In stark contrast to other planetary bodies in the solar system, the Earth is dynamic and constantly evolving in geological and biological terms. The sun drives the living and atmospheric process, while energy from the Earth’s core drives the geological processes. It would be difficult to overstate the importance of the energy flow from the Earth’s core. Without it there would be no continents, no volcanoes, no mountains, no oceans, and probably no life either. The Earth would be a dead planet like Mars or the moon. Energy from the Earth’s core produces the planet’s magnetic field that shields most of its inhabited parts from charged particles from the sun. That magnetic field also helps birds, bees, turtles and people navigate. Heat loss from the interior supports diverse and unique ecosystems around deep-sea hydrothermal vents.

Energy from the Earth’s core also sustains human life in a multitude of ways. The rock cycle contributes to the creation of commercial quantities of many important mineral such as phosphate and copper, the creation of fertile soil, the maintenance of global cycles, and source of geothermal energy that humans have tapped for thousands of years. A strong and relatively stable magnetic field also enables power grids and satellite communications to perform in more or less predictable ways.

Formation of the Earth’s Core

The story of the Earth’s core begins about 4.6 billion years ago the Earth formed when gravitational forces caused interstellar matter such as metals, rock, ice and dust to accrete (ball together). Centimeter size particles grew into meter and then kilometer scales, until a planet–sized object was formed. The accretion process dissipated such large amounts of energy that the initial Earth was molten. In a gravitational sorting process called differentiation, the denser, heavier parts were drawn to the center, and the less dense areas were displaced outwards. Denser materials such as nickel and iron thus flowed to the center, while lighter materials such as silicon and oxygen flowed to the surface. The Earth then began a cooling process that continues to this day. Cooling of the core causes growth of the inner core by solidification; the current rate of growth is about 1 millimeter per year.

Structure of the Earth’s Core

Structure of the Earth's interior. Source: NASA.

The Earth’s core has two parts: a solid inner core and a molten outer core. The inner core is found between about 5,100 and 6,400 kilometers depth, and has a mass of 0.09675x10²⁴ kilograms—its roughly the size of our moon, and has a density close to that of steel. The inner core remains in the solid state because of the intense pressure it is under. The outer core is an ocean of liquid metal found between about 2,900 and 5,100 kilometers depth, and has a mass of 1.835x10²⁴ kilograms. The outer core is composed mainly of a nickel-iron alloy, while the inner core is almost composed predominantly of iron.

One of the longest-running debates in Earth science is surrounds the estimation of the core’s temperature. There is no way to measure the temperature at the Earth’s core directly, so it must be inferred indirectly, which leaves plenty of room for disagreement. Recent work suggests that the temperate at the core-mantle boundary is about 3700ºC.

The Earth’s Heat Furnace

The internal heat energy of the Earth was much greater in the early stages of the Earth than it is today. The heat was produced by three distinct processes all of which were most intense during the first few hundred thousand years of the Earth’s history: (1) extraterrestrial impacts, (2) gravitational contraction of the Earth’s interior, and (3) the radioactive decay of unstable isotopes.

Extraterrestrial Impacts

As the proto-planet Earth began to increase in size, its growing gravitational field would have attracted extraterrestrial objects that traveled great velocities, typically 30,000 to 50,000 km/hr. When they collided with the Earth, their enormous kinetic energy was converted to heat energy upon impact, thus providing a component to the Earth’s internal heat source.

Gravitational Compression

The accretionary process in the Earth’s formation increased its gravitational attraction, forcing the Earth to contract into a smaller volume. Increased compaction resulted in the conversion of gravitational energy into heat energy, much like a bicycle pump heats up due to the compression of air inside it. Heat conducts very slowly through rock, so that the rapid build up of this heat source within the Earth was not accommodated by an equally rapid loss of heat through the surface.

Decay of Radioactive Elements

Radioactive decay processes release heat as a by-product. In its early stages of formation, the Earth had a greater number of radioactive elements than it does today, but many of these (e.g., aluminum-26) are short-lived and have decayed to near extinction. Others with a lengthier rate of decay and are still undergoing this radioactive process, thus still releasing heat energy. These include like Potassium-40, Uranium-238, Uranium-235, and Thorium-232.

Heat Loss from the Earth’s Interior

The total heat loss from the Earth’s interior is about 45x10¹² watts. Approximately 70% of this loss occurs through the oceanic crust, and the remaining 30% through the continental crust. The internal heat is lost in two major ways: (1) cooling of the ocean lithosphere, originally generated at mid-ocean ridges, and (2) conduction across the boundary between the mantle and the continental and oceanic lithospheres.

By comparison, the power of solar radiation intercepted by Earth (measured on the outer surface of the atmosphere) is about 1.74x10¹⁷ W, or nearly 3,900 times the heat loss from the planet’s interior.

The Earth’s Geodynamo

Gary Glatzmaier of UC Santa Cruz modeled Earth's geodynamo in 3-D on a supercomputer. Complex fields in the core contribute to a dipole field at the surface: inward flowing field lines (blue) at the North Pole, outward flowing field lines (yellow) at the South Pole.

Energy flows in the core also responsible for another of the planet’s unique features: a strong magnetic field. Scientists believe that the earth has had a magnetic field for at least 3.5 billion years. The basic mechanism of magnetic field generation is a dynamo process, in which the kinetic energy of convective motion in the Earth’s liquid core is converted into magnetic energy. The velocity of this fluid movement is on the order of 10 kilometers per year. The rotation of the earth couples this motion into a circulation that generates electric currents, and the electric currents in turn generate a magnetic field according to classical electromagnetic theory.

Since this process operates without an external energy source, the geodynamo is said to be self-sustaining. Without that regenerative process the electric currents and the associated magnetic field would dissipate in about 15,000 years.

Sources

• Department of Geological Sciences, San Diego State University, The earth's internal energy and internal structure, Accessed 27 June 2007.
• Lawrence Berkeley National Laboratory, The truth about Earth's core?, Accessed 27 June 2007.
• Pennsylvania State University, Probing Question: What heats the earth's core?, Accessed 27 June 2007.
• ScienceDaily, Discovery Of Earth's Inner, Innermost Core Confirmed, Accessed 21 August 2008.