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If not for Earth's magnetic field and plate tectonics, there would be no advanced life on earth. Plate tectonics brought gases to Earth's surface and Earth's magnetic field kept them from being blown away into interstellar space. Circulation in Earth's outer core and mantle cause the magnetic field and plate tectonics, respectively. Mars and Venus do not have significant magnetospheres, and oxygen and water were stripped from their atmosphere by solar wind.
The earth was too hot for plate tectonics in the first one or two billion years of Earth's history. The following video describes the period of plate tectonics.
The core of the earth (Figure 5-12) has radioactive uranium and other elements that release energy. This radioactivity has caused the inner core to maintain a high temperature for billions of years. Even though it is hot, the inner core of the earth is solid because of the high pressure exerted on it by the outer layers of the earth. The outer core of the earth is liquid. The mantle is a solid, but it moves very slowly, overturning every 500 million years. The mantle sets up circular currents (Figure 5‑13) as it carries energy from the core to the surface of the earth. The slow movement of the mantle circulation cells causes plate tectonics and volcanic activity (Figure 5‑14), which releases water and carbon dioxide into the atmosphere. Earth is the only planet in the solar system with plate tectonics.
Figure 5‑12. Core and mantle of the earth. Credit: KelvinSong. Used here per CC BY-SA 3.0.
Figure 5‑13. Circulation in the mantle. Credit: Forest Service – USDA.
Figure 5‑14. Boundaries of tectonic plates (black) and volcanic activity (red). Credit. USGS.
Circulating ionized metals in the earth's outer core create a magnetic field around the earth. The moving ions form electric currents, which then create magnetic fields. The uranium in the core of the earth produces the energy that causes the core to circulate. Some of the uranium might have come from the supernova that caused the collapse of the molecular cloud and formation of the sun.
Most scientists agree that the cause of flowing ionized metals in the outer core is a slow expansion of the inner core. It expands by 1 mm/year. As a result of this expansion, the inner core releases a small amount of lighter elements, such as sulfur, into the outer core. The lighter elements that are released into the outer core float to the outer part of the outer core. This flotation causes currents of flowing metal to form in the outer core.
Figure 5‑15. Simplified representation of the dynamo in Earth's outer core that creates Earth’s magnetic field. Credit: USGS.
The currents in the outer core are complicated and interact with the magnetic field. The flow starts wrapping the magnetic field into huge east west magnetic loops in the outer core (Figure 5‑15). The magnetic loops in the outer core induce helical electric currents in the outer core, and these electric fields form the North-South magnetic field through the central core of the earth and that wrap around the earth. The rotating magnetic loops are called a dynamo.
Geologic evidence shows that Earth had a magnetic field at least 3.5 billion years ago; however, Earth's inner core did not solidify prior to 1 billion years ago so the cause of the magnetic field prior to 1 billion years ago must have been different on the early Earth. In this article from MIT press, Jennifer Chu describes the research on the possible existence of a magnetic field during the Hadean and early Archaean eons.
Figure 5‑16. Earth's magnetosphere. 1 : Bow shock 2 : Magnetosheath 3 : Magnetopause 4 : Magnetosphere 5 : Northern tail lobe 6 : Southern tail lobe 7 : Plasmasphere. Solar wind coming from the left. Credit. Dennis Gallagher and Frederic Michel. Public domain.
Earth's magnetic field, the magnetosphere, deflects solar wind from the sun and protects the atmosphere (Figure 5‑16). It deflects a small amount of the solar wind to the north pole, which is what creates the Aurora Borealis. Earth is not the only planet with a dynamo and magnetosphere. Mercury's magnetosphere is also generated by a dynamo and is 1% as strong as Earth's magnetic field. Jupiter's magnetosphere is formed by a dynamo and dwarfs the magnetosphere of the earth. It forms an enormous aurora at Jupiter's north pole (Figure 5‑17).
Figure 5‑17. Aurora at north pole on Jupiter. Credit NASA, ESA, and Jay Nichols.
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