Example of a cometary impact with earth

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On June 30, 1908, a huge explosion rocked a remote area of central Siberia centered near the Podkamen-

naya (Lower Stony) Tunguska River, in an area now known as Krasnoyarsk Krai in Russia. After years of study and debate many geologists and other scientists think that this huge explosion was produced by a fragment of Comet Encke that broke off the main body and exploded in the air about five miles (eight km) above the siberian plains.

The early morning of June 30, 1908, witnessed a huge, pipelike fireball moving across the skies of Siberia, until at 7:17 a.m. a tremendous explosion rocked the Tunguska area and devastated more than 1,160 square miles (3,000 km2) of forest. The force of the blast is estimated to have been equal to 10-30 megatons (0.91-27 megatonnes), and is thought to have been produced by the explosion, six miles (10 km) above the surface of Earth, of an asteroid or comet with a diameter of 200 feet (60 m). The energy equivalence of this explosion was close to 2,000 times the energy released during the explosion of the Hiroshima atomic bomb. More energy was released in the air blast than the impact and solid earthquakes, demonstrating that the Tunguska impacting body exploded in the air. The pattern of destruction reflects the dominance of atmospheric shock waves rather than solid earthquakes that are estimated to have been about a magnitude 5 earthquake. Atmospheric shock waves were felt thousands of miles away, and people located closer than 60 miles (100 km) from the site of the explosion were knocked unconscious; some were thrown into the air by the force of the explosion. Fiery clouds and deafening explosions were heard more than 600 miles (965 km) from Tunguska.

For a long time one of the biggest puzzles at Tun-guska was the absence of an impact crater, despite all other evidence that points to an impact origin for this event. Many scientists now think that a piece of a comet, Comet Encke, broke off the main body as it was orbiting nearby Earth, and this fragment entered Earth's atmosphere and exploded about 5-6 miles (8-10 km) above the Siberian plains at Tunguska. This model was pioneered by Slovak astronomer Lubor Kresak, following earlier suggestions by the British astronomer F. J. W. Whipple in the 1930s that the bolide (a name for any unidentified object entering the planet's atmosphere) at Tunguska may have been a comet. Other scientists suggest the bolide may have been a meteorite, since comets are weaker than metallic or stony meteorites, and more easily break up and explode in the atmosphere before they hit Earth's surface. If the Tunguska bolide was a comet, it would likely have broken up higher in the atmosphere. In either case calculations show that, because of Earth's rotation, if the impact explosion happened only four hours and 47 minutes later, the city of Saint Petersburg would have been completely destroyed by the air blast. Air blasts from disintegrating meteorites or comets the size of the Tun-guska explosion occur about once every 300 years on Earth, whereas smaller explosions, about the size of the nuclear bombs dropped on Japan, occur in the upper atmosphere about once per year.

All the trees in the Siberian forest in an area the size of a large city were leveled by the explosion of Tunguska, which fortunately was unpopulated at the time. But a thousand reindeer belonging to the Evenki people of the area were reportedly killed by the blast. The pattern of downed trees indicates that the projectile traveled from the southeast to the northwest as it exploded. The height of the explosion over Tunguska is about the optimal height for an explosion-induced air burst to cause maximum damage to urban areas, and calculations suggest that if the area was heavily populated at the time of the impact, at least 500,000 people would have died. Despite the magnitude and significance of this event, the Tunguska region was very remote, and no scientific expeditions to the area to investigate the explosion were mounted until 1921, 13 years after the impact, and even then the first expedition reached only the fringes of the affected area. The first scientific expedition was led by geologist Leonid Kulik, who was looking for meteorites along the Podkamennaya Tunguska River basin, and he heard stories from the local people of the giant explosion that happened in 1908, and that the explosion had knocked down trees, blown roofs off huts, and knocked people over and even caused some to become deaf from the noise. Kulik then convinced the Russian government that an expedition needed to be mounted into the remote core of the Tunguska blast area, and this expedition reached the core of the blast zone in 1927. Kulik and his team found huge tracts of flattened and burned trees, but they were unable to locate an impact crater.

In June 2007 a team of scientists from the University of Bologna suggested that the small Lake Cheko, located about 5 miles (8 km) from the epicenter of the blast, may be the impact site. other scientists challenge this interpretation, noting that the lake has thick sediments, implying an older age than the age of the impact.

