If we could slice Earth in half, the first thing we would notice is that it has distinct layers. The heaviest materials (metals) are in the center. Less dense solids (rocks) are in the middle, and lighter liquids (mainly water) and gases are on top. Within Earth we know these layers as the iron-rich core, the rocky mantle and crust, the liquid ocean, and the gaseous atmosphere.

There are also variations in Earth’s composition and temperature with depth, which indicate that the interior of our planet is very dynamic. The rocks of the mantle and crust are in constant, if very slow, motion. In addition to plate tectonics, these motions cause continuous recycling between the surface and the deep interior. Furthermore, it is from Earth’s deep interior that the water and air of our oceans and atmosphere are replenished, allowing life to exist at the surface.

According to the nebular theory, Earth and the solar system began to form nearly  billion years ago due to the gravitational collapse of a huge cloud of dust and gases called a nebula. As material accumulated to form Earth (and for a short period afterward), the high-velocity impact of nebular debris and the decay of radioactive elements caused the temperature of our planet to increase steadily. During this time of intense heating, Earth became hot enough to melt iron and nickel. Melting produced dense blobs of liquid metal that sank toward the center of the planet. This process occurred rapidly on the scale of geologic time and produced Earth’s dense iron-rich core.

The early period of heating resulted in another process, called chemical differentiation, whereby melting formed buoyant masses of molten rock that rose toward Earth’s surface and solidified to produce a primitive crust. These rocky materials were rich in oxygen and “oxygen-seeking” elements, particularly silicon and aluminum, along with lesser amounts of calcium, sodium, potassium, iron, and magnesium. In addition, some heavy metals, such as gold, lead, and uranium, which have low melting points or were highly soluble in the ascending molten masses, were scavenged from Earth’s interior and concentrated in the developing crust. This early period of chemical differentiation established the three basic divisions of Earth’s interior: (1) the thin primitive crust, (2) the iron-rich core, and (3) Earth’s largest layer, called the mantle, which is located between the crust and core ▼.

Probing Earth’s Interior:
“Seeing” Seismic Waves

How do we know about the structure and properties of Earth’s deep interior? Because light does not travel through rock, we must find other ways to “see” into our planet. The best way to learn about Earth’s interior is to dig or drill a hole and examine it directly. Unfortunately, this is possible only at shallow depths. The deepest a drilling rig has ever penetrated is only 12.3 kilometers (7.6 miles), which is about 1/500 of the way to Earth’s center. This was an extraordinary accomplishment because temperature and pressure increase rapidly with depth.

Instead of drilling, much of what we know about Earth’s interior comes from detailed studies of seismic waves generated by large earthquakes. About 3000 earthquakes occur each year that are large enough (about MW 5.5) to travel all the way through Earth and be recorded by seismographs on the other side of the globe ▼. The P and S waves from these large earthquakes can be used to “see” into our planet in much the same way that medical technology uses ultrasound waves to generate sonograms.

Using the waves recorded on seismograms to visualize Earth’s interior structure is challenging. Seismic waves do not travel along straight paths; instead, they are reflected, refracted, and diffracted as they pass through our planet. They reflect (bounce) off boundaries between different layers, they refract (change direction) when passing from one layer to another layer, and they diffract (follow a curved path) around obstacles they encounter. These different wave behaviors have been used to identify the boundaries that exist within Earth.

One of the most noticeable behaviors of seismic waves is that they follow strongly curved paths (refer to Figure 5.40). Seismic waves also travel faster when rock is stiffer or less compressible. These properties of stiffness and compressibility can be used to interpret the composition and temperature of the rock. For instance, when rock is hotter, it becomes less stiff (imagine a chocolate bar left out in the Sun), and waves travel through it more slowly. Waves also travel at different speeds through rocks of different compositions. Thus, the speed at which seismic waves travel can help determine the types of rocks located in Earth’s interior, as well as how hot they are. Also, laboratory studies mimic Earth’s interior conditions of immense heat and pressure on rock to allow scientists to better understand the properties of minerals.

Let’s now look in more detail at how Earth’s three compositionally distinct layers—the crust, mantle, and core—can be further subdivided into zones based on physical properties. The physical properties used to define such regions include whether the layer is solid or liquid and how weak or strong it is. Knowledge of both the chemical composition and physical properties of Earth’s layers is essential to our understanding of basic geologic processes, such as volcanism, earthquakes, and mountain building ▼.

Earth’s Crust

The crust is Earth’s relatively thin, rocky outer skin. The two types of crust—continental and oceanic—have very different compositions, histories, and ages. In fact, oceanic crust is compositionally more similar to Earth’s mantle than to its continental crust.

Earth’s oceanic crust, which forms along mid-ocean ridges, is about 7 kilometers (4 miles) thick. The rocks of the oceanic crust are younger (about 180 million years old or less) and denser than continental rocks. Oceanic crust has a density of about 3.0 g/cm3 and is composed of the dark igneous rocks basalt and gabbro.

