3How do we know?Echo-sounding techniques are used to explore the Earth's crust. Images, similar to sonograms, are produced. A sonogram in the crust is called a seismic reflection. Seismic waves from "small explosions or thumper trucks" return echoes from rock layers. Seismographs pick up these echoes.
4How do we know? continued Seismic measurements from earthquakesP waves and S wavesP waves are primary waves, they travel faster than S waves, which are secondary wavesP waves are compression waves, exerting a force parallel to the direction of travelP waves can travel through liquid or solidS waves are shear waves that exert a force perpendicular to the direction of travelS waves can only travel through solid media
6Crust 5 – 80 km thick two types: Mostly made up of silicate rock oceanic - thinner and more dense5 – 10 km thickContinental – thicker and less dense15 – 80 km thickMostly made up of silicate rockComprises only 1% of Earth’s mass