The atmospheric blast from the Tunguska explosion raced around the planet two times before diminishing. Residents of Siberia who lived within about 50 miles (80 km) of the blast site reported unusual glowing light from the sky for several weeks after the explosion. It is possible that this light was being reflected by a stream of dust particles that were ripped off a comet as it entered the atmosphere before colliding with Earth's surface. The unusual nighttime illumination was reported from across Europe and western Russia, showing the extent of the dust stream in the atmosphere.

As the fireball from the Tunguska airburst moved through the atmosphere, the temperatures at the center of the fireball were exceedingly hot, estimated to be 30 million degrees Fahrenheit (16.6 million Celsius). On the ground trees were burned and scorched, and silverware utensils in storage huts near the center of the blast zone were melted by the heat. After the impact leveled the trees for a distance of about 25 miles (40 km) around the center of the impact, forest fires ravaged the area, but typically burned only the outer surface of many trees, as if the fires were a short-lived flash of searing heat.

The type of body that exploded above Tunguska has been the focus of much speculation and investigation. one of the leading ideas is that the impact was caused by a comet that exploded in the atmosphere above Tunguska, a theory pioneered by F. J. W. Whipple in a series of papers from 1930 to 1934. In the 1960s small silica and magnetite spherules that represent melts from an extraterrestrial source were found in soil samples from Tunguska, confirming that a comet or meteorite had exploded above the site. Further analysis of the records of the airblast indicated that several pressure waves were recorded by the event. The first was the type associated with the rapid penetration of an object into the atmosphere, and at least three succeeding bursts recorded the explosions of a probably fragmented comet about five miles (eight km) above the surface.

There have been other reported explosions, or possible explosions, of meteorites above the surface of Earth, creating air blasts since the Tunguska event, although none has been as spectacular. on August 13, 1930, a body estimated to be about 10 percent the size of the Tunguska bolide exploded above the Curuca River in the Amazonas area in Brazil, but documentation of this event is poor. On May 31, 1965, an explosion with the force equivalent of 600 tons (544 tonnes) of TNT was released eight miles (13 km) above southeastern Canada, and approximately 0.4 ounce (1 g) of meteorite material was recovered from this event. Similar sized events, also thought to be from meteorite explosions at about eight miles (13 km) above the surface, were reported from southeast Canada on May 31, 1965, over Lake Huron (Michigan) on September 17, 1966, and over Alberta, Canada, on February 5, 1967. No meteorite material was recovered from any of these events. Two mysterious explosions, probably meteorites exploding, with an equivalent of about 25 tons (23 tonnes) of TNT, were reported, strangely, over the same area of Sassowo, Russia, on April 12, 1992, and July 8, 1992. A larger explosion and airburst, esti mated to be equivalent to 10,000 tons of TNT, was reported over Lugo, Italy, on January 19, 1993, and another 25-ton (23-tonne) event over Cando, Spain, on January 18, 1994. Russia was struck again, this time in the Bodaybo region, by a 500-5,000 ton (450-4,500 tonne) equivalent blast on September 25, 2002, after a 26,000-ton (23,600-tonne) airburst from a meteorite explosion was recorded over the Mediterranean between Greece and Libya. The last reported airburst was from a high-altitude explosion, 27 miles (43 km) over Snohomish, Washington, on June 3, 2004. Clearly airbursts associated with the explosion of meteorites or comets are fairly common events, just as events as strong as the Tunguska explosion happen only about once every 300 years.

See also asteroid; astronomy; astrophysics; origin and evolution of the Earth and solar system; solar system.

FURTHER READING

Alvarez, Walter. T Rex and the Crater of Doom. Princeton, N.J.: Princeton University Press, 1997. Angelo, Joseph A. Encyclopedia of Space and Astronomy.

New York: Facts On File, 2006. Chaisson, Eric, and Steve McMillan. Astronomy Today.

6th ed. Upper Saddle River, N.J.: 2007. Chapman, C. R., and D. Morrison. "Impacts on the Earth by Asteroids and Comets: Assessing the Hazard." Nature 367 (1994): 33-39. Elkens-Tanton, Linda T. Asteroids, Meteorites, and Comets. New York: Facts On File, 2006. Spencer, John R., and Jacqueline Mitton. The Great Comet Crash: The Impact of Comet Shoemaker-Levy 9 on Jupiter. Cambridge: Cambridge University Press, 1995.

