Seismic Waves

Introduction

Earth is composed of three main layers.

1. Crust à the outermost and thinnest layer

2. Mantle à the middle layer that lies beneath the crust

3. Core à the innermost layer and is divided into a liquid outer core and a solid inner core

How do we know this?

Watch the video at the top of the page, then answer the questions in your science journal.

1. What is a seismic wave?

2. What is one human-made cause of seismic waves?

3. What is one natural cause of seismic waves?

4. How do seismic waves travel?

Scientists use instruments such as the seismograph and geophone to observe how energy travels through the Earth and determine information about the structure of our planet.

Let’s Practice: Review

Reflection and Refraction

The scientist who works to understand our planet's history is called a geologist. A geologist studies the Earth to predict the future better and explain current occurrences.

Geologists will use geophones which are sensors that are planted in the ground and convert ground movement into an electrical signal. The signals from a geophone are fed into sophisticated data processing equipment and arranged to produce a seismic reflection record.

Geologists create seismic waves by making an explosion or dropping a weight onto a vibration plate. As waves pass through layers of different densities, they change direction. This process is called refraction.

By measuring the refraction and reflection of the waves, geologists can determine the density, nature, depth, and thickness of the rock below our surface, helping them to locate things like fossil fuels, water, and minerals.

In your science journal, answer these questions.

1. What skills does a geologist need to be successful in their career?

2. What are three examples of fossil fuels?

3. What four things can a geologist determine by measuring the refraction and reflection of seismic waves?

Let’s Practice: Reflection and Refraction

Direct and Indirect Evidence

Geologists use two main types of evidence to learn about Earth's interior. They use direct evidence from rock samples and indirect evidence from seismic waves.

Direct Evidence

Observations you can make yourself are direct evidence. Scientists observe lava and rocks on Earth's surface. They study rocks from holes and mines drilled below the surface.

Watch the edpuzzle original "Patterns of Earth's Features (Volcanoes)."

Indirect Evidence

Indirect evidence is information scientists use to infer an answer when they can't make observations. It includes a recording of earthquake waves called seismic waves. Seismic waves produced by earthquakes allow scientists to measure the speed at which they travel, giving clues to the planet's structure.

Watch the puzzle original "Patterns of Earth's Features (Earthquakes)."

In your science journal, answer these questions.

1. Would you want to live near a volcano? Why or why not?

2. What can the Volcano Disaster Assistance Program do to stop future disasters?

3. Why do some buildings survive earthquakes?

Seismic Waves

During an earthquake, seismic waves move outward throughout our planet. Instruments worldwide record these seismic waves. The waves include:

· P waves à which can travel through solids and liquids

· S waves à which can travel through solids but NOT liquids

When seismic waves move from one material into a different material, their speed and direction change. Either their path bends, or the new material blocks them.

If Earth were made up of a single layer of the same material, the seismic waves would pass through Earth. Instead, seismic waves bend or stop. Using indirect evidence, geologists infer that Earth consists of different layers of different materials.

Study the image below, then answer these questions in your science journal.

1. What color are the S-waves?

2. What color are the P-waves?

3. Which wave went through the crust, mantle, outer, and inner core?

Let's Practice: Waves

Earth's Layers

The Earth's interior consists of three layers: crust, mantle, and core.

Geologists know more about the crust than the mantle or core because they can collect direct evidence.

Crust

The crust is broken into pieces called tectonic plates. Tectonic plates slowly move on the Earth's surface. The interaction and movement of these plates create and change landforms. Plate movements create:

· Mountains

· Trenches

· Volcanoes

The crust varies in thickness and is made up of different rock types. The continental crust comprises the continents, and the oceanic crust is under the oceans.

Earth's crust is made up of granite and basalt.

Mantle

Both P and S waves pass through the crust and mantle. Geologists infer that this likely means that both layers are solid. Seismic waves travel faster in the mantle.

Scientists study volcanoes to learn more about the mantle. During an eruption, mantle rock is forced up to the surface.

Although the mantle is mostly solid, it moves slowly over time. The tectonic plates in the crust sit on top of the mantle. The heat within the mantle moves by way of convection currents. These currents help move the plates and can change landforms on Earth's surface.

Earth's mantle is mostly made up of peridotite.

Outer Core

Below the mantle, geologists record a change in seismic waves at a depth of about 3,000 kilometers. The P waves change speed and direction, and the S waves stop. Since S waves can't pass through liquids, geologists can infer that a liquid layer exists deep inside the Earth. This layer is the outer core and is mostly made up of melted iron and nickel.

Convection currents move material around the outer core, and the movement of liquid iron produces an electric current. This current is the source of the Earth's magnetic field.

Convection currents in the outer core produce the Earth's magnetic field.

Inner Core

Geologists use seismic waves to make inferences about the inner core. The liquid outer core stops the S waves, but the P waves enter the inner core and change speed and direction. Their speed and path indicate that the inner core is a solid ball made mostly of iron.

Geologists believe that the inner core is made of solid material like iron.

In your science journal, fill in the blanks.

1. Geologists believe that the inner core is made up of material similar to _______.

2. Convection currents in the outer core produce the Earth's _______ field.

3. Earth’s mantle is mostly made up of _________.

4. Earth’s _______ is made up of granite and basalt.