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Answer. Seismic wave diffraction shows a boundary between more and less dense rocks that is commonly taken to be the crust-mantle boundary. It’s called the Mohorovicic discontinuity or Moho for short. It’s usually at 30-40 km depth underneath continents, but can be as deep as 80 km.
Figure 19.2a: P-waves generally bend outward as they travel through the mantle due to the increased density of mantle rocks with depth. When P-waves strike the outer core, however, they bend downward when traveling through the outer core and bend again when they leave. The bending of seismic waves is called refraction.
Because the earth’s mantle becomes more rigid and compressible as the depth below the asthenosphere increases, P-waves travel faster as they go deeper in the mantle. The density of the mantle also increases with depth below the asthenosphere. The higher density reduces the speed of seismic waves.
The experiments also showed that liquid iron begins to react with mantle substances at pressures of 20 to 30 gigapascals. Such pressures are far less than those at the core-mantle boundary (136 gigapascals).
The mantle is the mostly-solid bulk of Earth’s interior. The mantle lies between Earth’s dense, super-heated core and its thin outer layer, the crust. The mantle is about 2,900 kilometers (1,802 miles) thick, and makes up a whopping 84% of Earth’s total volume.
The special feature of the upper mantle is the asthenosphere. It is located just below the lithosphere and is made up of rock that is fluid and can move. Its chemical composition is very similar to the crust.
The lower mantle is an important player in the geological action we see above the surface, such as earthquakes and volcanoes. The core heats the lower mantle, and the warm lava rises into the upper mantle. As the upper mantle cools, the lava spreads and falls back to the center of the earth.
The core of the Earth is about the same thickness as the mantle, whereas the core of the apple is smaller than the white pulp of the apple. Earth’s crust is broken up into plates that move slowly around on the surface of the Earth. The apple skin is one whole layer that is not broken.
When you slice an apple, you find different layers. A small core is in the center. A thick layer is in the middle. A thin skin is all around the outside.