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Machac - Vertical Tectonics
The Traprock, Vol. 2, December 2003, pp 2 - 5
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Plate Tectonics: the Ups and Downs
Tamara Machac
The theory of Plate Tectonics helps explain many geological features on the Earth. However,
ordinary horizontal plate tectonics does not explain why Africa has been rising over the past 100
million years and why Australia had an inland sea when the global sea level was low, and was dry
when the global sea level was high. Research shows that subducting plates can pull other plates and
continents to lower elevations with them. Continents and plates can rise in elevation if they sit on top
of large hot rising regions in the mantle, or when a subducting slab loses its pull as it goes deeper into
the mantle. This vertical motion is really en extension of the horizontal theory of Plate Tectonics,
which evolved from Wegner’s theory of Continental Drift.
Geologists view the theory of Plate Tectonics
“…as the grand unifying theory of geology, because
it can successfully explain a great many geologic
phenomenon…(Marshak, 2001).” The process
towards validating this theory has a long history
because it was very difficult to gather enough
convincing evidence right away. Today, scientists
are still tweaking and finding out new things about
plate tectonics and the inner workings of the earth to
explain why some continents move vertically, in
addition to the horizontal plate motion that is more
commonly associated with plate tectonics.
This vertical motion is really en extension of the
horizontal theory of Plate Tectonics. Plate Tectonics
evolved from Wegner’s theory of Continental Drift,
which was one of the first theories to explain why
the continents have the jig-saw puzzle shape they do
today. He thought the continents might have once
been connected in the past as one big super
continent for several reasons. South America,
Africa, India and Australia all have glaciation
patterns that would make sense only if they were all
once connected as one landmass. If they were not
once connected, the striations from the glaciers seem
to show that glaciers moved from the sea onto the
land, which is not possible. Wegner studied
sedimentary rocks for clues on ancient climate, and
that found remnants of tropical climates (coal and
signs of coral reefs) are deposited in “belts” across
continents that are no longer in tropical zones
(Marshak, 2001). This proved that land masses must
have been in a different location in the past. He also
found that some continents had the same type of
fossils. The animals that made theses fossils could
not have possibly swum the great distances that
exists presently between the continents. Also, rock
types and mountain ranges from different continents
match up. Mountain ranges in the eastern U.S. have
rock components that match those in Greenland,
Great Britain, Scandinavia and North West Africa.
However, Wegner could not correctly explain the
mechanism that drives continental movement. He
thought that continents plowed through oceanic
rock, and that “under the action of forces associated
with the rotation of the earth, the continents had
broken apart, opening up the Atlantic and Indian
oceans (Wilson, 1963).
Wegner’s theory of continental drift was
abandoned for awhile because there was not enough
evidence yet to explain what drove this motion. In
the 1960’s, J.T. Wilson found further evidence that
supported Wegner’s idea. He realized that the sea
floor is a continuous system of ridges and trenches.
The convection of the earth’s interior is what drives
the movement of the continents; trenches pull the
ocean floor into the mantle and ridges form new
ocean floor and push and spread surrounding sea
floor. Because igneous rocks can easily be
magnetized at the time of their formation and the
polarity or the direction of the Earth’s magnetic field
shifts throughout history, “The rocks of any one
continent show consistent trends in change of
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Machac - Vertical Tectonics
The Traprock, Vol. 2, December 2003, pp 2 - 5
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orientation with age; those from other continents
show different shifts (Wilson, 1963).” This means
that if the rocks from different continents record the
same shifts in their magnetic fields, they were once
formed around the same time in the same place.
Wilson looked at rock ages for further supporting
evidence and found that oceanic rock, which is much
younger than continental rock, gets progressively
older as one moves farther away from the midoceanic ridges. The ages of the islands in the
Atlantic Ocean tend to increase with the distances
they are from the mid-oceanic ridges. When the
islands were young, they were closer to the ridges.
(Wilson, 1963)
Eventually, as more scientists found more
supporting evidence, Wegner’s original theory
evolved in the modern theory of Plate Tectonics.
This theory says that the Earth’s surface is divided
into many rigid plates, which move relative to each
other; they can drift apart, slide past each other and
under each other. Plate interactions usually take
place at plate boundaries, which are classified by the
movements of the plates relative to each other.
Ocean ridges or divergent boundaries create new
crust and cause sea-floor spreading. At divergent
boundaries, continents can rift apart and for a new
ocean. Subduction zones or convergent boundaries
(trenches) destroy the old crust by forcing it under
an overriding plate. The subducted matter then
sinks into the mantle. At convergent boundaries,
continents can collide to form mountain ranges.
Plate movement drives convection in the upper
mantle, but the not in the lower mantle. Also, the
plates move about 1-15 cm/year (Marshak, 2001).
Figure 1 (from Gurnis, 2001)
Ordinary horizontal plate tectonics that most of
us are familiar with is not enough to explain some
features on the Earth’s surface, such as why Africa
has been rising over the past 100 million years and
why Australia had an inland sea when the global sea
level was low, and was dry when the global sea level
was high. To find out what causes this vertical
motion, scientists use seismology, the study of
earthquakes, to study and map out the Earth’s
mantle. By measuring how fast seismic waves,
shock waves caused by earthquakes, travel through
different parts of the mantle during an earthquake,
we can find out where the mantle is hot and cold.
