Every year, earthquakes and volcaniceruptions cause much destruction

throughout the world. Such phenomenawere once thought to be unconnected, butsince the late 1960s, scientists have under-stood that these events are surface manifes-tations of the tremendous forces operatingin the Earth’s interior that are slowly butconstantly changing the face of our planet.

The Earth is divided into three zones.The crust, a brittle, low-density zone, over-lies the dense mantle. Separating the crustfrom the mantle is a distinct boundarycalled the Mohorovi?i> (or Moho) discon-tinuity. Enclosed by the mantle is theEarth’s core, which consists mainly of ironand nickel.

Temperatures inside the Earth rangefrom about 1,600°F in the upper mantle toperhaps 9,000°F in the core. Heat creates

convection currents in a semimolten partof the mantle called the asthenosphere.Above the asthenosphere is the lithosphere,a solid layer about 40 miles thick, consistingof the crust and part of the mantle. Thelithosphere is divided into rigid plates,moved around by the currents in the astheno-sphere, a process named plate tectonics.

The Earth was formed around 4.6 billionyears ago. Lighter elements floated towardthe surface, where they formed crustalrocks. The oldest rocks so far discoveredare about 4 billion years old, while the oldest fossils occur in rocks formed around3.5 billion years ago. An explosion of lifeoccurred at the start of the Cambrian period, 570 million years ago. The fossilrecord since the start of the Cambrian hasenabled scientists to piece together thestory of life on Earth.

COPYRIGHT PHILIP’S

Submarine volcanoes

Land volcanoes active since 1700

Boundaries of tectonic plates

L a u r a s i a

G o n d w a n a l a n

T e t h y sS e a

Transitional zone(370 miles)

Upper mantle(c. 230 miles) Crust (average 3–30 miles)

Solid inner core(840 miles)

180 million years ago 135 million years ago

Trench

Rift

New ocean floor

Zones of slippage

Present day

Lower mantle(1,050 miles)

Molten outer core(1,300 miles)

Andes

AMERICAN PLATE

Mid-AtlanticRidge

Asthenosphere

HimalayasTibetanPlateau

Mid-IndianOcean Ridge

Sea level

Sea-floor spreading in the Atlantic Ocean and plate collision

Atlantic OceanPeru–Chile

TrenchContinental crust

(sial)

Upwellingmagma

NAZCAPLATE

INDIAN–AUSTRALIAN

PLATE

IndiaAsia

Constructiveplate margin

Lithosphere

Oceanic crust(sima)

Collisionzone

Brazilian Plateau

Sea-floor spreading in the Indian Ocean and continental plate collision

The huge ridges that run through the oceans representboundaries between plates. Here plates are diverging andmolten magma from the mantle rises along a central riftvalley to form new crustal rock. These ocean ridges, which

are active zones where earthquakes and volcanic eruptionsare common, are called constructive plate margins. Des-tructive plate margins, which occur when two platesconverge, are marked by deep-ocean trenches as one plate

is forced under the other. The descending plate is melted toproduce the magma that fuels volcanoes alongside thetrenches. Movements of descending plates are often sudden,triggering earthquakes in overlying continental areas.

AFRICAN PLATE

South America

Diagrams not drawn to scale

Volcanoes occur when hot liquefied rock beneaththe Earth’s crust is pushed up by pressure to thesurface as molten lava. There are some 550 knownactive volcanoes, around 20 of which are eruptingat any one time.

Eruption at side ventLayers of cinders and

lava from previouseruptions

Ash and gas cloud

Neck or pipe

Lava flow

Volcanic bombs

Main vent

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For more information:

72 Types of rock

Mountain building

73 Surface processes

83 The carbon cycle

98 Minerals

In 1915, Alfred Wegener produced a series of world maps proposing that, around 200 millionyears ago, the continents had been joined togetherin a supercontinent that he called Pangaea. This

land mass started to break up about 180 millionyears ago and the parts drifted to their presentpositions. In the 1950s and 1960s, evidence fromstudies of the ocean floor suggested that the low-

density continents rest on huge slow-movingplates. The arrows on the present-day worldmap (below) show that the continents are stillon the move.

CONTINENTAL DRIFT

DISTRIBUTION OF VOLCANOES

PLATE TECTONICS

70 GEOLOGY OF THE EARTH

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