The outer shell
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Earth’s outermost, rigid, rocky layer is called the crust. It is composed of low-density, easily melted rocks; the continental crust is predominantly granitic rock (see granite), while composition of the oceanic crust corresponds mainly to that of basalt and gabbro. Analyses of seismic waves, generated by earthquakes within Earth’s interior, show that the crust extends about 50 km (30 miles) beneath the continents but only 5–10 km (3–6 miles) beneath the ocean floors.
At the base of the crust, a sharp change in the observed behaviour of seismic waves marks the interface with the mantle. The mantle is composed of denser rocks, on which the rocks of the crust float. On geologic timescales, the mantle behaves as a very viscous fluid and responds to stress by flowing. Together the uppermost mantle and the crust act mechanically as a single rigid layer, called the lithosphere.
The lithospheric outer shell of Earth is not one continuous piece but is broken, like a slightly cracked eggshell, into about a dozen major separate rigid blocks, or plates. There are two types of plates, oceanic and continental. An example of an oceanic plate is the Pacific Plate, which extends from the East Pacific Rise to the deep-sea trenches bordering the western part of the Pacific basin. A continental plate is exemplified by the North American Plate, which includes North America as well as the oceanic crust between it and a portion of the Mid-Atlantic Ridge. The latter is an enormous submarine mountain chain that extends down the axis of the Atlantic basin, passing midway between Africa and North and South America.
The lithospheric plates are about 60 km (35 miles) thick beneath the oceans and 100–200 km (60–120 miles) beneath the continents. (It should be noted that these thicknesses are defined by the mechanical rigidity of the lithospheric material. They do not correspond to the thickness of the crust, which is defined at its base by the discontinuity in seismic wave behaviour, as cited above.) They ride on a weak, perhaps partially molten, layer of the upper mantle called the asthenosphere. Slow convection currents deep within the mantle generated by radioactive heating of the interior drive lateral movements of the plates (and the continents on top of them) at a rate of several centimetres per year. The plates interact along their margins, and these boundaries are classified into three general types on the basis of the relative motions of the adjacent plates: divergent, convergent, and transform (or strike-slip).
In areas of divergence, two plates move away from each other. Buoyant upwelling motions in the mantle force the plates apart at rift zones (such as along the middle of the Atlantic Ocean floor), where magmas from the underlying mantle rise to form new oceanic crustal rocks.
Lithospheric plates move toward each other along convergent boundaries. When a continental plate and an oceanic plate come together, the leading edge of the oceanic plate is forced beneath the continental plate and down into the asthenosphere—a process called subduction. Only the thinner, denser slabs of oceanic crust will subduct, however. When two thicker, more buoyant continents come together at convergent zones, they resist subduction and tend to buckle, producing great mountain ranges. The Himalayas, along with the adjacent Plateau of Tibet, were formed during such a continent-continent collision, when India was carried into the Eurasian Plate by relative motion of the Indian-Australian Plate.
At the third type of plate boundary, the transform variety, two plates slide parallel to one another in opposite directions. These areas are often associated with high seismicity, as stresses that build up in the sliding crustal slabs are released at intervals to generate earthquakes. The San Andreas Fault in California is an example of this type of boundary, which is also known as a fault or fracture zone (see submarine fracture zone).
Most of Earth’s active tectonic processes, including nearly all earthquakes, occur near plate margins. Volcanoes form along zones of subduction, because the oceanic crust tends to be remelted as it descends into the hot mantle and then rises to the surface as lava. Chains of active, often explosive volcanoes are thus formed in such places as the western Pacific and the west coasts of the Americas. Older mountain ranges, eroded by weathering and runoff, mark zones of earlier plate-margin activity. The oldest, most geologically stable parts of Earth are the central cores of some continents (such as Australia, parts of Africa, and northern North America). Called continental shields, they are regions where mountain building, faulting, and other tectonic processes are diminished compared with the activity that occurs at the boundaries between plates. Because of their stability, erosion has had the time to flatten the topography of continental shields. It is also on the shields that geologic evidence of crater scars from ancient impacts of asteroids and comets is better-preserved. Even there, however, tectonic processes and the action of water have erased many ancient features. In contrast, much of the oceanic crust is substantially younger (tens of millions of years old), and none dates back more than 200 million years.
This conceptual framework in which scientists now understand the evolution of Earth’s lithosphere—termed plate tectonics—is almost universally accepted, although many details remain to be worked out. For example, scientists have yet to reach a general agreement as to when the original continental cores formed or how long ago modern plate-tectonic processes began to operate. Certainly the processes of internal convection, segregation of minerals by partial melting and recrystallization, and basaltic volcanism were operating more vigorously in the first billion years of Earth’s history, when the planet’s interior was much hotter than it is today; nevertheless, how the surface landmasses were formed and were dispersed may have been different.
