Page 60 - Ultimate Visual Dictionary (DK)

PREHISTORIC EAR TH
The Earth’s crust ELEMENTS IN THE EARTH’S CRUST

THE EARTH’S CRUST IS THE SOLID outer shell of the Earth. It includes Other elements 2%
continental crust (about 25 miles/40 km thick) and oceanic crust (about
four miles/6 km thick). The crust and the topmost layer of the mantle Potassium 2.6% Magnesium 2%
form the lithosphere. The lithosphere consists of semirigid plates Sodium 2.8%
Calcium 3.6%
that move relative to each other on the underlying asthenosphere (a
Iron 5%
partly molten layer of the mantle). This process is known as plate Aluminum 8%
tectonics and helps explain continental drift. Where two plates move Silicon 28%
apart, there are rifts in the crust. In mid-ocean, this movement results
in seafloor spreading and the formation of ocean ridges; on continents,
crustal spreading can form rift valleys. When plates move toward each other,
one may be subducted beneath (forced under) the other. In mid-ocean, this causes
Oxygen 46%
ocean trenches, seismic activity, and arcs of volcanic islands. Where oceanic crust
is subducted beneath continental crust or where continents collide, land may be
uplifted and mountains formed (see pp. 62–65). Plates may also slide past each
other—along the San Andreas fault, for example. Crustal movement on continents
may result in earthquakes, while movement under the seabed can lead to tidal waves.

FEATURES OF PLATE MOVEMENTS Ocean trench formed where oceanic Subduction zone
crust is forced under continental crust
Ridge where magma
is rising to form new
oceanic crust

Region of
seafloor
spreading

Magma rises to
form a hot spot Volcano develops Volcanic island Oceanic Magma rises
Rift formed where over hot spot and that originally crust melts to form a
two plates are Magma (molten rock) builds up to form formed over volcano
moving apart erupts at rift an island hot spot

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