Earth's differentiation

Earth formed from the debris of the solar nebula and the differentiation of layers in the earth's interior was initially driven by heat. This continuing process has had many parallels in other planetary and celestial bodies throughout the universe, particularly through the activities of tectonic plates.

Earth's Formation

Before the earth could differentiate, it had to form. The most widely accepted timeline involves the formation of the universe by the big bang, which occurred roughly 13.7 billion years ago. More than 9 billion years later, the solar system began to form from the solar nebula, a giant rotating cloud of big bang debris. Protoplanets and asteroids began to form from that debris by a process called accretion: the adding on of mass by pulling that mass in gravitationally. One of these protoplanets became Earth, whose temperature eventually increased to a critical point so that differentiation of interior layers could occur.

The big bang theory was first proposed in 1927 by Georges Lemaître, a Belgian astronomer, physicist, and priest, who called his theory a “hypothesis of the primeval atom.” (The term “big bang” was coined during a 1949 radio broadcast by Sir Fred Hoyle, an English mathematician and astronomer who supported an opposing theory of the universe's formation.)

According to the big bang theory, the universe was in an extremely hot, dense state 13.7 billion years ago and began rapidly expanding, resulting in cooling and in continuous expansion, even into the present day. Protons, neutrons, and electrons formed early on in this expansion, followed by nuclei and atoms. With these pieces of matter in existence, galaxies and stars began to form, some beginning only 480 million years after the big bang.

According to the nebular hypothesis, first described in 1734 by Swedish scientist Emanuel Swedenborg, the solar system and its sun formed from the gravitational collapse of a “tiny” portion of a giant molecular cloud (GMC) 8 to 9 billion years after the big bang. (“Tiny,” in this sense, means having a diameter of about 3.25 light-years.) GMCs are huge, dense, and gravitationally unstable collections of molecular hydrogen molecules; these clouds collapse to form stars. The mass of this particular GMC collided to form the presolar nebula, a rotating disc-shaped cloud of debris and gas, at the center of which was the sun in its earliest stages of formation. The bulk of the nebula's composition was hydrogen and helium, along with small amounts of lithium and other heavier elements.

As the nebula spun, it began compressing and spinning faster. The matter in the center, even more compressed than it had been earlier, collided more frequently, raising the temperature and bringing the proto-sun to the state of a T Tauri star, a young star not yet in the main-sequence, hydrogen-burning stage of life. During the next 50 million years, the sun's core temperature and pressure became high enough to launch it into its main sequence, propelled by hydrogen-to-helium nuclear fusion. The leftover debris from the sun's formation continued rotating as a solar nebula, and asteroids and protoplanets began to form within it, growing by accretion. One of these—a rocky, terrestrial protoplanet within the inner part of the solar system—was Earth.

Initially, the young earth's interior was a mostly heterogeneous mixture of silicates, nickel, and iron. When the center of the earth reached a critical temperature, one at which the silicates could melt, differentiation of internal layers, especially the core, began. Earth was under a period of bombardment by meteorites and other protoplanets at this time, and according to the giant impact hypothesis, one particularly large impact blew the barely formed mantle and crust from Earth, ultimately forming the moon. The forming core escaped unscathed. Most of the earth's existing continental crust is probably only 2 billion years old.

Sources of Heat in Earth's Interior

Heat provided the impetus for the differentiation of the earth's layers: As temperatures within the earth rose high enough to melt the substances within the early earth's interior, the molten substances were able to separate out because of density and other differences. The necessary heat came from multiple sources, including the decay of radioactive elements contained within the planet, gravitational compression as the earth continued to become more compact, and the ongoing impacts of meteorites.

Radioactive decay refers to the spontaneous emission of ionizing particles from unstable atoms, atoms whose nuclei have excess energy to release. The process likely contributed the most heat, leading up to the differentiation of the earth's layers. (Radioactive decay continues to occur within the earth.) At the time of the earth's differentiation, radioactive decay of potassium-40, uranium, and thorium drove this heating, and the decay of these elements continues to help drive convection in the molten outer core. Despite the high density of uranium and thorium, both elements are found abundantly in the crust rather than in the core.

Although most substances migrated based on density during the differentiation of the earth's layers, some migrated based on chemical affinity; uranium and thorium interact more readily with the silicates in the earth's upper layers than with the densely packed iron in the core. This process is called chemical stratification.

Another contributor to the earth's internal heat was gravitational compression, a process by which gravity forced the earth to become smaller and denser as it cooled, releasing pressure. Mathematically described by the Kelvin-Helmholtz mechanism, this process, which also occurs in other planets and stars, results in an increased temperature in the core. The gravitational compression of a planet ends when an opposing pressure gradient balances that compression. This same equilibrium is evident in the earth's atmospheric layers: Pressure keeps them from collapsing onto the earth and gravity holds them down enough so that they do not disappear into space.