Once major continental shields grew, plate tectonics was characterized by the cyclic assembly and breakup of supercontinents created by the amalgamation of many smaller continental cores and island arcs. Scientists have identified two such cycles in the geologic record. A supercontinent began breaking up about 700 million years ago, in late Precambrian time, into several major continents, but by about 250 million years ago, near the beginning of the Triassic Period, the continued drifting of these continents resulted in their fusion again into a single supercontinental landmass called Pangea. Some 70 million years later, Pangea began to fragment, gradually giving rise to today’s continental configuration. The distribution is still asymmetric, with continents predominantly located in the Northern Hemisphere opposite the Pacific basin.
Startlingly, of the four terrestrial planets, only Earth shows evidence of long-term, pervasive plate tectonics. Both Venus and Mars exhibit geology dominated by basaltic volcanism on a largely immovable crust, with only faint hints of possibly limited episodes of horizontal plate motion. Mercury is intrinsically much denser than the other terrestrial planets, which implies a larger metallic core; its surface is mostly covered with impact craters, but it also shows a global pattern of scarps suggesting shrinkage of the planet, associated perhaps with interior cooling. Apparently essential to the kind of plate tectonics that occurs on Earth are large planetary size (hence, high heat flow and thin crust), which eliminates Mars, and pervasive crustal water to soften the rock, which Venus lost very early in its history. Although Earth is indeed geologically active and hence possesses a youthful surface, Venus’s surface may have been completely renewed by global basaltic volcanism within the past billion years, and small portions of Mars’s surface may have experienced very recent erosion from liquid water or landslides.
The interior of Earth
More than 90 percent of Earth’s mass is composed of iron, oxygen, silicon, and magnesium, elements that can form the crystalline minerals known as silicates. Nevertheless, in chemical and mineralogical composition, as in physical properties, Earth is far from homogeneous. Apart from the superficial lateral differences near the surface (i.e., in the compositions of the continental and oceanic crusts), Earth’s principal differences vary with distance toward the centre. This is due to increasing temperatures and pressures and to the original segregation of materials, soon after Earth accreted from the solar nebula about 4.56 billion years ago, into a metal-rich core, a silicate-rich mantle, and the more highly refined crustal rocks. Earth is geochemically differentiated to a great extent (see below Planetary differentiation). Crustal rocks contain several times as much of the rock-forming element aluminum as does the rest of the solid Earth and many dozens of times as much uranium. On the other hand, the crust, which accounts for a mere 0.4 percent of Earth’s mass, contains less than 0.1 percent of its iron. Between 85 and 90 percent of Earth’s iron is concentrated in the core.
The increasing pressure with depth causes phase changes in crustal rocks at depths between 5 and 50 km (3 and 30 miles), which marks the top of the upper mantle, as mentioned above. This transition area is called the Mohorovic̆ić discontinuity, or Moho. Most basaltic magmas are generated in the upper mantle at depths of hundreds of kilometres. The upper mantle, which is rich in the olivine, pyroxene, and silicate perovskite minerals, shows significant lateral differences in composition. A large fraction of Earth’s interior, from a depth of about 650 km (400 miles) down to 2,900 km (1,800 miles), consists of the lower mantle, which is composed chiefly of magnesium- and iron-bearing silicates, including the high-pressure equivalents of olivine and pyroxene.
The mantle is not static but rather churns slowly in convective motions, with hotter material rising up and cooler material sinking; through this process, Earth gradually loses its internal heat. In addition to being the driving force of horizontal plate motion, mantle convection is manifested in the occurrence of temporary superplumes—huge, rising jets of hot, partially molten rock—which may originate from a deep layer near the core-mantle interface. Much larger than ordinary thermal plumes, such as that associated with the Hawaiian island chain in the central Pacific (see volcano: Intraplate volcanism), superplumes may have had profound effects on Earth’s geologic history and even on its climate. One outburst of global volcanism about 66 million years ago, which created the vast flood basalt deposits known as the Deccan Traps on the Indian subcontinent (see plateau), may have been associated with a superplume, though this model is far from universally accepted.
