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The Theory of Plate Tectonics
Transcript of The Theory of Plate Tectonics
By Anab Mohamed
and Stephanie Ross
What is Plate Tectonics?
Plate Tectonics is the theory that explains the features and movement of the Earth's surface in the present and the past.
It is said that Earth's outer shell is divided into several plates that glide over the mantle.
The plates act like a hard rigid shell which is called the lithosphere.
The History of Plate Tectonics Theory
Alfred Wegener is usually given credit for the discovery of Plate Tectonics.
The observation that South America and Africa seem to fit perfectly into one another, indicates that at one time they were part of the same land mass.
But how can this be?
It all has to do with the Theory of Plate Tectonics. It is actually believed that all the continents were originally one large one called Pangaea. Watch as the continents form in this video:
Using a pencil and a piece of paper, take all that you've learned from this presentation about the Theory of Plate Tectonics to create 5 steps to how this could have happened.
This is what your steps should look like:
Step 1: The core heated the part of the mantle closer to the core while the mantle nearer the surface was cooler.
Step 2: The hot part of the mantle rose towards the earth's crust, as the cooler part sank down towards the core this caused a cycle.
Step 3: The mantle pushed up into the rift zones and because they are weak the crust broke causing the tectonic plates to move away from the rifts.
Step 4: Once the break occurred, the convection currents caused the tectonic plates to move away from each other.
Step 5: Eventually the rift zone got so big that it formed a basin and water poured into the basin forming oceans.
The word Pangaea means "all lands" in ancient Greek
Dinosaurs and Vegetation
Something to prove continental drift and therefore the Theory of Plate Tectonics, is Dinosaurs and Vegetation.
If you look at this diagram, you will notice different types of species being found all over the world.
They were probably all once together on one land form until it was separated.
Similarities between plant and animal fossils have been found on land masses that are very far apart today.
For example many dinosaur fossils and footprints have been found around the world. If the land masses were separated like they are now, how did they get across the oceans?
J. Tuzo Wilson was a Canadian geophysicist who helped redefine the whole concept of Plate Tectonics
In 1965, his paper "A New Class of Faults and Their Baring on Continental Drift" explained his theory of Plate Tectonics
His theory helped explain continental drift and became almost universally accepted.
He was creditied with coining the term "plate"
It is believed that the
force moving the plates are convection currents in the mantle.
What happens is, the hot part of the mantle (down near the core of the Earth) is less dense than the mantle above it, this causes that hot mantle to rise towards the surface of the earth. Once it has raised to the surface of the Earth, it starts to cool which causes the density to increase which causes it to sink again, producing a flow of mantle in a circular pattern not unlike the movement of liquid in soup when it is cooking on the stove. Since the plates are floating on the moving mantle, they too move as their foundation is moving (the mantle).
Diverging or Spreading Plates
Sometimes it is because of seafloor spreading.
continents drift as a result of
plate tectonics. They separate
away from each other. How
does this happen though?
Plates made of oceanic crust pull apart from each other causing a hole. The magma then oozes out through the hole to fill the space.
This forms a ridge called mid-ocean ridge. The magma also spreads outward forming new oceanic floor and crust.
When two continental
plates diverge, a valley like rift
forms. The rift is a dropped zone
where the plates are pulling apart.
Eventually the widening crust will become
thin enough that a piece of the continent will
break off forming a new tectonic plate.
At this point, water from the ocean will
rush in creating a new sea basin.
Subduction zones are "Converging Boundaries" meaning the plates are coming together.
A subduction zone, would be an area where one plate is sliding under another plate. This can result in earthquakes and
tsunamis because the
movement of one
plate sliding under
another one will
move the top
the area of
Transform Plate Boundaries
Transform boundaries are where plates slide next to each other.
Transform boundaries cause
lots of earthquakes due to
their sliding motion.
The most famous transform
boundary in the world is
probably the San Andreas fault.
The west part of California is slowly
moving towards the rest of
California. Los Angelas gets about
6cm closer to San Francisco every
year because the motion of the
plates there are going sideways
rather than vertical.
In Fact, in 1906, the San Andreas Fault suffered a huge devastating that shattered buildings into pieces.
(Divergent or Spreading Boundaries)
Ring of Fire
The "Ring of Fire" is a 25, 000 mile horseshoe shaped ring of 452 volcanoes. From the southern tip of South America along the western coast of North America then accross the Berings Straight, down through Japan and into New Zeland
It is composed of convergent boundaries (Mount. St. Helens), divergent boundaries (the eastern pacific rise) and transform boundaries (State of California).
It is home to the most
active volcanoes on
Krakatoa (an island in
Indonesia) was almost
entirely destroyed by a
volcanic eruption in 1883.
Geologists have divided earth’s
history into four time periods
called eras. During each era,
there was major sediment
deposition (breaking down of materials).
Also during these
were formed and
often mass extinctions
of animals occurred.
The 4 main Geologic Eras are:
Cenozoic (recent life)
Mesozoic (middle life)
Paleozoic (ancient life)
Precambrian (earliest life)
- began, 66 billion years ago
-ice sheets cover most of North America
- continents take on their present shape
- formation of the Rocky Mountains completed
-human beings develop
-age of mammals
-modern forms of life evolve
-began, 245 million years ago
-ended, 66 million years ago
-formation of Rocky Mountains begins.
-Innuitian Mountains formed
-shallow seas in the interior of North
America at various times
-age of reptiles, such as dinsaurs
-first flowering plants
-first birds and mammals
-Began 570 million years ago
-Ended 245 million years ago
-periods when large parts of
North America are covered by
-Appalachians are formed
-age of the amphibians and fish
-large swamps - coal formed from this
-first plants and animals appear on
-began 4600 million years ago
-ended 570 million years ago
-Precambrian shields, such as
the Canadian Shield, Brazilian
Shield, African Shield, and
Australian Shield are formed
-first multi-celled organisms
Hope you learned something!