Lithosphere

Lithosphere - outer part of Earth, is consisted of the crust and the upper mantle. The term

lithosphere is derived from Greek λιθος and ζθαιρα which means ―rock layer.‖ The lithosphere

is approximately 65 to 100 km (40 to 60 mi) thick and lies above the layer known as the

asthenosphere, which consists of softer, less rigid rocky material. Geologists regard the

lithosphere as the relatively cool, outermost layer of the planet and view it as a rigid shell.

Although the rock compositions of the crust and the upper mantle differ, geologists prefer to

view the two parts as a single unit because both are brittle and they behave as a single mass in

the motion of the rocky plates that make up Earth’s surface layer.

Earth scientists distinguished the lithosphere from the underlying asthenosphere by studying seismic waves. Seismic waves originate from the movement of rock masses during an earthquake. The speed with

which seismic waves travel through Earth’s interior depends on the nature and density of the rocks

beneath the surface. Scientists have analyzed hundreds of seismic waves and found that such waves abruptly slow down at a depth of about 100 km (60 mi). At this depth lies the boundary between the

lithosphere and the underlying, softer asthenosphere. The plastic nature of the asthenosphere causes

seismic waves to slow down in this layer.

The crust is the outermost part of the lithosphere. Underlying the crust is the upper mantle—a relatively

brittle part of the earth’s interior. The rocks of the crust include granite and basalt. Granite makes up the crust of the continents, while basalt makes up the crust of the ocean floor. The rocks found in the crust

consist mostly of lighter elements such as silicon, potassium, and sodium. The density of these rocks is

about three times that of water. The rocks of the upper mantle are denser. Rocks found in the upper mantle include peridotite, a dark-colored rock composed of the minerals olivine and pyroxene. These

minerals contain the heavier elements iron and magnesium.

The lithosphere has been at the center of research in plate tectonics, one of the 20th century’s most

revolutionary scientific theories. This theory explains that the lithosphere consists of several large and

small tectonic plates, or thin, brittle pieces that move against one another. These plates are fragments of the lithosphere that are packed together, similar to pieces of a giant jigsaw puzzle. The tectonic plates

move at about the same speed at which fingernails grow. This rate may not seem to be significant, but on

a geological time scale of millions of years; such movements may shift entire continents all over the

earth’s surface.

The motion of the lithospheric plates results from their attachment to the plastic, slowly flowing

asthenosphere. Where two plates are in contact, three different types of boundaries (called plate

boundaries) are possible. A convergent boundary exists between two plates that are moving toward each

other. A divergent boundary exists between plates that are moving apart from each other (as in the middle of the Atlantic Ocean). A transform boundary refers to the boundary between two plates that are sliding

past each other (as in the region of the San Andreas Fault in California).

Most violent geologic events, such as earthquakes and volcanic eruptions, occur at plate boundaries where

two plates are slowly colliding or rubbing against each other. Indeed, the distribution of most of the major regions of the world that are prone to earthquakes and eruptions closely follows the boundaries between

the lithospheric plates.

Formation of Rocks

Minerals Mineral, in general, any naturally occurring chemical element or compound, but in mineralogy

and geology, chemical elements and compounds that have been formed through inorganic

processes. Petroleum and coal, which are formed by the decomposition of organic matter, are not

minerals in the strict sense. More than 3000 mineral species are known, most of which are

characterized by definite chemical composition, crystalline structure, and physical properties.

They are classified primarily by chemical composition, crystal class, hardness, and appearance

(color, luster, and opacity). Mineral species are, as a rule, limited to solid substances, the only

liquids being metallic mercury and water. All the rocks forming the earth's crust consist of

minerals. Metalliferous minerals of economic value, which are mined for their metals, are known

as ores.

Kinds of Rocks Igneous Rocks

Sedimentary Rocks

Metamorphic Rocks

Igneous Rocks

Igneous rock - any of various crystalline or glassy rocks formed by the cooling and solidification

of molten earth material. Igneous rocks comprise one of the three principal classes of rocks, the

others being metamorphic and sedimentary.

Igneous rocks are formed from the solidification of magma, which is a hot (600° to 1,300° C, or

1,100° to 2,400° F) molten or partially molten rock material. The Earth is composed

predominantly of a large mass of igneous rock with a very thin veneer of weathered material—

namely, sedimentary rock. Whereas sedimentary rocks are produced by processes operating

mainly at the Earth's surface by the disintegration of mostly older igneous rocks, igneous—and

metamorphic—rocks are formed by internal processes that cannot be directly observed and that

necessitate the use of physical-chemical arguments to deduce their origins. Because of the high

temperatures within the Earth, the principles of chemical equilibrium are applicable to the study

of igneous and metamorphic rocks, with the latter being restricted to those rocks formed without

the direct involvement of magma.

Magma is thought to be generated within the plastic asthenosphere (the layer of partially molten

rock underlying the Earth's crust) at a depth below about 60 kilometres (40 miles). Because

magma is less dense than the surrounding solid rocks, it rises toward the surface. It may settle

within the crust or erupt at the surface from a volcano as a lava flow. Rocks formed from the

cooling and solidification of magma deep within the crust are distinct from those erupted at the

surface mainly owing to the differences in physical and chemical conditions prevalent in the two

environments. Within the Earth's deep crust the temperatures and pressures are much higher than

at its surface; consequently, the hot magma cools slowly and crystallizes completely, leaving no

trace of the liquid magma. The slow cooling promotes the growth of minerals large enough to be

identified visually without the aid of a microscope (called phaneritic, from the Greek θανερος,

visible). On the other hand, magma erupted at the surface is chilled so quickly that the individual

minerals have little or no chance to grow. As a result, the rock is either composed of minerals

that can be seen only with the aid of a microscope (called aphanitic, from the Greek αθανης,

invisible) or contains no minerals at all (in the latter case, the rock is composed of glass, which is

a highly viscous liquid). These results in two groups: (1) plutonic intrusive igneous rocks that

solidified deep within the crust and (2) volcanic, or extrusive, igneous rocks formed at the Earth's

surface. Some intrusive rocks, known as subvolcanic, were not formed at great depth but were

instead injected near the surface where lower temperatures result in a more rapid cooling process;

these tend to be aphanitic and are referred to as hypabyssal intrusive rocks.