The third factor was external: the bombardment of Earth by meteors, which added heat locally through shock waves and impact melts. Upon impact, meteorites send physical energy through the earth in the form of propagating shock waves, causing a rapid increase in temperature and pressure at and around the point of impact. This resulted in the temporary melting of rock (impact melts).

With all three processes in place, especially radioactive decay, the earth's temperature increased past the melting point of the silicates and eventually past the melting point of iron (1,538 degrees Celsius [C] or 2,800 degrees Fahrenheit), allowing the iron catastrophe to begin; this triggered the formation of the earth's core and ultimately the differentiation of all of earth's interior layers.

Differentiation of Earth's Layers

Core formation began roughly 10 million years after Earth started to form; this time period is within the Hadean eon, which spans from Earth's formation approximately 4.6 billion years ago until an arbitrary point generally pinned at 3.8 billion years ago. While a primitive mantle and core likely differentiated around the same time, the giant impact hypothesis suggests that 4.52 billion years ago, a nearly Mars-sized protoplanet smashed into Earth, ejecting much of the crust and mantle (but not the core) from the planet and into orbit, ultimately forming the moon. Over the next 150 million years, a new mantle and crust formed on Earth.

Brought about by heat, particularly radioactive decay, the core's formation is often referred to as the iron catastrophe because iron was involved significantly in the process. As the temperature rose, the fairly well-mixed substances within the earth's interior—the silicates, the nickel-iron mixture (NiFe), and other elements—began to melt and separate from one another, mostly on the basis of density differences. In a process referred to as the rain-out model, the heavy NiFe rained out from the molten emulsion of silicates and fell toward the center of the earth, where it accumulated into what became known as the core.

Because temperature and pressure increase toward the center of the earth, further differentiation occurs: a solid inner core and a molten liquid outer core. The core spans from the earth's center to about 2,890 kilometers (1,790 miles) below the earth's surface, giving it a diameter about twice as large as the diameter of the moon. The temperature at the center of the earth is an estimated 6,373 kelvins (6,100 degrees C), about the same as at the surface of the sun.

However, not all elements differentiated by density. As in the case of radioactive decay, chemical stratification was most significant in some cases, such as with heavy elements like uranium. Because of its high affinity for silicates and its difficulty fitting within iron's dense structure, uranium rose toward the surface rather than sinking to the core.

With the differentiation of the earth's core also came the generation of the earth's magnetic field, which is sustained by the outer core. This is explained by the dynamo theory, which describes a magnetic field generated by a rotating, convecting, electricity-conducting liquid. The molten outer core's rotation is provided by the earth's Coriolis effect. Convection is a result of internal heat sources and it conducts electricity. The magnetic field is critical for many reasons, particularly protecting the earth's atmosphere from solar wind.

The mantle and the crust continue to differentiate, with old material being destroyed and new material being created at tectonic plate boundaries and faults. The mantle, mainly composed of silicates, magnesium, and some iron that did not end up in the core, is considered a solid layer, although its upper portion does move very slowly over time. Making up 84 percent of the volume of the earth, the upper mantle provides a moving platform for the crust's tectonic plates to ride upon.

The crust, which accounts for about 1 percent of the earth's volume, is a rocky, solid layer. There are two types of crust: continental crust and oceanic crust. Continental crust is thick, with a relatively low density, and consists of rocks such as granite. Oceanic crust is thinner, is higher in density, and consists mainly of rocks such as basalt. Continental crust has an average age of about 2 billion years, with some samples dating back as far as 4.3 billion years; most existing oceanic crust is 200 million years old at most.

Parallels in Other Planetary Bodies

In basic terms, planetary differentiation merely refers to the separation of layers in a planetary body caused by physical and chemical properties; density is typically a main factor in the separation. Planetary differentiation generally results in a distinct core and mantle, and sometimes in a crust as well. It is not necessarily a process with a definite end; the earth, for example, continues to differentiate as crust is destroyed and created through tectonic activity.

The formation and differentiation processes of Earth are not unique; indeed, Earth is just one planetary body among many that exists under similar external factors, so there are many examples of parallel formation and differentiation processes that have occurred or are still occurring throughout the universe. Earth's own moon, dwarf and regular planets, and asteroids are just a few examples of planetary and celestial bodies that share early similarities with Earth.

The moon, which is Earth's only natural satellite, is one of several hundred celestial bodies in the solar system that orbit a larger body. The giant impact hypothesis suggests that the formation of the moon occurred around 4.53 billion years ago, in a process that parallels Earth's formation by accretion from the debris of the solar nebula.

The moon's differentiated layers are also similar to those of Earth: a solid core rich in iron, a molten outer core also mainly composed of iron, a mantle, and a crust. Scientists believe that these layers were formed by a process called fractional crystallization, which also occurs now in the earth's upper layers as magma cools. Fractional crystallization refers to minerals precipitating from a melted substance, much like the formation of the earth's core during the iron catastrophe. When the moon accreted, the energy released by the impact probably caused the early moon to be covered by partially melted rock (a type of magma ocean), from which minerals precipitated, leading to the differentiation of internal layers.

