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Chapter 19
Plate Tectonics
Notes
(4 classes notes + 1 class Xword & review + 1 class test)
Objectives:
1. Explain early ideas about crustal movement. pp. 514-525
2. Describe and give examples of convergent, divergent and
transform plate boundaries. pp. 525-539
As the accuracy of world maps improved, a German scientist named
Alfred Wegener noticed that the continents seemed to fit together like
a jigsaw puzzle. Around 1915, he proposed a hypothesis, called
continental drift, which stated that about 200 million years ago, a
supercontinent called Pangaea (see Figure 19.2 p. 515 text) began to
break apart and that the continents somehow had drifted to their
present location. See continential drift for the last 3.3 billion years
here. See breakup of Pangaea here. See animation here for future
positions of continents. He put forth 4 pieces of evidence to support
his theory (see https://www.youtube.com/watch?v=_5q8hzF9VVE )
1. The way the continents fit together.
Wegener noted the remarkable similarity between the coast lines of
the continents, especially those of eastern South America and western
Africa. His opponents argued that shore lines were continually being
modified by erosion and the continents now could not possibly be the
same shape now as they would have been 200 million ago. Hence the
current fit between continents was just a coincidence. However, in the
early 1960's scientists investigated the fit between the edges of the
continental shelves and found it to be remarkably good (see Figure
19.3 p. 516 text). These underwater boundaries would have been less
susceptible to erosion and provided a much better picture of the true
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shape of the continents around the time that Pangaea existed. See map
here.
2. The existence of the same fossils in regions very far apart
(Fossil Correlation).
Wegener found that the fossils of several organisms were found on
different continents so far apart that they could not have occurred
there unless the land masses were joined at one point in time. He used
fossils of the Mesosaurus , which was found in eastern South
America and southern Africa (see Figure 19.4 p. 517 text), and fossils
of Glossopteris, which are found in Africa, Australia, India, South
America, and Antarctica, as support. His opponents argued that these
continents were once joined by land bridges. However, if a land
bridge large enough to reach from Africa to South America once
existed, its remnants would now have to be present below sea level.
No such remnants exist.
3. Rocks of the same type and structure in regions very far apart.
Wegener found that the type and age of rocks found on one continent
matched those on other continents. For example, the mountain belt
that contains the Appalachians extends through the eastern US and
ends in Newfoundland. However, rocks of similar type and age
reappear in the British Isles and extend through Scandinavia (see
Figure 19.6 p. 518 text). When current land masses are reassembled
into Pangaea, the mountain chains form a continuous belt.
4. Evidence for similar climates in regions that currently
experience very different climates.
Wegener found layers of glacial till in southern Africa, South
America, India and Australia, with striated and grooved bedrock
underneath. This suggested that these regions had once been covered
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by glaciers. The direction of the striations suggested that if the
continents were always where they are now, then these glaciers must
have moved in from the sea. Wegener explained that if Pangaea was
situated with South Africa centered on the South Pole, then the
presence of glaciers on Pangaea near the South Pole could explain the
existence of the glacial till and the striations and grooves that
currently exist on continents very far apart. A Pangaea in this position
would also place the current Northern Hemisphere in the tropical
region and would explain the presence of major coal fields in the US,
Europe, and Siberia (see Figure 19.7 p. 519 text).
One of the main objections or weaknesses to Wegener's hypothesis of
continental drift was that he could not explain how the continents
actually drifted or moved. He first proposed that the tidal influence
caused by the moon was responsible for moving the continents (i.e.
the force of gravity created by the moon moved the continents). This
was quickly proven not to be possible. He then explained that the
continents cut through the ocean crust much like an icebreaker, but no
evidence was found showing the ocean crust deformed where a
continent moved through it. By 1968, enough data had been gathered
to explain how the continents drifted apart after the break up of
Pangaea. The explanation for the movement of the continents is called
the theory of plate tectonics. It essentially states that the earth's outer
shell consists of about 20 rigid slabs called plates. These plates are
constantly in motion relative to one another (see Figure 19.17 pp.
