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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”?
Alfred Wegener
But first, let’s examine continental drift theory
He named the single landmass – Pangaea (“all lands”)
› His theory became known as continental drift theory
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.
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
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
› 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 sea-floor spreading theory.
Picture from USGS
http://pubs.usgs.gov/gip/dynamic/HHH.html
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 mid-ocean 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, add this to the end of your notes
› Due next class –there may be a quiz in your near future !
