Question

Explain how S waves "tell us" the mantle is solid.

Short answer

S waves move through solids only, so their travel through the mantle indicates it is solid.

Step-by-step solution

Understand the Nature of S Waves

S waves, or secondary waves, are a type of seismic wave that move through the Earth during an earthquake. They are characterized by their ability to move through solid materials but not through liquids.

Analyze S Waves Behavior in Earth's Layers

Since S waves can only travel through solids, when they pass through the Earth, they do not pass through the outer core, which is made of liquid iron and nickel. Instead, they are stopped or deflected.

Interpret Seismic Data

By studying the paths that S waves take through the Earth using seismic data, scientists can deduce which layers are solid and which are liquid. The continuous travel of S waves through the mantle indicates that it is in a solid state.

Key concepts

The Mantle

The mantle is an expansive layer situated between the Earth's crust and its outer core. It is mostly composed of silicate rocks rich in magnesium and iron, making it denser than the crust. This region extends to a depth of about 2,900 kilometers (1,800 miles) beneath the surface.

• This rock layer is primarily solid but behaves like a viscous fluid over geological timescales, which allows for slow creeping movements known as mantle convection.
• Mantle convection leads to the tectonic plate movements that shape the Earth's surface.

These convection currents are important because they drive processes such as continental drift, the formation of mountains, and volcanic activity. Understanding the solid nature of the mantle is crucial for comprehending plate tectonics and other earth sciences.

Seismic Waves

Seismic waves are energy waves that travel through the Earth's layers, generated by natural events like earthquakes or artificially through explosions. These waves are categorized broadly into two types: body waves and surface waves. Body waves are further divided into P waves and S waves.

• P waves (Primary waves): These are compressional waves that can travel through solids, liquids, and gases. They are the fastest type of seismic wave and the first to be detected by seismographs.
• S waves (Secondary waves): These are shear waves that move perpendicular to the direction of wave propagation and can only travel through solids. They are slower than P waves and follow them in arrival times.

By analyzing the behavior of seismic waves, scientists gain insights into the Earth's internal structures, such as differentiating between solid and liquid zones within the Earth.

Earthquakes

Earthquakes occur when there is a sudden release of energy in the Earth's crust, causing seismic waves to be generated. This release of energy often happens due to tectonic stress, which accumulates over time and causes faults or fractures in the crust.

• During an earthquake, stored energy is abruptly released, causing the ground to shake.
• Seismologists use instruments called seismographs to detect, record, and analyze seismic waves produced by earthquakes.

Understanding earthquakes helps in both natural disaster preparedness and the study of the Earth's internal processes. By studying seismic waves from these events, scientists can infer the properties and behaviors of different Earth layers.

Earth's Layers

The Earth is made up of several distinct layers, each with its own physical and chemical properties. These layers include the crust, mantle, outer core, and inner core.

• Crust: The outermost layer, composed of solid rock, both continental and oceanic. It is the thinnest layer, making up less than 1% of the Earth's volume.
• Mantle: Located beneath the crust, the mantle is solid yet behaves plastically, allowing for convection currents.
• Outer Core: Composed of liquid iron and nickel, which S waves cannot travel through.
• Inner Core: A solid sphere composed mainly of iron and some nickel, found at the center of the Earth.

The study of how seismic waves move through these layers provides important information about their composition and state. For instance, S waves traveling through the mantle but not the outer core supports the conclusion that the mantle is indeed solid.