Waves, when moving from one medium (material type) to another, will bend if they strike at an angle. They will bend one way if moving unto a less dense medium, the other way if entering a more dense medium. Earthquakes generate massive waves in the earth. By measuring how long those waves take to reach different seismographs (tools for measuring Earthquakes), scientists can determine how the wave moved through the earth, and through that path, the density of the materials the waves moved through.

Temperature data is hypothesized based on the material we know the upper mantle to be made of (from lava) and its density and then supported by deep holes we have dug, where thr Temperature increases in a fairly linear manner.