Page 21 - Oceans

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Crust carried sideways
Upper mantle by moving mantle continental drift
flows sideways Crust dragged apart
at midocean rift As the Earth’s crust is dragged around by the mobile mantle, the
Sinking current slabs of rock that form the continents are dragged around with it.
drags ocean This makes the continents and oceans change in size and shape.
crust down
pangaea
Some 200 million years ago there was just one huge supercontinent
that geologists call Pangaea. This was surrounded by a global ocean
known as Panthalassa.
Heat within the
Earth creates
convection currents
≤ THE cREEPing MAnTLE
Most of the Earth’s mantle is solid, but it is able
to flow extremely slowly, rather like the ice of a breakup
glacier. Heat generated inside the planet keeps By 100 million years ago, America and Africa had drifted apart, opening
it moving in currents that rise, flow sideways, up the Atlantic Ocean; India was moving north toward Asia.
cool, and then sink. These convection currents
drag sections of the Earth’s crust apart and carry
them sideways. They draw the thin, heavy ocean
crust back into the mantle, but the lighter rocks
of the continents stay on the surface.

Oceanic crust: solid
rock, 5 miles (8 km)
thick; up to 1,800°F
(1,000°C) today's oceans
continents During the next 100 million years the Atlantic became broader as
Ocean: water, America moved west. Panthalassa shrank to become the Pacific, while
average about 2.5 India drifted north to collide with Asia.
miles (4 km) deep

< Moving PLATEs
The creeping mantle effect has
noRTH made the Earth’s crust break into
AMERicAn many separate plates that are
EuRAsiAn PLATE constantly on the move. They
PLATE
are either slipping apart at the
divergent boundaries that form
midocean ridges, pushing together
okHoTsk at the convergent boundaries that
noRTH ARAbiAn PLATE
juAn AMERicAn PLATE are marked by
DE fucA PLATE PAcific the deep ocean trenches, or sliding
PLATE PLATE
cARibbEAn PLATE past each other at transform
boundaries. These plate boundaries
are the sites of frequent
PHiLiPPinE earthquakes and tsunamis, and
cocos PLATE AfRicAn PLATE
PLATE they are dotted with volcanoes
souTH and volcanic islands.
PAcific AMERicAn
PLATE nAzcA PLATE inDo-AusTRALiAn
PLATE PLATE
scoTiA kEY
PLATE
AnTARcTic Convergent boundary
PLATE
Divergent boundary
Transform boundary
Uncertain boundary