Thomas, Paul J., Christopher F. Chyba, and Christopher P. McKay, eds. Comets and the Origin and Evolution of Life. New York: Springer-Verlag. 1997.

constellation Human groupings of stars in the sky into patterns, even though they may be far apart and lined up only visibly, are known as constellations. About 6,000 stars (and other points of light such as distant galaxies, planetary nebulas, quasars, etc.) are visible to the naked eye from Earth, and an irresistible tendency to see patterns and figures in these points of light has persisted for generations going back thousands of years. Peoples of many cultures have grouped these apparent configurations of points of light in the sky into patterns called constellations, the most famous of which are named after ancient Greek mythological beings. Typically most stars that make up a constellation physically exist far apart in space, and appear grouped near to one another only when viewed from Earth. some of the earliest records of constellations date back to about 2500 b.c.e., from the Mesopotamian region, where early peoples used the patterns in the stars to help tell stories, mixing mythology, religion, cultural values, and tradition from generation to generation. There are some references to patterns of stars being grouped into constellations by the ancient

Chinese and Jewish cultures that may go back to 4000 b.c.e. The peoples and stories changed with each generation, but the constellations were always there to remind the new generations that their elders were watching from above. some cultures also used constellations for navigation and for marking the seasons. For instance, the star Polaris, which is part of the Little Dipper, indicates the north direction,

THE CONSTELLATiONS

Name/Meaning (Latin [English])

Genitive Form of Latin Name

Abbreviation

Approximate Position (Equatorial Coordinates)

RA(h)

§ (°)

Andromeda (name: princess)

Andromedae

And

1

+40

Aquarius (water bearer)

Aquarii

Aqr

23

-15

Aquila (eagle)

Aquilae

Aql

20

+5

Ara (altar)

Arae

Ara

17

-55

Argo Navis (ship of Argonauts), now split into the modern constellations: Carina, puppies, Pyxis, and Vela

Aries (ram)

Arietis

Ari

3

+20

Auriga (charioteer)

Aurigae

Aur

6

+40

Bootes (berdsman)

Boötis

Boo

15

+30

Cancer (crab)

Cancri

Cnc

9

+20

Canis Major (great dog)

Canis Majoris

CMa

7

-20

Canis Minor (little dog)

Canis Minoris

CMi

8

+5

Capricornus (sea goat)

Capricorni

Cap

21

-20

Cassiopeia (name: queen)

Cassiopeiae

Cas

1

+60

Centaurus (centaur)

Centauri

Cen

13

-50

Cepheus (name: king)

Cephei

Cep

22

+70

Cetus (whale)

Ceti

Cet

2

-10

Corona Austrina (southern crown)

Coronae Australis

CrA

19

-40

Corona Borealis (northern crown)

Coronae Borealis

CrB

16

+30

Corvus (crow)

Corvi

Crv

12

-20

Crater (cup)

Crateris

Crt

11

-15

Cygnus (swan)

Cygni

Cyg

21

+40

Delphinus (dolphin)

Delphini

Del

21

+10

Draco (dragon)

Draconis

Dra

17

+65

and, with its fairly constant position in the sky, has served as a navigational aid for ages. some cultures have used the first appearance of certain stars or constellations just above the horizon at daybreak to mark the start of different seasons, such as the harvest, spring, and end of winter. In other cases the relative positions of different constellations were used by some mythological cultures to form predic tions of a person's destiny, thus creating the field of astrology.

