High School Earth Science: Plate Tectonics - Inside Earth — Flashcards

How do scientists know what is inside Earth? They don’t have much direct evidence. Rocks yield some clues but mainly just about the upper crust. Only in rare instances does a mineral, such as diamond, come to the surface from the lower crust or mantle. Scientists know about Earth’s interior mainly from indirect evidence such as seismic waves.    
Seismic waves are caused by the energy from earthquakes traveling through the ground. Seismic waves travel outward in all directions from the point where the ground breaks in an earthquake. The waves are detected by seismometers around the world.

Two types of seismic waves are most useful for learning about Earth’s interior: P-waves and S-waves.   
- P-waves (primary waves) are the fastest seismic waves, so they are always the first waves to arrive at seismometers.    
P-waves travel more slowly through liquids than solids.    
- S-waves (secondary waves) are only about half as fast as P-waves, so they always arrive later at seismometers.    
S-waves cannot travel through liquids at all.    

By tracking seismic waves with seismometers all over Earth, scientists have learned important information about the planet’s interior. For example, P-waves slow down as they pass from the mantle to the outer core, so scientists know that the outer core is liquid whereas the mantle is solid. Another clue that the outer core is liquid is that S-waves stop at the mantle-core boundary.