Why does P-wave velocity decrease dramatically at the core-mantle boundary?

The Earth is composed of three main layers: the outer crust, the mantle, and the core. The core is further divided into two layers - the outer core and the inner core. The mantle is a solid layer, while the core consists of molten iron and nickel, with the outer core being partially liquid and the inner core being solid. The core-mantle boundary is the region where the mantle transitions into the core.

P-waves, or primary waves, are a type of seismic wave generated during earthquakes or man-made explosions. They are compressional waves that travel through the Earth's interior and can pass through both solid and liquid materials. P-waves have the highest velocity among all seismic waves and are typically the first to be detected by seismographs during an earthquake.

The velocity of P-waves depends on several factors such as the density, elasticity, and compressibility of the material through which they are propagating. Generally, P-wave velocity increases with increasing density and stiffness of the material. However, in the case of the Earth's interior, additional factors such as temperature, pressure, and the presence of partially molten materials also play a significant role in determining P-wave velocity.

The dramatic decrease in P-wave velocity at the core-mantle boundary is attributed to several factors: 1. Change in composition: As P-waves pass from the solid mantle to the liquid outer core, the material composition changes significantly. The mantle, which is composed mainly of solid silicate minerals, is much stiffer than the partially liquid outer core, which is made up of molten iron and nickel. This change in material properties leads to a decrease in P-wave velocity. 2. Presence of partially molten materials: The presence of partially molten iron and nickel in the outer core greatly affects the P-wave velocity. This is because the molten part of the material can flow and does not transmit the compressional forces efficiently. As a result, the P-wave velocity decreases. 3. Temperature and pressure: The core-mantle boundary represents a region with a significant temperature and pressure difference. The high temperature in the outer core reduces the stiffness and elasticity of the material, further decreasing the P-wave velocity. By considering these factors at the core-mantle boundary, we can understand why P-wave velocity decreases dramatically when crossing from the solid mantle into the partially liquid outer core.