Transcript Plate Tectonics What is it and what makes it work?

Plate Tectonics
What is it and what
makes it work?
Oceanography
The Earth’s interior
1000 C
4000 C
7000 C
Pea-soup analogy
mantle convection
Heat transfer from the core to the
mantle
Produces slow convection currents
of the mantle material
(in the order of centimetres per year)
Mantle convection currents produce
lateral forces at the boundary between
the mantle and the crust
Lateral forces at the crustmantle boundary push the crust
around – in different directions
leading to the motion of crustal
plates.
The plates are moving at rates of a few
centimeters per year
San Diego is moving at 5 cm/year vs
Imperial valley is moving 1 cm/year
Plate motions
Divergent boundary
Convergent boundary
Subduction
Transform boundary
Plates of the Earth’s crust
Plates around North America
The Theory of Plate Tectonics
Unifying principle of geology.
Evolved in the late 1960s, early 1970s.
1. Realization that certain parts of the Earth were more
affected by earthquakes and volcanoes.
2. Also noted that large mountainous regions occur on
margins of continents.
3. Changes in sea level were related to these events.
They were not a random occurrences.
Distribution of major volcanoes and earthquakes
“Ring of Fire”
The Theory of Plate Tectonics
Many hypotheses were presented to answer these
questions.
None answered all of the questions at all localities.
This led to the hypothesis of PLATE TECTONICS.
It became widely accepted in a few short decades.
The Theory of Plate Tectonics
Steps in Development
1. Benjamin Franklin (late 1700s)
Recognized that crust of Earth was a shell.
Surface could be broken and parts moved about.
2. Alfred Wegener (1912)
German meteorologist-geophysicist
Proposed theory of Continental Drift.
Proposed continents float on a denser
underlying interior of the Earth.
CONTINENTS periodically break up and
DRIFT apart.
The Theory of Plate Tectonics
Wegener believed all continents were joined together.
Supercontinent of Pangaea existed about 200 million
years ago.
Pangaea covered 40% of the Earth’s surface.
The Theory of Plate Tectonics
Majority of Pangaea was in Southern Hemisphere.
Pangaea was surrounded by a single ocean, the
Panthalassic Sea.
Pangaea broke up ~ 180 million years ago.
The Theory of Plate Tectonics
Wegener first published his theory 1912.
The Theory of Plate Tectonics
Evidence in support of Continental Drift
1. Continental Fit
Sir Francis Bacon (1620)
noted that the continents
might fit together.
Made observation after
seeing some newly made
maps.
The Theory of Plate Tectonics
2. Habitats of Modern
Organisms
Hippopotamus found in
Africa and Madagascar.
Marsupials in Australia.
Indicate some migration
and evolution took place
before and after drift
began.
Marsupial Mammals
Marsupials are the group of mammals commonly thought of as
pouched mammals. They give live birth, but they do not have
long gestation times like placental mammals. Instead, they give
birth very early and the young animal, essentially a helpless
embryo, climbs from the mother's birth canal to the nipples.
There it grabs on with its mouth and continues to develop, often
for weeks or months depending on the species. Like other
mammals, the marsupials are covered with hair. Mothers nurse
their young — a young kangaroo may nurse even when it has
grown almost to the mother's size.
The only naturally occurring marsupial in the United States is
the opossum, Didelphis marsupialis. In the past, however,
marsupials were quite common. During the Mesozoic
marsupials were very common in North America; more
common, in fact, than placental mammals. They persisted here
until the mid- to late-Tertiary. In South America and Australia,
however, marsupials continued to be an important group of land
mammals.
• Many South American forms
are similar to the North
American opossum. The
marsupials of South America
began to go extinct in the late
Miocene and Early Pliocene
when a land connection with
North America formed,
allowing placental mammals to
cross into South America. In
Australia, though, marsupials
continue to be very diverse, and
are the dominant native
mammals. They include
kangaroos, koalas, tasmanian
devils, wombats, and other
typical Australian mammals.
Until recently, they also
included the marsupial wolf,
Thylacinus. Like the quagga,
the marsupial wolf is now
extinct. The last individual was
seen in Tasmania in the 1950s.
The Theory of Plate Tectonics
3. Fossil Record
Wegener used the
fossil record.
Found fossils of
plants and
animals that were
found on several
continents.
Included animals, Cynognathus, Lystrosaurus, Mesosaurus,
and plants Glossopteris.
The Theory of Plate Tectonics
4. Similar Rock Types Across Ocean Basins
Mountains of Northern Hemisphere similar in Greenland, NA, and
Europe.
Also similar rocks between South America and Africa.
The Theory of Plate Tectonics
5. Ancient Climates
Glacial striations found in India, Australia, South America
and Africa.
Radiate from a point in southern Africa.
Also coal deposits found in presently cold regions,
such as Norway.
The Theory of Plate Tectonics
Wegener seemed to have a great idea, and lots of
supporting evidence.
However, Plate Tectonics was not widely accepted
until the early 1970’s.
Why?
Wegener could not explain how the continents drift.
He could provide no mechanism for the theory.
The Theory of Plate Tectonics
Captain Harry Hammond Hess
(later Professor of Geology at Princeton)
Believed in Wegener’s hypothesis
Conducted echo-soundings of the oceans during WWII
Mapped the mid-ocean ridges (3000 m high and 2000 m wide)
Located deep-sea trenches (10,000 m deep) associated with
large continental mountain belts (the Andes) and island arcs
(Aleutians, Japan)
Hess presented sea-floor spreading as a mechanism in 1960’s