Introducing the Earth’s Interior
At this point, we leave our fantasy space voyage and turn our attention inward to the materials that make up the solid Earth, because we need to be aware of these before we can discuss the architecture of the Earth’s interior. Let’s begin by reiterating that the Earth consists mostly of elements produced by fusion reactions in stars and by supernova explosions. Only four elements (iron, oxygen, silicon, and magnesium) make up 91.2% of the Earth’s mass; the remaining 8.8% consists of the other 88 elements (figure above). The elements of the Earth comprise a great variety of materials.
- Organic chemicals. Carbon-containing compounds that either occur in living organisms or have characteristics that resemble compounds in living organisms are called organic chemicals.
- Minerals. A solid, natural substance in which atoms are arranged in an orderly pattern is a mineral. A single coherent sample of a mineral that grew to its present shape is a crystal, whereas an irregularly shaped sample, or a fragment derived from a once-larger crystal or cluster of crystals, is a grain.
- Glasses. A solid in which atoms are not arranged in an orderly pattern is called glass.
- Rocks. Aggregates of mineral crystals or grains, or masses of natural glass, are called rocks. Geologists recognize three main groups of rocks. (1) Igneous rocks develop when hot molten (liquid) rock cools and freezes solid. (2) Sedimentary rocks form from grains that break off pre-existing rock and become cemented together, or from minerals that precipitate out of a water solution. (3) Metamorphic rocks form when pre-existing rocks change in response to heat and pressure.
- Sediment. An accumulation of loose mineral grains (grains that have not stuck together) is called sediment.
- Metals. A solid composed of metal atoms (such as iron, aluminium, copper, and tin) is called a metal. An alloy is a mixture containing more than one type of metal atom.
- Melts. A melt forms when solid materials become hot and transform into liquid. Molten rock is a type of melt geologists distinguish between magma, which is molten rock beneath the Earth’s surface, and lava, molten rock that has flowed out onto the Earth’s surface.
- Volatiles. Materials that easily transform into gas at the relatively low temperatures found at the Earth’s surface are called volatiles.
The most common minerals in the Earth contain silica (a compound of silicon and oxygen) mixed in varying proportions with other elements. These minerals are called silicate minerals. Not surprisingly, rocks composed of silicate minerals are silicate rocks. Geologists distinguish four classes of igneous silicate rocks based, in essence, on the proportion of silica to iron and magnesium. In order, from greatest to least proportion of silica to iron and magnesium, these classes are felsic (or silicic), intermediate, maﬁc, and ultramaﬁc. As the proportion of silica in a rock increases, the density (mass per unit volume) decreases. Thus, felsic rocks are less dense than maﬁc rocks. For now, we introduce the four rock types whose names we need to know for our discussion of the Earth’s layers that follows. These are
- granite, a felsic rock with large grains;
- gabbro, a maﬁc rock with large grains;
- basalt, a maﬁc rock with small grains; and
- peridotite, an ultramaﬁc rock with large grains.
People have speculated about what’s inside our planet since ancient times. What is the source of incandescent lavas that spew from volcanoes, of precious gems and metals found in mines, of sparkling mineral waters that bubble from springs, and of the mysterious forces that shake the ground and topple buildings? In ancient Greece and Rome, the subsurface was the underworld, Hades, home of the dead, a region of ﬁre and sulphurous fumes. Perhaps this image was inspired by the molten rock and smoke emitted by the volcanoes of the Mediterranean region. In the 18th and 19th centuries, European writers thought the Earth’s interior resembled a sponge, containing open caverns variously ﬁlled with molten rock, water, or air. In fact, in the popular 1864 novel Journey to the Centre of the Earth, by the French author Jules Verne, three explorers hike through interconnected caverns down to the Earth’s centre.
How can we explore the interior for real? We can’t dig or drill down very far. Indeed, the deepest mine penetrates only about 3.5 km beneath the surface of South Africa. And the deepest drill hole probes only 12 km below the surface of northern Russia compared with the 6,371 km radius of the Earth, this hole makes it less than 0.2% of the way to the centre and is nothing more than a pinprick. Our modern image of the Earth’s interior, one made up of distinct layers, is the end product of many discoveries made during the past 200 years.