Greek astronomers later adopted many of the ancient Mesopotamian constellations, to which they added their own culture and stories, establishing a now commonly used set of 48 constellations (see the table the Constellations). These were first codified by Eudoxus of Cnidus, then Hipparchus, and finally

Name/Meaning (Latin [English])

Genitive Form of Latin Name

Abbreviation

Approximate Position (Equatorial Coordinates)

RA(h)

§ (°)

Equuleus (little horse)

Equulei

Equ

21

+10

Eridanus (name: river)

Eridani

Eri

3

-20

Gemini (twins)

Geminorum

Gem

7

+20

Hercules (name: hero)

Herculis

Her

17

+30

Hydra (sea serpent; monster)

Hydrae

Hya

10

-20

Leo (lion)

Leonis

Leo

11

+15

Lepus (hare)

Leporis

Lep

6

-20

Libra (scale; balance beam)

Librae

Lib

15

-15

Lupus (wolf)

Lupi

Lup

15

-45

Lyra (lyre)

Lyrae

Lyr

19

+40

Ophiuchus (serpent bearer)

Ophiuchii

Oph

17

0

Orion (name: great hunter)

Orionis

Ori

5

0

Pegasus (name : winged horse)

Pegasi

Peg

22

+20

Perseus (name: hero)

Persei

Per

3

+45

Pisces (fishes)

Piscium

Psc

1

+15

Piscis Austrinus (southern fish)

Piscis Austrini

PsA

22

-30

Sagitta (arrow)

Sagittae

Sge

20

+10

Sagittarius (archer)

Sagittarii

Sgr

19

-25

Scorpius (scorpion)

Scorpii

Sco

17

-40

Serpens (serpent)

Serpentis

Ser

17

0

Taurus (bull)

Tauri

Tau

4

+15

Triangulum (triangle)

Trianguli

Tri

2

+30

Ursa major (great bear)

Ursae Majoris

UMa

11

+50

Ursa Minor (little bear)

Ursae Minoria

UMi

15

+70

Virgo (virgin; maiden)

Virginis

Vir

13

0

THE MODERN CONSTELLATiONS

Name/Meaning (Latin [English])

Genitive Form of Latin Name

Abbreviation

Approximate Position (Equatorial Coordinates)

RA(h)

§n

Antlia (air pump)

Antiae

Ant

10

-35

Apus (bird of paradise)

Apodis

Aps

16

-75

Caelum (sculptor's chisel)

Caeli

Cae

5

-40

Camelopardalis (giraffe)

Camelopardalis

Cam

6

+70

Canes Venatici (hunting dogs)

Canum Venaticorum

CVn

13

+40

Carina (keel)*

Carinae

Car

9

-60

Chamaeleon (chameleon)

Chamaeleontis

Cha

11

-80

Circinus (compasses)

Circini

Cir

15

-60

Columba (dove)

Columbae

Col

6

-35

Coma Berenices (Berenice's hair)

Comae Berenices

Com

13

+20

Crux (southern cross)

Crucis

Cru

12

-60

Dorado (swordfish)

Doradus

Dor

5

-65

Fornax (furnace)

Fornacis

For

3

-30

Grus (crane)

Gruis

Gru

22

-45

Horologium (clock)

Horologii

Hor

3

-60

Hydrus (water snake)

Hydri

Hyi

2

-75

Indus (Indian)

Indi

Ind

21

-55

Lacerta (Lizard)

Lacertae

Lac

22

+45

Leo Minor (little lion)

Leonis Minoris

LMi

10

+35

Lynx (lynx)

Lyncis

Lyn

8

+45

Mensa (table mountain)

Mensae

Men

5

-80

Microscopium (microscope)

Microscopii

Mic

21

-35

Monoceros (unicorn)

Monocerotis

Mon

7

-5

Musca (fly)

Muscae

Mus

12

-70

Norma (carpenter's square)

Normae

Nor

16

-50

Octans (octant; navigation device)

Octantis

Oct

22

-85

Pavo (peacock)

Pavonis

Pav

20

-65

Phoenix (Phoenix; mythical bird)

Phoenicis

Phe

1

-50

Pictor (painter's easel)

Pictoris

Pic

6

-55

Puppis (stern)*

Puppis

Pup

8

-40

Pyxis (nautical compass)*

Pyxidis

Pyx

9

-30

Name/Meaning (Latin [English])

Genitive Form of Latin Name

Abbreviation

Approximate Position (Equatorial Coordinates)

RA(h)

§n

Reticulum (net)

Reticuli

Ret

4

-60

Sculptor (sculptor's workshop

Sculptoris

Scl

0

-30

Scutum (shield)

Scuti

Sct

19

-10

Sextans (sextant)

Sextantis

Sex

10

0

Telescopium (telescope)

Telescopii

Tel

19

-50

Triangulum Australe (southern triangle)

Trianguli Australe

TrA

16

-65

Tucana (toucan)

Tucanae

Tuc

0

-65

Vela (sail)*

Velorum

Vel

9

-50

Volans (flying fish)

Volantis

Vol

8

-70

Vulpecula (fox)

Vulpeculae

Vul

20

+25

*Originally part of ancient constellation Argo Navis (ship of Argonauts)

*Originally part of ancient constellation Argo Navis (ship of Argonauts)

compiled in the work Syntaxis by Ptolemy about 150 b.c.e.

