stabilize the magnetic field generated by the liquid outer core. The average magnetic field in Earth's outer core is estimated to measure , 50 times stronger than the magnetic field at the surface. Seismology The layering of Earth has been inferred indirectly using the time of travel of refracted and reflected seismic waves created by earthquakes. The core does not allow shear waves to pass through it, while the speed of travel (seismic velocity) is different in other layers. The changes in seismic velocity between different layers causes refraction owing to Snell's law, like light bending as it passes through a prism. Likewise, reflections are caused by a large increase in seismic velocity and are similar to light reflecting from a mirror. See also Hollow Earth Geological history of Earth Lehmann discontinuity Rain-out model Travel to the Earth's center Solid earth References Further reading External links