The deep-seated plutonic rocks can be exposed at the surface for study only after a long period of

denudation or by some tectonic forces that push the crust upward or by a combination of the two

conditions. (Denudation is the wearing away of the terrestrial surface by processes including

weathering and erosion.) Generally, the intrusive rocks have cross-cutting contacts with the

country rocks that they have invaded, and in many cases the country rocks show evidence of

having been baked and thermally metamorphosed at these contacts. The exposed intrusive rocks

are found in a variety of sizes, from small veinlike injections to massive dome-shaped batholiths,

which extend for more than 100 square kilometres (40 square miles) and make up the cores of

the great mountain ranges.

Extrusive rocks occur in two forms: (1) as lava flows that flood the land surface much like a river

and (2) as fragmented pieces of magma of various sizes (pyroclastic materials), which often are

blown through the atmosphere and blanket the Earth's surface upon settling. The coarser

pyroclastic materials accumulate around the erupting volcano, but the finest pyroclasts can be

found as thin layers located hundreds of kilometres from the opening. Most lava flows do not

travel far from the volcano, but some low-viscosity flows that erupted from long fissures have

accumulated in thick (hundreds of metres) sequences, forming the great plateaus of the world

(e.g., the Columbia River plateau of Washington and Oregon and the Deccan Plateau in India).

Both intrusive and extrusive magmas have played a vital role in the spreading of the ocean basin,

in the formation of the oceanic crust, and in the formation of the continental margins. Igneous

processes have been active since the formation of the Earth some 4.6 billion years ago. Their

emanations have provided the water for the oceans, the gases for the primordial oxygen-free

atmosphere, and many valuable mineral deposits.

Sedimentary Rocks

Sedimentary rock - rock formed at or near the Earth's surface by the accumulation and

lithification of sediment (detrital rock) or by the precipitation from solution at normal surface

temperatures (chemical rock). Sedimentary rocks are the most common rocks exposed on the

Earth's surface but are only a minor constituent of the entire crust, which is dominated by

igneous and metamorphic rocks.

Sedimentary rocks are produced by the weathering of pre-existing rocks and the subsequent

transportation and deposition of the weathering products. Weathering refers to the various

processes of physical disintegration and chemical decomposition that occur when rocks at the

Earth's surface are exposed to the atmosphere (mainly in the form of rainfall) and the

hydrosphere. These processes produce soil, unconsolidated rock detritus, and components

dissolved in groundwater and runoff. Erosion is the process by which weathering products are

transported away from the weathering site, either as solid material or as dissolved components,

eventually to be deposited as sediment. Any unconsolidated deposit of solid weathered material

constitutes sediment. It can form as the result of deposition of grains from moving bodies of

water or wind, from the melting of glacial ice, and from the downslope slumping (sliding) of

rock and soil masses in response to gravity, as well as by precipitation of the dissolved products

of weathering under the conditions of low temperature and pressure that prevail at or near the

surface of the Earth.

Sedimentary rocks are the lithified equivalents of sediments. They typically are produced by

cementing, compacting, and otherwise solidifying pre-existing unconsolidated sediments. Some

varieties of sedimentary rock, however, are precipitated directly into their solid sedimentary form

and exhibit no intervening existence as sediment. Organic reefs and bedded evaporites are

examples of such rocks. Because the processes of physical (mechanical) weathering and

chemical weathering are significantly different, they generate markedly distinct products and two

fundamentally different kinds of sediment and sedimentary rock: (1) terrigenous clastic

sedimentary rocks and (2) allochemical and orthochemical sedimentary rocks.

Clastic terrigenous sedimentary rocks consist of rock and mineral grains, or clasts, of varying

size, ranging from clay-, silt-, and sand- up to pebble-, cobble-, and boulder-size materials. These

clasts are transported by gravity, mudflows, running water, glaciers, and wind and eventually are

deposited in various settings (e.g., in desert dunes, on alluvial fans, across continental shelves,

and in river deltas). Because the agents of transportation commonly sort out discrete particles by

clast size, terrigenous clastic sedimentary rocks are further subdivided on the basis of average

clast diameter. Coarse pebbles, cobbles, and boulder-size gravels lithify to form conglomerate

and breccia; sand becomes sandstone; and silt and clay form siltstone, claystone, mudrock, and

shale.

Chemical sedimentary rocks form by chemical and organic reprecipitation of the dissolved

products of chemical weathering that are removed from the weathering site. Allochemical

sedimentary rocks, such as many limestones and cherts, consist of solid precipitated nondetrital

fragments (allochems) that undergo a brief history of transport and abrasion prior to deposition

as nonterrigenous clasts. Examples are calcareous or siliceous shell fragments and oöids, which

are concentrically layered spherical grains of calcium carbonate. Orthochemical sedimentary

rocks, on the other hand, consist of dissolved constituents that are directly precipitated as solid

sedimentary rock and thus do not undergo transportation. Orthochemical sedimentary rocks

include some limestones, bedded evaporite deposits of halite, gypsum, and anhydrite, and banded

iron formations.

Sediments and sedimentary rocks are confined to the Earth's crust, which is the thin, light outer

solid skin of the Earth ranging in thickness from 40–100 kilometres (25 to 62 miles) in the

continental blocks to 4–10 kilometres in the ocean basins. Igneous and metamorphic rocks

constitute the bulk of the crust. The total volume of sediment and sedimentary rocks can be

either directly measured using exposed rock sequences, drill-hole data, and seismic profiles or

indirectly estimated by comparing the chemistry of major sedimentary rock types to the overall

chemistry of the crust from which they are weathered. Both methods indicate that the Earth's

sediment-sedimentary rock shell forms only about 5 percent by volume of the terrestrial crust,

which in turn accounts for less than 1 percent of the Earth's total volume. On the other hand, the

area of outcrop and exposure of sediment and sedimentary rock comprises 75 percent of the land

surface and well over 90 percent of the ocean basins and continental margins. In other words,

80–90 percent of the surface area of the Earth is mantled with sediment or sedimentary rocks

rather than with igneous or metamorphic varieties. The sediment-sedimentary rock shell forms

only a thin superficial layer. The mean shell thickness in continental areas is 1.8 kilometres; the

sediment shell in the ocean basins is roughly 0.3 kilometre. Rearranging this shell as a globally

encircling layer (and depending on the raw estimates incorporated into the model), the shell

thickness would be roughly 1–3 kilometres.