As the Roman Empire expounded, the Romans adopted the Greek constellations and spread their usage throughout the Western world. But, as the Roman Empire declined and the Dark Ages ensued, the light of the constellations was largely preserved only in the Arabic world, where the works were translated into The Almagest, in which many of the older observations were embellished and more detailed observations added. At the end of the Dark Ages the traditional Greek constellations experienced a revival in Europe during the Renaissance, initiating a period of rapid scientific inquiry. In 1603 the German astronomer Johann Bayer published Uranome-tria, the first major star catalog covering the entire celestial sphere visible from Earth. Bayer introduced the nomenclature of using Greek letters for the main stars in each constellation, assigning a (alpha) to the brightest, b (beta) to the second brightest, and so on, as well as named a dozen new constellations in the Southern Hemisphere. Since then other astronomers have named additional constellations, including several named by the Polish-German astronomer Johannes Hevelius, and the 18th-century French astronomer Nicolas-Louis de Lacaille. Astonomers today recognize 88 different constellations, including 47 of the 48 original Greek constellations. These are listed in the table the Modern Constellations.

See also astronomy; galaxies; galaxy clusters; universe.

FURTHER READING

Chaisson, Eric, and Steve McMillan. Astronomy Today. 6th ed. Upper Saddle River, N.J.: Addison-Wesley, 2007.

Comins, Neil F. Discovering the Universe. 8th ed. New

York: W. H. Freeman, 2008. Dibon-Smith, Richard. The Constellations Web Page. Available online. URL: http://www.dibonsmith.com/ index.htm. Updated November 8, 2007. Snow, Theodore P. Essentials of the Dynamic Universe: An Introduction to Astronomy. 4th ed. St. Paul, Minn.: West, 1991.

continental crust Continental crust covers about 34.7 percent of the Earth's surface, whereas exposed continents cover only 29.22 percent of the surface, with the discrepancy accounted for in the portions of continents that lie underwater on the continental shelves. Its lateral boundaries are defined by the slope break between continental shelves and slopes, and its vertical extent is defined by a jump in seismic velocities to 4.7-5 miles per second (7.6-8.0 km/s) at the Mohorovicic discontinuity. The continental crust ranges in thickness from about 12.5 to about 37 miles (20-60 km), with an average thickness of 24 miles (39 km). The continents are divided into orogens, made of linear belts of concentrated deformation, and cratons, marking the stable, typically older interiors of the continents. The distribution of elevation of continents and oceans can be portrayed on a curve showing the percentage of land at a specific elevation, versus elevation, known as the hypsometric curve, or the hypsographic curve. The curve is a cumulative frequency profile representing the statistical distribution of areas of the Earth's solid surface above or below mean sea level. The hypsometric curve is strongly bimodal, reflecting the two-tier distribution of land in continents close to sea level, and on ocean floor abyssal plains 1.9-2.5 miles (3-4 km) below sea level. Relatively little land surface is found in high mountains or in deep-sea trenches.

Most of the continental crust is now preserved in Archean cratons that form the cores of many continents. They are composed of ancient rocks that have been stable for billions of years, since the Archean. Cratons generally have low heat flow, few if any earthquakes, and no volcanism, and many are overlain by flat-lying shallow water sedimentary sequences. Continental shields are places where the cratonic crust is exposed at the surface, whereas continental platforms are places where the cratonic rocks are overlain by shallow-water sedimentary rocks, presently exposed at the surface.

