Download Theory of Plate Tectonics

The earth’s crust is made of twelve plates. These
plates float around on the mantle’s flowing rock.
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?
Observation: Continents seem to be able to fit together!!
Coincidence??? Or were they once connected
1858 Geographer Antonio Snider-Pellegrini made these maps
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 we’re very unsure of.
People often say, “Oh it’s 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”.
Alfred Wegener
1912
•German meteorologist.
•He was the first to suggest
the hypothesis of
continental drift to explain
Pellegrini’s observations
that continents fit together
like puzzle pieces.
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
Continental
“drift” as
Wegener
imagined it.
Evidence to support the hypothesis
of continental drift
• People don’t 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???
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.
Wegener’s evidence for Continental Drift
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
Glacial striations
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.
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
•Wegener’s 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
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.
New Evidence to Support Wegener’s 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 midocean 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!
• In the 1950’s 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 1950’s 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.
Mid Ocean Ridges:
Radioactive dating showed that rocks are youngest
near mid ocean ridges and that they get older the
farther you get from the ridge
Pangaea means supercontinent, one huge landmass.
 200 million years ago all of the present-day
continents combined to form a single
supercontinent called Pangaea.
 http://www.ucmp.berkeley.edu/geology/ani
m1.html
Divergent Boundary:
the boundary between 2 plates
that are spreading apart, known
as sea-floor spreading.
An example is the Mid-Ocean
Ridge.
Diagram of a Mid-Ocean Ridge.
Convergent Boundary:
the boundary between 2 plates
that are moving towards each
other.
Also called a Subduction Zone.
Cross-section of a Subduction Zone.
Diagram of what happens at a Subduction Zone.
Transform Boundary:
a boundary between 2 plates that
are sliding past one another.
An example is the San Andreas
Fault.
Diagram of a Transform Plate Boundary.
13 Major Tectonic Plates of the World.
Earthquakes
 Earthquakes- are caused when a plate
(part of the crust) lurches downward or
undergoes structural transformation
 http://www.abag.ca.gov/bayarea/eqmaps/ani
mation.html
 http://www.pbs.org/wnet/savageearth/anima
tions/earthquakes/
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.
 http://www.pbs.org/wnet/savageearth/anima
tions/volcanoes/index.html
Ring of Fire- A circle of violent geological
activity surrounding much of the pacific ocean