GY 111 Lecture Notes

D. Haywick (2016-17)

GY 111 Lecture Note Series

Intro to Plate Tectonics
Lecture Goals:
A) Alfred Wegener and Drifting Continents
B) The Plate Tectonic Revolution
C) The Earth's Interior (Seismic waves)
D) Plate Tectonics Mechanisms
Reference: Press et al. (2004), Chapters 2, 4, 20 and 21; Grotzinger et al. (2007) Chapters 2, 12 and 14

A) Alfred Wegener and Drifting Continents

In the early 1890s, a rather perceptive German
meteorologist by the name of Alfred Wegener began
to look at the world in an unorthodox manner. This was
a time when scientists of all disciplines were analyzing
every aspect of the Earth and the beasties 1 that
inhabited the Earth. They followed standard scientific
procedures (or at least they should have). They
observed they hypothesized, they tested, they refined
their hypotheses and lastly, they formulated theories. It
was not that long before that Darwin came up with the
theory of evolution. Geologists, always a careful bunch,
were still coming to grips with geological processes. They knew about volcanoes and
earthquakes, they knew what mountains were composed of and that fossils were important
components of a lot of rocks, they just couldnt come up with the methods by which all these
observations could be united. They were looking for a geological theory of everything. (note: the
geologists eventually found it. Physicists are still looking for their Grand Unification Theory
this will unite all known physical forces).
Back to Wegener. He observed that the shorelines of some continents looked like they could fit
into the shorelines of others almost like a jigsaw puzzle. Take for example the coastlines of
Africa and South America:
Wegener also realized that there were geological
elements common to both shorelines. Rocks of
around 250-300 millions year of age (250-300 Ma)
were similar on both sides and they contained
identical land-based beasties (e.g. dinosaurs).
Wegener hypothesized that in the past, Africa and
South America were united together. Not only that,
he was able to put most of the continents together
into a supercontinent he called Pangaea. He
published a map very much like this one in 1912 and
his idea soon became known as Continental Drift:

Note: the close fit

of the coastlines!
1

Dr. Haywick refers to all living creatures as beasties. There are big beasties (elephants and whales),
small beasties (dogs and cats) and micro beasties (plankton etc).

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Source: Windley, B.F. 1977. The Evolving Continents. John Wiley 385p.
To put it bluntly, the geological world did not take kindly to Wegeners radical idea.
Remember, by this time, scientists expected hard evidence and testing of hypotheses rather than
simple ideas. Moreover, a very famous geologist at the time (Dana) had recently argued very
convincingly that the oceans and continents were firmly rooted in place (e.g., no drifting was
permitted). So if you were a geologist at the time, who would you believe? A well established
geo-God (Dana) or a non-geologist newbee (Wegener). True, there was some evidence for
continental separation (e.g., the fossils), but argued the scientists, maybe the animals just rafted
across the sea on top of a tree. Today, the terrestrial fossil content of rocks on both sides of the
south Atlantic Ocean when taken in conjunction with lots of other data is considered vital
evidence of the earlier existence of super continents; however, 90+ years ago, the fossils alone
could not do it. Continental Drift was delegated to the quasi-science shelf of libraries because it
did not have enough supporting evidence. I collect old geology books and it is interesting how
some researchers ridiculed Wegeners idea. I wonder what they thought when Wegener was
proven almost right.
Are you interested about what geologists of the time believed in? Are you curious about what the
accepted theory of mountain building was in 1912? The check out the link below (its to a GY
112 lecture).
http://www.usouthal.edu/geology/haywick/GY112/112lect6.pdf
Wegeners ideas remained purely speculative for about 40 years. In the interim, the world went to
war twice. Most people remember these World Wars as a time of death, destruction and overall
nastiness. They did do one thing that would prove invaluable to science in general (and geology
in particular): they produced new technology. Wegeners ideas were born again following World
War II.
B) The Plate Tectonic Revolution
Did you see the movie U571? Remember that scene when the submarine is diving beneath the
German destroyer and they almost hit the bottom of the boat? Scary stuff huh? That was fiction.
In World Wars I and II, it was more likely that a sub would run into a submarine mountain than

