Page 44 - (DK) Ocean - The Definitive Visual Guide

42 OCEAN GEOLOGY

The Origin of Oceans

and Continents

zircon crystals,
among the earliest
continental crust
EARTH’S OCEANS FORMED MORE THAN 4 billion years ago, mainly from materials
water vapor that condensed from its primitive atmosphere but also from water
ZIRCON
brought from space by comets. Initially, after acquiring a layered internal structure, primitive
the Earth had a uniform crust that was enriched in lighter elements and floated continental crust
thickens above
on an upper mantle made of denser materials. Later, the crust became sinking mantle
flow, without
differentiated into two types as continents began to form, made from mantle
interference
rocks that were chemically distinct from those underlying the oceans.

Continental Crust
sedimentary
The continents include a wide range of rock THE OLDEST ROCKS rocks
types, including granitic igneous rocks, sedimentary These sedimentary
rocks, and the metamorphic rocks formed by the rocks on Baffin Island primitive
lie on the Canadian oceanic
alteration of both. They contain a lot of quartz, a crust
Shield. The stable
mineral absent in oceanic crust. The first continental continental shields
rocks were the result of repeated melting, cooling, contain the world’s
and remixing of oceanic crust, driven by volcanic most ancient rocks,
activity above mantle convection cells, which were which are around
4 billion years old.
much more numerous and vigorous than today’s.
Each cycle left more of the heavier components in
the upper mantle and concentrated more of the lighter components in
the crust. The first microcontinents grew as lighter fragments of crust
collided and fused. Thickening of the crust led to melting at its base
and underplating with granitic igneous rocks. Weathering accelerated
volcanic activity
the process of continental rock formation, retaining the most resistant adds igneous rocks
components, such as quartz, while washing solubles into the ocean. to surface above
rising flows
basaltic rift basalt sheets
lava (dikes)
sediment Oceanic Crust
ocean
surface
The oceanic crust has a higher density than the
continental crust, making it less buoyant. Both types
of crust can be thought of as floating on the “plastic”
ocean upper mantle, and the oceanic crust lies lower due to
crust
gabbro its lower buoyancy. It is relatively thin, with a depth
peridotite of never more than 7 miles (11 km), compared with a
thickness of 15–43 miles (25–70 km) for most continental
crust. It consists mainly of basalt, an igneous rock that is
lithosphere
low in silica compared with continental rocks, and richer
in calcium than the mantle. Basalt lava is created when
Moho
hot material in the upper mantle is decompressed, allowing
top layer
magma
asthenosphere rises to of upper it to melt and form liquid magma. The decompression
surface mantle occurs beneath rifts in the crust, such as those found at the
mid-ocean ridges, and it is through these rifts that lava is
OCEAN-FLOOR STRUCTURE
Three layers of basalt in the crust (basaltic extruded onto the surface to create new ocean crust.
lava, dikes, and gabbro) are separated from
the mantle by the Mohorovic ˇic ´ discontinuity
(the Moho). The top layer of the upper mantle
is fused to the base of the crust to form the
rigid lithosphere, which makes up tectonic
plates.The asthenosphere is the soft zone
INTRODUCTION MANTLE ROCKS
over which the plates of the lithosphere glide.

Peridotite is the dominant rock type
found in the mantle, consisting of
silicates of magnesium, iron, and
other metals. Sometimes it is
brought to the surface when parts
of the ocean floor are uplifted, as
here in Newfoundland, Canada, or
as fragments from volcanic activity.