3 How 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.
4 How 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
6 Crust 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