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another boat. The surface of the ocean may appear to be flat, but the bottom of the ocean is NOT.
The earliest mariners suspected this and WWI/WWII submariners knew this (thank God for
sonar!), but until the end of WWII, no one made any attempt to map out the ocean floors. The
technology was not available yet. Once the war ended, a number of surveys were initiated and the
submarine highs and lows were finally mapped out. Surprise, surprise; instead of a random
distribution, topographic highs and lows were arranged in linear fashion. There were underwater
mountain ranges (called mid oceanic ridges) and deep valleys (called trenches or troughs).
Most mid oceanic ridges were dominated by volcanoes, thermal vents and active lava flows. One
in particular (the Mid Atlantic Ridge) ran right down the middle of the Atlantic

This was pretty neat stuff. A few years later, a new discipline of geology was initiated that would
forever change the way geologists looked at the world. Researchers learned that some ironbearing minerals and rocks preserved the Earths magnetic orientation at the time of their
formation. Igneous rocks formed 100 million years ago (100 Ma) were like compasses. Provided
that you could read the paleomagnetic signature, you could determine the orientation of the
rocks relative to the north-south poles at that time. Well it is pretty easy to read the paleomagnetic
signature of rocks. The interesting thing is that the poles were not consistent over time. In the past
(and presumably this will also occur in the future), the north and south poles reversed themselves
every few 10s of thousands of year (10s of Ka). Successive lava flows recorded these reversals:

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When marine geologists started to sample across the Mid Atlantic and other oceanic ridges, they
made an amazing discovery. The paleomagnetic signatures of the rocks recorded numerous
reversals, but in a striped pattern (see image to right from http://www.calstatela.edu)
The youngest rocks occurred down the
middle of the oceanic ridges (remember the
middle of the oceanic ridges were
characterized by active volcanoes) and the
oldest rocks occurred along the continental
margins. Most importantly, the age pattern
was symmetrical around the ridges. The
explanation for these data is pretty clear.
The volcanically active mid-oceanic ridges
must be areas were new oceanic crust is
being formed and from here, the crust
spreads out laterally. Wegener was kind of
right. The continents are drifting apart, but
it is not just the continents that are moving.
The continents and large parts of the
oceans are moving relative to one another. We now envision the Earths surface as being broken
up into a series of plates (officially tectonic plates) and refer to the motion of continents/oceans
plate tectonics. It wasnt until the middle to late1960s that plate tectonic theory was generally
accepted by the majority of the planets geologists. Plate tectonics, which is as important to
geologists as evolution is to biologists is less than 50 years old. We are still refining it. These are
interesting times to be a geoscientist.

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Once the idea of plate tectonics was widely accepted, geologists sat down and really looked at the
Earth. The evidence of plate tectonics started to jump out at people:
1) Fossil and rock suites that match up on opposite sides of continental shorelines (Wegener was
right!)
2) Localization of mountains, volcanoes, earthquakes and trenches along lines on the Earths
surface (they show the location of plate boundaries)
Well be spending a good
chunk of an upcoming
lecture or two discussing
volcanoes
and
earthquakes. For now,
consider just where most
of them take place. The
map to the right comes
from http://pubs.usgs.gov
and shows the distribution
of earthquakes from 19781987:

And this one shows the distribution of volcanoes (from http://www.intute.ac.uk):

There is nothing random about where most of the volcanoes and earthquakes occur; they are on
plate boundaries. If Wegener had this information when he proposed continental drift, it likely
would have be accepted a lot earlier that it was.

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There is more morphological evidence for plate tectonics.

