Transcript Science 7: Unit E: Planet Earth

Science 7: Unit E: Planet Earth
Topic 4 – The Moving Crust
What makes a good Scientific
Theory?
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Two things:
I. It can explain why something in the universe
is the way it is, acts the way it does, etc.
II. It can make a testable prediction about
some yet untested question.
Eg. I can explain that decomposers are
necessary for an ecosystem, but unless I can
make a testable prediction (an ecosystem with
the decomposers removed will become full of
waste and dead organisms) it is useless.
The Structure of the Earth
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From outside to centre:
I. Crust – 5-60 km thick: This is the rocky outside of
the Earth where all organisms live. We’ve never seen
anything but the crust.
II. Mantle – 2885 km thick: Rock and magma under
the crust. Temperatures range from 1000-4000C.
III. Outer Core – 2270 km thick: Liquid iron and
nickel. Temperatures here are 5500C.
IV. Inner Core – 1216 km thick. Even though
temperatures here are higher than in the outer core
(6000C), the pressure of the weight of the Earth
keeps the metal (iron and nickel) a solid ball.
NOTE: Lithosphere – The rocky part of the Earth
(upper Mantle and the Crust).
Why is the Earth arranged the way
it is?
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In 1910, Alfred Wegener noticed that the
continents had shapes that fit together
like pieces in a jigsaw puzzle. Based on
this he made his theory:
Continental Drift Theory: It is
hypothesized that the continents were at
one time part of a single super-continent
called Pangaea. They then drifted apart.
They continue to drift today.
Evidence for Continental Drift
Theory
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To support his theory, Wegener collected the
following evidence:
Continents’ edges fitting together.
Similar mountain and land formations in
different continents.
Fossils of similar species of animals and plants in
different continents, even Antarctica.
In the end, Wegener could not explain WHY the
continents were moving and so his theory was
not taken seriously.
Advances in Technology
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Since Wegener, society has developed new
technology that has helped map the ocean floor
and provided new clues about Earth’s interior
(which can’t be seen directly), tech. such as:
Sonar – Sound waves emitted by a ship or sub
bounce off the ocean floor and help give the
distance and shape of the floor.
Deep Sea Subs – scientists can travel and
observe ocean trenches and ridges directly in
the safety of their vehicle.
Sea Floor Spreading Theory
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With the new information provided by sonar,
deep sea vehicles and magnetic readings near
the Mid-Atlantic Ridge, scientists noted that the
evidence pointed to a sea floor in the Atlantic
that was creating new rock.
Sea Floor Spreading Theory – Because of the
creation of new igneous rock at the Mid-Atlantic
Ridge, the sea floor is continuously spreading.
This theory was a step in the right direction, but
it couldn’t explain everything. That would come
with the next theory.
Theory of Plate Tectonics
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Based on all of the collected evidence, scientists
have created a new theory that explains all of the
evidence so far and makes predictions about what
areas of the Earth are likely to have volcanoes and
earthquakes.
Theory of Plate Tectonics - the Earth’s crust is
actually broken up into pieces called plates which
float on the magma mantle. Plates moving towards
each other are called converging plates, plates
moving away are called diverging plates, and
plates sliding past each other are called shearing
plates.
The plates are usually named after the continents
they contain.
Diverging Boundary
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The boundary between two diverging plates. Eg.
Iceland near the Mid-Atlantic Ridge.
This area is characterized by volcanic eruptions
and earthquake activity.
The diverging boundaries are caused by
convection currents in the magma pushing
warmer magma up, until the pressure causes an
eruption (most of the time it cools enough and
sinks back down.)
Diverging boundaries will eventually form ridges
like the Mid-Atlantic Ridge.
Converging Boundary
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The boundary between two converging plates.
Eg. The Himalayas.
These areas are characterized by high mountain
ranges, earthquakes, and the occasional
volcano.
If a continental tectonic plate (lighter and
thicker) bumps into an ocean plate (heavier and
thinner), the ocean plate will slide under the
continental plate and become magma, while the
continental plate will rise and form mountains.
This process of one plate sliding under the other
is called subduction.
Converging Boundaries Cont’d
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If two continental plates collide, you’ll get
mountains.
If a continental and oceanic plate collide,
you’ll get subduction and mountains
volcanoes.
If two oceanic plates collide, you’ll get
islands and a deep ocean trench.