The earth

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Plate Tectonics

• The earths crust is made of twelve plates. These
  plates float around on the mantles flowing rock.

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Plate Tectonics
A theory of Earth Science that explains both the
shape of the sea floor and of land forms. This
theory was developed largely from observations of
the sea floor.
So how did the theory of Plate tectonics develop?
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Observation Continents seem to be able to fit
together!! Coincidence??? Or were they once
connected
1858 Geographer Antonio Snider-Pellegrini made
these maps
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The Development of a theory

• What is a theory?
• In science, it is an explanation for observations
  of nature.
• A theory in science must be supported by a great
  deal of evidence.
• In everyday language the word theory may be
  used to mean a guess,
• a hypothesis, a prediction or something that
  were very unsure of. People often say, Oh
  its just a theory!. But that is not its
• meaning in science. A scientific theory is one
  that most scientists accept.
• Theories in science
• A scientific theory may be a currently accepted
  theory, a widely supported theory, a
  discarded theory, or a new theory.

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• Alfred Wegener 1912
• German meteorologist.
• He was the first to suggest the hypothesis of
  continental drift to explain Pellegrinis
  observations that continents fit together like
  puzzle pieces.

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The Hypothesis of Continental Drift

• Continents were once together in a super
  continent he called Pangea
• Then broke up into two super continents call
  Laurasia and Gondwanaland
• Continents can move by plowing through the crust
  like ships plowing through water in the sea
• Continents are still moving today

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Continental drift as Wegener imagined it.
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Evidence to support the hypothesis of continental
drift

• People dont just propose a hypothesis without
  evidence to support their ideas!
• What evidence besides the puzzle pieces
  observation could have led Wegener to propose
  this hypothesis???

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1. Evidence from Fossils

• Animals living today on distant continents (ex
  Africa South America) are all different from
  one another.
• Fossils show that animals and plants long ago
  were similar. This implies the continents were
  connected.
• In the millions of years since separation,
  species evolved differently on different
  continents.
• Tropical fossils are found in many polar areas
  suggesting that those places were once closer to
  the equator.

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Wegeners evidence for Continental Drift
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2. Evidence from Glaciation

• Scratches made on bedrock by glaciers on
  different continents line up when the
  continents are placed together.
• On some continents, glacial striations visible
  today make no sense. The run east-west rather
  than north-south.
• Evidence of glaciers is found in tropical areas
  suggesting that those areas were once further
  north or south.
• This suggests that the continents were connected
  when the striations were made

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Glacial striations
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3. Evidence from rock and mineral formations

• Coal deposits had been found in Antarctica.
• Coal deposits only form in the tropics suggesting
  that Antarctica had once been in the tropics.
• Rock composition in certain areas does not match
  surrounding rock but does match rock type of a
  distant continent.
• Ex Rocks of Northern Newfoundland do not match
  NA but are similar in structure and composition
  to those of Scotland. This suggests that a small
  bit of what is now Scotland became attached to NA
  when the two split.

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Unfortunately for Wegener, the scientific
community did not accept his ideas. His proposed
theory was rejected because

• Evidence was considered too circumstantial
• Not enough evidence
• Wegeners theory could not explain what forces
  could cause continents to travel large distance
  over the surface of the earth.
• Without harder evidence and without a believable
  reason why continents might be moving, people
  were not convinced

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1930 Wegener left for Greenland to find more
evidence to support his hypothesis. He was never
seen again. This is the last photo of Wegener
taken with a native guide. His continental drift
hypothesis was hotly debated for the next 30
years.
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New Evidence to Support Wegeners hypothesis

• After World War I the newly invented sonar
  showed the ocean floor to be much more rugged
  than was previously thought.
• In 1947 the survey ship Atlantis found that the
  sediment layer on the floor of an ocean was very
  thin near mid-ocean ridges but that it became
  thicker as you got farther from the ridge.
• Deep sea sediments are produced by the
  accumulation of dead plants and animals over long
  periods of time. So very little sediment implies
  very new sea bottom!

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• In the 1950s a great undersea mountain range
  (named the mid-ocean ridge system) that extends
  through all oceans was discovered. It zigzags
  between the continents with an average height of
  4,500 m. The mountain range was found to be very
  seismically active.
• Also in the 1950s many very deep ocean trenches
  were discovered, mapped and found to also be very
  seismically active.
• Radioactive dating showed that no ocean rocks are
  older than about 200 million year whereas
  continental rocks close to 4 billion years old
  are common.

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Mid Ocean Ridges
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Radioactive dating showed that rocks are youngest
near mid ocean ridges and that they get older the
farther you get from the ridge
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Pangaea means supercontinent, one huge landmass.

• 200 million years ago all of the present-day
• continents combined to form a single
• supercontinent called Pangaea.

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• http//www.ucmp.berkeley.edu/geology/anim1.html

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Divergent Boundary

• the boundary between 2 plates that are spreading
  apart, known as sea-floor spreading.
• An example is the Mid-Ocean Ridge.

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Diagram of a Mid-Ocean Ridge.
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Convergent Boundary

• the boundary between 2 plates that are moving
  towards each other.
• Also called a Subduction Zone.

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Cross-section of a Subduction Zone.
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Diagram of what happens at a Subduction Zone.
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Transform Boundary

• a boundary between 2 plates that are sliding past
  one another.
• An example is the San Andreas Fault.

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Diagram of a Transform Plate Boundary.
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13 Major Tectonic Plates of the World.
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Earthquakes

• Earthquakes- are caused when a plate (part of the
  crust) lurches downward or undergoes structural
  transformation
• http//www.abag.ca.gov/bayarea/eqmaps/animation.ht
  ml
• http//www.pbs.org/wnet/savageearth/animations/ear
  thquakes/

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Volcanoes

• Volcanoes- some of the crust will melt when it
  subducts (plunges downward) forming magma. In
  some places this magma rises through to the
  surface and causes a volcanic eruption.

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• http//www.pbs.org/wnet/savageearth/animations/vol
  canoes/index.html

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Ring of Fire- A circle of violent geological
activity surrounding much of the pacific ocean