Mountain belts and their inverted counterparts trenches are
also mostly located along plate boundaries. Trenches are deep
canyons in the ocean floor and they occur where the
subducting plate is dragged under the overriding plate at
convergent plate boundaries (see image to the right showing
the trench between the South American and Nazca Plates
from: http://www.platetectonics.com/. More on this shortly
when we get to plate tectonic mechanisms
There is still more evidence supporting plate tectonics:
3) Hot spots and chains of volcanic islands within ocean
basins
etc.
Hawaii is the
classic
hot
spot
(see
image to left from http://visearth.ucsd.edu)
There is lots of other evidence (e.g., accurately
measured spreading rates through the use of laser),
but you should appreciate the strong case for plate
tectonics. Text books usually go into more detail
about the evidence supporting plate tectonics and
you are HIGHLY encouraged to read the relevant
chapters pertaining to this topic. As I have tried to
convey to in this lecture, the story of the evolution
of plate tectonics is at least as fascinating as the theory itself.

C) The Earth's interior & seismic waves (very detailed notes; more on this topic later in the semester)
If we all lived in a Star Trek universe, exploring the interior of the Earth would be comparatively
easy. All you'd have to do is use scanners or beam a chunk of it up to the Enterprise and let Data
or Spock examine it with a tricorder. Unfortunately, we are a long way from that type of
exploration.
Today, there are really only a few ways that we can explore the interior
of the Earth. I can think of 3 ways:
1) Drill a hole: Geologists have been drilling holes into the Earth since
the late 1800's. This is easy. All you need is a drilling rig, some drilling
pipe and a drill bit. If I have the time (and remember to bring them in),
will see some examples of drill bits in the lecture. They look pretty
impressive (especially the big ones!), but they really are simple devices.
They have rotating tungsten-carbide cutters that grind away the rock as
they turn. A bit is attached to a length of drill pipe and the whole thing
is turned in the rig complex. As the bit descends deeper into the Earth,
pipes are added to the assemblage extending its penetration. Today,
petroleum geologists regularly drill holes that exceed 20,000 feet

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(almost 4 miles) to obtain natural gas (e.g.,

Mobile Bay). They can go deeper, but there
would have be pretty good economic reasons
to do so because of the cost involved in
drilling. There are a few academic holes
out there and I had the opportunity to visit
one of them in the fall of 2006. I was at a
conference in Bavaria, German and the intraconference field trip was to the German
Deep Borehole project (see image at bottom
of this page). In the 1980s, the Germans
began a project to drill the worlds deepest
well in order to study high temperature geochemical/geophysical processes. They planned on
going down to 12 km (almost 7 miles), but stopped at 9 km (5 miles) when they hit temperatures
around 325C (their target). Apparently they miscalculated the geothermal gradient in this area.
So their hole is NOT the worlds deepest (that record is held by a well in California), but it is still
the deepest currently open borehole. Indeed, the project is still active and geologists from around
the world come here to study subsurface conditions not otherwise possible. They kept the drilling
complex up (see photo to right) and added a nice visitors center to the site where you can buy all
manner of crap, including rock cuttings (the remnants of the rock drilled through to make the
hole).
If you are wondering how far down can we go, well at the present time, it really is not possible to
exceed more than 10 miles by conventional drilling. The metal drill bits simply start to melt if
you go too deep. So until we develop new technology (where's Captain Kirk when you need
him!), we will have to rely on other sources of deep Earth information.
2) Get physical samples from volcano: Volcanoes are simply holes at the Earth's surface where
molten rock escapes from the interior. It follows that the stuff erupting from a volcano tells you
about the nature of rocks, fluids and gases below the surface (there is more erupted from a
volcano than just lava; there are horrendous amounts of gases including water vapor). Each
active volcano is underlain by one or more magma chambers 5 to 20 km or so below the surface.
In some cases (e.g., above hot spots), the magma may be derived from much deeper. But as useful
as these samples are, they are isolated to specific points on the Earth. What's really needed to sort
out the Earth's interior is some sort of technique that allows us to build up a coherent picture of
the whole darn planet. What we need is rock version of radar or medical X-rays. Luckily for us,
such a technique exists. It's called geophysics.
Geophysics, as the name implies, is a combination of geology and
physics. Specifically, it is the study of how shock waves (officially
called seismic waves) travel through the Earth. To explain how this
works, it is best to once again turn to petroleum geology (see cartoon
to right from http://www.naturalgas.org). Since the early 1900's
geologists have used seismic waves to look for petroleum
reservoirs. What they do is install a series of microphones (called
geophones) and listen to how seismic waves travel through the rock
over time. The seismic waves used to be generated through
explosions, but now we simply use thumpers, devices that repeatedly
lift and drop heavy metal plates on the ground. The result is similar to
the effect of smashing a hammer on a cement floor. Seismic waves
can also be generated at sea through the use of "pingers" (these