With a radius of almost 3,500 km (2,200 miles), Earth’s core is about the size of the entire planet Mars. About one-third of Earth’s mass is contained in the core, most of which is liquid iron alloyed with nickel and some lighter, cosmically abundant components (e.g., sulfur, oxygen, and, controversially, even hydrogen). Its liquid nature is revealed by the failure of shear-type seismic waves to penetrate the core. A small, central part of the core, however, below a depth of about 5,100 km (3,200 miles), is solid iron. This inner core is itself divided into two layers known only by the polarity differences of the iron crystals found within them. The polarity of the iron crystals of the innermost layer is oriented in an east-west direction, whereas that of the outermost layer is oriented north-south. Temperatures in the core are extremely hot, ranging from 4,000–5,000 K (roughly 6,700–8,500 °F; 3,700–4,700 °C) at the outer part of the core to 5,000–7,000 K (8,500–12,100 °F; 4,700–6,700 °C) in the centre, comparable to the surface of the Sun. Large uncertainties in temperature arise from questions as to which compounds form alloys with iron in the core, and more recent data favour the lower end of the temperature estimates for the inner core. The core’s reservoir of heat may contribute as much as one-fifth of all the internal heat that ultimately flows to the surface of Earth. The basic structure of Earth—crust, mantle, and core—appears to be replicated on the other terrestrial planets, though with substantial variations in the relative size of each region.
The geomagnetic field and magnetosphere
Helical fluid motions in Earth’s electrically conducting liquid outer core have an electromagnetic dynamo effect, giving rise to the geomagnetic field. The planet’s sizable, hot core, along with its rapid spin, probably accounts for the exceptional strength of the magnetic field of Earth compared with those of the other terrestrial planets. Venus, for example, which has a metallic core that may be similar to Earth’s in size, rotates very slowly and has no detected intrinsic magnetic field. Mercury and Mars have only small intrinsic magnetic fields.
Earth’s main magnetic field permeates the planet and an enormous volume of space surrounding it. A great teardrop-shaped region of space called the magnetosphere is formed by the interaction of Earth’s field with the solar wind. At a distance of about 65,000 km (40,000 miles) outward toward the Sun, the pressure of the solar wind is balanced by the geomagnetic field. This serves as an obstacle to the solar wind, and the flow of charged particles, or plasma, is deflected around Earth by the resulting bow shock. The magnetosphere so produced streams out into an elongated magnetotail that stretches several million kilometres downstream from Earth away from the Sun.
Plasma particles from the solar wind can leak through the magnetopause, the sunward boundary of the magnetosphere, and populate its interior; charged particles from the Earth’s ionosphere also enter the magnetosphere. The magnetotail can store for hours an enormous amount of energy—several billion megajoules, which is roughly equivalent to the yearly electricity production of many smaller countries). This occurs through a process called reconnection, in which the Sun’s magnetic field, dragged into interplanetary space by the solar wind, becomes linked with the magnetic field in Earth’s magnetosphere. The energy is released in dynamic structural reconfigurations of the magnetosphere, called geomagnetic substorms, which often result in the precipitation of energetic particles into the ionosphere, giving rise to fluorescing auroral displays.
Converging magnetic field lines fairly close to Earth can trap highly energetic particles so that they gyrate between the Northern and Southern hemispheres and slowly drift longitudinally around the planet in two concentric doughnut-shaped zones known as the Van Allen radiation belts. Many of the charged particles trapped in these belts are produced when energetic cosmic rays strike Earth’s upper atmosphere, producing neutrons that then decay into electrons, which are negatively charged, and protons, which are positively charged. Others come from the solar wind or Earth’s atmosphere. The inner radiation belt was detected in 1958 by the American physicist James Van Allen and colleagues, using a Geiger-Müller counter aboard the first U.S. satellite, Explorer 1; the outer belt was distinguished by other U.S. and Soviet spacecraft launched the same year. Earth’s magnetosphere has been extensively studied ever since, and space physicists have extended their studies of plasma processes to the vicinities of comets and other planets. (For additional information on the interaction of the Sun and Earth’s charged particles and magnetic fields, see plasma: Solar-terrestrial forms.)
An important characteristic of Earth’s magnetic field is polarity reversal. In this process the direction of the dipole component reverses—i.e., the north magnetic pole becomes the south magnetic pole and vice versa. From studying the direction of magnetization of many rocks, geologists know that such reversals occur, without a discernible pattern, at intervals that range from tens of thousands of years to millions of years, though they are still uncertain about the mechanisms responsible. It is likely that during the changeover, which is believed to take a few thousand years, a nondipolar field remains, at a small fraction of the strength of the normal field. In the temporary absence of the dipole component, the solar wind would approach much closer to Earth, allowing particles that are normally deflected by the field or are trapped in its outer portions to reach the surface. The increase in particle radiation could lead to increased rates of genetic damage and thus of mutations or sterility in plants and animals, leading to the disappearance of some species. Scientists have looked for evidence of such changes in the fossil record at times of past field reversals, but the results have been inconclusive.
Clark R. Chapman Jonathan I. Lunine