Journey to the center of the Earth

This exercise is a part of Educator Guide: Earth’s Inner Core is Relatively Young / View Guide

Directions for teachers: After students read “Earth’s inner core is relatively young,” have them create diagrams to explain core concepts including Earth’s inner structure, the creation of a magnetic field and thermal convection. After sharing those diagrams with the class, students can work together to create a class diagram of the geodynamo and answer the discussion questions provided.

Suggestion for structuring diagramming and discussion: Divide the class into five groups and review the five diagramming prompts listed below. Assign each group one prompt and allow students to do additional research as needed. You might want to assign specific roles to group members: a drawer, a writer, a presenter and so on. For a 60-minute class period, consider allowing about 20 minutes for each group to diagram the core concept and then 10 minutes for all of the group presentations. Then you can allow 20 minutes to create a class diagram of the geodynamo, with another 10 minutes to discuss the follow-up questions.

Notes to the teacher: Introduction to Geomagnetism by the U.S. Geological Survey is a useful background resource for information on the geodynamo.

Directions for students: The Science News article “Earth’s inner core is relatively young” explores how a change in the Earth’s inner structure created the relatively strong magnetic field that exists today. The concepts below all relate to the geodynamo, which sustains this magnetic field. Follow your teacher’s instructions to draw a diagram that explains one of the concepts. Be as detailed as possible and label your diagram appropriately.

Group prompts:

1. Creation of a magnetic field

Magnetic fields are generated by moving charged particles, or an electric current. As an electric current flows through a wire, magnetic field lines are formed in concentric circles perpendicular to the wire. In their diagram, students should indicate the direction of the magnetic field around the current flow. If the current is flowing in an upward direction through a wire perpendicular to a piece of paper, the magnetic field lines are drawn as concentric circles in the counterclockwise direction on the piece of paper.

2. Thermal convection (include the concept of density)

In thermal convection, heat is transferred through movement of materials in the liquid or gaseous state. As a material heats up from some outside heat source, its molecules get farther apart, which gives the material a lower density. Its lower density causes it to rise and move farther from the heat source. As it rises, cooler materials take its place near the heat source. Those cooler materials heat and rise. This movement creates a circular system of heat transfer known as a convection current. In their diagram, students should include a heat source and a visual representation of this circular system. Students should also show the changes in molecular spacing throughout the circular process.

3. Heat conduction (include the concept of thermal equilibrium)

In heat conduction, heat is transferred through contact between materials. Temperature is a measurement of the intensity or degree of heat present in a material. Heat energy is the result of molecular movement (vibrations, translations and rotations). When contact occurs between objects with different temperatures, heat energy is transferred between moving molecules until thermal equilibrium is met (both objects have the same temperature). In their diagram, students should indicate the temperature of two different substances before and after they are in contact. The initial temperature of one substance should be higher than the other, and once in contact, the higher temperature would decrease and the lower temperature would increase until the temperatures are the same (thermal equilibrium is met).

4. Crystallization (include the concept of heat transfer and use water as an example)

In crystallization, a liquid substance physically changes state to become a solid. As a liquid substance at its freezing point loses thermal energy, molecular motion slows and intermolecular attraction strengthens, which allows molecules to align themselves into a crystalline structure. In their diagram, students should show unorganized molecules of water in a liquid phase and the transition to an organized alignment of water molecules in the solid phase, as ice. Students should indicate the loss of heat energy during the process and show the increase of molecular attraction in the transition. The freezing point conditions should be noted and will remain the same before and after the phase change (for H2O, 0° C at 1 atm).

5. Earth’s inner structure (include composition and approximate depth ranges)

Earth’s inner structure is made up of the crust, mantle and core. The Earth’s crust can be one of two kinds: thinner crust under oceans and thicker crust under continents. The thinner, oceanic crust is made up of basalt and has a depth of about 8 km. The thicker, continental crust is made up of granite and has a depth of up to about 70 km.

The mantle is divided into the upper and lower mantle, which are primarily made up of silicate rocks. The upper mantle, closer to the surface, is up to about 700 km thick and the lower mantle is found between about 700 km and 2,800 km below the surface.

The Earth’s core is divided into the liquid outer core and the solid inner core, both primarily made up of iron and nickel. The outer core is from about 2,800 km below the surface to approximately 5,000 km deep, and the inner core is a sphere approximately 2,500 km wide.

In their diagrams, students should show a cross section of Earth with the correct placement and relative depths of all the components. Composition and depth information should be labeled.

Class prompts:

1. How do the individual diagrams inform the larger geodynamo process, as it is described in the article? How does Earth’s core generate a magnetic field? Draw a diagram of the geodynamo as a class, using group diagrams to inform the larger diagram. Discuss how energy flows and transforms throughout the geodynamo process.

The inside of the Earth is very, very hot because the planet retains thermal energy left over from the cosmic collisions that formed it and those that followed. The inner core is solidifying through crystallization (fourth concept). This crystallization releases heat energy and, along with the leftover heat from the planet’s formation, provides an energy source for heat conduction (third concept) and convection (second concept). Thermal convection creates cyclical movement of the molten iron and nickel in the core (Earth’s structure, fifth concept), and the resulting flow of electrons produces the magnetic field (first concept). The magnetic field builds because of the complex flow of fluids, which are not only rising and falling but also being twisted thanks to the Coriolis effect.

2. Explain why the geodynamo is considered to be self-sustaining. Why do you think geophysicist Peter Olson says “all planets lose heat”? How would you represent the general energy flow between Earth and space? Will the geodynamo effect last forever?

The interplay of the inner and outer core makes the dynamo self-sustaining over billions of years. Changes in one layer drive changes in another layer and vice versa. All planets lose heat, as Olson says, because planets are not closed systems. They are connected to the vast, empty and cooler space around them. As Earth cools and the inner core grows and crystallizes, heat is transferred up into the mantle and ultimately away from the planet. Encourage students to think about where on Earth they can witness this heat loss and how heat might be added back into the Earth system. If Earth’s core ever fully solidified, the geodynamo would shut down. But that process would take so long that our sun probably will have engulfed the Earth by then anyway.

3. How does life on Earth benefit from the magnetic field and thus the geodynamo? What technologies depend on the field? What could happen to life and these systems if the field weakens or changes?

The magnetic field protects Earth from the dangerous radiation of the solar wind and solar storms. Encourage students to think about why this is important for life today, but also why it mattered when life was first evolving. Without the magnetic field, the radiation would eat away at our atmosphere, including the ozone layer that protects life from ultraviolet radiation. Many organisms rely on the magnetic field for navigation and orientation, including humans. Cellphone GPS systems and military navigation technologies depend on the field, for example. The field also protects our satellites and power grids from damaging radiation.

If the field weakened or flipped, this protection could be temporarily lost. Your compass might think North is in a different direction. Technologies might be damaged. Rates of cancer might increase. What’s more, other animals that navigate via the field could get lost on their journeys. But remind students that the strength of Earth’s magnetic field is already quite variable and a big weakening or flip would likely occur gradually, perhaps over thousands of years.