Early Earth: Formation of a Layered Planet
We know that Earth is a layered planet with a core, mantle, and crust surrounded by a fluid ocean and a gaseous atmosphere (see Chapter 1). How was Earth transformed from a hot, rocky mass into a living planet with continents, oceans, and a pleasant climate? The answer lies in gravitational differentiation: the transformation of random chunks of primordial matter into a body whose interior is divided into concentric layers that differ from one another both physically and chemically. Gravitational differentiation occurred early in Earth’s history, as soon as the planet got hot enough to melt.
Earth Heats Up and Melts
Although Earth probably started out as an accretion of planetesimals and other remnants of the solar nebula, it did not retain this form for long. To understand Earth’s present layered structure, we must return to the time when Earth was still subject to violent impacts by planetesimals and larger bodies. As these objects crashed into the primitive planet, most of their energy of motion (kinetic energy) was converted into heat—another form of energy—and that heat caused melting. A planetesimal colliding with Earth at a typical velocity of 15 to 20 km/s would deliver as much kinetic energy as 100 times its weight in TNT. The impact energy of a body the size of Mars colliding with Earth would be equivalent to exploding several trillion 1-megaton hydrogen bombs (a single one of which would destroy a large city)—enough to eject a vast amount of debris into space and to melt most of what remained of Earth.
Many scientists now think that such a cataclysm did occur during the middle to late stages of Earth’s accretion. A giant impact by a Mars-sized body created a shower of debris from both Earth and the impacting body and propelled it into space. The Moon aggregated from that debris (Figure 9.4). According to this theory, Earth re-formed as a body with an outer molten layer hundreds of kilometers thick—a magma ocean. The huge impact sped up Earth’s rotation and changed the angle of its axis, knocking it from vertical with respect to Earth’s orbital plane to its present 238 inclination. All this occurred about 4.51 billion years ago, between the beginning of Earth’s accretion (4.56 billion years ago) and the formation of the oldest Moon rocks brought back by the Apollo astronauts (4.47 billion years ago).
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Another source of heat that contributed to melting early in Earth’s history was radioactivity. When radioactive elements decay, they emit heat. Although present in only small amounts, radioactive isotopes of uranium, thorium, and potassium have continued to keep Earth’s interior hot.
Differentiation of Earth’s Core, Mantle, and Crust
As a result of the tremendous impact energy absorbed during Earth’s formation, its entire interior was heated to a “soft” state in which its components could move around. Heavy material sank to become the core, releasing gravitational energy and causing more melting, and lighter material floated to the surface and formed the crust. The rising lighter matter brought heat from the interior to the surface, where it could radiate into space. In this way, Earth differentiated into a layered planet with a central core, a mantle, and an outer crust (Figure 9.5).
Earth’s Core
Iron, which is denser than most of the other elements, accounted for about a third of the primitive planet’s material (see Figure 1.12). This iron and other heavy elements, such as nickel, sank to form a central core, which begins at a depth of about 2890 km. By probing the core with seismic waves, scientists have found that it is molten on the outside but solid in a region called the inner core, which extends from a depth of about 5150 km to Earth’s center at about 6370 km. Today the inner core is solid because the pressures deep in Earth’s interior are too high for iron to melt.
Earth’s Crust
Other molten materials that were less dense than iron and nickel floated toward the surface of the magma ocean. There they cooled to form Earth’s solid crust, which today ranges in thickness from about 7 km on the seafloor to about 40 km on the continents. We know that oceanic crust is constantly generated by seafloor spreading and recycled into the mantle by subduction. In contrast, continental crust began to accumulate early in Earth’s history from silicates of relatively low density with a felsic composition and low melting temperatures. This contrast between dense oceanic crust and less dense continental crust is what helps drive oceanic crust into subduction zones, while continental crust resists subduction.
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The 4.4-billion-year-old zircon grains recently found in Western Australia (see Chapter 8) are the oldest terrestrial material yet discovered. Chemical analysis indicates that they formed near Earth’s surface under relatively cool conditions and in the presence of water. This finding suggests that Earth had cooled enough for a crust to exist only 100 million years after the planet re-formed following the giant impact that produced the Moon.
Earth’s Mantle
Between the core and the crust lies the mantle, the layer that forms the bulk of the solid Earth. The mantle is made up of the material left in the middle zone after most of the denser material sank and the less dense material rose toward the surface. It is about 2850 km thick and consists of ultramafic silicate rocks containing more magnesium and iron than crustal silicates do. Convection in the mantle removes heat from Earth’s interior (see Chapter 2).
Because the mantle was hotter early in Earth’s history, it was probably convecting more vigorously than it does today. Some form of plate tectonics may have been operating even then, although the “plates” were probably much smaller and thinner, and the tectonic features were probably very different from the linear mountain belts and long mid-ocean ridges we now see on Earth’s surface. Some scientists think that Venus today provides an analog for these long-vanished processes on Earth. We will compare tectonic processes on Earth and Venus shortly.
Earth’s Oceans and Atmosphere Form
The oceans and atmosphere can be traced back to the “wet birth” of Earth itself. The planetesimals that aggregated into our planet contained ice, water, and other volatiles, such as nitrogen and carbon, locked up in minerals. As Earth differentiated, water vapor and other gases were freed from these minerals, carried to the surface by magmas, and released through volcanic activity.
The enormous volumes of gases spewed from volcanoes 4 billion years ago probably consisted of the same substances that are expelled from present-day volcanoes (though not necessarily in the same relative abundances): primarily hydrogen, carbon dioxide, nitrogen, water vapor, and a few others (Figure 9.6). Almost all of the hydrogen escaped into space, while the heavier gases enveloped the planet. Some of the air and water may also have come from volatile-rich bodies from the outer solar system, such as comets, that struck the planet after it had formed. Countless comets may have bombarded Earth early in its history, contributing water, carbon dioxide, and gases to the early oceans and atmosphere. The early atmosphere lacked the oxygen that makes up 21 percent of the atmosphere today. Oxygen did not enter the atmosphere until oxygen-producing organisms evolved, as we will see in Chapter 11.
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