Here's why:
* Seismic waves travel at different speeds through different materials. Denser materials, like solid rock, transmit seismic waves faster than less dense materials, like molten rock or liquids.
* P-waves (primary waves) are compressional waves that travel through both solids and liquids. They are the fastest type of seismic wave.
* As P-waves travel through the Earth, their velocity changes. This change in velocity can be measured using seismographs.
* A significant drop in P-wave velocity indicates a major change in material density.
The two major density discontinuities inside the Earth are:
1. The Mohorovičić discontinuity (Moho): This boundary separates the Earth's crust from the mantle. The P-wave velocity increases significantly at the Moho, indicating a denser mantle.
2. The Gutenberg discontinuity: This boundary separates the Earth's mantle from the outer core. The P-wave velocity decreases drastically at this point, indicating a much less dense outer core composed mainly of molten iron and nickel.
Therefore, the sudden decrease in P-wave velocity at the Gutenberg discontinuity is a clear indication of a dramatic decrease in material density inside the Earth.
