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Active PlanetEarth is a dynamic planet that is always changing its form. Heat generated by nuclear reactions deep below the surface creates hugely powerful currents that keep Earths rocks on the move, triggering earthquakes and volcanic eruptions. Meanwhile, solar energy striking the planet in different ways creates currents in the air, driving the atmospheric turmoil of the weather. This changes with the seasons and from place to place, creating an enormous range of climates and habitats for the most dynamic element of alllife.

THE PLATES OF EARTHS CRUSTHeat generated deep within the planet creates currents in the mobile mantle rock beneath the crust. These currents drag some sections of the cool, brittle crust apart while pushing other parts together, fracturing the crust into separate plates. The biggest of these span oceans and continents, but there are many smaller plates. At their boundaries the plates may be diverging (pulling apart), converging (pushing together), or sliding past each other at transform faults.

WHERE MOVING PLATES MEETThe boundaries between the plates are volcanic earthquake zones. The plates move very slowly, pulling apart at divergent boundaries. This allows hot rock below to melt, erupt, and cool to form new crust especially at the spreading rifts that form mid-ocean ridges. Meanwhile, at convergent boundaries, one plate slides beneath another, pushing up mountain ranges and making volcanoes erupt. Other volcanoes erupt over hot spots in the mantle below the crust.

CaribbeanPlate

Cocos Plate

North American Plate North American Plate

Eurasian Plate

African Plate

Antarctic Plate

Indo-AustralianPlate

PacificPlate

PacificPlate South American

Plate

NazcaPlate

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Divergent boundary

Convergent boundary

Transform fault

Uncertainboundary

Lower atmosphere, 10 miles (16 km) thick

Crust, 545 miles (870 km) thick

Continental crust, much thicker than oceanic crust

Broad basin formed near uplifted area

Ancient converging boundary, now inactive

Mountains created when plate boundary was active

Oceanic crust formed from heavy basalt rock

Upper mantle, mostly solid but very hot

Mantle, solid but mobile owing to heat currents

Spreading rift forming a mid-ocean ridge

Hot-spot volcano erupting over mantle plume

Mantle, 1,800 miles (2,900 km) thick Liquid outer

core, 1,400 miles (2,250 km) thick

Solid inner core, 1,515 miles (2,440 km) across

ACTIVE PLANET

DOWN TO THE COREEarth formed from iron-rich asteroids that

smashed together to build the planet. Early in its history it, melted, allowing

the heavy iron to sink and create a metallic core. The core is

surrounded by lighter rock, with the lightest forming Earths crust. Most of the water on the planet lies in huge oceans, and above

them is the layer of air that forms the atmosphere.

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Ocean trench marking convergent plate boundary

Volcano erupting over convergent boundary

Earthquake zoneone plate grinding under another

Plates pulling apart, creating a rift valley

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(c) 2011 Dorling Kindersley. All Rights Reserved.

ACTIVE PLANET

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Key to map

Less than 20 cm (50 in)

2079 in (50200 cm)

More than 79 in (200 cm)

December is the southern summer and

northern winter

Arctic Circle, where Suns rays

are dispersed

Equator, where Suns rays are

concentrated

March is the northern spring

June is the northern summer and southern winter

Sahara Desert

Atacama Desert, Chile

The Sun

Rising warm, moist air near Equator

Rainforest, Borneo

Tropic of Capricorn

THE SEASONSEarth spins on a tilted axis, so as it orbits the Sun once a

year, the North Pole points toward the Sun in June and away from it in December. This means that in

regions north of the Tropics it is summer in June but winter in Decemberand the opposite is

true to the south of the tropics. Near the Equator it is always warm and there are

annual wet and dry seasons.

COLD POLEThe Tropics are the hottest parts of the planet because the Suns rays directly

strike them, concentrating the heat energy. Near the poles the same amount

of heat energy is spread out over a broader area, so it does not have as much

heating effect, even in the summer. At midwinter, the entire polar region is in

permanent darkness, so it gets no solar energy at all and is bitterly cold.

JUNGLE AND DESERTConcentrated sunlight near the Equator heats Earths surface, warming the air above. The warm air rises, carrying moisture with it. This forms huge clouds that spill tropical rain, fueling the growth of rainforests. The dry, cooling air then flows north and south and sinks over the subtropics, creating deserts. Similar air-circulation patterns affect the climate in the far north and south.

RAINFALLSome parts of the world get much more rain than others.

The wettest regions are mostly rainforest zones, where year-round rain and warmth promote lush plant growth.

Regions of moderate rainfall are naturally forests and grasslands, although most of this land is now used for

farming. The driest regions may be too dry for many plants to grow, creating desertsbut they also include

some northern forest zones and polar tundra.

Descending cool, dry air over desert zone

Cold air (in blue) becomes chilled in upper atmosphere

Warm air (in red) heats up near Earths surface

Cool, dry air sinks over desert zone

Tropic of Cancer

Earths axis

South Pole

North Pole

(c) 2011 Dorling Kindersley. All Rights Reserved.