Morphology of the Mid-Ocean Ridge
21
are carried to the ocean by rivers and winds. Here they build up continental margins,
with the left-overs accumulating in abyssal plains.
Much of the sediment comes to the sea through quasi-catastrophic events: floods,
storms, earthquakes or - on a longer time scale - ice advances. Other types of
sediment arrive at the sea floor as a more or less continuous rain of particles: shells
of plankton organisms, wind-bome dust, cosmic spherules. Through geologic time
these gradually accumulating pelagic sediments built up a layer of a few hundred
meters thickness, which forms a veneer on the oceanic crust. This veneer contains a
detailed history of the evolution of ocean circulation and of pelagic organisms, for the
last 100 to 150 million years.
Although exogenic processes tend to level the Earth, by erosion and deposition,
they also can build mountains. The outstanding example is the Great Barrier Reef off
eastern Australia, whose mountain tops rise thousands of meters above the floor of
the Coral Sea. The mesa-like reef mountains are made from the calcium carbonate
secreted by corraline algae, stony corals, mollusks, and small unicellular organisms
called foraminifera. The algae, of course, depend on sunlight. The corals and foraminifera contain unicellular algae within their bodies, in symbiosis: they too depend on
sunlight for growth.
With this briefest of all introductions to the opposing effects of endogenic processes (which wrinkle Earth's surface) and exogenic processes (which mainly smooth
it), let us now return to the nature of the grand morphology of the sea floor - and to
sea-floor spreading.
How does this concept explain the major features of the sea floor?
1.4 Morphology of the Mid-Ocean Ridge
Ultimately, it took geophysical evidence based on crustal magnetism to compel acceptance of the mobility of the ocean floor, and to tum the hypothesis of sea-floor
spreading into the ruling theory. The most obvious achievement of the new theory is
the explanation of the origin of the Mid-Ocean Ridge, the central morphologic feature
of the sea floor. The Mid-Ocean Mountain Range is more than 60000 km long and
takes up one third of the ocean floor, that is, about one fourth of the Earth's surface.
In the Atlantic and along certain other portions, the crest is marked by a central rift,
a 30- to 50-km-wide steep-walled valley 1 km deep, or more (see Figs. 1.2 and 1.3).
The crestal morphology is usually very rugged and complicated, while the flanks tend
to be smoothed by sediment (Fig. 1.7). The following is a brief account of how the
theory of sea-floor spreading explains the character of the Mid-Ocean Ridge.
The crest is characterized by shallow earthquakes (centers at less than about 60 km
deep), by active volcanism, and by high heat flow values. The upwelling and spreading of mantle material pulls apart the crust, producing the central rift, and generating
the earthquakes. It also brings up heat from the Earth's interior. The spreading rate,
that is, the rate at which sea floor on one side moves away from that on the other, is
on the order of 1 to 10 cm per year. The hot mantle material filling the gap is less
dense than old oceanic crust, because of thermal expansion. Away from the central
21
are carried to the ocean by rivers and winds. Here they build up continental margins,
with the left-overs accumulating in abyssal plains.
Much of the sediment comes to the sea through quasi-catastrophic events: floods,
storms, earthquakes or - on a longer time scale - ice advances. Other types of
sediment arrive at the sea floor as a more or less continuous rain of particles: shells
of plankton organisms, wind-bome dust, cosmic spherules. Through geologic time
these gradually accumulating pelagic sediments built up a layer of a few hundred
meters thickness, which forms a veneer on the oceanic crust. This veneer contains a
detailed history of the evolution of ocean circulation and of pelagic organisms, for the
last 100 to 150 million years.
Although exogenic processes tend to level the Earth, by erosion and deposition,
they also can build mountains. The outstanding example is the Great Barrier Reef off
eastern Australia, whose mountain tops rise thousands of meters above the floor of
the Coral Sea. The mesa-like reef mountains are made from the calcium carbonate
secreted by corraline algae, stony corals, mollusks, and small unicellular organisms
called foraminifera. The algae, of course, depend on sunlight. The corals and foraminifera contain unicellular algae within their bodies, in symbiosis: they too depend on
sunlight for growth.
With this briefest of all introductions to the opposing effects of endogenic processes (which wrinkle Earth's surface) and exogenic processes (which mainly smooth
it), let us now return to the nature of the grand morphology of the sea floor - and to
sea-floor spreading.
How does this concept explain the major features of the sea floor?
1.4 Morphology of the Mid-Ocean Ridge
Ultimately, it took geophysical evidence based on crustal magnetism to compel acceptance of the mobility of the ocean floor, and to tum the hypothesis of sea-floor
spreading into the ruling theory. The most obvious achievement of the new theory is
the explanation of the origin of the Mid-Ocean Ridge, the central morphologic feature
of the sea floor. The Mid-Ocean Mountain Range is more than 60000 km long and
takes up one third of the ocean floor, that is, about one fourth of the Earth's surface.
In the Atlantic and along certain other portions, the crest is marked by a central rift,
a 30- to 50-km-wide steep-walled valley 1 km deep, or more (see Figs. 1.2 and 1.3).
The crestal morphology is usually very rugged and complicated, while the flanks tend
to be smoothed by sediment (Fig. 1.7). The following is a brief account of how the
theory of sea-floor spreading explains the character of the Mid-Ocean Ridge.
The crest is characterized by shallow earthquakes (centers at less than about 60 km
deep), by active volcanism, and by high heat flow values. The upwelling and spreading of mantle material pulls apart the crust, producing the central rift, and generating
the earthquakes. It also brings up heat from the Earth's interior. The spreading rate,
that is, the rate at which sea floor on one side moves away from that on the other, is
on the order of 1 to 10 cm per year. The hot mantle material filling the gap is less
dense than old oceanic crust, because of thermal expansion. Away from the central