The ﬁrst clue that led away from Jules Verne’s sponge image came when researchers successfully measured the mass of the whole Earth, and from this information derived its average density. They found that the average density of our planet far exceeds the density of common rocks found on the surface. Thus, the interior of the Earth must contain denser material than its outermost layer and can’t possibly be full of holes. In fact, the mass of the Earth overall is so great that the planet must contain a large amount of metal. Since the Earth is close to being a sphere, the metal must be concentrated near the centre. Otherwise, centrifugal force due to the spin of the Earth on its axis would pull the equator out, and the planet would become a disk. (To picture why, consider that when you swing a hammer, your hand feels more force if you hold the end of the light wooden shaft, rather than the heavy metal head.) Finally, researchers realized that, though molten rock occasionally oozes out of the interior at volcanoes, the interior must be mostly solid, because if it weren't, the land surface would rise and fall due to tidal forces much more than it does.
|An early image of Earth’s internal layers.|
Eventually, researchers concluded that the Earth resembled a hard-boiled egg, in that it had three principal layers: a not-so-dense crust (like an eggshell, composed of rocks such as granite, basalt, and gabbro), a denser solid mantle in the middle (the “white,” composed of a then-unknown material), and a very dense core (the “yolk,” composed of an unknown metal) (figure above a, b). Clearly, many questions remained. How thick are the layers? Are the boundaries between layers sharp or gradational? And what exactly are the layers composed of?
Clues from the Study of Earthquakes: Reﬁning the Image
|Faulting and earthquakes.|
When rock within the outer portion of the Earth suddenly breaks and slips along a fracture called a fault, it generates shock waves (abrupt vibrations), called seismic waves, that travel through the surrounding rock outward from the break. Where these waves cause the surface of the Earth to vibrate, people feel an earthquake, an episode of ground shaking. You can simulate this process, at a small scale, when you break a stick between your hands and feel the snap with your hands (figure above a, b).
In the late 19th century, geologists learned that earthquake energy could travel, in the form of waves, all the way through the Earth’s interior from one side to the other. Geologists immediately realized that the study of earthquake waves travelling through the Earth might provide a tool for exploring the Earth’s insides, much as ultrasound today helps doctors study a patient’s insides. Speciﬁcally, laboratory measurements demonstrated that earthquake waves travel at different velocities (speeds) through different materials. Thus, by detecting depths at which velocities suddenly change, geoscientists pinpointed the boundaries between layers and even recognized subtler boundaries within layers. For example, such studies led geoscientists to subdivide the mantle into the upper mantle and lower mantle, and subdivide the core into the inner core and outer core.
Pressure and Temperature Inside the Earth
In order to keep underground tunnels from collapsing under the pressure created by the weight of overlying rock, mining engineers must design sturdy support structures. It is no surprise that deeper tunnels require stronger supports: the downward push from the weight of overlying rock increases with depth, simply because the mass of the overlying rock layer increases with depth. In solid rock, the pressure at a depth of 1 km is about 300 atm. At the Earth’s centre, pressure probably reaches about 3,600,000 atm.
Temperature also increases with depth in the Earth. Even on a cool winter’s day, miners who chisel away at gold veins exposed in tunnels 3.5 km below the surface swelter in temperatures of about 53°C (127°F). We refer to the rate of change in temperature with depth as the geothermal gradient. In the upper part of the crust, the geothermal gradient averages between 20°C and 30°C per km. At greater depths, the rate decreases to 10°C per km or less. Thus, 35 km below the surface of a continent, the temperature reaches 400°C to 700°C, and the mantle-core boundary is about 3,500°C. No one has ever directly measured the temperature at the Earth’s centre, but calculations suggest it may exceed 4,700°C, close to the Sun’s surface temperature of 5,500°C.