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devices generate sound waves of a specific

frequency that are capable of traveling through
rock layers). My favorite type of seismic survey
are the convoy type systems whereby large
truck like components simply drive across the
countryside recording the seismic stratigraphy
below the surface as they go. The Lithoprobe
project
(see
image
to
left
from
http://www.geop.ubc.ca/ Lithoprobe/transect/SOSS)
is an example of this type of survey. Five or six
vehicles that each do something (one pings,
one records, the others listen) can quickly do a seismic line many hundreds of km long. Hey;
this is almost Star Trek-scale science!
No matter how the seismic waves are generated, they all travel through the earth following
specific physical rules. As they travel from one medium to another (one rock type to another),
they can speed up or slow down, bounce back toward the surface, or even stop entirely. Before
we focus on the behavior of seismic waves, it is perhaps best to first consider something that you
are more familiar with through high school physics classes; light waves.
When light passes from one medium (e.g., air) to another (e.g., water),
it changes speed. From air to water, the light rays slow down. To your
eye which requires reflected light to see things, the change in speed is
perceived as a change in the orientation of objects that pass from air
into water. This is what causes the "broken arm" appearance when you
stick your arm into a pool of water or the displaced fish perception (see
image to right from http://www.iop.org), when you go to clean your
aquarium. This property is called refraction. Light waves can also
bounce or reflect off of surfaces which is the reason why we can
perceive them with our eyes in the first place. Seismic waves do exactly
the same thing and if you are a clever petroleum geologist, you can use
the pattern of reflections combined with refractions to build up a coherent picture of the rock
layers below the surface (see nasty image to
left from http://www.geo.uu.nl). Of course you
do need a computer to time the arrival of all of
the reflected seismic waves and to sort these
data into a picture of the rock layers. Even
still, there is a significant amount of skill that
is required to "read" these images. Like all
scientific techniques, becoming an expert in
seismic stratigraphy (this is the science of
resolving rock layers via geophysical
techniques; see image at the top left of the
next page from http://www.cpfieldinstitute.org)
requires practice, practice, practice.
Now let's turn back to the Earth's interior. A
thumper or small explosion is sufficient to generate a seismic pulse that can be recorded by
geophones in the vicinity of the source of the waves, but there is only so far that these waves can
travel before they diminish to nothing. In order to resolve structures near the center of the Earth,
we need either really large explosions (which are fun, but difficult to obtain licenses for), or