528-529 text). The plates vary in size and include the very large
Pacific plate and the adjacent, smaller Nazca plate. Note that these
plates often contain both continental and oceanic crust. This contrasts
with Wegener's continental drift hypothesis that said that only the
continental crust moved.
Recall that the earth's rigid outer shell is called the lithosphere and
includes both continental and oceanic crust as well as part of the
upper mantle. Below the lithosphere is the plastic-like asthenosphere.
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It is this fluid like region that allows for the movement of the solid
plates above it. Keep in mind that the plates in the theory of plate
tectonics are rigid so any two places on the same plate should always
remain the same distance apart. However, the distance between places
on separate plates should vary as the plates move relative to each
other.
The edges the plates are called the plate boundaries. It is along the
plate boundaries that most of the earth's seismic activity, volcanoes,
earthquakes, and mountain building occur. There are three (3) main
types of plate boundaries.
1. Divergent boundaries - occur where plates move apart. These
boundaries occur mostly at ocean ridges and allow molten material to
come from inside of the earth and create new sea floor (see Figure
19.17A p. 529, Figure 19.19 p. 533, and Figure 19.18 p. 531 text).
See animation here.
2. Convergent boundaries - occur where plates move together. The
denser plate will move under the less dense plate (subduction) and
this usually results in volcanic activity (see Figure 19.21 p. 535 text)
or mountain building (see Figure 19.7B p. 529 & Figure 19.23 p. 538
text).
A trench is usually formed where one plate moves under the other.
See animation here.
3. Transform plates - occur where plates move laterally (grind past
each other). The movement of these plates is often accompanied by
earthquakes (see Figure 19.7C p. 529 & Figure 19.24 p. 539 text). See
animation here.
See all boundaries here.
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Sample Exam Questions
1. What was the name of the single "supercontinent" which Alfred
Wegner hypothesized that today's continents were once part of?
(A) Atlantis
(B) Gondwanaland
(C) Laurasia
(D) Pangaea
2. What is the primary reason that continental drift was initially
rejected by a majority of geologists?
(A) Continental reconstruction showed that continents fitted together
very poorly.
(B) Fossil evidence suggested that the continents have always been
where they are today.
(C) Glacial evidence suggested that the continents have always been
where they are today.
(D) There was no proposed mechanism to explain continental
movement.
Do #'s 1, 2, 3, 4, 11, 14 p. 551 text.
Read pp. 523-539 for next day
Divergent Boundaries
Most of these are located at the top of ocean ridges (see Figure 19.17
pp. 528-529), for example the Mid-Atlantic Ridge and the Mid-Indian
Ridge. Tensional forces cause adjacent plates to move away from
each other and molten rock rises from the asthenosphere and cools to
form new ocean floor. See animations here and here. This process is
called seafloor spreading. The relatively narrow area where the
plates are moving apart is called the spreading center. Most spreading
centers exist on the sea floor, but some are found on land. When a
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spreading center develops on land, the land mass breaks up into
smaller pieces. This was how Wegener explained the breakup of
Pangaea. As the pieces at the spreading center move apart, the upward
force of the rising magma creates cracks in the crust (see Figure
19.19A p. 533 text). As the plates move in opposite directions, the
broken slabs fall downward creating rifts or rift valleys (see Figure
19.19B p. 533 text) see animations here and here. As the spreading
continues, the rift valley will lengthen and widen and eventually reach
the sea, creating a narrow linear sea like the Red Sea (see Figure
19.19C p. 533 text). Eventually, the sea will widen to create a wide
ocean basin (see Figure 19.19D p. 533 text). See animation here.
Sample Exam Questions
1. Where would you find a divergent boundary?
(A) Andes mountains
(B) Japan trench
(C) Mid-Atlantic ridge
(D) San Andreas fault
Convergent Boundaries
New lithosphere is created at divergent boundaries. Since the amount
of lithosphere is roughly constant, lithosphere must also be consumed.
This occurs at convergent boundaries. At a convergent boundary,
compressional forces cause two plates to collide and one plate is
forced under (or subducted under) the other into the mantle. See
animation here. This area is called the subduction zone. What
happens at each boundary depends on the type of crust making up
each plate. As a result, there are three (3) types of convergent zones.
