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Canadian Shield

GuianaShield

AmazonianShield

IndianShield

AngaraShield

Arabian Shield

BalticShield

AustralianShield

ChineseShield

Ethiopian Shield

Valley

Equator

Tropic of Cancer

Arctic Circle

Tropic of Capricorn

Antarctic Circle

2

12

Holocene 10,000 BP to presentPleistocene

Pliocene

26

37

53

65

135

190

225

280

345

395430

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570

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Miocene

Oligocene

Eocene

Paleocene

Jurassic

Cretaceous

Triassic

Permian

Carboniferous

Devonian

Silurian

Ordovician

Cambrian

Tertiary

Quaternary

CEN

OZO

ICM

ESO

ZOIC

PALE

OZO

ICPR

E-CA

MB

RIA

N

ERA PERIOD EPOCH

COPYRIGHT PHILIP’S

Pre-Cambrian shields

Sedimentary cover on Pre-Cambrian shieldsPaleozoic (Caledonian and Hercynian) foldingSedimentary cover on Paleozoic folding

Mesozoic folding

Sedimentary cover on Mesozoic folding

Cenozoic (Alpine) folding

Sedimentary cover on Cenozoic foldingIntensive Mesozoic and Cenozoic vulcanism

Principal faults

Oceanic marginal troughs

Midoceanic ridges

Overthrust faults

Earthquake magnitude is usually rated according to either the Richter or the Modified Mercalli scale, both devised by seismologists in the1930s. The Richter scale measures absolute earthquake power withmathematical precision: each step upward represents a tenfold increasein the amplitude of the shockwave. Theoretically, there is no upper limit,but most of the largest earthquakes measured have been rated atbetween 8.8 and 8.9. The 12-point Mercalli scale, based on observedeffects, is often more meaningful, ranging from I (earthquakes noticedonly by seismographs) to XII (total destruction); intermediate pointsinclude V (people awakened at night; unstable objects overturned), VII(collapse of ordinary buildings; chimneys and monuments fall), and IX(conspicuous cracks in ground; serious damage to reservoirs).

Notable Earthquakes Since 1900Year Location Mag. Deaths1906 San Francisco, USA 8.3 3,0001906 Valparaiso, Chile 8.6 22,0001908 Messina, Italy 7.5 83,0001915 Avezzano, Italy 7.5 30,0001920 Gansu (Kansu), China 8.6 180,0001923 Yokohama, Japan 8.3 143,0001927 Nan Shan, China 8.3 200,0001932 Gansu (Kansu), China 7.6 70,0001933 Sanriku, Japan 8.9 2,9901934 Bihar, India/Nepal 8.4 10,7001935 Quetta, India* 7.5 60,0001939 Chillan, Chile 8.3 28,0001939 Erzincan, Turkey 7.9 30,0001960 S. W. Chile 9.5 2,2001960 Agadir, Morocco 5.8 12,0001962 Khorasan, Iran 7.1 12,2301964 Anchorage, USA 9.2 1251968 N. E. Iran 7.4 12,0001970 N. Peru 7.8 70,0001972 Managua, Nicaragua 6.2 5,0001974 N. Pakistan 6.3 5,2001976 Guatemala 7.5 22,5001976 Tangshan, China 8.2 255,0001978 Tabas, Iran 7.7 25,0001980 El Asnam, Algeria 7.3 20,0001980 S. Italy 7.2 4,8001985 Mexico City, Mexico 8.1 4,2001988 N.W. Armenia 6.8 55,0001990 N. Iran 7.7 36,0001992 Flores, Indonesia 6.8 1,8951993 Maharashtra, India 6.4 30,0001994 Los Angeles, USA 6.6 511995 Kobe, Japan 7.2 5,0001995 Sakhalin Is., Russia 7.5 2,0001996 Yunnan, China 7.0 2401997 N. E. Iran 7.1 2,4001998 Takhar, Afghanistan 6.1 4,2001998 Rostaq, Afghanistan 7.0 5,0001999 Izmit, Turkey 7.4 15,0001999 Taipei, Taiwan 7.6 1,7002001 Gujarat, India 7.7 14,0002002 Baghlan, Afghanistan 6.1 1,0002003 Boumerdes, Algeria 6.8 2,2002003 Bam, Iran 6.6 30,0002004 Sumatra, Indonesia 9.0 250,0002005 N. Pakistan 7.6 74,0002006 Java, Indonesia 6.4 6,200

