Deformation, Mountain Building, and the Continents - The Changing Earth: Exploring Geology and Evolution - page 265

◗ Figure 10.20 Orogeny and the Origin of a Volcanic Island Arc at an Oceanic-Oceanic
Plate Boundary

Volcanic
island arc

Back-arc basin

Subduction of an oceanic plate and the origin
of a volcanic island arc and a back-arc basin.

a

Back-arc
spreading

Continued subduction and back-arc spreading.

b

Deformation of back-arc basin
and continental margin

Back-arc basin begins to close, resulting in
deformation of back-arc basin and continental
margin deposits.

c

Island arc “plastered”
onto continental margin

Accretionary
wedge

Fore-arc
basin

Back-arc fold
and thrust belt

Metamorphism

Thrusting of back-arc sediments onto the
adjacent continent and suturing of the island arc
to the continent.

d

Earth's crust is similar to the iceberg in that it sinks into
the mantle to its equilibrium level. Where the crust is thick-
est, as beneath mountains, it sinks farther down into the
mantle and it also rises higher above the surface. And because
continental crust is thicker and less dense than oceanic crust,
it stands higher than the ocean basins. Remember, the man-
tle is hot, yet solid, and under tremendous pressure, so it be-
haves in a fl uid-like manner.

( Figure 10.24). Ice is slightly less dense than water, so it

fl oats. According to Archimedes' principle of buoyancy, an
iceberg sinks in water until it displaces a volume of water
whose weight is equal to that of the ice. When the iceberg
has sunk to an equilibrium position, only about 10% of its
volume is above water level. If some of the ice above water
level should melt, the iceberg rises to maintain equilibrium
with the same proportion of ice above and below the water.

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