1. Oceanic-Continental Convergence - this type of convergence
occurs when a plate made of oceanic crust collides with a plate of
continental crust. In this case, the more dense oceanic crust will sink
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into the asthenosphere. As the oceanic plate slides under the
overriding continental crust, the ocean plate bends, producing a deepocean trench. Eventually, the ocean plate will melt as it moves down
into the mantle. This new magma is less dense than the magma
surrounding it and will rise and intrude into the continental crust and
cool. Sometimes the magma will reach the surface and volcanic
eruptions will occur (igneous rock is formed). When this occurs, a
belt of volcanic activity called a continental volcanic arc is formed
(see Figure 19.21A p. 535 text). Examples of continental volcanic
arcs are the Rocky and Andes Mountains. See animations here, here,
here, and here
Sample Exam Questions
1. What plate boundary activity is presently occurring at the western
edge of North America?
(A) continent - continent collision
(B) ocean - ocean collision
(C) seafloor spreading
(D) seafloor subduction
2. Oceanic-Oceanic Convergence - this type of convergence occurs
when a plate made of oceanic crust collides with another plate of
oceanic crust. The more dense crust is again subducted but in this
case the volcanoes occur on the ocean floor instead of on the
continents. If the volcanoes reach the ocean surface volcanic island
arcs are formed (see Figure 19.21B p. 535 text). See animation here.
Examples are the Aleutian, Mariana, and Tonga islands. Continued
volcanic activity can cause some of the sediment to reach the deep sea
trench where they are deformed and metamorphosed. This will
eventually lead to a mature island arc composed of volcanic rocks,
metamorphosed sedimentary rocks, and intrusive igneous rocks.
Examples are the Alaskan Peninsula, the Philippines, and Japan.
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3. Continental-Continental Convergence - this type of convergence
occurs when a plate made of continental crust collides with another
plate of continental crust. Since both crusts have about the same
density, neither will bend below the other. As a result, the plates ram
into each other and are forced to rise upward, creating mountain
ranges such as the Himalayas (See animation here and here), the Alps,
the Appalachians, and the Ural mountains (see Figure 19.21C p. 535
& Figure 19.23 p. 538 text).
Transform Boundaries
The third type of boundary is the transform fault, where plates grind
past each other without producing new crusts (divergent boundaries)
or destroying crust (some convergent boundaries). See animations
here and here. The direction of these faults roughly parallels the
direction in which the plate is moving. These faults are called
transform faults because the relative motion of plates can change or
be transformed along the faults. Note that in Figure 19.25 p. 541 text,
an entire portion of the Juan de Fuca ridge is being transported
towards the west coast of North America, while on the other sides of
the two faults, the sea floor is spreading but the position of the rift
valley stays the same. The Mendocino transform fault at the southern
end of the Juan de Fuca plate allows the oceanic crustal material
created at the Juan de Fuca ridge to be transported to the subduction
zone beneath western North America.
Most transform faults lie beneath the oceans, but a few occur on the
continents. The most famous is the San Andreas Fault in California.
This region experiences many earthquakes because of the movement
along this fault. See picture here. Another transform fault is the
Alpine Fault in New Zealand. See picture here.
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Sample Exam Question
1. Match each type of boundary to the appropriate activity. Some
types may be used more than once.
Continental crust is folded to
A. Transform Fault
form mountains (this is not
accompanied by subduction)
B. Divergent
San Andreas fault is an example.
C. Convergent
Volcanic island arcs form.
Himalayas are an example.
Mid-Atlantic ridge is an example.
2. With the aid of well labeled diagrams, explain what happens at:
(i) a divergent plate boundary, and
(ii) any one of the convergent plate boundaries.
Do #'s 8, 12, 17 p. 551 text.
Read pp. 540-544, 547-549 for next day.