Despite the relatively insignificant volume of the sedimentary rock shell, not only are most rocks

exposed at the terrestrial surface of the sedimentary variety, but many of the significant events in

Earth history are most accurately dated and documented by analyzing and interpreting the

sedimentary rock record instead of the more voluminous igneous and metamorphic rock record.

When properly understood and interpreted, sedimentary rocks provide information on ancient

geography, termed paleogeography. A map of the distribution of sediments that formed in

shallow oceans along alluvial fans bordering rising mountains or in deep, subsiding ocean

trenches will indicate past relationships between seas and landmasses. An accurate interpretion

of paleogeography and depositional settings allows conclusions to be made about the evolution

of mountain systems, continental blocks, and ocean basins, as well as about the origin and

evolution of the atmosphere and hydrosphere. Sedimentary rocks contain the fossil record of

ancient life-forms that enables the documentation of the evolutionary advancement from simple

to complex organisms in the plant and animal kingdoms. Also, the study of the various folds or

bends and breaks or faults in the strata of sedimentary rocks permits the structural geology or

history of deformation to be ascertained.

Finally, it is appropriate to underscore the economic importance of sedimentary rocks. For

example, they contain essentially the world's entire store of oil and natural gas, coal, phosphates,

salt deposits, groundwater, and other natural resources.

Several subdisciplines of geology deal specifically with the analysis, interpretation, and origin of

sediments and sedimentary rocks. Sedimentary petrology is the study of their occurrence,

composition, texture, and other overall characteristics, while sedimentology emphasizes the

processes by which sediments are transported and deposited. Sedimentary petrography involves

the classification and study of sedimentary rocks using the petrographic microscope. Stratigraphy

covers all aspects of sedimentary rocks, particularly from the perspective of their age and

regional relationships as well as the correlation of sedimentary rocks in one region with

sedimentary rock sequences elsewhere.

Metamorphic Rocks

Metamorphic rock - any of a class of rocks that result from the alteration of pre-existing rocks in

response to changing environmental conditions, such as variations in temperature, pressure, and

mechanical stress, and the addition or subtraction of chemical components. The pre-existing

rocks may be igneous, sedimentary, or other metamorphic rocks.

The word metamorphism is taken from the Greek for ―change of form‖; metamorphic rocks are

derived from igneous or sedimentary rocksthat have altered their form (recrystallized) as a result

of changes in their physical environment. Metamorphism comprises changes both in mineralogy

and in the fabric of the original rock. In general, these alterations are brought about either by the

intrusion of hot magma into cooler surrounding rocks (contact metamorphism) or by large-scale

tectonic movements of the Earth's lithospheric plates that alter the pressure-temperature

conditions of the rocks (regional metamorphism). Minerals within the original rock, or protolith,

respond to the changing conditions by reacting with one another to produce a new mineral

assemblage that is thermodynamically stable under the new pressure-temperature conditions.

These reactions occur in the solid state but may be facilitated by the presence of a fluid phase

lining the grain boundaries of the minerals. In contrast to the formation of igneous rocks,

metamorphic rocks do not crystallize from a silicate melt, although high-temperature

metamorphism can lead to partial melting of the host rock.

Because metamorphism represents a response to changing physical conditions, those regions of

the Earth's surface where dynamic processes are most active will also be regions where

metamorphic processes are most intense and easily observed. The vast region of the Pacific

margin, for example, with its seismic and volcanic activity, is also an area in which materials are

being buried and metamorphosed intensely. In general, the margins of continents and regions of

mountain building are the regions where metamorphic processes proceed with intensity. But in

relatively quiet places, where sediments accumulate at slow rates, less spectacular changes also

occur in response to changes in pressure and temperature conditions. Metamorphic rocks are

therefore distributed throughout the geologic column.

Because most of the Earth's mantle is solid, metamorphic processes may also occur there. Mantle

rocks are seldom observed at the surface because they are too dense to rise, but occasionally a

glimpse is presented by their inclusion in volcanic materials. Such rocks may represent samples

from a depth of a few hundred kilometres, where pressures of about 100 kilobars (3,000,000

inches of mercury) may be operative. Experiments at high pressure have shown that few of the

common minerals that occur at the surface will survive at depth within the mantle without

changing to new high-density phases in which atoms are packed more closely together. Thus, the

common form of SiO2, quartz, with a density of 2.65 grams per cubic centimetre, transforms to a

new phase, stishovite, with a density of 4.29 grams per cubic centimetre. Such changes are of

critical significance in the geophysical interpretation of the Earth's interior.

In general, temperatures increase with depth within the Earth along curves referred to as

geotherms. The specific shape of the geotherm beneath any location on Earth is a function of its

corresponding local tectonic regime. Metamorphism can occur either when a rock moves from

one position to another along a single geotherm or when the geotherm itself changes form. The

former can take place when a rock is buried or uplifted at a rate that permits it to maintain

thermal equilibrium with its surroundings; this type of metamorphism occurs beneath slowly

subsiding sedimentary basins and also in the descending oceanic plate in some subduction zones.

The latter process occurs either when hot magma intrudes and alters the thermal state of a

stationary rock or when the rock is rapidly transported by tectonic processes (e.g., thrust faulting

or large-scale folding) into a new depth-temperature regime in, for example, areas of collision

between two continents. Regardless of which process occurs, the result is that a collection of

minerals that are thermodynamically stable at the initial conditions are placed under a new set of

conditions at which they may or may not be stable. If they are no longer in equilibrium with one

another under the new conditions, the minerals will react in such a way as to approach a new

equilibrium state. This may involve a complete change in mineral assemblage or simply a shift in

the compositions of the pre-existing mineral phases. The resultant mineral assemblage will

reflect the chemical composition of the original rock and the new pressure-temperature

conditions to which the rock was subjected.

Because protolith compositions and the pressure-temperature conditions under which they may

be placed vary widely, the diversity of metamorphic rock types is large. Many of these varieties

are repeatedly associated with one another in space and time, however, reflecting a uniformity of

geologic processes over hundreds of millions of years. For example, the metamorphic rock

associations that developed in the Appalachian Mountains of eastern North America in response

to the collision between the North American and African lithospheric plates during the Paleozoic

are very similar to those developed in the Alps of south-central Europe during the Mesozoic-

Cenozoic collision between the European and African plates. Likewise, the metamorphic rocks

exposed in the Alps are grossly similar to metamorphic rocks of the same age in the Himalayas

of Asia, which formed during the continental collision between the Indian and Eurasian plates.