Most cratons have a thick mantle root or tecto-sphere, characterized seismologically and from xeno-lith studies to be cold and refractory, having had basaltic melt extracted from it during the Archean. Seismological studies have shown that many parts of the tectosphere are strongly deformed, with most of the minerals oriented in planar or linear fabrics. Current understanding about the origin of stable continental cratons and their roots hinges on recognizing which processes change the volume and composition of continental lithosphere with time, and how and when juvenile crust evolved into stable continental crust. Despite decades of study, several major unresolved questions remain concerning Archean tectosphere: How is it formed? Large quantities of melt extraction (ultramafic in composition, if melting occurred in a single event) are required from petrological observations, yet little of this melt is preserved in Archean cratons, which are character

Ocean Shelf Eroded Orogen Shield Orogen

Ocean Shelf Eroded Orogen Shield Orogen

Melting Continental Crustal
G Infobase Publishing

General crustal structure of different provinces as determined by seismology. Numbers in boxes represent densities in grams/cm3, and other numbers represent the seismic velocity of P waves in kilometers per second: BG = basalt-gabbro, GAG = amphibolite and granulite, Pe = peridotite, d = diorite, An Ga = anorthositic gabbro, S = sediments, Gr = granitic-gneiss upper crust, M = Mohorovicic discontinuity.

ized by highly evolved crust compositions. In what tectonic settings is it formed? Hypotheses range from intraplate, plume-generated settings to convergent margin environments. Finally, once formed, does the chemical buoyancy and inferred rheological strength of the tectosphere preserve it from disruption? Until recently most scientists would argue that cratonic roots last forever—isotopic investigations of mantle xenoliths from the Kaapvaal, Siberian, Tanzanian, and slave cratons document the longevity of the tectosphere in these regions. However, the roots of some cratons are now known to have been lost, including the North China craton, and the processes of the loss of the tectosphere are as enigmatic as the processes that form the roots.

Orogens and orogenic belts are elongate regions that are eroded mountain ranges, and typically have abundant folds and faults. Young orogens are mountainous and include such familiar mountain ranges as the Rockies, Alps, and the slightly older Appalachians. Many Archean cratons are welded together by Proterozoic and younger orogens. In fact many Archean cratons can be divided into smaller belts that represent fragments of the planet's oldest oro-genic belts.

orogens have been added to the edges of the continental shield and cratons through processes of mountain building related to plate tectonics. Mountain belts are of three basic types, including fold and thrust belts, volcanic mountain ranges, and fault-block ranges. Fold and thrust belts are contractional mountain belts, formed where two tectonic plates collided, forming great thrust faults, folds, metamor-phic rocks, and volcanic rocks. Detailed mapping of the structure in the belts can enable geologists to reconstruct their history and essentially pull them back apart. Investigations have revealed that many of the rocks in fold- and thrust-belt types of mountain ranges were deposited on the bottom of the ocean, continental rises, slope, shelves, or on ocean margin deltas. When the two plates collide, many of the sediments get scraped off and deformed, forming the mountain belts. Thus fold and thrust mountain belts mark places where oceans have closed.

Volcanic mountain ranges include Japan's Fuji and Mount st. Helens in the Cascades of the western united states. These mountain ranges are not formed primarily by deformation, but by volcanism associated with subduction and plate tectonics.

Fault-block mountains, such as the Basin and Range Province of the western united states, are formed by the extension or pulling apart of the continental crust, forming elongate valleys separated by tilted fault-bounded mountain ranges.

Every rock type known on Earth is found on the continents, so averaging techniques must be used to determine the overall composition of the crust. Estimates suggest that continental crust has a composition equivalent to andesite (or granodio-rite) and is enriched in incompatible trace elements, the elements that do not easily fit into lattices of most minerals and tend to get concentrated in magmas.

The continents exhibit a broadly layered seismic structure that is different from place to place, and different in orogens, cratons, and parts of the crust with different ages. In shields the upper layer may typically be made of a few hundred meters of sedimentary rocks underlain by generally granitic types of material with seismic velocities of 3.5 to 3.9 miles per second (5.7-6.3 km/s) to depths of a couple to 6 miles (a few to 10 km), then a layer with seismic velocities of 3.9-4.2 miles per second (6.4-6.7 km/s). The lower crust is thought to be made of layered amphibolite and granulite with velocities of 4.2-4.5 miles per second (6.8-7.2 km/s). Orogens tend to have thicker, low-velocity upper layers and a lower-velocity lower crust.

