Chapter 3
Plate Tectonics
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Famous people in Geology: (1) Alfred Wegener, a German meteorologist.
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Wegener proposed the theory of continental drift in the early part of the 20th
century. He wasn’t the first to notice the curious fit of the continental margins.
He was the first, however, to provide convincing evidence to support this idea.
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Observing a simple world map, he noticed that the coastal outlines of different
continents aligned with one another, as if they had been a single land mass at
some time in the past and had subsequently moved apart.
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Wegener formed his theory of continental drift based on his observations as he
traveled around the world. One observation was that glacial till deposits were
found in regions as unalike to each other as southern Africa, southern India,
Antarctica and southern Australia.
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Other observations supporting the continental drift theory included the
alignment of mountain ranges, coal deposits, and fossil distributions that
were located on different continents. His questioned how these very similar
geologic features, many caused by geological/environmental deposition over
many eons, could now be found on totally different land masses? The only
logical answer was that at some time in the past, all the landmasses that we
know today as separate continents, had come together as one giant land
mass.
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He named this primordial, supercontinent “Pangaea”. Pangaea is thought to
have existed 245-65 mya.
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Later, the continents slowly drifted apart. When Wegener presented his
theory in 1926, most scientists discounted it because at the time, no one,
including Wegener, understood how continents could move.
The distribution of late
Paleozoic glacial till deposits
on a present day map.
The distribution of these same
deposits on a map of
Wegener’s Pangaea (260-280
mya).
Additional support of
Wegener’s theory was
provided by historical
climate evidence that
showed coal deposits,
sand dunes, coral reefs,
and other climate-driven
variables also were
aligned when the
continents were made to
come together in one
land mass.
Wegener noticed that belts of rocks
in the Appalachian mountains of
the U.S. and Canada closely
resembled similar rock belts in
Great Britain, Scandinavia, and NW
Africa. These regions were adjacent
to each other in Pangaea.
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He proposed (incorrectly), several ideas to account for the
movement of continents.
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One idea used centrifugal force due to the Earth’s rotation to
move the continents, while a second idea had a continent
“plowing” through oceanic crust. The instrumentation and
technology that we use today to track continental or “plate”
movement hadn’t been invented yet to provide a plausible
explanation for continental drift.
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Later scientists would validate Wegener’s theory using
sophisticated instrumentation that could measure various present
and past earth processes.
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Two scientists named Hess and Dietze discovered, in 1960, that
new oceanic crust is continually being created at mid-ocean
ridges by sea-floor spreading.
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Sea-floor spreading or divergence, pushes continents further
apart from each other.
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Continents move closer during the process of subduction. A
subduction zone is where old ocean floor is being consumed,
or subducted, at very deep canyon-like structures called ocean
trenches.
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The plate tectonic theory (Tuzo Wilson, a Canadian scientist,
coined the term “plate”) is an overall geologic theory that
describes continental drift, sea floor spreading, and
subduction. It also links plate movements with other earth
processes such as earthquakes, volcanism, and mountain
building.
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Geologists now believe that the continents formed and
dispersed not just once, but several times over Earth’s history.
Harry Hess, in the late l950’s, deduced that new oceanic crust was being created along
mid-ocean ridges and that old crust was being consumed at trenches. This process is what
causes the continents to drift apart or move closer together and provides the explanation
of why we have continental drift.
Ridge push forces
Slab pull forces
Rocks retain a record of the Earth’s magnetic field at the time they were formed. This record of
ancient magnetism is called paleomagnetism. We refer to the Earth as a dipole magnet: one
end points to the north magnetic pole and one to a south magnetic pole. The Earth’s dipole tilts
at about a 11o angle to the rotational axis (the imaginary line through the center of the Earth
around which the planet spins). Therefore, the geographic poles of the Earth do not coincide
with the magnetic poles. The angle between the direction that a compass needle points and the
direction to “true” geographic north is called the magnetic declination. The magnetic poles
tend to vary over time.
Convective flow in the
Earth’s outer core generates
an electromagnetic field.
The angle of
inclination is
illustrated here.
This is when a
compass needle tilts
in the direction of
the Earth’s
magnetic field,
which is curvilinear.
Rocks develop paleomagnetism when the iron atoms inside magnetite crystals within a
rock (for example, basalt) cool, and lock permanently into position aligned with the
Earth’s magnetic pole. That’s how scientists know that the Earth’s magnetic field has
reversed many times in the past.
Polar-wandering was
originally thought to
occur because the
magnetic poles drifted
while the continents
were fixed. The
magnetic poles do
move around some,
but not more than 15o
from the geographic
pole. Geologists
realized that the real
reason why there are
polar-wander paths is
because the continents
are moving.
Scientists use sonar to develop a bathymetric profile of the ocean floor.
Sea-floor spreading
The floor of the oceans includes abyssal plains (4.5 km below sea level) and mid-ocean
ridges (2-2.5 km below sea level). Examples of mid-ocean ridges include the Mid-Atlantic
Ridge in the Atlantic Ocean, the East Pacific Rise in the South Pacific Ocean and the
Southeast Indian Ocean Ridge in the Southern Ocean. The top of the ridge is called the
ridge axis. A narrow depression that runs along the ridge axis is called an axial trough.
Deep-ocean trenches border volcanic arcs. Seamounts are mostly inactive volcanoes that
lie below the ocean’s surface. Parallel bands of fracture zones that segment the ridges into
pieces also exist on the ocean floor.
A map that
shows the
distribution of
earthquakes in
the ocean
basins.
Marine magnetic anomalies as revealed by a magnetometer being dragged across the
ocean floor showed that the Earth’s magnetic field has reversed, for reasons still being
investigated, many times in the past.
The dark bands indicate
positive anomalies, the
uncolored bands
negative anomalies.
The earth behaves like a giant magnet, and
thus is surrounded by a magnetic field.
Earth’s magnetic field can be represented
by a dipole that points from the north
magnetic pole to the south. Every now
and then, the magnetic polarity reverses.
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Normal polarity
Reversed polarity
The rock of oceanic crust preserves a record
of the Earth’s magnetic polarity at the time
the crust formed. Eventually, a symmetric
pattern of polarity stripes develops.
The age of oceanic crust varies with location. The youngest crust lies
along a mid-ocean ridge, and the oldest along the coasts of continents.
Here, the different color stripes correspond to different ages of
oceanic crust. Red is youngest, purple is oldest.
Marine magnetic anomalies are stripes
representing alternating bands of oceanic
crust that differ in the measured strength
of the magnetic field above them. Stronger
fields are measured over crust with normal
polarity, while weaker fields are measured
over crust with reversed polarity.
Normal Reversed Mid-ocean ridge
polarity polarity
(normal polarity)
Magnetic reversals are recorded in a succession of
lava flows. Lavas with normal polarity are colored
red, those with reverse polarity are yellow.
Magnetic reversals
Using radiometrically dated
samples of lava flows,
scientists developed the age
of magnetic reversals over the
past 4.5 million years. These
intervals are called polarity
chrons. Shorter intervals are
called subchrons. By relating
this information with the
magnetic anomaly data from
the sea floor, geologists have
determined the spreading
rate of the ocean crust (about
2 cm per year at the midAtlantic Ridge, for example)
and the sea floor age (4.5
million yrs.). In the Pacific,
the oceanic crust is moving at
the rate of about 10 cm per
year. At a rate of 5 cm per
year, sea-floor spreading can
produce a 5,000-km wide
ocean in 100 million years.
End of Chapter 3