Science · 7th Grade · Earth's Changing Surface · Weeks 28-36

Earth's Interior and Layers

Students explore the composition and characteristics of Earth's layers, from the crust to the core, and how scientists study them.

• Common Core State Standards
• MS-ESS2-2

About This Topic

Earth is composed of four concentric layers with distinct compositions and physical properties. The crust is the thin, solid outer shell where we live, ranging from about 5 km thick under oceans to 70 km beneath major mountain ranges. Below the crust lies the mantle, a roughly 2,900 km thick layer of silicate rock that behaves as a solid over short time scales but flows slowly over millions of years. At the center are the liquid outer core and solid inner core, both composed primarily of iron and nickel. The MS-ESS2-2 standard asks students to develop a model to describe the uneven distribution of Earth's mineral resources as the result of past and current geoscience processes.

Scientists cannot drill to Earth's core -- the deepest borehole ever, the Kola Superdeep Borehole in Russia, reached only 12 km. Most knowledge of Earth's interior comes from analyzing seismic waves. P-waves (compressional) travel through both solid and liquid material; S-waves (shear) cannot pass through liquid. The absence of S-waves on the far side of Earth from an earthquake provided the first clear evidence for a liquid outer core.

Active learning tasks that ask students to reason from seismic data to structural inferences model the actual practice of geophysics and make the layers feel like genuine discoveries rather than facts to memorize.

Key Questions

1. Differentiate between the layers of Earth based on their composition and physical properties.
2. Explain how seismic waves provide evidence about Earth's interior.
3. Analyze the role of convection currents in Earth's mantle.

Learning Objectives

• Compare and contrast the physical properties and chemical composition of Earth's crust, mantle, outer core, and inner core.
• Explain how the behavior of P-waves and S-waves changes as they pass through different layers of Earth's interior.
• Analyze seismic wave data to infer the location and state (solid or liquid) of Earth's internal layers.
• Model the process of convection currents within the mantle and explain their role in plate tectonics.

Before You Start

Properties of Matter

Why: Students need to understand concepts like solid, liquid, and density to differentiate the states and compositions of Earth's layers.

Plate Tectonics

Why: Prior knowledge of plate movement provides context for understanding the mantle's role and convection currents.

Key Vocabulary

Lithosphere	The rigid outer part of the earth, consisting of the crust and upper mantle. It is broken into tectonic plates.
Asthenosphere	The upper layer of the Earth's mantle, below the lithosphere, in which there is relatively low resistance to plastic flow and convection is thought to occur.
Seismic Waves	Waves of energy that travel through Earth's layers, generated by earthquakes or explosions. They provide information about Earth's interior.
P-waves (Primary waves)	Fastest seismic waves that compress and expand the rock they move through. They can travel through solids, liquids, and gases.
S-waves (Secondary waves)	Slower seismic waves that move rock particles side to side. They can only travel through solids.

Watch Out for These Misconceptions

Common Misconception

Earth's mantle is liquid lava.

What to Teach Instead

The mantle is solid but ductile -- it flows very slowly under sustained pressure over millions of years, similar to how silly putty deforms slowly under weight. Magma exists only in specific locations such as mid-ocean ridges and hotspots. Using putty to show how a solid material deforms under pressure without becoming liquid helps students separate 'flowing' from 'melting.'

Common Misconception

We cannot know anything about Earth's interior because we cannot see it.

What to Teach Instead

Seismic wave patterns give scientists clear, quantitative evidence about the density, state (solid vs. liquid), and composition of each layer. Walking students through the logic of using S-wave shadow zones to infer the presence of a liquid outer core demonstrates how indirect evidence can be just as compelling as direct observation.

Active Learning Ideas

Inquiry Circle: Seismic Wave Detectives

Groups receive maps showing P-wave and S-wave arrival times at seismic stations worldwide after a simulated earthquake. Students identify the shadow zone where S-waves are absent, develop a claim about what this pattern indicates about Earth's interior structure, and sketch a cross-section model consistent with their evidence.

45 min·Small Groups

Think-Pair-Share: Why Are Dense Materials at the Center?

Present data cards showing the density, temperature, and seismic wave speed for each of Earth's four layers. Students individually rank layers by density, compare with a partner, and discuss why the densest materials are at the center rather than evenly distributed throughout the planet.

20 min·Pairs

Stations Rotation: Models of Earth's Interior

Students rotate through three stations: a hard-boiled egg as a structural Earth model (identify the crust, mantle, and core analogs), a density column demonstration showing how liquids of different densities layer by depth, and a spring toy (Slinky) to model P-wave and S-wave propagation. At each station, students identify what the model shows well and what it fails to represent accurately.

50 min·Small Groups

Real-World Connections

• Geophysicists use seismic data from earthquake monitoring stations worldwide, like those managed by the Incorporated Research Institutions for Seismology (IRIS), to create detailed maps of Earth's interior structure, aiding in the search for mineral resources and understanding geological hazards.
• Engineers designing earthquake-resistant structures in seismically active regions, such as Los Angeles or Tokyo, must consider the properties of different soil and rock layers, which are influenced by the underlying Earth's structure.
• Volcanologists study the movement of magma, which originates from the mantle, to predict volcanic eruptions. Understanding mantle convection helps explain where and why volcanoes form, like those in the Hawaiian Islands or along the Ring of Fire.

Assessment Ideas

Quick Check

Provide students with a simplified diagram showing seismic wave paths (P and S waves) through Earth. Ask them to label the Earth's layers (crust, mantle, outer core, inner core) based on where the waves are detected or blocked. Include a question: 'What evidence does this diagram provide about the state of the outer core?'

Discussion Prompt

Pose the question: 'Imagine you are a geophysicist who has just received seismic data from a major earthquake. How would you use the arrival times and paths of P-waves and S-waves to determine if a layer deep inside Earth is solid or liquid?' Facilitate a class discussion where students share their reasoning.

Exit Ticket

Students draw a simple cross-section of Earth showing the four main layers. For each layer, they write one key characteristic (e.g., composition, state of matter, relative thickness). They should also write one sentence explaining how scientists learned about this layer without visiting it.

Frequently Asked Questions

What are the four layers of the Earth?

The four layers are the crust (thin, solid, rocky outer shell), the mantle (thick, mostly solid silicate rock that flows slowly over geologic time), the outer core (liquid iron-nickel), and the inner core (solid iron-nickel kept solid by extreme pressure). The boundary between the crust and mantle is called the Mohorovicic discontinuity, or Moho.

How do scientists know what the inside of the Earth is made of?

Scientists primarily use seismic waves from earthquakes. P-waves and S-waves travel at different speeds through different materials, and S-waves cannot pass through liquids. By analyzing arrival times and shadow zones at monitoring stations worldwide, scientists infer the layered structure and composition of Earth's interior without ever drilling there.

What causes convection currents in the mantle?

Heat from Earth's core and from radioactive decay in the mantle causes rock near the core-mantle boundary to heat up, become less dense, and rise slowly. As it nears the upper mantle, it cools, becomes denser, and sinks again. This slow circulation over millions of years is the primary driver of tectonic plate movement.

How does active learning help students understand Earth's interior?

Since no student can observe Earth's interior directly, strategies that reconstruct scientific reasoning from data are especially valuable here. When students analyze seismic shadow zone data themselves and derive the conclusion that Earth has a liquid outer core, they practice the same inferential reasoning geologists use -- making the science process as visible as the science content.