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The
Evidence for Plate Tectonics
Age
and Nature of the Ocean Bottom The basaltic lavas that make up
the ocean floor were discovered to be less than 180 million years old, and the
amount of sediments in the ocean basins were much thinner than expected.
The absence of old oceanic rocks was disconcerting given that the oldest
continental rocks were over 3 billion years old (the oldest continental
crust is now known to be 3.96 billion years old)! In addition, the ocean
floor was not flat and featureless as expected. Numerous oceanographic
surveys finally led to the conclusion that a great submarine mountain range more
than 50,000 kilometers long, up to 800 kilometers across and 4,500 meters or
more high virtually encircled the Earth. This mountain range is often
called the mid-oceanic ridge, although not all parts of the ridge are
located near the middle of the ocean.
The young age of the
oceanic crust in comparison to the ancient continental crust suggested that
recycling of oceanic crust was occurring.
Zebra-Striped
Magnetic Patterns of the Ocean Basalts In the 1950s,
geophysicists, using magnetometers to determine the polarity of magnetite
crystals in the basalt lavas that made up the ocean bottoms, made some other
important discoveries. It was known that as magnetite grains crystallize
in lavas, the magnetite grains align themselves with the orientation of the
Earth's magnetic field. The geophysicists recognized that there was a
striped pattern of alternating normal polarity, reversed polarity, normal
polarity in the basalts of the oceans. The basalt lavas marked by reverse
polarity magnetism must have crystallized during a period when the Earth's
magnetic field was the opposite of what it is today (at those times, a compass
would point to the south pole instead of the north pole). The discovery
that the linear pattern of magnetism was symmetrical across the submarine
mountain range, and that the basalt lavas became increasingly older on either
side of the ridge with distance suggested a solution. The solution was
sea-floor spreading. This theory suggested that new oceanic crust in the
form of basaltic lavas was produced at the ocean ridges, cooled, crystallized
and moved away from the ridges as newer oceanic crust replaced it at the ridges.
Earthquake
and Volcanic Activity Earthquakes and volcanic activity on
the Earth is concentrated in a linear band that snakes around the world.
This band is particularly evident around the edge of the Pacific Ocean where it
is known as the Ring of Fire. Within the ocean basins near the Ring of
Fire are some of the deepest oceanic waters on Earth. These linear areas
of anomalously deep water are called trenches. In the late 1920s,
seismologists had identified earthquake zones parallel to the trenches that were
inclined 40 to 60° from the horizontal and extended several hundred kilometers
into the Earth. These Wadati-Benioff zones, named for the scientists that
first recognized them, marked the descent of the oceanic plates back into the
mantle at the oceanic trenches. The Princeton University geologist,
Henry Hess, realized that at the same time that new sea floor is being created
at the ridges, old sea floor is being consumed by subduction at the trenches.
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