Assessing the age of Earth’s core

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

Directions: Ask students to answer question No. 1 by looking at only the headline of the article “Earth’s inner core is relatively young.” Then have students read the full article and answer the questions that follow.

1. Based on the headline alone, what terms or concepts do you expect to encounter in this article?

The headline reveals that the article is about the inner structure of the Earth and magnetic fields. Given these topics, I expect to read about the inner and outer core, and perhaps the mantle and crust. I expect to read about different types and temperatures of rock. Concepts I expect to encounter include energy and heat transfer, viscosity and convection, magnetism and magnetic poles.  

2. Use the dates and time frames mentioned in the article to construct a timeline covering Earth’s history. Be sure to incorporate and/or note uncertainties.

4.54 billion years ago: Earth formed.

4.2 billion years ago: Clear signs of magnetic field.

2.5 billion to 500 million years ago: Previously proposed ages for the solidification of the inner core.

2 billion to 1 billion years ago: Core and mantle were still molten, assuming the new research is true.

900 million to 600 million years ago: Earth’s magnetic field was weak, according to simulations.

565 million years ago: Earth’s original magnetic field was on the point of collapse, according to the new research.

After 565 million years ago: Inner core solidified and geodynamo began.

3. The article distinguishes between the Earth’s original magnetic field and the magnetic field that exists today. Relate each of the fields to the Earth’s structure. How does what’s happening in the Earth drive the fields?

The original field resulted from heat within the planet driving circulation within the molten core. Today’s field results from the geodynamo, which is driven by the interplay between the solid inner core and molten outer core.

4. Describe what causes the ongoing circulation, called the geodynamo, at the center of the Earth. What phenomenon does the ongoing circulation generate?

Within the Earth, there is a solid inner core and a molten outer core, both composed mainly of iron and nickel, but there is not a hard line between the two. As material cools and crystallizes, it sinks toward the inner core. This crystallization affects the composition of the remaining fluid. More buoyant liquid rises and cooling liquid crystallizes to continue the process. This self-sustaining circulation is called the geodynamo and it generates a strong magnetic field with two opposing poles.

5. How do scientists gain clues to Earth’s past magnetic field? Name and explain the use of two techniques mentioned in the article.

Scientists look for traces of magnetism in ancient rocks and can gain clues to the intensity of the field and times when the magnetic poles switched. These clues come from “magnetic inclusions,” iron-rich grains that line up with the orientation of the magnetic field. Scientists also use computer simulations to gain clues to how circulation would have changed as the planet cooled.

6. Geophysicist Peter Olson says “all planets lose heat.” Name as many instances as you can where heat transfer is mentioned in the article.

In addition to Peter Olson’s mention: As the planet loses heat over time, it cools and its composition changes. The cooling of the inner core leads to crystallization at its center and the geodynamo. Heat-driven circulation within the early Earth’s molten core led to a magnetic field that weakened over time.

7. What gaps in knowledge still exist for scientists studying the history of the Earth’s inner structure and magnetic field?

Scientists don’t know for sure how long the period of a weak magnetic field might have lasted, and they don’t know how recently Earth’s inner core solidified, just that it was after 565 million years ago. Scientists also don’t know how to reconcile new findings with data from the rock record about how fast Earth cooled.

8. What data might help fill in those gaps?

More data from rocks with magnetic inclusions might fill in the time gaps from around a billion to two billions years ago, between 900,000 and 600,000 years ago and since then. Scientists might also look for rocks with magnetic inclusions from other areas of the globe to support or refute the evidence from Canada.

9. Based on the article, why is Earth’s magnetic field important to life on the planet?

Earth’s magnetic field protects it from solar winds, the charged particles constantly ejected by the sun.