Plate Tectonic Theory

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Plate Tectonic Theory
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Plate Tectonic Theory

• Plate tectonics is a relatively new theory that
  has revolutionized the way geologists think about
  the Earth.
• Plate Large slab of solid rock
• Tectonics from the Greek root to build
• Plate tectonics is a combination of two earlier
  ideas, continental drift and sea-floor spreading

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Tectonic Plates

• According to the theory, the surface of the Earth
  is broken into large plates.

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Earths Interior

• The lithosphere is the rigid, upper part of the
  mantle and all of the crust
• It is broken into 30 plates which vary greatly
  in size and shape.
• The lithosphere floats on the asthenosphere which
  is the flowing part of the upper mantle.

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Historical Theories

• 1596-
• Abraham Ortelius (April 14, 1527 June 28,
  1598) was a Flemish cartographer (map maker) and
  geographer.

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Continental Drift

• Ortelius is believed to be the first person to
  imagine that the continents were joined together
  before drifting to their present positions.

Ortelius was the first to see that the shape of
the coasts of South America and Europe-Africa
were similar, and to propose continental drift as
an explanation
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Nicolaus Steno 1636-1686

• Nicolaus Steno observed the changes in a
  sequence of rock layers in the mountains of
  Italy.
• Steno's observations became known as the Law of
  Superposition

Danish Anatomist and Geologist
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Law of Superposition
in a sequence of sedimentary rock layers, each
layer of rock is older than the layer above it
and younger than the rock layer below it.
Youngest rock layer
Oldest rock layer
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James Hutton

• 1785-
• James Hutton (1726-1797) was a Scottish
  geologist.
• In 1785 he presented his uniformitarian principle
  to the Royal Society of Edinburgh.

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Uniformitarian principleThe present is the key
to the past

• suggests that catastrophic processes were not
  responsible for the landforms that existed on the
  Earth's surface.
• This idea was opposed to the ideas of that time
  period which were based on a biblical
  interpretation of the history of the Earth.

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Unitarianism

• means the same throughout
• Scientists believed that the rates of all
  geologic processes had been the same throughout
  time.
• The theory of uniformitarianism suggested that
  the landscape developed over long periods of time
  through a variety of slow geologic and geomorphic
  processes.

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The Grand Canyon in Arizona shows how geologic
processes such as weathering and erosion happen
slowly over time.
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Alfred Wegener

• Alfred Wegener (1880-1930)
• German geophysicist
• Professor of meteorology and geophysics at the
  University of Marburg
• Studied fossils on different continents

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1912- Supercontinent Theory

• around 200 million years ago, the supercontinent
  Pangaea began to split apart .
• Pangaea comes from the Greek all the Earth

• Wegener's theory was based in part on what
  appeared to him to be the remarkable fit of the
  South American and African continents, first
  noted by Abraham Ortelius three centuries
  earlier.

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Fossil Evidence

• Wegener was also intrigued by the occurrences of
  unusual geologic structures and of plant and
  animal fossils found on the matching coastlines
  of South America and Africa, which are now widely
  separated by the Atlantic Ocean.
• He reasoned that it was physically impossible for
  most of these organisms to have swum or have been
  transported across the vast oceans. To him, the
  presence of identical fossil species along the
  coastal parts of Africa and South America was the
  most compelling evidence that the two continents
  were once joined.

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Pangaea
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Continental Drift
225 million years ago- Supercontinent
Pangaea Over millennia, continents drifted
apart. Present Day 7 continents which are
continuing to drift (separate) at a rate of
about 10 cm per year.
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A Theory Contended

• The theory of continental drift would become the
  spark that ignited a new way of viewing the
  Earth. But at the time Wegener introduced his
  theory, the scientific community firmly believed
  the continents and oceans to be permanent
  features on the Earth's surface.
• Wegeners proposal was not well received, even
  though it seemed to agree with the scientific
  information available at the time. 

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The Fatal Weakness

• A fatal weakness in Wegener's theory was that it
  could not satisfactorily answer the most
  fundamental question raised by critics of his
  time
• What kind of forces could be strong enough to
  move such large masses of solid rock over such
  great distances?