Unlike oceanic crust, which has a relatively homogeneous chemical composition, continental crust consists of many rock types. Although the upper crust has an average composition of a granitic rock, called granodiorite, its composition and structure vary considerably from place to place.

Continental crust averages about 40 kilometers (25 miles) thick but can be more than 70 kilometers (40 miles) thick in mountainous regions, such as the Himalayas and the Andes. It has an average density of about 2.7 g/cm3 which is much lower than the density of mantle rock. The low density of the continents relative to the mantle explains why continents are buoyant—acting like giant rafts, floating atop the mantle—and why they cannot be readily subducted into the mantle. The discovery of continental rocks more than  billion years old provides evidence that continental rocks cannot be easily recycled into the mantle.

Earth’s Mantle

More than 82 percent of Earth’s volume is contained in the mantle, a solid, rocky shell that extends to a depth of about 2900 kilometers (1800 miles) beneath Earth’s crust. The boundary between the crust and mantle represents a marked change in chemical composition. The dominant rock type in the uppermost mantle is peridotite, which is richer in the metals iron and magnesium than the rocks found in either the continental or oceanic crust.

The upper mantle extends from the crust–mantle boundary down to a depth of about 660 kilometers (410 miles). Recall that the upper mantle is divided into two different parts. The top portion of the upper mantle, along with the Earth’s crust, makes up the stiff lithosphere. The lower unit of the upper mantle contains the weak asthenosphere.

The lithosphere (“sphere of rock”) forms Earth’s relatively cool, rigid outer shell that is broken into sections of various sizes, called tectonic plates. The lithosphere is more than 250 kilometers (155 miles) thick below the oldest portions of the continents and about 100 kilometers (60 miles) thick below the seafloor ▼.

Beneath Earth’s lithosphere is a solid, but comparatively weak, layer termed the asthenosphere (“weak sphere”). Because the asthenosphere and lithosphere are mechanically detached from each other, the lithosphere is able to move independently of the asthenosphere.

From 660 kilometers (410 miles) deep to the top of the core, at a depth of 2900 kilometers (1800 miles), is the lower mantle. Because the pressure in the mantle increases with depth (due to the weight of the overlying rock), the mantle becomes stronger with depth. Despite their strength, the rocks in the lower mantle are extremely hot, and as a result, they are capable of very gradual flow.

Inner and Outer Core

The core is thought to consist mainly of iron combined with an unknown quantity of nickel, as well as minor amounts of oxygen, silicon, and sulfur—elements that readily form compounds with iron. Because of the extreme pressure found in the core, this iron-rich material has an average density of more than 10 times the density of water, or 10 g/cm3.

The outer core is a liquid, iron-rich layer 2770 kilometers (1410 miles) thick. The liquid nature of the outer core was discovered when researchers found that S waves do not penetrate the outer core. Scientists concluded that because S waves do not pass through liquids, and S waves do not pass through the outer core, Earth’s outer core must be liquid.

At Earth’s center lies the inner core, a solid dense metallic sphere with a radius of 1216 kilometers (755 miles). Despite its higher temperature, the inner core is solid, not molten like the outer core, due to the immense pressures that exist at the center of our planet. The density at Earth’s center is about 13 times that of water, or 13 g/cm3.

• The layered internal structure of Earth developed due to gravitational sorting of Earth materials early in the history of the planet. The densest material settled to form Earth’s core, while the least dense material rose to form Earth’s crust, oceans, and atmosphere.

• Seismic waves allow geoscientists to “look into” Earth’s interior. Like sonograms used to image human organs, seismic waves generated by large earthquakes reveal details about Earth’s layered structure.

• Earth has two distinct kinds of crust: oceanic and continental. Oceanic crust is thinner, denser, and younger than continental crust. Oceanic crust also readily subducts, whereas the less dense continental crust does not.

• The uppermost mantle and crust make up Earth’s rigid outer shell, called the lithosphere, which overlies the asthenosphere—a solid but relatively weak layer. The lower mantle is a strong solid layer but capable of very gradual flow.

• Earth’s core is very dense and composed of a mixture of iron and nickel, with minor amounts of lighter elements. The outer core is liquid, whereas the inner core is solid.

• asthenosphere: A subdivision of the mantle situated below the lithosphere. This zone of weak material exists below a depth of about 100 kilometers (60 miles) and in some regions extends as deep as 700 kilometers (430 miles). The rock within this zone is easily deformed.

• core: The innermost layer of Earth, located beneath the mantle. The core is divided into an outer core and an inner core.

• crust: The very thin, rocky, outermost layer of Earth.

• inner core: The solid innermost layer of Earth, about 1300 kilometers (800 miles) in radius.

• lithosphere: The rigid outer layer of Earth, including the crust and upper mantle.

• mantle: The 2900-kilometer- (1800-mile-) thick layer of Earth located below the crust and above the core.

• outer core: A layer beneath the mantle about 2200 kilometers (1364 miles) thick that has the properties of a liquid.