Transcript 2.2 Plate Tectonics

Think of something that humans used to believe
but that they no longer hold to be true.
(an opinion or “fact”)
e.g. -flat earth
-animal machines
-alchemy (that you could turn purified metals into gold)
What changed?
-new evidence
-new ideas about existing evidence
“…we have to be prepared always for
the possibility that each new discovery,
no matter what [scientific discipline]
furnishes it, may modify the
conclusions we draw.”
-Alfred Wegener
CONTINENTAL DRIFT
AND
PLATE TECTONICS
Continental Drift
In 1912, Alfred Wegener put forward a paper expressing
his belief that the earth’s continents were moving
(however, very slowly).
Wegener suggested
that a single
“supercontinent” (a
continent made up of
all, or most, of the land
on earth) called
Pangaea had existed at
some point in the past.
*Turns out, Pangaea (meaning “entire earth”) did exist, between
200 and 300 million years ago.
“WHY DID HE BELIEVE THIS?”
Evidence for Continental Drift
1) The fit of the continents
2) Fossil Evidence
3) Rock-type similarities
4) Evidence of Glaciers in Warm Climates
1.Fit of the Continents
Wegener noticed that the continents are shaped such
that they could fit together like jig-saw puzzle pieces.
e.g. East Coast of South America and West Coast of Africa
2.Fossil Evidence
• Fossils of similar plants and animals were found on different
continents, suggesting that at one point, they shared the same
landmass.
e.g. Glossopteris, a prehistoric (now extinct) plant in Antarctica,
South America and Africa, could not have spread seeds across
the ocean.
3.Rock Type Similarities
Similar rock types
(age and structure)
were found along
mountain ranges on
different
continents.
e.g. Caledonian Mountains in Scandinavia and Ireland, the Atlas
Mountains in Northwestern Africa, and the Appalachian mountains
in Eastern North America.
4.Glacial Evidence
• There is evidence of prehistoric ice sheets having been present in
Africa, South America, India, and Australia. Wegener believed that
the only way this could have happened was if these continents had
been located at higher (colder) latitudes historically.
• e.g. glacial striations (scrapings), in hot desert landscapes.
The Problem was…
• Wegener’s contemporaries did not share his
enthusiasm for the theory of Continental Drift.
• They mocked and ridiculed him, because he
could not explain “HOW” the continents
moved.
• In other words, they would not believe him
until he presented a cause (a force) that could
reasonably and logically explain the
movement of the continents.
Plate Tectonics
• In 1968, John Tuzo Wilson, a
Canadian geophysicist who
studied continental drift
discovered the “HOW” that
had evaded Wegener.
• Wilson proposed that
“convection currents” in the
earth’s “mantle” provided
adequate energy to displace
landmasses.
The Mantle
The Earth’s mantle is a layer below the crust that descends to 2900 km below the
surface.
The upper part of the mantle is less dense than the crust above it, and the crust
therefore “floats” on top of the mantle.
The movement of the continents (tectonic plates) can be compared to broken
slabs of ice floating on a lake in winter time.
Convection
Convection is the transfer of heat though a material (i.e. water,
magma, air). It creates cycles (or currents), with warmer areas
rising and cooler areas falling.
In the mantle, lower-lying magma is hotter and less dense, therefore
it tends to rise. Magma located at higher points in the mantle is
cooler and more dense, and therefore descends relative to its
surrounding material.
Convection Currents
Think of convection currents as giant conveyor belts.
Their movement pushes and pulls the earth’s crust.
(continental movement: 2-10 cm/year)
Plate Tectonics
The edges of tectonic plates, where one plate meets
another, are referred to as Plate Boundaries.
There are three types.
Convergent boundaries involve tectonic plates moving
towards each other. It is typical for one plate to
descend under the other in a process known as
subduction.
Divergent boundaries involve tectonic plates moving
away from each other.
Transform boundaries involve two plates scraping
eachother while moving in opposite directions
(or sometimes moving in the same direction at different speeds)
Plate Boundaries
Plate Boundaries
India
The Indian subcontinent is subducting
under the Eurasian Plate, resulting in
the building of the Himalaya
Mountain Range.
Question from Yesterday
Fastest-moving Plate:
NAZCA: 160mm/year
or 16 cm/year.
Why might it be moving
faster than other
continents?
Subduction Drag
Increasing weight as
plate subducts.