Metamorphic rocks produced during collisions between oceanic and continental plates from

different localities around the world also show striking similarities to each other yet are markedly

different from metamorphic rocks produced during continent-continent collisions. Thus, it is

often possible to reconstruct tectonic events of the past on the basis of metamorphic rock

associations currently exposed at the Earth's surface.

Rock Cycle Geologic materials—mineral crystals and their host rock types—are cycled through various forms. The process depends on temperature, pressure, time, and changes in environmental conditions in the Earth's crust and at its surface. The rock cycle illustrated in Figure 1 reflects the basic relationships among igneous, metamorphic, and sedimentary rocks. Erosion includes weathering (the physical and chemical breakdown of minerals) and transportation to a site of deposition. Diagenesis is, as previously explained, the process of forming sedimentary rock by compaction and natural cementation of grains, or crystallization from water or solutions, or recrystallization. The conversion of sediment to rock is termed lithification.

Fig. 1

Theories about Changes in the Lithosphere

Continental Drift Theory

In 1910 American geologist Frank B. Taylor proposed that lateral (sideways) motion of

continents caused mountain belts to form on their front edges. Building on this idea in 1912,

German meteorologist Alfred Wegener proposed a theory that came to be known as Continental

Drift: He proposed that the continents had moved and were once part of one, large

supercontinent called Pangaea. Wegener was attempting to explain the origin of continents and

oceans when he expanded upon Taylor’s idea. His evidence included the shapes of continents,

the physics of ocean crust, the distribution of fossils, and paleoclimatology data.

Continental drift helped to explain a major geologic issue of the 19th century: the origin of

mountains. Theories commonly called on the cooling and contracting of the earth to form

mountain chains. The mountain-building theories of German geologist Leopold von Buch and

French geologist Leonce Elie de Beaumont were catastrophic in nature. American geologists

James Hall and James Dwight Dana proposed the geosynclinal theory of mountain building—a

theory based on the downward bending of the earth’s crust (a geosyncline). Austrian geologist

Eduard Suess developed a related theory. Hall, Dana, and Suess believed that continents and

ocean basins were ancient, permanent features on earth and that mountain belts formed at their

edges.

Most geologists did not accept the theory of continental drift in the 1920s and 1930s. British

geologist Arthur Holmes supported continental drift and proposed that convection (a type of heat

movement) inside the earth drove continental drift. Others who favored the idea included South

African geologist Alex du Toit, who studied geologic evidence for the southern continents of

Gondwanaland, part of the hypothetical supercontinent Pangaea. Other scientists, such as British

geophysicist Harold Jeffreys, argued that continental drift was physically impossible.

Paleontologists, such as American George Gaylord Simpson, said that the distribution of fossils

could be explained by other means.

Pangaea

Pangaea, name given the single supercontinent that existed on Earth during the late Paleozoic

and early Mesozoic eras (about 300 million to 200 million years ago). Pangaea was made up of

two connected continental masses: Gondwanaland to the south and Laurasia to the north. The

modern continents are the result of the breakup of Pangaea, followed by the breakup of

Gondwanaland and Laurasia. The processes that formed Pangaea and later broke it apart are

known as plate tectonics, sometimes called continental drift.

The name Pangaea (Greek παν, entire, all,+ Latin Gaea, fr. Greek Γαια, the earth) was coined in

1912 by Alfred Wegener, the German meteorologist who published the first scientifically argued

theory of continental drift. Utilizing geological and fossil evidence, Wegener postulated the

existence of Pangaea as a supercontinent that had existed throughout the early history of Earth.

According to Wegener, Pangaea broke up to form the present continents beginning during the

Cretaceous Period in the late Mesozoic Era and continuing through the following Cenozoic Era.

Seafloor Spreading Theory

After World War II, geophysical evidence began to accumulate that confirmed the lateral motion

of continents and indicated the young age of oceanic crust. This evidence led to the theories of

seafloor spreading and plate tectonics in the 1960s. American marine geologists Robert S. Dietz

and Harry H. Hess proposed the seafloor spreading hypothesis, the concept that the oceanic crust

is created as the seafloor spreads apart along midocean ridges. American oceanographers Bruce

C. Heezen, Marie Tharp, and others prepared detailed maps of the ocean floors and the mid-

Atlantic ridge and rift system, a mountainous chain found throughout the ocean. These maps

provided additional evidence that seemed to support the continental drift theory. Further

evidence came from paleomagnetism, the record of the orientation of earth's magnetic field

recorded in rocks. In the 1950s, British geophysicist S. Keith Runcorn determined that this

evidence indicated that the continents had moved relative to the earth’s magnetic poles and to

each other. British marine geophysicists Fred J. Vine and Drummond Matthews described the

record of changes in the earth’s magnetic field when they discovered ―magnetic stripes‖ formed

at spreading centers of the mid-ocean ridges, leading to the Vine-Matthews hypothesis. Magnetic

stripes were also independently described by Canadian geophysicist Lawrence Morley and

confirmed by American marine geologist Walter Pitman and others. These stripes indicated

reversals of the direction of the earth’s magnetic field recorded in rock as new ocean crust was

created at mid-ocean ridges. Scientists used paleomagnetism and seafloor spreading to determine

that the continents had moved relative to the magnetic poles and to each other.

Plate Tectonics Theory Canadian geophysicist J. Tuzo Wilson and American geophysicist Jason Morgan, among others,

proposed the outline of the theory of plate tectonics in the 1960s. This theory stated that the

earth’s lithosphere is made up of several rigid plates. These plates slide and move over a less-

rigid layer called the asthenosphere. A plate may be composed entirely of oceanic crust, like the

Pacific Plate, or of part ocean crust and part continental crust, like the North American Plate.

