Earth’s structure and plate tectonics · Grade 6
The Earth is made of layers: a thin rocky crust, a thick mantle of hot rock, a liquid outer core and a solid inner core. The crust and the top of the mantle are cracked into tectonic plates that move a few centimetres a year, causing earthquakes, volcanoes and mountains where they meet.
Key facts
- Crust: thin, solid rock (about 5 to 70 km thick). Mantle: hot rock that can flow very slowly (about 2900 km thick). Outer core: liquid iron and nickel. Inner core: solid iron and nickel, hotter than 5000°C.
- Tectonic plates move a few centimetres a year, carried by convection currents in the mantle.
- Plates moving apart: magma rises and makes new crust (the Mid-Atlantic Ridge). An ocean plate sinking under a continental plate: volcanoes and earthquakes (the Andes). Two continental plates colliding: fold mountains (the Himalayas). Plates sliding past each other: earthquakes (the San Andreas Fault).
- Alfred Wegener (1912) suggested continental drift: coastlines that fit together, matching fossils and rocks on different continents, and coal in Antarctica.
- Earthquakes are recorded with seismometers. In a volcano, magma rises from the magma chamber up the vent and comes out of the crater as lava and ash.
- Earthquake waves showed us the layers: some waves cannot pass through the liquid outer core.