Unit 5: Plate Tectonics
6.1 Structure of the Earth: (17.1 pg 493-496)
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The earth is nearly spherical with a circumference of 12,800 Km at the equator. It is composed of layers.
o Crust is the outside layer of hard rock. It is only about 100 km thick.
o Mantle is the layer below the crust. It is about 3000km thick. It has three layers within itself:
i. The solid outer mantle that is part of the lithosphere.
ii. The asthenosphere which is the soft and syrupy molten layer that while being solid is
under so much pressure that it acts like thick syrup.
iii. And finally the super dense inner mantle.
o Outer Core is the mainly liquid iron and nickel and is about 5100Km thick, the inner core is solid
dense iron and is 6378 Km thick.
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Geologists have learned about the Structure of the Earth through direct and indirect observations.
o Direct observations:
i. Volcano sampling – taking samples of the lava to determine its composition.
ii. Drill samples – from oil rigs and other activities (e.g. mining) that drill deep holes into the
earth.
iii. Modern Mapping – has allowed us to get detailed images of the earth to study.
o Indirect observations:
i. Magnetic field – tells use when rocks have formed and what the magnetic field of earth
used to look like (more on this in Chap 18).
ii. Gravity strength – by measuring very carefully, scientists can get an idea about which
layers of the Earth are thicker or denser based on small changes in the strength of gravity.
6.2 Evidence for a Dynamic Earth: (17.2 pg 497-500)
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The lithosphere of the earth is divided up into tectonic plates, which are pieces of the crust that float
on the denser asthenosphere.
By 1912 enough evidence had been accumulated for Alfred Wegner to come up with the theory of
continental drift. Some of the most convincing evidence was:
o The way continents fit together like a puzzle (mainly south America and Africa)
o Match of geological features (e.g. mountain ranges) on distant coastlines (eastern America
matches western Europe and Africa)
o Identical ancient fossils on distant coastlines (Africa, south America, Australia)
o The presences of coal deposits in non-tropical areas (Antarctica)
o Evidence of glaciers near the equator (Africa)
The idea of continental drift was met with a lot of opposition. The main problems with it were:
o The age of the Earth would have to be billions of years, not just thousands of years. At the
time, scientists believed it was only 6000 to 400 million years old.
o More importantly, Wegner could not explain HOW the plates moved.
6.3 New evidence for a Dynamic Earth: (17.3 Pg 501-504)
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During World War II, Henry Hess was in charge of a submarine that discovered mid-ocean ridges. A
mid-ocean ridge is a massive undersea mountain range that runs along the bottom of the ocean. Since
this initial discovery, a total length of 75,000 km of mid-ocean ridges has been discovered.
Hess proposed a process called sea-floor spreading to explain why the sea floor was widening at midocean ridges. Molten magma from the Earth’s core rises and fills the ridges.
Future work suggested the sea floor was cycled back into the Earth to create ocean trenches.
Some of the Earth’s heat is created by radioactive decay. Radioactive dating (using half-life!) suggests
age of the rock further from the mid-ocean ridge is older.
Magnetic lava that has cooled to form rock has taken on the alignment of the magnetic field of the
Earth. Bands that show opposite magnetization along the ocean floor suggest the rock has formed at
different geological times.
Lava sampling = http://www.youtube.com/watch?v=egEGaBXG3Kg
Magnetic evidence = http://www.youtube.com/watch?v=BCzCmldiaWQ
6.4 Theory of Plate Tectonics: (17.4 Pg 505-508)
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The theory of plate tectonics states that the Earth’s crust is divided up into about 12 large plates and 20
smaller plates. These plates float on the denser asthenosphere below it. The movement of these plates
act to form mountains, trenches, rifts, etc.
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There are three kinds of plate boundaries:
1. Divergent boundary, where plates pull apart; if this happens on land a rift valley is formed.
2. Convergent boundary, where plates press against each other.
o Oceanic-oceanic convergent boundaries create islands like the Philippines
o Oceanic-continental convergent boundaries create deep-ocean trenches called subduction
zones, and form mountain ranges on continents, like in BC’s coastal mountains.
o Continental-continental convergent boundaries create inland mountains, such as the
Himalayas.
3. Transform boundaries are where the two plates slide past one another. These are also known as
strike-slip faults.
6-5 Cause and Effects of plate movement: (18.1 pg 517-521)
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Scientists believe tectonic plates are moved by three forces:
o Mantle convection is the idea that there is a treadmill-like current in the asthenosphere that
moves the crust that sits above it. (hot magma rises in one area and cool magma sinks in another)
o Ridge Push is the idea that the hot magma pushing up the mid ocean ridges also is pushing the
ocean plates apart.
o Slab Pull is when one plate slides underneath the other. The subduction zone causes the bottom
plate to be pushed back into the mantle and this pulls the plate away from the mid ocean ridge.
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Mountains form at convergent boundaries.
o Ocean – continent boundary causes mountains to form along the coast. These often push land
that was once underwater up high causing ocean fossils to be found in the mountains.
o Continent – continent boundary causes mountains to form from the uplifting of the two continent
edges. This can also cause former coast lines and marine fossils to be found in the mountains.
Rifts form at divergent boundaries. If this happens on land a rift valley forms. Often volcanoes also form
along these rifts since the crust is thinner.
Trenches form at a convergent boundary.
They are where one plate is subducted under
the other. If there are mountain chains along
a coast then you know that a few hundred
kilometers out into the ocean that there must
be a trench.
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6-6 Geological Events: (18.2 pg 522-528)
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Plate tectonics cause many of the geological events and features that we are familiar with. They cause:
a. Volcanoes
b. Earthquakes
A Volcano marks a spot where there is a crack in the lithosphere.
a. They can form at mid-ocean ridges where plates are being pulled apart, and magma rises up.
b. They can also form about 200 km from a trench, when trapped water becomes steam and melts
through the crust. This forms volcano belts on continents and volcanic island arcs in the ocean.
c. They can form above a hot spot in the mantle, caused by heated concentrations of radioactive
substances near the Earth’s core, causing hot columns of rising mantle that melt through the crust.
Earthquakes happen when two tectonic plates move past each other. The plates do not have smooth
edges so there is tremendous friction that keeps them from sliding...until enough pressure builds up, and
then the plates move suddenly. This sudden movement is what we call an earthquake.
The boundary where the two plates move past each other is called a fault.
The location of the earthquake is called the focus. The focus can be deep in the earth or relatively shallow
down. It depends which part of the two plates slide past each other. The epicenter is the spot above the
focus, on the surface of the Earth. Earthquakes are categorized according to the depth of their focus.
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Earthquakes travel through the Earth in wave form. These are called body waves and there are two kinds:
a. Primary waves (P-waves) are waves that travel parallel to the direction of the wave. They are a
‘compression’ wave. They are like pulling on a spring and letting the end go. It stretches and
compresses. They can travel through all parts of the Earth.
b. Secondary waves (S-waves) are waves that travel sideways (shear) to the direction of the wave.
They are like a slithery snake. They can only travel through the mantle and the crust. They cannot
travel through the core.
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The most destructive kind of earthquake wave is called a surface wave. They travel along the surface of
the crust like a wave on the ocean. They are what destroy buildings, bridges, etc.
Scientists use instruments called seismographs to measure the strength of earthquakes. They use the
Richter scale. Each increase of 1 on the scale means that the earthquake is 10 times as strong. An
earthquake of 5 is 10 times stronger than a 4.
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