Download 8.9A the historical development of evidence that supports plate

8.9A the historical development of evidence
that supports plate tectonic theory
› Theory of Plate Tectonics – the theory that pieces of
Earth’s lithosphere are in constant motion on the
asthenosphere.
› The motion in the mantle (the asthenosphere) is
caused by convection currents.
› But how did this theory become the “one”?
But first, let’s examine continental drift theory
Alfred Wegener
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Previously, geologists
believed that the continents
remained fixed in their
position
In 1912 Alfred Wegener
hypothesized that all the
continents had once been
joined together in a single
landmass and have since
drifted apart.

He named the single landmass – Pangaea (“all lands”)
› His theory became known as continental drift theory
› Pangea to now
› Describe
Pangea now in
your notes.
The continents are still slowly moving, at about the
speed your fingernails grow. Satellite measurements
have confirmed that every year the Atlantic Ocean
gets a few inches wider (at the Marianas Trench)!
› Wegener supported his
theory with evidence from:
– Landforms
– Fossils
– Climate and Glacial
› Wegener said that if we looked the shapes of the
continents on a map, it seemed reasonable to
suspect that they had all once been joined
• Matching coastlines on different continents

Today, we have a much better idea of the true outer
boundary of each continent by examining its
continental shelf.
◦ The seaward edge of a continental shelf lies
submerged below the ocean. When the edges of the
continental shelves are placed together, the fit is
remarkable.
› Mountain ranges and other
features on the continents
provided further evidence
– For example, the
Appalachian mountains of
eastern North America
matched with the Scottish
Highlands
– When these landmasses are
reassembled on a map of
Pangaea the mountain
chains form a nearly
continuous belt
› Wegener found
identical fossils on
the matching
coastlines of both
South America and
Africa.
– Wegener argued
that South
America and
Africa must have
at one time been
joined.
› He found evidence for dramatic global climatic
changes.
– Deep scratches on bedrock in Africa indicated that
at one time it was covered in glaciers, which means
it must have been much closer to the South Pole
› He also found that the fossils found in a certain place
often indicated a climate utterly different from the
climate of today
– i.e. tropical fossils in climates that currently have a
harsh polar climate
› Glacial evidence showed up in places where the
climate did not reflect such environments
› As compelling as this evidence was, Wegener’s theory
was not accepted by the scientific community.
– It would take another 50 years for this to occur.
› He could not provide a satisfactory explanation for the
force that pushes or pulls the continents
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
Oceanographer
First to map details of the
ocean floor on a global
scale
Her observations became
crucial to the eventual
acceptance of the theories
of plate tectonics and
continental drift
Marie Tharp's "World Ocean Floor Map”
1977
Picture from USGS
http://pubs.usgs.gov/gip/dynamic/HHH.html
› In the 1960’s, a scientist
named Harry Hess made a
discovery that would
vindicate Wegener.
› Using new technology,
radar, he discovered that
the seafloor has both
trenches and mid-ocean
ridges.
› Harry Hess proposed the seafloor spreading theory.
Echo Sounding Device Used By Hess
› Hess proposed that hot,
less dense material
below Earth’s crust rises
toward the surface at
the mid-ocean ridges.
› Then, it flows sideways,
carrying the seafloor
away from the ridge in
both directions.
Picture from http://library.thinkquest.org/17457/platetectonics/4.php
› As the seafloor spreads apart at a mid-ocean ridge, new
seafloor is created.
› The older seafloor moves away from the ridge in opposite
directions.
› This helped explain how the crust could move something
that the continental drift hypothesis could not do.
› Strange rocks shaped like
pillows or like toothpaste
squeezed from a tube
have been found
– These rocks can form only
when molten material
hardens quickly after
erupting under water
– These rocks showed that
molten material has
erupted again and again
from cracks along the
central valley of the midocean ridge
› Scientists discovered that the rock that makes up
the ocean floor lies in a pattern of magnetized
“stripes”
› 780,000 years ago, magnetic poles reversed
themselves
– If they reversed today, the needle in a compass would
point south instead of north
› The rock in the ocean is made of iron, which
began as molten material
Magnetic Reversals
› Plates move apart
› Magma rises and cools
forming new sea-floor
– Youngest rock is near the
ridge, it gets older as you
move away
› The earth’s magnetic field
is recorded in the rock as
it forms
› We have a record of the
earth’s magnetic reversals
on either side of the ridge.
100 Greatest
Discoveries:
Magnetic Reversals
› The final proof of sea-floor spreading came
from rock samples obtained by drilling into
the ocean floor. The Glomar Challenger, a
drilling ship built in 1968, gathered the
samples.
› Then the scientists determined the age of
the rocks in the samples. They found that
the farther away from a ridge the samples
were taken, the older the rocks were. The
youngest rocks were always in the center
of the ridges. This showed that sea-floor
spreading really has taken place.
As distance from the ridge
increases, the age and
thickness of the sea-floor
increases.
› Theory of sea floor spreading - hot, less dense
material below the Earth’s crust rises toward
the surface at the mid-ocean ridges. Then it
flows sideways, carrying the seafloor away
from the ridge in both directions
› As distance from the ridge increases, the age
of the sea-floor increases.
Sea Floor Spreading (8 min)
Hydrothermal Vents (5 min)
How the continents were formed (15 min)
› Use colored pencils
to color the key at
the bottom
› Then color in the
map with your fossil
evidence
› When done, glue in
to the end of your
notes
› Due next class –
there may be a
quiz in your near
future !