New ocean crust is generated at ocean ridges (underwater mountain chains formed by the young

ocean crust). Older ocean crust sinks down, or subducts, into the earth’s mantle at subduction

zones, which are found at the deepest parts of the ocean, called trenches. As the plates move,

they collide and form mountains. The plates recycle crust, generate volcanoes, and move past

each other along faults. Using satellites, scientists can now measure movement of the continental

plates in centimeters per year. Plate boundaries are the sites of most of the earth's earthquakes

and the majority of earth's volcanoes. The continents are made of remelted sediments and

partially melted oceanic crust, forming a lower density layer that has collected through time. The

mechanism that drives the earth’s crustal plates is still not known, but geologists can use plate

tectonics to explain most geologic activity.

Fig. 2

Production and destruction of Earth's crust according to the theory of plate tectonics.

Oceanic crust is continually generated at divergent plate boundaries (typified by

midocean ridges and their rift zones) from upwelling mantle material, and it is consumed

in the subduction process at convergent plate boundaries (marked by deep-sea trenches).

Areas of convergence are sites of mountain building or of formation of volcanic island

arcs. At transform, or strike-slip, boundaries, two plates slide past each other laterally;

these areas are often associated with a high frequency of earthquakes.

Fig. 3

Fig. 4

The principal tectonic plates that make up Earth's lithosphere. Also located are orogenic

belts, or great mountain ranges, that have been produced comparatively recently at the

boundaries of converging plates. The several dozen hot spots locate sites where plumes of

hot mantle material are upwelling beneath the plates.

Geochronological Maps:

Cambrian Period

Late Ordovician Period

Early Silurian Period

Early Devonian Period

Carboniferous Period

Permian Period

Early Triassic Period

Late Jurassic Period

Late Cretaceous Period

Tertiary Period

Atmosphere

Atmosphere – [Greek αημος, vapor + ζθαιρα, sphere] – mixture of gases surrounding any

celestial object that has a gravitational field strong enough to prevent the gases from escaping;

especially the gaseous envelope of Earth. The principal constituents of the atmosphere of Earth

are nitrogen (78 percent) and oxygen (21 percent). The atmospheric gases in the remaining 1

percent are argon (0.9 percent), carbon dioxide (0.03 percent), varying amounts of water vapor,

and trace amounts of hydrogen, ozone, methane, carbon monoxide, helium, neon, krypton, and

xenon.

The mixture of gases in the air today has had 4.5 billion years in which to evolve. The earliest

atmosphere must have consisted of volcanic emanations alone. Gases that erupt from volcanoes

today, however, are mostly a mixture of water vapor, carbon dioxide, sulfur dioxide, and

nitrogen, with almost no oxygen. If this is the same mixture that existed in the early atmosphere,

then various processes would have had to operate to produce the mixture we have today. One of

these processes was condensation. As it cooled, much of the volcanic water vapor condensed to

fill the earliest oceans. Chemical reactions would also have occurred. Some carbon dioxide

would have reacted with the rocks of Earth’s crust to form carbonate minerals, and some would

have become dissolved in the new oceans. Later, as primitive life capable of photosynthesis

evolved in the oceans, new marine organisms began producing oxygen. Almost all the free

oxygen in the air today is believed to have formed by photosynthetic combination of carbon

dioxide with water. About 570 million years ago, the oxygen content of the atmosphere and

oceans became high enough to permit marine life capable of respiration. Later, some 400 million

years ago, the atmosphere contained enough oxygen for the evolution of air-breathing land

animals.

The water-vapor content of the air varies considerably, depending on the temperature and

relative humidity. With 100 percent relative humidity, the water-vapor content of air varies from

190 parts per million (ppm) at -40°C (-40°F) to 42,000 ppm at 30°C (86°F). Minute quantities of

other gases, such as ammonia, hydrogen sulfide, and oxides of sulfur and nitrogen, are temporary

constituents of the atmosphere in the vicinity of volcanoes and are washed out of the air by rain

or snow. Oxides and other pollutants added to the atmosphere by industrial plants and motor

vehicles have become a major concern, however, because of their damaging effects in the form

of acid rain. In addition, the strong possibility exists that the steady increase in atmospheric

carbon dioxide, mainly as the result of the burning of fossil fuels since the mid-1800s, may affect

Earth’s climate.

Similar concerns are posed by the sharp increase in atmospheric methane. Methane levels have

risen 11 percent since 1978. About 80 percent of the gas is produced by decomposition in rice

paddies, swamps, and the intestines of grazing animals, and by tropical termites. Human

activities that tend to accelerate these processes include raising more livestock and growing more

rice. Besides adding to the greenhouse effect, methane reduces the volume of atmospheric

hydroxyl ions, thereby curtailing the atmosphere’s ability to cleanse itself of pollutants.

The study of air samples shows that up to at least 88 km (55 mi) above sea level the composition

of the atmosphere is substantially the same as at ground level; the continuous stirring produced

by atmospheric currents counteracts the tendency of the heavier gases to settle below the lighter

ones. In the lower atmosphere, ozone, a form of oxygen with three atoms in each molecule, is

normally present in extremely low concentrations. The layer of atmosphere from 19 to 48 km (12

to 30 mi) up contains more ozone, produced by the action of ultraviolet radiation from the sun.

Even in this layer, however, the percentage of ozone is only 0.001 by volume. Atmospheric

disturbances and downdrafts carry varying amounts of this ozone to the surface of Earth. Human

activity adds to ozone in the lower atmosphere, where it becomes a pollutant that can cause

extensive crop damage.

The ozone layer became a subject of concern in the early 1970s, when it was found that

chemicals known as chlorofluorocarbons (CFCs), or chlorofluoromethanes, were rising into the

atmosphere in large quantities because of their use as refrigerants and as propellants in aerosol

dispensers. The concern centered on the possibility that these compounds, through the action of

sunlight, could chemically attack and destroy stratospheric ozone, which protects Earth’s surface

from excessive ultraviolet radiation. As a result, industries in the United States, Europe, and

Japan replaced chlorofluorocarbons in all but essential uses.

The atmosphere may be divided into several layers. In the lowest one, the troposphere, the

temperature as a rule decreases upward at the rate of 5.5°C per 1,000 m (3°F per 3,000 ft). This

is the layer in which most clouds occur. The troposphere extends up to about 16 km (about 10 mi)

in tropical regions (to a temperature of about -79°C, or about -110°F) and to about 9.7 km (about

6 mi) in temperate latitudes (to a temperature of about -51°C, or about -60°F). Above the

troposphere is the stratosphere. In the lower stratosphere the temperature is practically constant

or increases slightly with altitude, especially over tropical regions. Within the ozone layer the

temperature rises more rapidly, and the temperature at the upper boundary of the stratosphere,

almost 50 km (about 30 mi) above sea level, is about the same as the temperature at the surface

of Earth. The layer from 50 to 90 km (30 to 55 mi), called the mesosphere, is characterized by a

marked decrease in temperature as the altitude increases.

