Fueling the Earth

Fueling the Earth

The Hidden Heat Beneath Our Feet

Begin Investigation

Begin Investigation

Learning Objectives

Students will...

Model radioactive decay to demonstrate thermal energy generation within Earth.

Analyze decay patterns to understand radioactive decay and cumulative energy production.

Evaluate evidence for Earth’s two primary heat sources.

Apply understanding of heat generation to explain geological phenomena.

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Mission Briefing

GEOLOGICAL SURVEY HEADQUARTERS CASE ASSIGNMENT: URGENT

Dear Scientist, Yesterday, our deep-Earth monitoring stations detected something unusual. Heat readings from Earth’s core showed levels of activity that should not be possible after 4.5 billion years. The board of directors is demanding answers. How can a planet this old still be generating heat? Where is this energy coming from, and will it last? We have compiled all available data, but we need someone with your investigative skills to make sense of it. The evidence is scattered across several fields of geological research. Each mission will take you deeper into the mystery of Earth’s internal heat. Time is critical. The International Geological Conference is in three days, and the organizers are expecting a full report. We're counting on you. Your investigation begins now. Director Sarah Chen Earth Sciences Division

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Select Your Mission

Explore B

Explore A

Engage

Explain

Review

Select Your Mission

Engage

Explore

Origins

Explain

Review

Nebula to Solar System

To understand Earth's interior heat, we need to go back to the very beginning — when our planet was born.

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Nebula to Solar System

To understand Earth's interior heat, we need to go back to the very beginning — when our planet was born.

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Nebula to Solar System

Approximately 4.6 billion years ago, a giant cloud of gas and dust called a solar nebula began to collapse under its own gravity.

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Nebula to Solar System

As the cloud collapsed, it began to spin faster and flatten into a disk.

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Nebula to Solar System

Most of the material was drawn toward the center, where pressure and temperature increased, eventually forming the Sun.

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Nebula to Solar System

Surrounding the young Sun was a rotating disk of leftover gas and dust.

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Nebula to Solar System

Within this disk, tiny dust particles began to collide and stick together through electrostatic forces, forming planetesimals—small, solid objects that were the building blocks of planets.

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Nebula to Solar System

Over time, planetesimals continued to collide and merge through a process called accretion, forming larger bodies called protoplanets.

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The Birth of a Hot Planet

In the region near the Sun, temperatures were extremely high, allowing only materials made of metal and silicate minerals to remain solid and form planets.

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The Birth of a Hot Planet

This is where the inner planets, Mercury, Venus, Earth, and Mars, formed.

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The Birth of a Hot Planet

Farther from the Sun, where temperatures were cooler, ice and gas condensed to form the gas giants Jupiter, Saturn, Uranus, and Neptune.

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Forged to Heat

Earth formed from repeated collisions of planetesimals. Each impact released energy, adding heat to the growing planet.

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Forged to Heat

The energy from impacts, along with pressure from gravity, caused much of Earth’s surface to melt, creating a global ocean of magma.

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Forged to Heat

As the molten planet cooled, dense metals like iron sank to form the core, while lighter rocks rose to form the mantle and crust.

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Forged to Heat

After 4.6 billion years, you'd expect Earth to have cooled off by now — just like smaller rocky worlds have. But it hasn't. Why not?

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The Cooling Mystery

Long ago, both Mars and Earth were born from the same fiery beginning. But 4.6 billion years is a long time.

Click on each planet to discover what Earth and Mars look like today — then decide for yourself what's different.

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Engage-Build Your Case

Working with your team, discuss what you observed. Develop three possible scientific hypotheses to explain the differences between Earth and Mars today. Record your ideas in your student journal.

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Whole Class Share

Building the Case

Once each team has developed its hypotheses, one representative will share their group’s strongest idea with the class.As you listen to other teams:

• Note similarities between ideas — which explanations seem most common?
• Identify new ideas or explanations your team hadn’t considered.
• Record the most compelling explanations in your Student Journal.

These shared ideas will help guide our investigation as we look for evidence to explain why Earth’s interior remains hot.

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Select Your Mission

Now it’s time to test your team's hypotheses and begin your mission to uncover the evidence.

Explore B

Explore A

Engage

Explain

Review

What Lies Beneath

Let’s start by examining what science reveals about Earth’s internal heat.

Click the magnifying glass to explore what lies beneath Earth’s surface.

What Lies Beneath

Click the magnifying glass to see how evidence from Earth’s rocks—made up of tiny atoms—led to a surprising discovery about the source of heat inside our planet.

What Lies Beneath

This is an atom — the smallest building block of matter.

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What Lies Beneath

At the center of every atom is a nucleus, made of protons (p+) and neutrons (n) tightly packed together.

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What Lies Beneath

All atoms of the same element have the same number of protons. But they can have different numbers of neutrons. These different versions are called isotopes.

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What Lies Beneath

Some isotopes have too many or too few neutrons, making them unstable. These are radioactive.

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What Lies Beneath

To become stable, unstable atoms release particles and energy (heat) — this is radioactive decay.

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What Lies Beneath

Billions of these tiny events deep inside Earth generate enormous internal heat.

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What Lies Beneath

This is Potassium-40, an unstable isotope with too many neutrons.

