Download Magnetic striping and polar reversals (See CD Tect ppt)

The Story of the development of the theory of Plate Tectonics
The ideas developed between the 1930s and late 60’s represent a
revolution in thinking about the Earth’s structure. At the time the theory
caused lots of arguments among scientist. There is now overwhelming
evidence for the theory of plate tectonics.
 Continental Drift to Plate tectonics
The idea that continents can drift about is called, not surprisingly,
Continental drift. This
theory was developed by
Alfred Wegener
When Wegener first put forward the idea in 1912 people thought he idea
was ridiculous. He believed that the continents were once joined forming
a ‘supercontinent’ Pangaea, however the big problem was that he knew
the continents had drifted but he couldn't explain how they drifted. The
old theory before this time was the "Contraction theory" which
suggested that the planet was once a molten ball and in the process of
cooling the surface cracked and folded up on itself. Others thought that
places like the Grand Canyon had been formed by a great catastrophe.
The first idea to have a big impact came form Arthur Holmes in 1929.
Holmes is widely regarded as the greatest British Earth Scientist of the
20th Century. Amongst his many contributions, he pioneered the study of
the age of the Earth, using the radioactive decay of uranium to lead. He
suggested that radioactive decay inside the Earth could produce heat
which would set up convection currents in the mantle. The mantle
behaves like ‘silly putty’ and can flow. This could be the driving force for
the movement of the continents.
1940 Charles Richter mapped out the occurrence of earthquakes and
noticed that they occurred in belts under volcanic ranges and in the
oceans. (See CD tect.ppt)
In the 1950s, oceanic exploration greatly expanded. Data gathered by
oceanographic surveys conducted by many nations led to the discovery
that a great mountain range on the ocean floor virtually encircled the
Earth. (SEE CD Tect ppt.) Called the global mid-ocean ridge, this immense
submarine mountain chain -- more than 50,000 kilometers (km) long and,
in places, more than 800 km across -- zig-zags between the continents,
The Story of the development of the theory of Plate Tectonics
winding its way around the globe like the seam on a baseball. Rising an
average of about 4,500 meters(m) above the sea floor, the mid-ocean
ridge is nearly as high as Everest. Though hidden beneath the ocean
surface, the global mid-ocean ridge system is the most prominent
topographic feature on the surface of our planet.
Magnetic striping and polar reversals (See CD Tect ppt)
Later in the 1950s, scientists, using magnetic instruments
(magnetometers) adapted from airborne devices developed during World
War II to detect submarines, began recognizing odd magnetic variations
across the ocean floor. Basalt -- the iron-rich, volcanic rock making up the
ocean floor-- contains a strongly magnetic mineral (magnetite) and can
locally distort compass readings. When the rock is formed it takes on the
magnetic field direction prevailing at the time of formation. These newly
discovered magnetic variations provided another means to study the deep
ocean floor.
1962 Harry Hess
1963 Vine and Matthews
Hess proposed that new ocean
floor was made in the centres of
oceans and then carried sideways
on convection current conveyor
belts. This became know as sea
floor spreading.
Fred Vine and Drum Matthews
from
Cambridge
University
realised that the magnetic stripes
on either side of the ridge
matched
up.
This
provided
convincing evidence that seafloor
spreading was taking place
Look at the animation on the CD.
The Story of the development of the theory of Plate Tectonics
1965 J Tuzo Wilson. Proposed that the Earth is divided into several
large rigid plates, these are moved apart as sea floors spread. See CD
 The Theory of Plate Tectonics
1967 Dan McKenzie at Cambridge University developed all the earlier
ideas into the theory of plate tectonics. He calculated that the Earth’s
circumference was not getting bigger, yet new crust was being developed
by sea floor spreading in the oceans. He looked and found places were
there was evidence that plates were being destroyed at the same rate.
There is now overwhelming evidence for the theory of plat tectonics.
Geologists now have a clear idea of what happens on Earth and many are
looking for evidence of planets tectonics taking place on other planets.
There is some evidence that Mars may not have plates. Astronomers have
observed a huge volcano, several times the height of Everest, which
suggests that the margins are static.