Objectives:
1. Relate convection theory to plate movement. pp. 547-549
2. Relate the formation of rocks and minerals to movement at plate
boundaries. Notes
Recall that the role of a theory is to describe. The theory of plate
tectonics describes the motion of the plates which make up the
lithosphere but it does not explain what actually causes the plates to
move (i.e. the driving mechanism). One hypothesis that may explain
why the plates move is called the convection current hypothesis.
This hypothesis suggests that large convection currents within the
mantle cause plate motion. The warm, less dense material in the
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mantle rises very slowly where the ocean ridges occur (divergent
boundary). It then moves laterally (sideways) and drags the plate with
it. This causes seafloor spreading. Eventually the material will cool
and sink back into the mantle (convergent boundary), where it is
reheated and the process begins again (see Figure 19.32A p. 549 text).
See animation here
Research Activity: Briefly outline the contribution(s) of the following
scientists to the development of the theory of plate tectonics.
 Frank Taylor
 Alfred Wegener
 Alexander DuToit
 Arthur Holmes
 Harry Hess & Robert Deitz
 Fredrick Vine & Drummond Matthews
 J. Tuzo Wilson
 Xavier Le Pichon & Dan McKenzie
Rocks and Minerals Formed at Plate Boundaries
Different types of rocks form at divergent and convergent plate
boundaries, depending on the rock type that is melted to form magma
and the things that get added to the magma before it cools.
At mid-ocean ridges (divergent boundary), pressure on the underlying
rocks is lessened as the plates move apart. This lowers the melting
point of the underlying rocks and the mantle is melted to form mafic
magma with the composition of basalt. This is the reason that basalt is
the main rock type found in ocean basins.
At subduction zones (convergent boundary), partially molten basalt
(from the subducting plate), mantle (from the overlying mantle
wedge), and crust (melted as the magma nears the surface) together
form magma of intermediate composition. This magma cools to form
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rocks like andesite (see Figure 3.8 p. 69 text). When continental crust
melts, the result is a felsic magma, which tends to explode rather than
erupt quietly. Rocks like granite result from this type of magma (see
Figure 3.8 p. 69 text).
Note that metamorphism can also occur in the subduction zone.
Therefore it is possible that metamorphic rocks can be produced at a
subduction zone (convergent boundary).
Sample Exam Questions
1. Sediments can be metamorphosed at convergent boundaries. Where
do they come from?
a) From deep sea sediments and continental sediments.
b) From continental sediments only.
c) From deep sea sediments only.
d) From plutonic and volcanic rocks formed by the melting of the
ocean floor.
2. Regional metamorphism can happen where continental and oceanic
plates collide. Why?
a) The crusts tend to pile up at the subduction zones.
b) The convergence results in high pressures.
c) The combination of stress from convergence and the high extent of
local igneous activity cause it.
d) The high extent of local igneous activity causes it.
Objectives:
1. Relate the rock cycle to plate tectonics. Notes
2. Relate plate tectonics to the geology of the local area. p. 518, 529
3. Outline the evidence for plate tectonics theory. pp. 540-544
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The Rock Cycle and Plate Tectonics
Material on the continents is weathered and transported to the
continental margins. As these layers of sediment get thicker, the
bottom layers become lithified (turned to rock) because of the
pressure of the sediment above. When the region containing the
sediment becomes a convergent plate boundary, the oceanic crust
moves downward and the pressure and heat may transform the
sedimentary rock into metamorphic rock. Some of the oceanic crust
may go deep enough and begin to melt. When this occurs, the magma
will move upward and some will cool below the surface to form
igneous rock. Some magma will reach the surface to form a different
type of igneous rock. The igneous rocks on the surface are eroded and
the sediments transported to the continental margin where the cycle
begins again.
Plate Tectonics and Local Geology
Recall that the mountains found in western Newfoundland have the
same rock type and rock structure as those found in the British Isles
and Scandinavia (see Figure 19.6 p. 518 text). These mountains were
formed when Pangaea existed and Newfoundland was close to what
is now Europe. When the North American and Eurasian plates moved
apart (see Figure 19.17 pp. 528-529 text), these land masses were
transported to their current locations, but still have similar geological
features.
