Seismic waves and the evidence for Earth's layers

No one has drilled through Earth's crust (the deepest hole reaches about 12 kilometers), so scientists study the interior using seismic waves from earthquakes. P waves, or primary waves, are compressional waves that travel fastest and can pass through solids, liquids, and gases, while S waves, or secondary waves, are slower shear waves that can travel only through solids. Wave speed depends on the density and stiffness of the rock, so waves bend (refract) and reflect at boundaries between layers. Seismometers around the world record an S-wave shadow zone on the far side of Earth from an earthquake, beyond about 103 degrees away, which shows that the outer core is liquid, and P waves bend sharply at the core, creating their own shadow zone between about 103 and 142 degrees. A sudden jump in wave speed at the base of the crust, the Mohorovičić discontinuity (Moho), marks the boundary between crust and mantle, and in 1936 Danish seismologist Inge Lehmann used faint P waves arriving inside the shadow zone to discover the solid inner core. Other evidence supports this model: Earth's average density, about 5.5 grams per cubic centimeter, is about twice that of surface rocks, so denser material must lie deep inside; iron meteorites suggest what the core is made of; and Earth's magnetic field is best explained by flowing liquid iron in the outer core.

This explanation is AI-generated and can occasionally be wrong. 🚩 Report