From investigations of the propagation and reflection of radio waves, it is known that beginning

at an altitude of 60 km (40 mi), ultraviolet radiation, x rays, and showers of electrons from the

sun ionize several layers of the atmosphere, causing them to conduct electricity; these layers

reflect radio waves of certain frequencies back to Earth. Because of the relatively high

concentration of ions in the air above 60 km (40 mi), this layer, extending to an altitude of about

1000 km (600 mi), is called the ionosphere. At an altitude of about 90 km (55 mi), temperatures

begin to rise. The layer that begins at this altitude is called the thermosphere, because of the high

temperatures reached in this layer (about 1200°C, or about 2200°F). The region beyond the

thermosphere is called the exosphere, which extends to about 9,600 km (about 6,000 mi), the

outer limit of the atmosphere.

The density of dry air at sea level is about 1/800 the density of water; at higher altitudes it

decreases rapidly, being proportional to the pressure and inversely proportional to the

temperature. Pressure is measured by a barometer and is expressed in millibars, which are related

to the height of a column of mercury that the air pressure will support; 1 millibar equals 0.75 mm

(0.03 in) of mercury. Normal atmospheric pressure at sea level is 1,013 millibars, that is, 760

mm (29.92 in) of mercury. At an altitude of 5.6 km (about 3.5 mi) pressure falls to about 507

millibars (about 380 mm/14.96 in of mercury); half of all the air in the atmosphere lies below

this level. The pressure is approximately halved for each additional increase of 5.6 km in altitude.

At 80 km (50 mi) the pressure is 0.009 millibars (0.0069 mm/0.00027 in of mercury).

The troposphere and most of the stratosphere can be explored directly by means of sounding

balloons equipped with instruments to measure the pressure and temperature of the air and with a

radio transmitter to send the data to a receiving station at the ground. Rockets carrying radios

that transmit meteorological-instrument readings have explored the atmosphere to altitudes

above 400 km (250 mi). Study of the form and spectrum of the polar lights gives information to a

height possibly as great as 800 km (500 mi).

Layers of Earth’s atmosphere Fig. 5

The layers of Earth's atmosphere. The yellow line shows the response of air temperature

to increasing height.

Fig. 6

The Van Allen radiation belts contained within Earth's magnetosphere. Pressure from the

solar wind is responsible for the asymmetrical shape of the magnetosphere and the belts.

Hydrosphere

Hydrosphere – [Greek ύδορ, water + ζθαιρα, sphere] – discontinuous layer of water at or near

the Earth's surface. It includes all liquid and frozen surface waters, groundwater held in soil and

rock, and atmospheric water vapour.

Water is the most abundant substance at the surface of the Earth. About 1.4 billion cubic

kilometres (326 million cubic miles) of water in liquid and frozen form make up the oceans,

lakes, streams, glaciers, and groundwaters found there. It is this enormous volume of water, in its

various manifestations, that forms the discontinuous layer, enclosing much of the terrestrial

surface, known as the hydrosphere.

Central to any discussion of the hydrosphere is the concept of the hydrologic cycle. This cycle

consists of a group of reservoirs containing water, the processes by which water is transferred

from one reservoir to another (or transformed from one state to another), and the rates of transfer

associated with such processes. These transfer paths penetrate the entire hydrosphere, extending

upward to about 15 kilometres (nine miles) in the Earth's atmosphere and downward to depths on

the order of five kilometres in its crust.

This article examines the processes of the hydrologic cycle and discusses the way in which the

various reservoirs of the hydrosphere are related through the hydrologic cycle. It also describes

the biogeochemical properties of the waters of the Earth at some length and considers the

distribution of global water resources and their utilization and pollution by human society.

Details concerning the major water environments that make up the hydrosphere are provided in

the articles ocean, lake, river, and ice. See also climate for specific information about the impact

of climatic factors on the hydrologic cycle. The principal concerns and methods of hydrology

and its various allied disciplines are summarized in Earth sciences.

Hydrologic cycle Hydrologic cycle - cycle that involves the continuous circulation of water in the Earth-

atmosphere system. Of the many processes involved in the hydrologic cycle, the most important

are evaporation, transpiration, condensation, precipitation, and runoff. Although the total amount

of water within the cycle remains essentially constant, its distribution among the various

processes is continually changing.

Evaporation, one of the major processes in the cycle, is the transfer of water from the surface of

the Earth to the atmosphere. By evaporation, water in the liquid state is transferred to the gaseous,

or vapour, state. This transfer occurs when some molecules in a water mass have attained

sufficient kinetic energy to eject themselves from the water surface. The main factors affecting

evaporation are temperature, humidity, wind speed, and solar radiation. The direct measurement

of evaporation, though desirable, is difficult and possible only at point locations. The principal

source of water vapour is the oceans, but evaporation also occurs in soils, snow, and ice.

Evaporation from snow and ice, the direct conversion from solid to vapour, is known as

sublimation. Transpiration is the evaporation of water through minute pores, or stomata, in the

leaves of plants. For practical purposes, transpiration and the evaporation from all water, soils,

snow, ice, vegetation, and other surfaces are lumped together and called evapotranspiration, or

total evaporation.

Water vapour is the primary form of atmospheric moisture. Although its storage in the

atmosphere is comparatively small, water vapour is extremely important in forming the moisture

supply for dew, frost, fog, clouds, and precipitation. Practically all water vapour in the

atmosphere is confined to the troposphere (the region below 6 to 8 miles [10 to 13 km] altitude).

The transition process from the vapour state to the liquid state is called condensation.

Condensation may take place as soon as the air contains more water vapour than it can receive

from a free water surface through evaporation at the prevailing temperature. This condition

occurs as the consequence of either cooling or the mixing of air masses of different temperatures.

By condensation, water vapour in the atmosphere is released to form precipitation.