Considerable debate and uncertainty surrounds the timing and processes responsible for the growth of the continental crust from the mantle. Most scientists agree that most of the growth occurred early in Earth history, since more than half of the continental crust is Archean in age, and about 80 percent is Precambrian. some debate centers on whether early tectonic processes resembled those currently operating, or whether they differed considerably. The amount of current growth and how much crust is being recycled back into the mantle are currently poorly constrained. Most petrological models for the origin of the crust require that it be derived by a process including partial melting from the mantle, but simple mantle melting produces melts that are not as chemically evolved as the crust. Therefore the crust is probably derived through a multistage process, most likely including early melts derived from seafloor spreading and island arc magmatism, with later melts derived during collision of the arcs with other arcs and continents. Other models seek to explain the difference by calling on early higher temperatures leading to more evolved melts.

See also craton; greenstone belts; orogeny.

FURTHER READING

Taylor, Stuart Ross, and Scott M. McLennan. Planetary

Crusts. Cambridge: Cambridge University Press,

2008.

continental drift The theory of continental drift was a precursor to plate tectonics. Proposed most clearly by Alfred Wegener in 1912, continental drift states that the continents are relatively light objects that are floating and moving freely across a substratum of oceanic crust. The theory was largely discredited because it lacked a driving mechanism, and seemed implausible if not physically impossible to most geologists and geophysicists at the time. But many of the ideas of continental drift were later incorporated into the paradigm of plate tectonics.

Early geologists recognized many of the major tectonic features of the continents and oceans. Cratons are very old, stable portions of the continents that have been inactive since the Precam-brian. They typically exhibit subdued topography, including gentle arches and basins. Orogenic belts are long, narrow belts of structurally disrupted and metamorphosed rocks, typified (when active) by volcanoes, earthquakes, and folding of strata. Abyssal plains are stable, flat parts of the deep oceanic floor, whereas oceanic ridges are mountain ranges beneath the sea with active volcanoes, earthquakes, and high heat flow. To explain the large-scale tectonic features of the Earth, early geologists proposed many hypotheses, including popular ideas that the Earth was either expanding or shrinking, forming ocean basins and mountain ranges. In 1910-25 Wegener published a series of works including his 1912 treatise on The Origin of Continents and Oceans. Wegener proposed that the continents were drifting about the surface of the planet, and that they once fit back together to form one great supercontinent, Pangaea. To fit the coastlines of the different continental masses together to form his reconstruction of Pangaea, Wegener defined the continent/ocean transition as the outer edge of the continental shelves. The continental reconstruction proposed by Wegener showed remarkably good fits between coastlines on opposing sides of ocean basins, such as the Brazilian Highlands of South America fitting into the Niger delta region of Africa. Wegener was a meteorologist, and since he was not formally trained as a geologist, few scientists at the time believed his findings, although we now know that he was largely correct.

Most continental areas lie approximately 985 feet (300 m) above sea level, and if we extrapolate present erosion rates back in time, we find that continents would be eroded to sea level in 10-15 million years. This observation led to the application of the principle of isostasy to explain the elevation of the continents. Isostasy, which is essentially Archimedes' principle, states that continents and high topography are buoyed up by thick continental roots floating in a denser mantle, much like icebergs floating in water. The principle of isostasy states that the elevation of any large segment of crust is directly proportional to the thickness of the crust. It is significant that geologists working in scandinavia noticed that areas that had recently been glaciated were rising quickly relative to sea level, and they equated this observation with the principle of isostatic rebound. The flow of mantle material within the zone of low viscosity beneath the continental crust accommodates isostatic rebound to compensate for the rising topography. These observations revealed that mantle material can flow at rates of a couple of inches (several centimeters) per year.

In The Origin of Continents and Oceans Wegener fit all the continents back together to form a Permian supercontinent, Pangaea (or "all land"). Wegener also used indicators of past climates, such as locations of ancient deserts and glacial ice sheets, and distributions of certain plant and animal species to support his ideas. Wegener's ideas found support from a famous south African geologist, Alex L. Du Toit, who in 1921 matched the stratigraphy and structure across the Pangaea landmass. Du Toit found the same plants, such as the Glossopterous fauna, across Africa and south America. He also documented similar reptiles and even earthworms across narrow belts of Wegener's Pangaea, supporting the concept of continental drift.