Evidence for Plate Tectonics Theory
There are four (4) pieces of evidence which resulted in the overall
acceptance of the theory of plate tectonics. They are paleomagnetism,
earthquake patterns, ocean drilling, and hotspots.
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1. Paleomagnetism - some rocks contain minerals (like magnetite)
which can indicate the direction of the earth's magnetic field. When
magma which contains a mineral like magnetite begins to cool, the
mineral becomes magnetized in a direction parallel to the direction of
the earth's magnetic field. When the magma hardens, the magnetic
field of the minerals will be locked in place and will not change, even
if the rock containing it is moved by faulting or folding. Therefore
rocks formed millions of years ago will still indicate the direction of
the earth's magnetic field when the rocks were formed.
Paleomagnetism was used in two ways to support plate tectonics.
i. Polar wandering - scientists found that the magnetic alignment in
lava flows of different ages varied widely. They plotted the location
of the north magnetic pole over a period of 500 million years and
found that the location of magnetic pole had shifted with time (see
Figure 19.11 p. 522 text). This apparent movement of the pole was
called polar wandering.
Although the magnetic poles do move slightly, the location of the
magnetic pole is always near that of the geographic pole. So if the
position of the magnetic pole had changed over 500 million years,
then the geographic pole must have moved as well. This meant that
the land mass containing the geographic pole had to be moving
during this time, which supported plate tectonics.
ii. Magnetic Reversals and Seafloor Spreading- scientists also
learned that the earth's north and south magnetic poles also switch
direction periodically. Therefore, if a rock containing magnetic
minerals like magnetite has formed relatively recently, the direction
of the magnetic field of the rock will be opposite to that of a similar
rock formed when the magnetic poles were reversed. When direction
of the magnetic field of the rock is the same as the direction of the
earth's magnetic field, the rock is said to possess normal magnetism.
If the direction of the magnetic field is opposite, the rock is said to
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have reverse polarity. If seafloor spreading occurs at divergent plates,
then the sequence of magnetic field reversal should mirror each other
on opposite sides of the ridge. The discovery that the magnetic field
did mirror each other on opposite sides of the ridge supported seafloor
spreading a therefore the theory of plate tectonics (see Figure 19.15 p.
526 & Figure 19.16 p. 527 text). See animations here and here
2. Earthquake Patterns - According to the theory of plate tectonics,
deep-focus earthquakes are associated with convergent boundaries but
not with divergent boundaries. Scientists found that the distribution of
deep-focus earthquakes closely parallels the locations of convergent
plates (compare Figure 19.26 p. 542 and Figure 19.22 p. 536) but not
the locations of divergent plates. This discovery again added support
to plate tectonics.
3. Ocean drilling - when scientists drilled into the ocean floor near an
ocean ridge, they found that the age of the drill samples increased as
the distance from the crest of the ridge increased. This agrees with
seafloor spreading, which suggests that the youngest layers of ocean
crust would occur near the ridge crest and the oldest at the continental
margins.
4. Hotspots - when the locations of seamounts in the Pacific Ocean
were mapped, a chain of volcanic structures was found to extend from
the Hawaiian Islands to the Midway Islands to the Aleutian trench.
The age of the seamounts was found to increase as one moved away
from Hawaii. It was proposed that a rising plume of magma is located
under Hawaii which generates a volcanic area or hotspot. As the
Pacific plate moves over this hotspot, successive volcanic mountains
are formed which are then carried away as the plate moves farther
(see Figure19.29 p. 544 text). Another volcanic mountain is then
generated. The formation of this chain of volcanic mountains
indicates that the Pacific plate moves over time and therefore supports
plate tectonics. See animation here.
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Sample Exam Questions
1. Why was the discovery of parallel zones of magnetic reversals in
rocks on the seafloor, a crucial turning point in the development of
plate tectonic theory?
(A) It explained what was making the plates move.
(B) It explained why Earth's magnetic field occasionally reverses
polarity.
(C) It provided confirmation that shallow earthquakes occur in the
middle of the oceans.
(D) It provided confirmation that the entire seafloor is moving.