Precipitation that falls to the Earth is distributed in four main ways: some is returned to the

atmosphere by evaporation, some may be intercepted by vegetation and then evaporated from the

surface of leaves, some percolates into the soil by infiltration, and the remainder flows directly as

surface runoff into the sea. Some of the infiltrated precipitation may later percolate into streams

as groundwater runoff. Direct measurement of runoff is made by stream gauges and plotted

against time on hydrographs.

Most groundwater is derived from precipitation that has percolated through the soil.

Groundwater flow rates, compared with those of surface water, are very slow and variable,

ranging from a few millimetres to a few metres a day. Groundwater movement is studied by

tracer techniques and remote sensing.

Ice also plays a role in the hydrologic cycle. Ice and snow on the Earth's surface occur in various

forms such as frost, sea ice, and glacier ice. When soil moisture freezes, ice also occurs beneath

the Earth's surface, forming permafrost in tundra climates. About 18,000 years ago glaciers and

ice caps covered approximately one-third of the Earth's land surface. Today, about 12 percent of

the land surface remains covered by ice masses.

Hydrologic cycle: processes

Fig. 7

In the hydrologic cycle, water is transferred between the land surface, the ocean, and the

atmosphere. The numbers on the arrows indicate relative water fluxes.

Fig. 8

The present-day surface hydrologic cycle, in which water is transferred from the oceans

through the atmosphere to the continents and back to the oceans over and beneath the

land surface. The values in parentheses following the various forms of water (e.g., ice)

refer to volumes in millions of cubic kilometres; those following the processes (e.g.,

precipitation) refer to their fluxes in millions of cubic kilometres of water per year.

Biosphere

Fig. 9

Earth's Biosphere

The earth’s biosphere contains numerous complex ecosystems that collectively contain

all of the living organisms of the planet. Unique perspectives of the earth help suggest the

immensity and complexity of the planet’s biosphere. En route to the moon in December

1972, the Apollo 17 spacecraft took this image of the earth, showing Arabia and the

continent of Africa.

Biosphere - relatively thin life-supporting stratum of the Earth's surface, extending from a few

kilometres into the atmosphere to the deep-sea vents of the ocean. The biosphere is a global ecosystem

composed of living organisms (biota) and the abiotic (nonliving) factors from which they derive energy

and nutrients.

Before the coming of life, the Earth was a bleak place, a rocky globe with shallow seas and a thin

band of gases—largely carbon dioxide, carbon monoxide, molecular nitrogen, hydrogen sulfide,

and water vapour. It was a hostile and barren planet. This strictly inorganic state of the Earth is

called the geosphere; it consists of the lithosphere (the rock and soil), the hydrosphere (the water),

and the atmosphere (the air). Energy from the Sun relentlessly bombarded the surface of the

primitive Earth, and in time—millions of years—chemical and physical actions produced the

first evidence of life: formless, jellylike blobs that could collect energy from the environment

and produce more of their own kind. This generation of life in the thin outer layer of the

geosphere established what is called the biosphere, the ―zone of life,‖ an energy-diverting skin

that uses the matter of the Earth to make living substance.

The biosphere is a system characterized by the continuous cycling of matter and an

accompanying flow of solar energy in which certain large molecules and cells are self-

reproducing. Water is a major predisposing factor, for all life depends on it. The elements carbon,

hydrogen, nitrogen, oxygen, phosphorus, and sulfur, when combined as proteins, lipids,

carbohydrates, and nucleic acids, provide the building blocks, the fuel, and the direction for the

creation of life. Energy flow is required to maintain the structure of organisms by the formation

and splitting of phosphate bonds. Organisms are cellular in nature and always contain some sort

of enclosing membrane structure, and all have nucleic acids that store and transmit genetic

information.

All life on Earth depends ultimately upon green plants, as well as upon water. Plants utilize

sunlight in a process called photosynthesis to produce the food upon which animals feed and to

provide, as a by-product, oxygen, which most animals require for respiration. At first, the oceans

and the lands were teeming with large numbers of a few kinds of simple single-celled organisms,

but slowly plants and animals of increasing complexity evolved. Interrelationships developed so

that certain plants grew in association with certain other plants, and animals associated with the

plants and with one another to form communities of organisms, including those of forests,

grasslands, deserts, dunes, bogs, rivers, and lakes. Living communities and their nonliving

environment are inseparably interrelated and constantly interact upon each other. For

convenience, any segment of the landscape that includes the biotic and abiotic components is

called an ecosystem. A lake is an ecosystem when it is considered in totality as not just water but

also nutrients, climate, and all of the life contained within it. A given forest, meadow, or river is

likewise an ecosystem. One ecosystem grades into another along zones termed ecotones, where a

mixture of plant and animal species from the two ecosystems occurs. A forest considered as an

ecosystem is not simply a stand of trees but is a complex of soil, air, and water, of climate and

minerals, of bacteria, viruses, fungi, grasses, herbs, and trees, of insects, reptiles, amphibians,

birds, and mammals.

Stated another way, the abiotic, or nonliving, portion of each ecosystem in the biosphere includes

the flow of energy, nutrients, water, and gases and the concentrations of organic and inorganic

substances in the environment. The biotic, or living, portion includes three general categories of

organisms based on their methods of acquiring energy: the primary producers, largely green

plants; the consumers, which include all the animals; and the decomposers, which include the

microorganisms that break down the remains of plants and animals into simpler components for

recycling in the biosphere. Aquatic ecosystems are those involving marine environments and

freshwater environments on the land. Terrestrial ecosystems are those based on major

vegetational types, such as forest, grassland, desert, and tundra. Particular kinds of animals are

associated with each such plant province.

Ecosystems may be further subdivided into smaller biotic units called communities. Examples of

communities include the organisms in a stand of pine trees, on a coral reef, and in a cave, a

valley, a lake, or a stream. The major consideration in the community is the living component,

the organisms; the abiotic factors of the environment are excluded.

A community is a collection of species populations. In a stand of pines, there may be many

species of insects, of birds, of mammals, each a separate breeding unit but each dependent on the

others for its continued existence. A species, furthermore, is composed of individuals, single

functioning units identifiable as organisms. Beyond this level, the units of the biosphere are those

of the organism: organ systems composed of organs, organs of tissues, tissues of cells, cells of

molecules, and molecules of atomic elements and energy. The progression, therefore, proceeding

upward from atoms and energy, is toward fewer units, larger and more complex in pattern, at

each successive level.

