Question

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

Short answer

The dramatic decrease in P-wave velocity at the core-mantle boundary is due to several factors: 1) change in composition from the solid mantle made of silicate minerals to the partially liquid outer core consisting of molten iron and nickel, 2) presence of partially molten materials in the outer core which hinders the efficient transmission of compressional forces, and 3) significant temperature and pressure differences at the boundary, leading to reduced stiffness and elasticity of the materials. These factors result in a decrease in P-wave velocity when transitioning from the solid mantle to the partially liquid outer core.

Step-by-step solution

Earth's Interior Structure

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

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.

P-Wave Velocity and Material Properties

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.

Decrease in P-Wave Velocity at the Core-Mantle Boundary

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.

Key concepts

Earth's Interior Structure

Delving into the complex construction of Earth reveals a fascinating multi-layered composition, akin to an onion with several distinct layers. At the very surface, we have the crust, which is relatively thin and brittle, like the skin of an apple. Below the crust lies the mantle, a massive, solid layer composed primarily of silicate minerals, which accounts for approximately 84% of Earth's total volume. Imagine a thick, solid shell enveloping a much denser, hotter core.

The core itself is an enigma that exists in two different states and is mainly composed of iron, nickel, and some light elements. The outer core is molten, a sea of roiling, electrically conductive liquid metal, while the inner core is solid, due to the immense pressures that overcome the high temperatures. These layers don't exist in isolation; they interact at their boundaries, with the core-mantle boundary being particularly crucial for understanding seismic activities.

Seismic Waves

Imagine the Earth shaking under your feet, the energy from an earthquake rippling outward in all directions. These vibrations, known as seismic waves, come in two main types: body waves and surface waves. The body waves, consisting of P-waves (Primary waves) and S-waves (Secondary waves), plunge into the depths of our planet.

P-waves are compressional waves, somewhat similar to the way sound waves travel through the air. They push and pull the rock as they move through the Earth's layers, being fast travellers able to navigate through both liquid and solid materials. This adaptability makes them the first to arrive at seismic stations following an earthquake, serving as messengers that reveal glimpses of Earth's interior. Their characteristics and the speed at which they travel offer scientists clues about the nature of the materials they pass through, painting a picture of variations in density and state of the internal layers.

Core-Mantle Boundary

The core-mantle boundary is akin to the frontier between two distinct worlds within our planet. It's a transition zone located roughly 2,900 kilometers beneath our feet, marking the end of the mantle and the beginning of the outer core. At this boundary, the solid rocky material of the mantle gives way to the liquid iron-nickel alloy of the outer core.

Core-Mantle Boundary's Effect on P-Wave Velocity

When P-waves reach this boundary, they encounter a dramatic shift in material properties. The dense, stiff mantle converts to the more fluid, molten outer core, resulting in a sudden reduction in wave velocity. This is because the molten outer core is less proficient at transmitting the compressional forces of P-waves, causing them to decelerate sharply. Understanding this sudden decrease in velocity provides geologists and seismologists with essential data for deciphering Earth's internal structure. It also underscores the boundary's role as a defining characteristic that influences geophysical phenomena.