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another recurring source

of
powerful
seismic
waves. Depending upon
your point of view, we are
lucky (or unlucky) to have
just a source of these
waves. They are called
earthquakes.
Earthquakes occur when
stresses build up beyond
the ability of rock layers
to resist them. There is a
sudden break, a release of
built up energy and
formation of seismic waves. Earthquakes will be discussed shortly, but right now, we need to
discuss what happens when an earthquake occurs in order to better understand geophysics and the
Earth's interior.
Earthquakes are capable of generating 2 major Earth-penetrating types of seismic waves (also
called body waves; see image at the bottom of this page):
1) P-Waves and 2) S-Waves
Once generated, these seismic waves can travel throughout much of the Earth's interior. We
record their passing through the use of a seismograph. Seismographs and the technique of
locating earthquake epicenters, will also be discussed in depth later.
P-Waves (or primary waves) are associated with compressive deformation (compression). They
travel through all states of matter (liquid, solid), are the fastest of the three seismic waves and
range in speed from 6 km/sec (granite) to 7 km/sec (gabbro) in crustal rocks. P-waves
dramatically increase in velocity the deeper they penetrate into the Earth. The reason is that
seismic waves travel faster through more dense materials.
S waves (or secondary waves) are associated with
shear. They are significantly slower than P-waves
(commonly 4-5 km/sec) and can only past through
solid materials. Should they encounter rock with
liquid properties (e.g., magma), they simply die out.
This is an important thing to remember because it
will be one of the facts that allow us to resolve the
Earth's interior.
First let's talk reflection and refraction on a grand
scale. P- and S-waves literally bounce all through
the Earth following a major earthquake. In fact, the
Earth literally "rings" like a bell due to the almost
constant vibrations caused by earthquakes. The
figure at the top of the next page is from
http://www.bbc.co.uk and illustrates the trajectories
of P and S-waves from an earthquake near the

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10

surface of the Earth.

As the seismic waves penetrate into
the Earth, they pick up speed or slow
down depending upon the density and
physical state of the rock layers that
they are passing through. It is that
reason for the bending that takes
place in the cartoon). The waves also
bounce off of specific layers in the
interior (not shown on the simplified
cartoon) and it is these major levels of
seismic reflection that geologists use
to divide up the Earth's interior. If you get the feeling that keeping track of all of the reflected,
refracted and primary seismic waves is a difficult task you are correct and that is why
seismologists came up with the strict nomenclature that is used to label the waves in the sketch
above.
Another more detailed diagram that illustrates the
changes in wave transit speed versus depth is
necessary at this point in our lecture (see image to
the right). It comes from http://geoweb.tamu.edu. In
it you will see several sudden changes in wave
velocity. The first is hard to see on the scale of this
diagram as it occurs a mere 5 to 35 km below the
Earth's surface. It was first identified by a Istrian
(part of Croatia) seismologist named Andriji
Mohorovicic in 1909 which explains why the
transition goes by the official name of Mohorovicic
discontinuity. However, since most of us cant
pronounce it, it is usually just called the Moho. At
the Moho, the velocity of S-waves suddenly
decreases from about 5 km/s to just over 4 km/s and
across the Moho, the velocity of P-waves suddenly increases by about 15 to 30 %. Clearly
something weird is going on at this level. Mohorovicic's interpretation was that there was a sharp
transition from rocks of lower density to rocks of higher density. Experiments established the
travel time of seismic waves through different types of rocks and through these data, geologists
confirmed his conclusion. Rocks above the Moho are primarily granite (6 km/s) and gabbro (7
km/s), and rocks below the Moho are primarily peridotite (8 km/s). These are rock terms that we
havent heard for a while in GY 111 which just goes to show you that you cant afford to forget
anything while at University (especially geology stuff!). For now, it is sufficient just to recognize
that there is a transition in rock types at shallow depths below the Earth's surface, that it is
resolved on geophysical grounds, and that it is the first division of the Earth's interior; the crust.
We have already discussed the Earths interior, so there is no need to do it again. However, there
are a couple of new aspects of seismic waves that we need to deal with given the remaining parts
of this lecture. The first are seismic shadow zones. If you look at the bottom diagram on the
previous page (the one from the BBC) you will see that on the opposite side of the Earth from
where an earthquake occurred, there is a region where seismic waves do not occur (hence the
term shadow zone). The one for S-waves (S-wave shadow zone) is particularly obvious
because it is so wide. It was this very fact that led geologists to conclude that the outer core of the