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Potassium-40 19 Protons, 21 Neutrons

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What Lies Beneath

Beta Particle

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To become stable, a neutron transforms into a proton — releasing a beta particle and heat.

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Replay

What Lies Beneath

Beta Particle

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To become stable, a neutron transforms into a proton — releasing a beta particle and heat.

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Replay

What Lies Beneath

With the added proton, Potassium-40 now becomes Calcium-40 — a stable atom with a new identity. The original unstable atom is called the parent. The new stable atom is called the daughter.

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Calcium-40 20 Protons, 20 Neutrons

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Explore:Modeling Radioactive Decay

Scientists cannot directly observe atoms decaying inside Earth, but they can model this process.

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Explore:Modeling Radioactive Decay

Materials

Procedure

Graphing

In this hands-on simulation, you will work with your team to model radioactive decay using coins to represent atoms.

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Analyze Your Data

Work with your team to analyze the results of your "Explore: Modeling Radioactive Decay." Use your data table and graphs to explain the patterns you observed in how the number of parent isotopes and total energy changed over time. Answer the Analysis Questions in your Student Journal.

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Select Your Mission

The investigation continues... You discovered that radioactive decay releases heat. Now let's explore the other sources that keep Earth’s interior hot.

Explore B

Engage

Explore A

Elaborate

Explain

Earth's Heat Sources

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Earth's interior stays hot because of two main energy sources: radioactive decay deep inside the planet, and residual heat left over from Earth's violent formation.

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Earth's Radioactive Fuel

The elements shown in the table — uranium, thorium, and potassium — release energy as their unstable atoms transform into more stable forms. Because their half-lives are measured in billions of years, they continue to produce heat today.

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Earth's Heat Over Time

By modeling decay rates, scientists can estimate how much heat radioactive elements have generated throughout Earth's history (Ga = billion years). This graph shows the heat produced by uranium, thorium, and potassium over time. What patterns do you notice in the heat flow over time? Answer the Analysis Questions in your Student Journal.

Back to Heat Sources

A Violent Beginning

Earth's formation 4.5 billion years ago generated enormous quantities of heat through two primary processes. First, accretional heating occurred as countless asteroids, comets, and planetesimals collided to form the growing Earth. Each impact converted the kinetic energy of the incoming object into thermal energy, as when a hammer strikes metal, warming both the object and the hammer.

Sinking Metal

Second, planetary differentiation released additional heat as Earth's materials separated by density. During Earth's first 100 million years, dense iron and nickel sank toward the center while lighter silicate minerals rose toward the surface. This massive reorganization converted gravitational potential energy into thermal energy through friction and compression, similar to how a sliding object generates heat through friction.

Less Dense Rock Rises

More Dense Metal Sinks

Iron-Nickel Core

Trapped Heat

This ancient heat remains trapped within Earth because rock acts as an excellent insulator. Heat moves extremely slowly through thousands of kilometers of solid rock, requiring millions of years to escape to the surface. Scientists estimate that Earth still retains approximately half of its original heat from formation.

Select Your Mission

The investigation continues... You discovered how radioactive decay produces heat inside Earth. Now is the time to explore how that heat has changed over time and what it reveals about Earth’s thermal evolution.

Explore B

Engage

Explore A

Elaborate

Explain

Earth’s Changing Heat

Over Earth’s history, the balance between internal heat generation and surface heat loss has shifted. Early in Earth’s history, total heat flow was nearly twice what it is today. As short-lived isotopes decayed away and primordial heat slowly escaped through the crust, the planet’s overall heat flow declined.

Earth’s Changing Heat

Even after billions of years, Earth’s interior is far from quiet. Heat rises from deep within the planet, but somewhere between the intensely hot core and the cold surface, the process becomes uncertain. But the questions is how does this heat move through thousands of kilometers of solid rock to reach the surface?

Team Analysis

Work with your team to analyze how Earth’s internal heat has changed over time. Use the heat flow graph, your notes on isotope decay, and the Trapped Heat data to explain how the balance between heat generation and heat loss has shifted throughout Earth’s history. Discuss what evidence suggests that Earth’s interior is still active today. Answer the Analysis Questions in your Student Journal.

Earth stayed alive—its core still burning after 4.5 billion years.

Procedure:

1. Begin with 250 coins (Earth's initial radioactive isotopes). 2. Place all tokens in the container and shake thoroughly. 3. Empty contents onto work surface. 4. Sort: HEADS (parent isotopes) vs TAILS (daughter isotopes). 5. Record data in Table 1 in your Student Journal.* Parent Isotopes Remaining = number of heads * Total Energy Released = total tails removed so far (each decay = 1 energy unit) 6. Remove daughter isotopes, return parent isotopes to the container. 7. Repeat until all coins have decayed.

Mars grew cold and silent, its surface frozen in time.

Using the Radioactive Decay Graph Analysis Sheet, plot your data from Table 1.

For Figure 1: Time versus Parent Isotopes Remaining

• Plot Time (x-axis) versus Parent Isotopes Remaining (y-axis).
• Connect the points with a smooth curve.

For Figure 2: Time versus Total Energy Released

• Plot Time (x-axis) versus Total Energy Released (y-axis).
• Connect the points with a smooth curve.

Materials:

• 250 coins or colored discs
• Covered container
• Radioactive Decay Graph Analysis Sheet

Earth stayed alive—its core still burning after 4.5 billion years.