2. With the aid of a labeled diagram explain how the Hawaiian Island
Chain formed.
3. What do the magnetic stripes on the ocean floor tell you about the
earth's surface?
4. Geoscientists have mapped the position of the poles relative to
continents over time. What information do the resulting patterns give?
Do #'s 16, p. 551 text.
Objectives:
1. Describe the sequence of plate tectonic events from Rodinia to the
present day continental arrangement.
2. Explain the formation of Newfoundland using plate tectonics.
3. Demonstrate an understanding that present day tectonic activity is
the continuation of a process that started with the breakup of Rodinia.
p. 530
4. Use present day ongoing plate tectonic activity to explain the
various types of plate margin activity.
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From Rodinia to Present
Rodinia formed about 1 billion years ago, survived for 300 million
years, came apart around 550 million years ago, and continued to
expand. It reformed into Pangaea, starting around 250 million years
ago, during the Carboniferous period. Pangaea started to break up
during the late Triassic, about 210-220 million years ago. During this
breakup, the North Atlantic formed first with the South Atlantic
opening later (see Box 19.1 p. 530 text).
See http://www.uwgb.edu/dutchs/platetec/plhist94.htm See future
animation here
The Geological Formation of Newfoundland
The island of Newfoundland is divided into three main zones created
by tectonic movement:
1. the Humber Zone
2. the Central Volcanic Belt (sometimes separated into Dunnage and
Gander Zones)
3. the Avalon zone
See maps here and here.
About five hundred million years ago, the area that now forms central
North America was under a warm tropical sea called the Iapetus
Ocean. It was surrounded by the land masses we now know as
Europe, Africa and North America. For a hundred million years,
forces within the earth's mantle slowly carried these continents on a
collision course. As the continents drifted together, the ocean floor
was squeezed and then pushed upward to form huge mountain ranges,
one of which we now call the Appalachians. The Humber Zone of
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Western Newfoundland is actually the northernmost part of the
Appalachians in North America. This range, although known by
another name, continues through most of the British Isles and on to
Norway. This is further proof, geologists believe, to support the
theory of drifting continents.
An area called the Tablelands in Gros Morne National Park has rocks
that are so rare on the earth's surface that the region has been named a
UNESCO World Heritage Site. The Tablelands were once part of the
earth's mantle but were pushed on top of the earth's crust during the
collision of the continents many millions of years ago. These mantle
rocks, normally dark green in colour, are now brown because they
have been open to the atmosphere for such a long time. The soil on
the Tablelands is so poor in nutrients, very little grows in this area. It
has often been described as a moonscape. See picture here.
The central geological zone of Newfoundland is known as the Central
Volcanic Belt. This zone was formed from the remains of the Iapetus
Ocean after the land masses squeezed together to form the
Appalachian Mountains. Recall that ocean floors are typically flat,
and this is why the central part of Newfoundland is flatter than either
the Western or Eastern regions.
The Avalon Zone on the east coast of Newfoundland contains rocks
equally spectacular rocks. They are not from the earth's mantle like
the Tablelands but from Africa! This too is explained by continental
drift. The story begins when the continental collisions finally ended
some 300 million years ago. Instead of seven continents, all of the
earth's landmass formed one super-continent called Pangaea, meaning
"all lands". Some 225 million years ago, the forces in the earth's
mantle that brought the continents together now slowly began to pull
them apart. In the process a remarkable thing happened. A small bit
of Africa got left behind! When you stand on Signal Hill in St, John's,
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you are standing on rocks that are identical to ones in the country of
Morocco in North Africa!
As the continents drifted further and further apart, the gap that was
created filled with water to form the Atlantic Ocean. Note that the
northern Atlantic opened first followed by the southern Atlantic.
Geologists believe continental drifting is still occurring. Europe and
North America, for example, are moving apart about three
centimeters every year.
Read The Newfoundland Story here and The Geology of
Newfoundland here.
STSE: The Geology of Newfoundland
CBC program Geologic Journey - The Atlantic Coast
Read pp. 583-600, 602-611, 614 for next day.
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