Plate Tectonics Climate

Modification of Alfred Wegener's reconstruction of Pangaea, originally from Origin of Continents and Oceans

Even with evidence such as the matching of geological belts across Pangaea, most geologists and geophysicists doubted the idea due to the lack of a conceivable driving mechanism, thinking it mechanically impossible for relatively soft continental crust to plow through the much stronger oceans. Early attempts at finding a mechanism were implausible and included such ideas as tides pushing the continents. Because of the lack of credible driving mechanisms, continental drift encountered stiff resistance from the geologic community, as few could understand how continents could plow through the mantle.

In 1928 British geologist Arthur Holmes suggested a driving mechanism for moving the continents. He proposed that heat produced by radioactive decay caused thermal convection in the mantle, and that the laterally flowing mantle dragged the continents with the convection cells. He reasoned that if the mantle can flow to allow isostatic rebound following glaciation, then maybe it can flow laterally as well. The acceptance of thermal convection as a driving mechanism for continental drift represented the foundation of modern plate tectonics. In the 1950s and 1960s the paleomagnetic data were collected from many continents and argued strongly that the continents had indeed been shifting, both with respect to the magnetic pole and also with respect to one another. When seafloor spreading and subduction of oceanic crust beneath island arcs was recognized in the 1960s, the model of continental drift was modified to become the new plate tectonic paradigm that revolutionized and unified many previously diverse fields of the earth sciences.

See also Du Toit, Alexander; Holmes, Arthur; plate tectonics; supercontinent cycles; Wegener, Alfred.

FURTHER READING

Moores, Eldridge M., and Robert Twiss. Tectonics. New

continental margin Continental margins are the transition zone between thick, buoyant continental crust and the thin, dense submerged oceanic crust. There are several different types, depending on the tectonic setting. Passive, trailing, or Atlantic-type margins form where an extensional boundary evolves into an ocean basin, and new oceanic crust is added to the center of the basin between continental margins that originally faced one another. These margins were heated and thermally elevated during rifting and gradually cooled and thermally subsided for several tens of millions of years, slowly accumulating thick sequences of relatively flat sedi ments, forming continental shelves. Continental slopes and rises succeed these shelves seaward. The ocean/continent boundary typically occurs at the shelf/slope break on these Atlantic-type margins, where water depths average fewer than a thousand feet (a couple of hundred meters). Passive margins do not mark plate boundaries but rim most parts of many oceans, including the Atlantic and Indian, and form around most of Antarctica and Australia. young, immature passive margins are beginning to form along the Red sea.

Convergent, leading, or Pacific-type margins form at convergent plate boundaries. They are characterized by active deformation, seismicity, and vol-canism, and some have thick belts of rocks known as accretionary prisms scraped off of a subducting plate and added to the overriding continental plate. Convergent margins may have a deep sea trench up to seven miles (11 km) deep marking the boundary between the continental and oceanic plates. These trenches form where the oceanic plate is bending and plunging deep into the mantle. Abundant folds and faults in the rocks characterize convergent margins. other convergent margins are characterized by old eroded bedrock near the margin, exposed by a process of sediment erosion where the edge of the continent is eroded and drawn down into the trench.

A third type of continental margin forms along transform or transcurrent plate boundaries. These are characterized by abundant seismicity and deformation, and volcanism is limited to certain restricted areas. Deformation along transform margins tends to be divided into different types, depending on the orientation of bends in the main plate boundary fault. Constraining bends form where the shape of the boundary restricts motion on the fault, and are characterized by strong folding, faulting, and uplift. The Transverse Ranges of southern California form a good example of a restraining bend. sedimentary basins and subsidence characterize bends in the opposite direction, where the shape of the fault causes extension in areas where parts of the fault diverge during movement. Volcanic rocks form in some of these basins. The Gulf of California and salton trough have formed in areas of extension along a transform margin in southern California.

See also convergent plate margin processes; divergent plate margin processes; plate tectonics; transform plate margin processes.

FuRTHER READING

Davis, R., and D. Fitzgerald. Beaches and Coasts. malden, mass.: Blackwell, 2004. Moores, Eldridge M., and Robert Twiss. Tectonics. New york: W. H. Freeman, 1995.

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