PLATE TECTONICS
PLATE TECTONICS
• With all our talk of glaciers melting and slowly
making the sea swallow our land whole, we
have to keep our minds open.
• Is it possible for land to grow?
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Of course, it is!
Anyone heard of Hawaii?
Those islands are growing all the time!
The volcanoes release magma which hardens
and grows the land.
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• It might be easy to think of the earth as a still
and dead thing.
• It’s not though! The earth is always changing!
• Over the years, the earth has been lifted up,
pushed down, bent and broken.
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• The deepest mine in the world is a gold mine
in South Africa and it is 3.8 km deep.
• That barely even scratches the earth’s surface.
• The core of the earth is over 6000 km deep.
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• If we haven’t ever been there how do we
know it’s there?
• Scientists use two main types of evidence to
learn about Earth’s interior.
– Direct evidence from rock samples.
– Indirect evidence from seismic waves.
PLATE TECTONICS
• Rock samples are great clues to what’s deep
within the earth.
• We have drilled up to 12 km below the surface
and taken samples.
• From these samples, we can make a good
guess as to what happens at deeper depths.
PLATE TECTONICS
• Seismic waves are a bit more interesting.
• When an earthquake occurs, it produces
what’s called a seismic wave.
• By studying the way these travel through the
earth, we can discover things about the layers
of the earth and the changes in composition
among them.
PLATE TECTONICS
PLATE TECTONICS
• The three main layers of the earth are the
crust, the mantle, and the core.
• These layers vary greatly in size, composition,
temperature and pressure.
PLATE TECTONICS
• Temperature changes as you near the earth’s
core.
• At first the rock is cool, but about 20 m down,
the rock would begin to get warmer.
• The core is the warmest part of the earth.
The heat is from the radioactive materials
inside the earth and leftovers from the
formation of the planet.
PLATE TECTONICS
• Pressure, as we go deeper into the earth
would also increase.
• Because there is more rock pushing down on
the deeper layers, the pressure increases.
PLATE TECTONICS
• As far as composition and thickness, let’s
discuss that layer by layer.
• The Crust is a layer if solid rock that includes
both dry land and the ocean floor.
PLATE TECTONICS
• The crust is the thinnest layer of the earth by
far.
• You can think of the crust as that paper-thin
outer layer of an onion.
• In most places, the crust is 5-40 km thick.
• It is thickest under mountains and thinnest at
the ocean floor.
PLATE TECTONICS
• The crust beneath the ocean is called oceanic
crust and this consists mostly of basalt.
– That’s dark rock with a very fine texture.
• Continental crust is the part that makes up
the continents and consists mostly of granite.
– This is rock of light color and coarse texture.
PLATE TECTONICS
• The layer beneath the crust is called the
mantle.
• The mantle is a layer of rock that is very hot
but still solid.
• The mantle is divided into three layers.
• All tolled, the mantle is nearly 3000 km thick.
PLATE TECTONICS
• The outermost layer of the mantle is called
the lithosphere.
– This is 100 m thick in most places and quite hard.
• The middle layer is called the asthenosphere.
– This layer is slightly softened by the heat.
• The lower layer is called the lower mantle.
– This layer is solid again.
PLATE TECTONICS
• The center of the earth is called the core.
• The core is mostly iron and nickel.
• The core is made of two layers…the outer core
and inner core.
• The outer layer is molten metal
• The inner layer is solid metal.
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• It is the movement of the liquid outer layer of
the core that create the earth’s magnetic
fields.
PLATE TECTONICS
• Let’s return to the crust for the next section of
our lesson which will be over continental
drift.
• When North and South America were
discovered, it was noticed how neatly the
continents could fit together, predominantly
South America and Africa.
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• It was 1910 before Alfred Wegener, a young
German Scientist hypothesized that the
continents had indeed once been joined, but
just has moved.
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• Wegener’s idea became known as continental
drift.
• He called the supercontinent made up of all
our lands Pangaea which literally means “all
lands.”
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• Over the years, Wegener gathered evidence
for his theory.
• The evidence he collected included…
– Land Features
– Fossils
– Evidence of Climate Change
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• The land features Wegener found were
exemplified by mountain ranges.
• When he pieced together the maps of Africa
and South America, he saw that the mountain
ranges line up!
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• The fossils Wegener studied were equally eyeopening.
• Nearly identical fossils of plants and animals
can be found on areas now separated by
oceans!
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PLATE TECTONICS
• The climate change evidence was poignant
too.
• The fossils and rocks would be different were
they in a warmer or colder climate.
• Wegener noticed that there were tropical
plant fossils on Spitsbergen, an island in the
Arctic Ocean.
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• Even with all this Wegener’s hypothesis was
rejected.
• They said he couldn’t provide enough
evidence to prove it to be fact.
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• The way mountains form is one reason
Wegener was shot down.
• Scientists assumed at the time that the reason
mountains happen was because the earth is
cooling and “shriveling up” so to speak.
• Wegener said that if this were so, mountains
would be everywhere and not in the thin,
narrow bands where the really are.
PLATE TECTONICS
• This argument brought about discussion of
plate tectonics.
• J. Tuzo Wilson was a Canadian scientist who
began to notice cracks in the earth.
• His theory was that the lithosphere was
broken up into sections called plates.
PLATE TECTONICS
• Wilson then combined what scientists knew
the sea-floor spreading, continental drift and
the Earth’s plates into a single theory.
• REMEMBER: A scientific theory is a welltested concept that explains a wide range of
observations.
PLATE TECTONICS
• The Theory of Plate Tectonics states that
pieces of the earth’s lithosphere are in slow,
constant motion, driven by convection
currents in the mantle.
• The theory explains the formation,
movement, and subduction of Earth’s Plates.
PLATE TECTONICS
• The edges of the plates are called faults.
• There are three types of these boundaries.
– Convergent
– Divergent
– Transform
• Different types of movement occur at each
type of boundary.
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• A divergent boundary is where two plates
come apart.
• Most of these are on the ocean floor.
• When they occur on land, the gap is called a
rift valley.
PLATE TECTONICS
• A convergent boundary is where two plates
come together or converge.
• When plates collide, the denser of the two
will slide under the less dense one.
PLATE TECTONICS
• Since oceanic crust is more dense than
continental crust, oceanic crust will always
sink beneath.
• When two continental crust plates meet, no
subduction takes place.
• Instead both plates smash upwards and a
mountain range is formed.
PLATE TECTONICS
• Transform boundaries are ones where two
plates slide past each other.
• The grinding created of the two plates is what
causes earthquakes.
• Crust is neither created or destroyed here.
PLATE TECTONICS
• Plates are moving all the time.
• Now, we have evidence that, before Pangaea,
there were other supercontinents.
• Perhaps one day, North America and Europe
will once again be next-door neighbors!!!