5000–6000 °C, as hot as the Sun’s surface |
- Continental crust is thicker (up to 70 km under mountains) and less dense.
- Oceanic crust is thinner (about 5–10 km) and denser.
- Swirling liquid iron in the outer core creates Earth’s magnetic field.
Lithosphere and asthenosphere
- The lithosphere is the rigid outer shell: the crust plus the uppermost mantle.
- Below it, the asthenosphere is a softer part of the upper mantle that can flow very slowly, like warm toffee.
Plate tectonics
The lithosphere is broken into about a dozen large tectonic plates and many smaller ones. They move about 2–10 cm per year, about as fast as your fingernails grow.
What moves the plates?
- Convection currents: hot mantle rock rises, cools, spreads and sinks, dragging plates along.
- Ridge push: new, hot crust at mid-ocean ridges sits higher and slides downhill.
- Slab pull: cold, dense oceanic plates sink into the mantle at trenches, pulling the rest of the plate behind them. This is thought to be the strongest force.
Plate boundaries
A plate boundary is the place where two plates meet. What happens there depends on how the plates move.
| Boundary | Motion | What happens | Example |
|---|---|---|---|
| Divergent | Plates move apart | New crust forms; rift valleys and mid-ocean ridges | Mid-Atlantic Ridge; East African Rift |
| Convergent: ocean–continent | Plates collide; ocean plate sinks (subduction) | Deep trenches, volcanic mountains, earthquakes | Juan de Fuca Plate under British Columbia; the Andes |
| Convergent: ocean–ocean | One plate subducts | Volcanic island arcs, trenches | Japan; the Mariana Trench |
| Convergent: continent–continent | Plates crumple | Huge mountain ranges | The Himalayas (India hitting Asia) |
| Transform | Plates slide past each other | Earthquakes | San Andreas Fault; Queen Charlotte Fault off Haida Gwaii |
Earthquakes and volcanoes
Most earthquakes and volcanoes occur along plate boundaries. The Ring of Fire around the Pacific Ocean has about 75% of the world’s active volcanoes and about 90% of its earthquakes.
Some volcanoes form at hot spots in the middle of plates, where plumes of hot mantle rise. The Hawaiian Islands formed as the Pacific Plate moved over a hot spot.
Canada’s plate setting
Most of Canada sits on the stable North American Plate. But the west coast is active: the Juan de Fuca Plate is subducting beneath Vancouver Island along the Cascadia Subduction Zone. It last produced a magnitude 9 earthquake in January 1700, and scientists expect another in the future, which is why British Columbia has earthquake preparedness programs.
Common mistakes
- Thinking the mantle is liquid lava. It’s solid rock that flows extremely slowly.
- Thinking plates float on the core. They move over the asthenosphere.
- Thinking continents and plates are the same. Plates include ocean floor too.
Try it yourself
- Draw Earth’s layers to scale on a long strip of paper (1 cm = 100 km). How thin is the crust?
- Heat water with a few drops of food colouring at one end of a pan. Watch convection currents form.
- Use a world map of earthquakes. Trace the plate boundaries.
Quick recap
- Earth has a crust, mantle, outer core and inner core.
- The lithosphere is broken into tectonic plates that move a few centimetres a year.
- Convection, ridge push and slab pull move the plates.
- Plates diverge, converge or slide past each other at boundaries.
- Most earthquakes and volcanoes happen at plate boundaries.