This article focuses on the makeup of the biosphere and examines the relationships between its

principal components, including man. The characteristics and dynamics of biological populations

and communities are dealt with, as are the interactions that constitute the primary stabilizing

links among the constituent organisms. Due attention is also given to the distribution patterns of

these biotic units and to the processes that produced such patterns. The major aquatic and

terrestrial ecosystems of the Earth are treated in some detail. Other points include energy

transformations and transfers within the biosphere and the cyclic flow of materials needed for

life. For the development, methodology, and applications of the study of interrelations of

organisms with their environment and each other, see ecology. Further treatment of the various

aquatic and terrestrial environments is provided in ocean, lake, river, continental landform,

Arctic, and Antarctica. For a discussion of the origin of life on Earth and the varieties of and

commonalities among organisms, see life and Earth, pregeologic history of. The characteristics

and classifications of living organisms are covered in detail in algae, amphibian, angiosperm,

animal, annelid, arachnid, arthropod, aschelminth, bacteria, bird, bryophyte, chordate, cnidarian,

crustacean, dinosaur, echinoderm, fern, fish, flatworm, fungus, gymnosperm, insect, lamp shell,

mammal, mollusk, moss animal, plant, protist, protozoa, reptile, sponge, and virus.

Five Kingdoms of Life

Kingdom Monera Unicellular prokaryotic organisms lacking distinct nuclei and membrane-bound organelles;

nutrition principally by absorption but some are photosynthetic and chemosynthetic.

Phylum Cyanophyta blue-green algae

Phylum Schizophyta bacteria

Kingdom Protista Unicellular or colonial eukaryotic organisms with distinct nuclei and organelles; nutrition by

photosynthesis, absorption, or ingestion.

Phylum Chrysophyta golden algae

Phylum Pyrrophyta dinoflagellates

Phylum Xanthophyta yellow-green algae

Phylum Protozoa protozoans (Trypanosoma, Chilomonas)

Kingdom Plantae Multicellular eukaryotic organisms with rigid cell walls and chlorophyll; nutrition principally by

photosynthesis.

Phylum Chlorophyta green algae (Spirogyra, Volvox)

Phylum Rhodophyta red algae (predominantly marine; seaweeds)

Phylum Phaeophyta brown algae (almost entirely marine; kelp)

Phylum Bryophyta liverworts, hornworts, mosses

Phylum Trachaeophyta vascular plants

Subphylum Lycopsida club mosses

Subphylum Sphenopsida horsetails

Subphylum Pteropsida ferns

Subphylum Spermopsida seed plants

Class Gymnospermae conifers, cycads, ginkoes

Class Angiospermae flowering plants

Subclass Dicotyledoneae grasses, lilies, orchids

Subclass Monocotyledoneae shrubs and trees

Kingdom Fungi Multinucleate plantlike organisms lacking photosynthetic pigments; nutrition absorptive.

Phylum Myxomycophyta slime molds

Phylum Eumycophyta true fungi

Class Deuteromycetes imperfect fungi

Class Ascomycetes sac fungi (yeast and mildews)

Class Basidiomycetes club fungi (mushroom and rusts)

Class Zygomycetes bread molds

Kingdom Animalia Multicellular organism without cell walls or chlorophyll; nutrition principally ingestive with

digestion in an internal cavity.

Phylum Porifera sponges

Phylum Coelenterata radially symmetrical marine mammals

Class Hydrozoa Portuguese man-of-war (Hydra)

Class Scyphozoa jellyfish

Class Anthozoa sea anemones and corals

Phylum Ctenophora comb jellies

Phylum Platyhelminthes flatworms

Class Turbellaria free-living flatworms (Planaria)

Class Trematoda parasitic flukes

Class Cestoda parasitic tapeworms

Phylum Aschelminthes roundworms (Trichina, Necator)

Phylum Rotifera rotifers

Phylum Bryozoa moss animals

Phylum Mollusca soft-bodied, unsegmented animals

Class Amphineura chitons

Class Gastropoda snails and slugs

Class Scaphopoda tooth shells

Class Pelecypoda clams and mussels

Class Cephalopoda squids and octopuses

Phylum Annelida segmented worms

Class Polychaeta sand worms

Class Oligochaeta earthworms

Class Hirudinea leeches

Phylum Anthropoda joint-legged animals; with exoskeleton

Class Crustacia lobsters, crabs, barnacles

Class Arachnida spiders, scorpions, ticks

Class Chilopoda centipedes

Class Diplopoda millipedes

Class Insecta grasshoppers, termites, beetles

Phylum Echinodermata marine; spiny radially symmetrical animals

Class Crinoidea sea lilies and feather stars

Class Asteroidea starfish

Class Ophluroidea brittle stars

Class Echinoidea sea urchin and sand dollar

Class Holothuroidea sea cucumbers

Phylum Hemichordata acorn worms

Phylum Chordata dorsal supporting rod (notochord) at same

stage; dorsal hollow nerve cord; pharyngeal

gill slits

Subphylum Urochordata tunicates

Subphylum Cephalochordata lancelets

Subphylum Vertebrata vertebrates

Class Agnatha jawless fishes (lampreys, hagfishes)

Class Chondrichthyes cartilaginous fishes (sharks, rays)

Class Ostelchthyes bony fishes

Class Amphibia frogs, toads, salamanders

Class Reptilia snakes, lizards, turtles

Class Aves birds

Class Mammalia mammals

Order Monotremata duckbill platypus, spiny anteater

Order Marsupialia opossums, kangaroos

Order Insectivora shrews, moles

Order Chiroptera bats

Order Edentada anteaters, armadillos

Order Rodentia rats, mice, squirrels

Order Lagomorpha rabbits and hares

Order Cetacea whales, dolphins, porpoises

Order Carnivora dogs, bears, skunks

Order Proboscidea elephants

Order Sirenia manatees

Order Perrisodactyla horse, hippopotamus, zebra

Order Artiodactyla pigs, deer, cattle

Order Primates monkeys, apes, humans

Full Classifications of Humans

Kingdom Animalia

Phylum Chordata

Subphylum Vertebrata

Superclass Tetrapoda

Class Mammalia

Subclass Theria

Infraclass Eutheria

Order Primates

Suborder Anthropoldea

Superfamily Hominoidea

Family Hominidae

Subfamily Homininae

Genus Homo

Species sapiens