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11

Earth was liquid in the first place. There is also a P-wave shadow zone, but it is much less
obvious on the BBC figure.
I am going to leave you hanging at this point with respect to the consequences of seismic wave
and earthquakes. We need to return to plate tectonics. However, later in the course, we will return
to earthquakes when we go into detail about structural geology and faulting. We will also talk
about the damage caused by seismic waves in a lecture I call Death and Destruction 101.
So back to plate tectonics. Seismic waves allowed geologists back in the 1930's to resolve several
layers inside of the Earth. The discoverer of the core (a female seismologist named Inge
Lehmann) observed wave refractions that she deduced had to be coming off a more dense layer
below the surface. We now recognize 4 major layers 2. From top to bottom, these are:

1) inner core,
2) outer core
3) mantle
4) crust

The thickness and properties of each of these layers are summarized in the table and figure below:
Layer
Name
Depth (Thickness)
Composition
State
Crust
0-35 km (5 - 35 km)
Rock
Solid
4
Mantle
35-2900 km (2865 km)
"Rock"
Solid-Ductile
3
Outer Core
2900-5100 km (2200 km) Iron/nickel
Liquid
2
Inner
Core
5100-6370
km
(1270
km)
Iron/nickel
Solid
1

The most important thing to note about these

layers is that they are distinguished on the
basis of geophysical properties. We now know
that there are many more finer scaled layers in
the Earths interior (the Earth is more like a
CD than an onion in terms of the scale of the
layering), but the four principle layers
identified through geophysics are though to be
at least partly responsible for plate tectonics.
The core is hot. The outer mantle and the crust
are relatively cool. Heat exchange is known to
produce convection currents from deep
2

New studies (post 2010) are starting to resolve even more subtle layering within the Earth's core. Some
are suggesting a possible sulfur-rich inner-inner core. Others imply an inner-inner core enriched in other
metals like potassium or even uranium. Both of these metals have radioactive isotopes which might also
explain the continued high temperatures of the inner Earth.

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12

portion of the Earths mantle that flow up toward the crust and then down again as the currents
cool. There is still considerable discussion about how convection actually . Some studies suggest
that convection currents originate at the mantle-core interface. Others suggest a stratified mantle
where convection occurs at different levels. A new study (2002) has questioned the ultimate
source of the heat from the core. It cant be left over heat from the origin of the solar system. The
Earth is over 4.5 billion years old (4.6 Ga) and would have cooled to a solid if there was not an
additional source of heat. We now know what that source is; radioactivity. As mentioned in the
footnote on the previous pages, new research has suggested that the Earth may contain an innerinner core composed of an iron-uranium or potassium alloy rather than iron/nickel as previously
concluded. Alternatively the radioactivity might just be spread throughout the Earth's interior. I
personally dont go for the uranium core idea because we do not find evidence of this in the solar
system (e.g., some iron/nickel meteorites would be highly enriched in uranium were uranium a
common element in planetary cores). But then again, who knows. It will be a long time before we
ever get to directly sample the core of the Earth (or any other planet for that matter).
What ever the ultimate source of the heat, convection currents rise toward the surface of the Earth
and when it gets near the surface, it spreads out:

The upper 100 km of the Earth (this

contains the crust and the uppermost
mantle) behaves relatively rigidly because
it is cool. The mantle that lies below this
rigid interval is much hotter and behaves in
a more ductile fashion. The gist of all this
is that the rigid interval (known as the
lithosphere) tends to break when subjected
to forces produced by convection. The
ductile layer (called the asthenosphere)
tends to stretch and flow.
Wegener proposed that the continents were
moving relative to one another. He was right, but he did not realize that the movement involved
100 km thick plates of lithosphere rather than just the continental masses. There are many large
and small lithospheric plates (see the colour
diagram below) and in typical scientific fashion,
most of them are named. North America for
example, is riding on the North American plate
as is the western portion of the northern Atlantic
ocean. Everything from Oakland, CA to western
Iceland is moving slowly westward at 2.5.
cm/year relative to the other side of the MidAtlantic Ridge. Every year London and New
York move about an inch further apart. The
distance between Mobile and London is also
increasing, but the distance between Mobile and
New York stays the same (we are on the same
plate).