An earthquake off the coast of Sumatra on December 26, 2004, triggered a deadlytsunami that swept across the IndianOcean, causing devastation in manycountries, in particular Sri Lanka, India,Thailand, and Indonesia, where the loss of life was greatest.* now Pakistan

Time, in millions of years before thepresent, is shown on a sliding scale,greatly compressed in the distant past.

Geologists devised their timescale on the basis of relative, not calendar, ages.Accurate dating was impossible and estimates were often bitterly disputed, but the

order in which the rocks were formed could be deduced from careful observation.The advent of radioactive dating – culminating in the 1950s with the

development of a mass spectrometer capable of accurately measuring tinyquantities of isotopes – appears to have settled the arguments. The Earth is

far older than geologists first imagined, but their painstakingly-createdstructure of geological time has withstood the advent of high technology.

The 4.6 billion (4,600 million) years since the formation of the Earth aredivided into four great eras, further split into periods and, in the case

of the most recent era, epochs. The present era is the Cenozoic (“newlife”), extending backward through “middle life” and “ancient life”

to the Pre-Cambrian, named after the Latin word for Wales, the location of some of the earliest known fossils. Most

of the Earth’s geological history is encompassed by the Pre-Cambrian: though traces of ancient life have since

been found, it was largely the proliferation of fossils from the beginning of the Paleozoic era onward, some

570 million years ago, which first allowed precisesubdivisions to be made.

Like the Cambrian, most are named after regionsexemplifying a period’s geology. Others – such

as the Carboniferous (“coal-bearing”) or theCretaceous (“chalk-bearing”) – are more directly

descriptive.

Anchorage1964

San Francisco1906

Managua1972

Mexico City1985

N. Peru1970

S.W. Chile1960

Chillan1939

Valparaiso1906

El Asnam1980

Agadir1960

Avezzano1915

Messina1908

Erzincan1939

N. Iran1990

Tabas1978

Quetta1935 Maharashtra

1993

Bihar1934

Tangshan1976

Gansu1920/1932

Nan Shan1927

Yokohama1923

Los Angeles1994 Kobe

1995

Sakhalin1995

Equator

Tropic of Cancer

Arctic Circle

Tropic of Capricorn

Antarctic Circle

Flores1992

Takhar/Rostaq1998

Izmit1999

Taipei1999

Gujarat2001

Guatemala1976

Sanriku1933

N.E. Iran1968/1997

Yunnan1996

N.W. Armenia1988

S. Italy1980

Khorasan1962

N. Pakistan1974

Baghlan2002

Boumerdes2003

Bam2003

Sumatra2004

N. Pakistan2005

Java2006

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Shockwaves reachthe surface

Epicenter – point onthe surface directlyabove the origin

Origin orfocus

Shockwavestravel outward

Mobile land areas

Submarine zones of mobile land areas

Stable land platforms

Submarine extensions of land platforms

Midoceanic volcanic ridges

Oceanic platforms

Principal earthquakes and dates(since 1900)

1976

X

Subduction zone

Earthquakes are a series of rapidvibrations originating from the slipping or faulting of parts of the Earth’s crustwhen stresses within build up to breakingpoint. They usually happen at depthsvarying from 5 to 20 miles. Severeearthquakes cause extensive damagewhen they take place in populated areas,destroying structures and severingcommunications. Most initial loss of lifeoccurs due to secondary causes such as falling masonry, fires, and flooding.

EARTHQUAKES

GEOLOGICAL TIME

GEOLOGY OF THE EARTH 71

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