<P> The interior structure of the Earth is layered in spherical shells: an outer silicate solid crust, a highly viscous mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core . Scientific understanding of the internal structure of the Earth is based on observations of topography and bathymetry, observations of rock in outcrop, samples brought to the surface from greater depths by volcanoes or volcanic activity, analysis of the seismic waves that pass through the Earth, measurements of the gravitational and magnetic fields of the Earth, and experiments with crystalline solids at pressures and temperatures characteristic of the Earth's deep interior . </P> <P> The force exerted by Earth's gravity can be used to calculate its mass . Astronomers can also calculate Earth's mass by observing the motion of orbiting satellites . Earth's average density can be determined through gravitometric experiments, which have historically involved pendulums . </P>

The two principal ways to describe earths structure are based on the
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