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13

Source: University of
North Dakota

Most geological action is concentrated along the edges where the plates rub past one another.
These are the so-called plate boundaries and 3 distinct types are recognized:

1) Divergent plate boundaries are ridges

where new oceanic crust is generated
(e.g., mid oceanic ridges like the Mid
Atlantic Ridge and the East Pacific Rise).
These are areas where the dominant force
between the plates is tension.

2) Convergent plate boundaries are linear

belts where plates are pushed together
(e.g., western South America, India-Asia
etc.). Here the dominant force is
compression and the ultimate result is
major uplift resulting in mountain belts. As
will be discussed in an upcoming lecture,
oceanic crust is destroyed through a
process called subduction along these
plate boundaries. Subduction also results in
deep marine trenches.

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14

3) Transform plate boundaries are areas where plates

slide past one another. Here, oceanic crust is neither
produced nor destroyed. An excellent example of this type
of plate boundary is the San Andreas fault in Southern
California. Another is the even more impressive Alpine
Fault of New Zealand as shown on the diagram at the top
of the next page.

Source: http://www2.nature.nps.gov)

It should be noted that each boundary has subdivisions. For

example, convergent plate boundaries may involve the
collision of two plates that both contain oceanic crust (ocean
plate-ocean plate convergence; example- Kermandec
Subduction Zone north of New Zealand; see image from
www.geosci.usyd.edu.au to right), or that both contain
continental crust (continental plate-continentental plate
convergence; example- Africa colliding with Europe), or
where one contains oceanic crust and the other continental
crust (you should be able to figure the name out for this one
by know; example- South America colliding with the Pacific
Plate).
As previously mentioned, each of the major tectonic plates
has an official name, usually consisting of the major
physiographic feature that is riding on the plate(e.g., we
live on the North American Plate). You probably noticed
on the last figure that residents of New Zealand live on two
separate plates (the Australian Plate 3 and the Pacific Plate).
This is because a major plate boundary runs right through the middle of the country. Its actually
a transform fault boundary and is dragging New Zealand into (as the Australians love to say),
the largest semicolon in the South Pacific.
If you look at the named plates, you might initially be
surprised to learn that there is no Atlantic Plate. The
boundary of the North American Plate is in the middle of
the Atlantic Ocean along the Mid-Atlantic Ridge. As
mentioned previously, everything west of this mountain
range (including the west half of Iceland), is moving in
the same direction as the rest of North America whereas
everything to the east, is moving east with Eurasia. Since
there is no independent movement of the Atlantic, it is not
considered a separate plate. By the way, Iceland, which
lies directly on the divergent plate boundary that
3

The actual name for the Australian Plate is the Australian-Indian Plate as both of these continent
bearing sections (as well as the ocean between them) are moving as a single body. At least for now. Your
humble instructor has some ideas about what might happen in the future about this tectonic plate. Why not
ask him in an upcoming class?

GY 111 Lecture Notes

D. Haywick (2016-17)

15

comprises the Mid-Atlantic Ridge, is getting wider every year in the same way that New Zealand
is getting thinner. There is good marketing potential for a weight loss product using plate
tectonics is any of you are majoring in advertising.

Important terms/concepts from todays lecture

(Google any terms that you are not familiar with)

Alfred Wegener
continental drift
hypothesis versus theory
super continent(s)
trenches and troughs
mid oceanic ridges (incl. Mid Atlantic Ridge, East Pacific Rise)
Paleomagnetism
plates (tectonic)
plate tectonics
Crust, mantle, inner and outer core
geophysics
convection (currents)
lithosphere
asthenosphere
divergent, convergent and transform plate boundaries
subduction
various tectonic plates (e.g., North American, Australia-Indian, Pacific etc.)
Island Arc
Magma
Country rock
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