OCEAN FLOOR SPREADING
In the decade following W.W.II, spurred by all the technological advances
made during the war, exploration of this planet made great strides. By
the late 1950s, there had been a tremendous accumulation of facts and startling
new discoveries had been made about the ocean floor.
Investigating the ocean floor
While direct exploration by drilling and by manned submersibles may
be the best way of exploring the ocean floor, such explorations are slow,
expensive and dangerous. Most of today's work uses remote sensing devices.
They include sonar, especially the newer side-scan sonars, magnetometers,
heat detectors, cameras taking still and movie pictures both underwater
and from satellites in space, and such time honored techniques as dredging
and seismic surveys.
All these techniques provided new facts indicating that oceans and continents
were clearly different. Among others we list: ·
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A distinct topography (see section on the earth's surface features).
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Oceans are comparatively young. None is older than about 170 million years.
In contrast, continents are hundreds of millions or billions of years old.
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Oceans are youngest near ridges, and become progressively older outward
Continents are oldest in their center and are younger at their margins.
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Oceans are areas of relatively large heat loss, especially along the rift
areas. Heat loss is lower in continental areas.
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Oceans are basaltic, continents granitic.
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Oceanic mountains, (the ridge), are usually located near the center of
oceans. Continental mountains are commonly located near the margins of
continents.
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Oceanic ridges are basaltic and have rifts running along their axes. Continental
mountains are deformed wedges of sediments, and have no rifts.
The question then, was how to explain all these contrasting features. It
had long been known that the ocean floor is made of basaltic lava flows.
In 1960, Hess suggested that a newly mapped mountain range that
underlies the Atlantic Ocean, called the Mid-Atlantic Ridge, was a place
where the ocean floor is literally being ripped apart before moving laterally
away from this ridge. Into this tear in the ridge, called a rift, new lava
was rising from the mantle, filling the gap, and forming new ocean floor,
and moving sideways as rifting continued. Perhaps the strongest evidence
that came to support this startling idea came from paleomagnetism.
As lavas cool, the iron oxides they contain acquire the magnetization
of the earth's magnetic field. This magnetic field constantly changes over
time, and at times, the field's polarity has been reversed compared to
the present polarity. During such times of reversal, a compass needle would
point South instead of North. Any newly forming lava acquires the unique
magnetic pattern prevalent at the time of its formation.
Investigation of the magnetization of the ocean floor by Vine and
Matthews showed that the ocean floor had a symmetrical pattern of magnetization
on either side of the mid-ocean rift in site after site. Such a pattern
can only be explained if we accept the idea that a commonly magnetized
layer has been pulled apart. In turn, this could only be true if the ocean
floor was rifting and spreading laterally as Hess suggested, and thus leads
to these layers of similar magnetism hundreds and thousands of miles apart
on either side of the rift.
By the late sixties, there was little doubt in most investigators’ mind
that the earth’s lithosphere was not a static rigid shell, but that it
was made up of pieces of varying sizes called plates all moving on top
of the asthenosphere, perhaps being driven by the convection currents Holmes
has envisioned. Because it is difficult to conceive of moving ocean floors
but static continents, it became clear that continents also were part of
this moving system of plates. While many of the details were yet to be
worked out, Wegener’s basic idea that continents had moved became the accepted
view. Since that time, plate motion has been confirmed by geodetic satellites.
More detailed views of the oceanic layers have also been received from
an ambitious ocean-drilling program that has been going on for over two
and a half decades (since 1968). This drilling has allowed us to determine
the ages of the oceanic materials and confirmed spreading rates and magnetostratigraphy.
Thus, a little over a century after organic evolution had been accepted
by the biological community, the geological community in turn, came to
envision an active and changing world, whose upper surface is composed
of plates in continual motion. This modern unifying model that includes
both ocean floor spreading and continental drift as its components is called
the Plate tectonics theory or model. |