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Arthur Holmes Lived 18901965 British Geologist

• 1929 - Convection Currents Theory about the
  time Wegener's ideas began to be dismissed for
  lack of a mechanism of movement, Holmes
  elaborated on the idea that the mantle undergoes
  thermal convection

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Thermal Convection in the Mantle

• As magma is heated, it tends to rise, then cools
  and sinks again. This repeated heating and
  cooling results in a current which may be enough
  to cause continents to move.

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Convection Conveyor Belt

• Arthur Holmes compared this thermal convection to
  a conveyor belt. The pressure from the upwelling
  of magma could break apart a continent and then
  force the broken continent in opposite directions
  carried by the convection currents. This idea
  received very little attention at the time.

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What We Know Today

• Even though the theory of continental drift was
  proposed in 1912 by Alfred Wegener, the idea of
  moving continents wasn't generally accepted until
  the early 1960s.
• That's when Wegener's theory was resurrected by
  Harry Hess, Robert Dietz, Fred Vine, and Drummond
  Matthews

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Forces that Shape the Earth

• Geologists now understand the forces that cause
  tectonic plates to move.
• The force that causes plates to move is called
  convection.
• This convection force occurs in the asthenosphere
  of the mantle

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Convection Currents

• The lithospheric plates are thought to be moved
  around by circulating motions.
• This process is similar to what you see in a lava
  lamp.
• The material in the lamp is heated by the bulb.
  The material then rises and is replaced by the
  cooler material that sinks to the bottom.
• This causes circulating movement

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Convection in the Earth
convection currents in the mantle move tectonic
plates as the plastic-like asthenosphere
circulates due to the heat present in the core.
The large scale circulations (motions) in the
asthenosphere move the lithospheric plates on the
surface of the Earth leading to the continental
drift observed today.
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Harry Hess Lived 19061969 American Geologist

• 1962 - Sea Floor Spreading Theory idea that the
  seafloor itself moves and carries continents with
  it, as it expands from a central point
• caused by convection currents in the molten, very
  weak upper mantle, or asthenosphere.

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Sea Floor Spreading

• molten magma from beneath the earth's crust could
  ooze up between the plates in the rift in the
  ocean floor.
• As the hot magma cooled in the ocean water, it
  would expand and push the plates beside it.

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Mid Atlantic Ridge

• North and South America would move to the west
  and Eurasia and Africa would move to the east.
• The Atlantic Ocean would get wider, but the
  coastlines of the landmasses would not change
  dramatically.

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Dan McKenzie Lives 1942 English Geophysicist

• 1968 - Theory of Plate Tectonics is a
  combination of two earlier ideas continental
  drift and sea-floor spreading.

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Plate Tectonics

• Earth's outermost layer, the lithosphere, is
  broken into 7 large, rigid pieces called plates.
• Several minor plates also exist.
• The plates are all moving in different
  directions and at different speeds, from 2 cm to
  10 cm per year in relationship to each other.

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Main Features of Plate Tectonics

• The Earth's surface is covered by a series of
  crustal plates.
• The ocean floors are continually moving,
  spreading from the center, sinking at the edges,
  and being regenerated.
• Convection currents beneath the plates move the
  crustal plates in different directions.
• The source of heat driving the convection
  currents is from radioactivity deep in the
  Earths mantle.

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Plate Boundaries

• Stay tuned for our next topic
• Plate Boundaries

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References

• USGS http//pubs.usgs.gov/publications/text/histor
  ical.html
• BBC Schools
• http//www.bbc.co.uk/schools/gcsebitesize/science
  /21c/earth_and_space/continentaldriftrev1.shtml
• University of Oregon
• http//jersey.uoregon.edu/imamura/121/lecture-9/
  tectonic.html
• PBS
• http//www.pbs.org/wgbh/aso/tryit/tectonics/
• University of California, Berkeley
• http//www.ucmp.berkeley.edu/history/wegener.html