Geology Reference
In-Depth Information
reference points for determining paleolatitude, an impor-
tant tool when reconstructing the location of continents
in the geologic past.
BRITISH
COLUMBIA
How fast and in what direction are Earth's plates moving?
Do they all move at the same rate? Rates of plate movement
can be calculated in several ways. The least accurate method
is to determine the age of the sediments immediately above
any portion of the oceanic crust and then divide the distance
from the spreading ridge by that age. Such calculations give
an average rate of movement.
A more accurate method of determining both the
average rate of movement and relative motion is by dat-
ing the magnetic anomalies in the crust of the seafloor.
The distance from an oceanic ridge axis to any magnetic
anomaly indicates the width of new seafl oor that formed
during that time interval. For example, if the distance be-
tween the present-day Mid-Atlantic Ridge and anomaly 31
is 2010 km, and anomaly 31 formed 67 million years ago
(
Seattle
JUAN
DE FUCA
PLATE
WASHINGTON
OREGON
NORTH AMERICAN
PLATE
CALIFORNIA
NEVADA
Figure 2.23), then the average rate of movement during
the past 67 million years has been 3 cm per year (2010 km,
which equals 201 million cm divided by 67 million years;
201,000,000 cm/67,000,000 years = 3 cm/year). Thus, for a
given interval of time, the wider the strip of seafl oor, the
faster the plate has moved. In this way, not only can the
present average rate of movement and relative motion be
determined (Figure 2.15), but the average rate of move-
ment in the past can also be calculated by dividing the dis-
tance between anomalies by the amount of time elapsed
between anomalies.
Geologists use magnetic anomalies not only to calcu-
late the average rate of plate movement, but also to deter-
mine plate positions at various times in the past. Because
magnetic anomalies are parallel and symmetric with respect
to spreading ridges, all one must do to determine the posi-
tion of continents when particular anomalies formed is to
move the anomalies back to the spreading ridge, which will
also move the continents with them (Figure 2.23). Unfortu-
nately, subduction destroys oceanic crust and the magnetic
record that it carries. Thus, we have an excellent record of
plate movements since the breakup of Pangaea, but not
as good an understanding of plate movement before that
time.
The average rate of movement, as well as the relative
motion between any two plates, can also be determined by
satellite-laser ranging techniques. Laser beams from a station
on one plate are bounced off a satellite (in geosynchronous
orbit) and returned to a station on a different plate. As the
plates move away from each other, the laser beam takes more
time to go from the sending station to the stationary satellite
and back to the receiving station. This difference in elapsed
time is used to calculate the rate of movement and the rela-
tive motion between plates.
Plate motions derived from magnetic reversals and
satellite-laser ranging techniques give only the relative mo-
tion of one plate with respect to another. Hot spots allow
◗
San Francisco
P A C I F I C
O C E A N
Los Angeles
PACIFIC
PLATE
Oceanic
ridge
Zone of
subduction
Transform
faults
◗
Figure 2.21
The San Andreas Fault—A Transform Plate Boundary
The San Andreas fault is a transform fault separating the Pacifi c
plate from the North American plate. It connects the spreading
ridges in the Gulf of California with the Juan de Fuca and Pacifi c
plates off the coast of northern California. Movement along the
San Andreas fault has caused numerous earthquakes. The insert
photograph shows a segment of the San Andreas fault as it cuts
through the Carrizo Plain, California.
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