The differentiation of one of the solar system's largest asteroids, 4 Vesta, also bears some similarities to Earth's differentiation. Asteroid 4 Vesta accounts for about 9 percent of the total mass of the asteroid belt, an asteroid-heavy region between Mars and Jupiter. Much can be inferred about 4 Vesta's differentiated interior because some of its material has fallen to Earth in the form of meteorites from an impact that hit 4 Vesta probably within the last 1 billion years. These meteorites (called Howardite-Eucrite-Diogenite [HED] meteorites) provide ample evidence of igneous processes, which involve the cooling of melted substances like magma.

From the HED meteorites, scientists have determined a rough timeline of 4 Vesta's differentiation. About 2 million years after forming by accretion, the asteroid likely experienced a process like Earth's iron catastrophe: Radioactive decay of an aluminum isotope provided enough heat to partially or fully melt the forming body, leading to the differentiation of a heavy metal core and a molten convecting mantle, most of which slowly crystallized as the asteroid cooled. The remaining molten material extruded to the surface, either by flowing or erupting, ultimately cooling to form a crust.

Earth's Differentiation Continues

Earth's differentiation did not end billions of years ago. It continues in the present, as parts of the lithosphere (crust and upper mantle) continually break down and rebuild. The lithosphere contains about seven major “chunks” called tectonic plates, and many minor chunks. These plates move very slowly, about one centimeter each year, and interact at transform faults, convergent boundaries, and divergent boundaries. This movement causes the formation of mountains and ocean trenches and also causes volcanic and seismic activity. These processes make the earth a living, ever-differentiating planetary body.

To understand plate tectonics, one must understand the three main types of boundaries that occur between plates. Convergent boundaries (also known as collision boundaries or destructive plate boundaries) are areas in which tectonic plates move toward each other, resulting in either a direct collision or subduction (when one plate slides under another). Because of density differences, plates made of oceanic crust tend to slide under continental crust plates, whereas a convergence of two continental crust plates often, but not always, results in a collision. Any of these actions is accompanied by a great deal of pressure, friction, and melting, resulting in earthquakes, volcanic activity, and the formation of mountain ranges.

Tectonic plates move from each other at divergent boundaries (also known as extensional or constructive boundaries). Molten magma flows up from the convecting mantle to fill the space. When boundaries of this type exist between two plates of continental crust, rift valleys form. Between oceanic plates, one can find the mid-ocean ridges (essentially underwater mountains) and volcanic islands.

Transform faults, the third boundary type, are also known as conservative plate boundaries. Unlike the other boundary types, transform faults do not involve the creation or destruction of crust. Instead, transform faults relieve stress caused by other boundaries. Typically present in a zigzag shape, they are often found within mid-ocean ridges and between continents.

To interact at these boundaries, tectonic plates ride on the slow-moving, denser asthenosphere, driven by a variety of forces, particularly convection within the mantle. These convection currents slowly bring heat from the interior of the earth, a process that exerts frictional and gravitational forces on tectonic plates. Other gravitational forces also are involved. At spreading ridges, for example, oceanic lithosphere is created in a way that the new material adds on to the ridge side of the plate. As that side becomes thicker than the other side of the plate, it sinks to create a slight lateral incline, resulting in gravity-driven sliding. The moon and sun also create a tidal drag that influences plate movement, and the Coriolis effect exerts some influence, too. Finally, the small wobbles that occur in Earth's imperfect rotation affect the movement of tectonic plates.

Some other terrestrial planets show evidence of plate tectonics as well. In general, it is expected that dry planets and celestial bodies larger than Earth have tectonic plate activity, while bodies around Earth's size might have activity if water is present. Tectonic activity is closely tied to other processes on Earth, particularly sea level rise.

Principal Terms

accretion: a process by which planetary bodies grow by gravitationally pulling in matter

chemical stratification: differentiation of layers of a planetary body based on chemical affinities and characteristics rather than physical properties

fractional crystallization: the process of minerals precipitating from a molten substance

giant impact hypothesis: the widely accepted hypothesis about the moon's formation, which is believed to have occurred when a protoplanet collided with the earth and ejected parts of the earth's mantle and crust, which entered orbit and formed the moon by accretion

gravitational compression: the process by which an object becomes smaller and denser because of the force of gravity acting on it; it produces heat in the center of a planetary body

iron catastrophe: the event leading up to the formation of the earth's iron core, when iron precipitated out of the early earth's molten silicate mixture and sank to the center of the planet

planetary differentiation: the separation of interior layers of a planetary body because of chemical and physical differences

protoplanet: the early stage of a planet, often referred to as a planetary embryo; it often collides with other protoplanets to form planets

radioactive decay: spontaneous emission of ionizing radiation from the nucleus of an unstable atom

tectonic plate: a slowly moving chunk of the earth's uppermost layer, the lithosphere

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