Solid Earth: layers and plates
Earth has layers, found from the way earthquake waves travel through it. The crust is thin, about 5 to 70 km. The mantle is thick, about 2,900 km, hot, solid rock that slowly flows. The outer core is liquid iron and nickel. The inner core is solid, because the pressure is huge.
Plate tectonics
The crust and the top of the mantle form the lithosphere, broken into plates. They move a few centimetres a year, pushed by heat flow in the mantle. At convergent boundaries plates collide (mountains, volcanoes, deep trenches). At divergent boundaries they move apart (new crust forms, as in mid-ocean ridges). At transform boundaries they slide past each other (earthquakes).
Rocks
Igneous rocks form from cooled magma, sedimentary rocks from pressed layers of sediment, and metamorphic rocks when heat and pressure change old rock.
Atmosphere: four layers
The atmosphere is held by gravity. From the ground up:
- Troposphere (0 to about 12 km): weather happens here; temperature falls with height.
- Stratosphere (to about 50 km): holds the ozone layer, which absorbs harmful ultraviolet light, so temperature rises.
- Mesosphere (to about 85 km): temperature falls again; meteors burn up here.
- Thermosphere (above): very thin, so a few particles can be very hot; auroras and satellites are here.
Greenhouse effect
Gases like carbon dioxide and water vapour trap heat. This keeps Earth warm enough for life; extra gases warm it more.
Ocean and air: heat on the move
The Sun heats the equator more than the poles. Warm air rises and cooler air sinks, so air moves in convection loops. Because Earth spins, moving air and water curve (the Coriolis effect), which sets the pattern of winds and surface ocean currents.
Why the ocean matters
Water holds a lot of heat and releases it slowly, so oceans calm the climate. Currents carry warm water to colder places. In El Nino years the winds weaken over the Pacific and warm water spreads east, changing the rain in many countries.
Space: orbit, tilt and seasons
Earth orbits the Sun once in about 365 days and spins once in about 24 hours. Earth's axis is tilted by about 23.5 degrees and keeps pointing in the same direction in space.
Why seasons happen
When the north end leans towards the Sun, sunlight hits the north more directly and days are longer: summer. Six months later the north leans away: winter. The south has the opposite seasons. It is not about distance.
Bigger picture
The Sun is a star. It is one of hundreds of billions in the Milky Way galaxy. Stars shine by fusing hydrogen into helium.
Try it: seasons with a torch
In a dark room, hold a torch and shine it on a globe or ball. Tilt the ball a little and walk around the torch, keeping the tilt pointing the same way. See how the light falls straighter on the top half in one place and on the bottom half on the opposite side. In the 3D, move the slider through the months.
Key formulas and definitions
- Earth axis tilt: about 23.5 degrees
- Orbit time: about 365.25 days; spin time: about 24 hours
- Troposphere: about 0-12 km; stratosphere: to about 50 km; mesosphere: to about 85 km
- Plate speed: a few cm per year
Worked examples
1. A plate moves 4 cm per year. How far does it move in 25 million years?
4 cm x 25,000,000 = 100,000,000 cm = 1,000 km.
2. Why is the stratosphere warmer at the top than at the bottom?
The ozone layer in the stratosphere absorbs ultraviolet light and turns it into heat, mostly in the upper part.
3. Why is it summer in the southern hemisphere when it is winter in the north?
Earth's axis is tilted. When the north leans away from the Sun, the south leans towards it, so the south gets more direct sunlight.
Common mistakes
- Thinking seasons come from Earth being closer to or farther from the Sun. They come from the tilt.
- Thinking the mantle is liquid. It is solid rock that flows very slowly; only the outer core is liquid.
- Saying plates are the crust only. Plates include the crust and the top of the mantle (lithosphere).
- Thinking weather happens in all layers. It happens in the troposphere.