When rocks are warm, they become soft, which
lowers wave speed. When rocks are cold, they
become hard, which increases wave speed. In the
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The Traprock, Vol. 2, December 2003, pp 2 - 5
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1970’s Clement Chase discovered that gravity in the
mantle is lowest over cool spots and highest over hot
spots and noticed that bands of low gravity surround
subduction zones, which has helped other scientists
track mantle convection changes (Gurnis, 2001)
(Fig. 1). By the mid 1980’s, Bradford Hager found
when low density fluids rise, they push the matter on
top, which creates an excess of mass. When cold
matter sinks, it drags the surrounding mass down,
creating a decrease in mass. Hager’s worked helped
to show that hot spots in the mantle push continents
up and cold spots, which are created by sinking
subducting plates, pull continents down (Gurnis,
2001). More research revealed that in the mantle,
cold dense material is swept away from
downwelling or cold, sinking regions, to the base of
upwellings or hot, rising regions (Gurnis et al.,
2000).
Scientists studied the vertical motion of
Australia and Africa to learn how the mantle can
cause continents to move up and down. In the case
of Australia, the researchers mostly used computer
simulations to model how a ghost, or the remains of
a subducting slab pulled Australia down, and how
the slab gradually lost its pull over time. Scientists
used seismic data to model the properties of the
Earth’s mantle and the history of continental
movement around Australia, which was determined
from geological evidence, as guidelines for their
computer simulations. From the models, they found
that Australia had an inland sea 120mya ago when
the global sea levels were low, because Australia,
which was still attached to Antarctica, was near a
subduction zone. A nearby subducting plate pulled
Australia along with it down towards the mantle.
Australia lost its inland sea 70mya ago when the
global sea level was high because the subducting
slab was sinking farther into the mantel, losing its
pull on the Australian plate. Alt the same time,
Australia was starting to drift away from this
subducting zone (see Fig 2). The remains of this
slab are now in between Australia and Antarctica
(Australian-Antarctic Discordance) (Moresi, 2003).
However, if Australia had not been under the
influence of the subducting plate, Australia would
have had no inland sea at the time the global sea
levels were low, and then would have a had an
inland sea or a least some flooding on the coasts
when the global sea levels were high. Today,
Australia is sinking because of a tug in the mantle
under Indonesia, which is also sinking. (Gurnis,
2001, Gurnis et al., 1998, and Moresi, 1999)
Figure 2: from Gurnis, 2001)
Rift Valley. Over the past 100 million years
it has been rising despite the fact it has not had a
tectonic collision in at least 400 million years.
When scientists tried to understand Africa’s vertical
motion, they again looked at computer simulation
The Southern half of Africa is an extensive
plateau over 1000 miles across and nearly one mile
high. It includes South Africa, the East African
Plateau and the highlands around the East African
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models, which were based on seismic data. The
models and seismic data suggest that there is a
superplume or very large hot, rising region in the
mantle underneath South Africa. When compared to
all the other hot regions in the mantle, Africa is over
the largest low shear velocity anomaly in the lower
mantle. This means that the seismic waves in this
region are slow, indicating a hot area in the mantle.
Africa’s uplift occurred mostly during the break up
of Gondwanaland (see Fig. 3) and also during the
Cenozoic at rates of 5-30 m/Myr (Gurnis et al.,
2000). Africa has this superplume because it was
once the center of Gondwanaland and was therefore
not affected by sinking plates that surrounded the
edges of the super continent. Today, while Africa is
rising up, Indonesia is sinking down because
upwelling mantle sweeps material away from
downwelling zones. (Anderson, 1995, Gurnis,
2001, and Gurnis et al., 2000)
Even though the theory of plate tectonics
explains many geologic features, it is far from being
complete. It took a long time to develop this theory
because there were and continue to be more parts
than previously thought. Scientists are still finding
out new things about plate tectonics and the inner
workings of the earth to explain why some
continents, such as Africa and Australia, move
vertically. Subducting plates can pull other plates
and continents to lower elevations with them.
Continents and plates can rise in elevation if they sit
on top of large hot rising regions in the mantle, or
when a subducting slab loses its pull as it goes
deeper into the mantle.
Figure 3: from http://www.nsf.gov/od/opp/support/gondwana.htm
Gurnis, Michael; Moresi, Louis; Müller, R. Dietmar.
Cretacious Vertical Motion of Australia and the
Australian-Antarctic Discordance. Science,. March 6,
1998, vol 279, p. 1499-1504.
Marshak, Stephen Earth: Portrait of a Planet, W.N.
Norton & Co., New York, 2001
Moresi, Louis Australia: modeling how a wandering
continent lost its inland sea. CSIRO E&M –
Geoscience and Geoengineering. 1999.
http://www.csiro.au/research/solidMech?Outreach/
SG-World/SGI-world3.html (10-19-200
Wilson, Tuzo,Continental Drift Scientific American,
New York, 1963, vol 208, p. 86-100
References
Anderson, Don L. Enhanced: Top-Down Tectonics?,
Science Magazine, 1995, vol 293, p.2016
Gurnis, Michael. Sculpting the Earth from the Inside
Out, Scientific American, New York, March 2001,
vol 284, p. 40-47
Gurnis, Michael; Mitrovia, Jerry X.; Ritsema, Jeroen;
van Heijsy, Hendrik-Jan., Constraining Mantle
Density Structure Using Geological evidence of
Surface Uplift Rates: The Case of the African
Superplume, Geochemistry, Geophysics,
Geosystems, vol 1, paper no. 1999GC000035, 2000
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