Tikalon Blog by Dev Gualtieri

Earth's Core

August 2, 2021

Humans owe our existence to certain felicitous properties of the Earth that allow life to exist on its surface. Some of these, such as air and water, are obvious. One overlooked property is thermal conductivity. The Earth is a ball of molten metal whose heat is insulated by its rocky crust.

Granite, as a representative of igneous rock, has a thermal conductivity of about 2 W/m/K. This is about 200 times smaller than that of copper (400 W/m/K), which is an excellent thermal conductor. It's the low thermal conductivity of rock, coupled with the cold of space, that keeps Earth's surface at the modest temperature of about 300 K.

In the aptly named Hadean eon, named for the underworld of Greek mythology, Earth's surface was "as hot as hell." The Hadean began with Earth's formation, about 4.6 billion years ago, and it extended until the formation of the first rocks, 4 billion years ago. After 4 billion years of cooling, Earth maintains a thermal gradient from its surface at about 300K to its core of estimated temperature 5,700K (see figure). The first few miles below Earth's surface have a thermal gradient of about 25°C/km. Miners at the Stillwater Mining Company platinum mine in Nye, Montana, work about a kilometer underground.

Thermal gradient of the Earth from crust to core. (Adapted from a Wikimedia Commons image by Bkilli1. Data from Boehler, ref. 1.[1] Click for larger image.)

Sitting atop a giant heat source has its advantages. Since Iceland is located at a rift between continental plates, it has access to huge quantities of geothermal power. According to Iceland's National Energy Authority, its geothermal power facilities generate 25% of Iceland's total electric production, and geothermal sources accounts for 66% of Iceland's primary energy use.[2] Somewhat more far-fetched, and a different take on the idea of hot money, is El Salvador's idea of using geothermal electric energy for bitcoin mining.[3] El Salvador's President, Nayib Bukele, has stated,

"I've just instructed the president of... our state-owned geothermal electric company... to put up a plan to offer facilities for #Bitcoin mining with very cheap, 100% clean, 100% renewable, 0 emissions energy from our volcanos."[3]

The Hellisheiði Geothermal Plant in Iceland (Hansueli Krapf).

The Hellisheiði Geothermal Plant in Iceland.

In the 20th century, Iceland transitioned from a country of subsistence agriculture and some fishing to industrial fishing and consumer products. As a consequence, Iceland has become one of the wealthiest nations in the world.

(Wikimedia Commons image by Hansueli Krapf)

Since it's hidden from us except by advanced scientific measurement techniques, our understanding of the inner Earth has evolved over many years. In the late 1770s, it was noted that the average density of material at Earth's surface is only 3 g/cm³, but the average density of Earth is 5.515 3 g/cm³. This was discovered by an experiment funded by the Royal Society to measure the deflection of a plumb bob at the base of a large mountain in Scotland, the deflection angle giving the relative densities of the mountain and the Earth.[4] The immediate conclusion was that the core was metallic (liquid iron has a density of about 7 g/cm³ at its melting point).

Differentiation in the early, molten Earth resulted in denser materials sinking to its center and less-dense materials floating to the crust. The dense material, believed to be 80% iron with the balance as mostly nickel, formed Earth's core. The development of seismology, which detects transitions between densities, found a transition at approximately 5,150 km (3,200 mi) beneath the Earth's surface between a density of about 10 g/cm³ and 12.5 g/cm³. This revealed the existence of an inner core of solid material with a radius of about 1,220 km (760 miles), and an outer core of liquid metal with an outer boundary about 2,890 km (1,800 miles) beneath Earth's surface.

Cross-section of the Earth. The inner core, which is about 20% of the Earth's radius, is solid because of pressure. The outer core is a liquid composed mostly of iron and nickel. Rotation of the outer core causes the Earth's magnetic field. (Modified Wikimedia Commons image by Surachit.)

As could be expected, the Earth is not precisely spherical, and it has a bulge at the equator that's a consequence of its rotation. Its shape is an oblate spheroid, but just by a little. Sea level at either pole is just 21 km closer to Earth's center than that at the equator, a difference of just 21/6370 = 0.33%. This leads to the question of whether or not the core is perfectly spherical.

An international team of seismologists with members from the University of California (Berkeley, California), the Université de Nantes (CNRS, Nantes, France), the Tokyo Institute of Technology (Tokyo, Japan), the Collège de France (Paris, France), and the Université de Paris (Paris, France) have discovered that Earth's solid inner core is growing faster on one side than the other.[5-6] The reason for this is unknown, but it's thought that this asymmetrical process has been operating since the core's first solidification more than half a billion years ago.[6] The inner core is growing faster under Indonesia's Banda Sea, but this hasn't resulted in a lopsided core, since gravity distributes the new growth to maintain a spherical inner core growing in radius by a millimeter per year.[6]

Many hands make light work, as this international team of seismologists must have discovered. This map shows the seismometer locations (triangles) at which seismic waves from earthquakes (circles) were measured to study Earth's inner core. The cyan colored stations are where new measurements were made for this study. These mostly sampled the inner core between Earth's poles. (UC Berkeley graphic by Daniel Frost)

such a growth asymmetry of the inner core suggests that something in the outer core or mantle under Indonesia's Banda Sea is removing heat from that region at a more rapid rate than on the opposite side, under Brazil. This heat loss would accelerate hexaferrum iron crystallization from the outer core, causing growth of the inner core on that side.[6] Such a process would also affect the Earth's magnetic field, since the dynamo action of the outer core that creates this field is driven by convection caused by the release of heat from the inner core.[6]

The discovery of this asymmetric growth explains why the crystallized iron in the core is more aligned with Earth's rotational axis in the west than in the east, despite the expectation that the crystals should be randomly oriented.[6] The research team developed a model that describes how asymmetric growth, which is about 60% higher in the east than the west, will preferentially orient iron crystals along the rotation axis, with more alignment in the west than in the east.[6] The model also reveals that the inner core is just a billion years old, plus or minus 500 million years.[5]

As iron crystals grow,they're redistributed by gravity, which moves the excess growth from the east to the west within the inner core.[6] The inner core is somewhat soft, having a viscosity that's greater than 10¹⁸ Pa-s, so the movement of the crystals aligns their crystal lattice structure with the rotation axis of Earth, but with a greater alignment in the west than the east.[5-6] The model also predicts the proportion of nickel to iron in the center of the earth, with the result that the inner core has between 4% and 8% nickel.[6] This is about the proportion in metallic meteorites.[6] Funding for this research was provided by the National Science Foundation.[6]

Earth's Core

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