Unit 20
The dynamic Earth.
Inside the Earth: seismic waves and a liquid core, drifting plates, and a magnetic field that flips.
Before this unit
The unit, lesson by lesson
Lessons
4 lessons · 24 practice questions · about 2 hours
Seismic waves
An earthquake sends out two kinds of wave, P and S, at different speeds. The gap between their arrivals gives the distance to the source, and three stations locate it on the map.
What is inside the Earth
Nobody has drilled deeper than twelve kilometres, but seismic waves cross the whole Earth. The S-wave shadow reveals a liquid outer core, the P-wave shadow the refraction in it, and faint waves in the shadow a solid inner core.
Plate tectonics
The Earth's shell is broken into plates that move a few centimetres a year on a slowly flowing mantle. Distance is speed times time, and each kind of boundary builds something different: a ridge, a trench and volcanoes, mountains, or a fault.
The Earth's magnetic field
Liquid iron flowing in the core makes the magnetic field like a dynamo, which is why the field can flip. The sea floor keeps the flips in stripes, and from their width the spreading rate is measured.