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Oceanography 10, T. James Noyes, El Camino College
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Plate Tectonics
“Plate tectonics” is the name given to our modern theory of how the Earth works. It helps us
understand how and why earthquakes, volcanoes, mountains, trenches, islands, oil deposits,
and much more occur where they do. The basic idea of
The lithosphere is made up of two parts.
plate tectonics is that the Earth is covered by a cool,
The upper part is made of lower-density,
thin outer layer called the lithosphere which floats on
lighter-colored
granite or higher-density,
a warmer, denser layer which can slowly flow called
darker-colored basalt. The lower part is
the mantle. The lithosphere is broken into pieces called
made of solidified mantle rock. The
plates. The semi-solid rock down in the mantle moves
bedrock of the continents is made of
due to the heat of the Earth’s core and pushes the
granite while the ocean floor is made of
plates 1 away from and into one another, resulting in the basalt, so continental lithosphere has a
earthquakes, volcanoes, mountains, trenches, and so on. lower density than oceanic lithosphere.
There are two kinds of lithosphere, lower-density
continental lithosphere with a granite “top” and higherdensity oceanic lithosphere with a basalt “top.”
Granite. Lower-density rock
making up the continents.
Basalt. Higher-density rock
making up the ocean floor.
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The density of one side of a plate may be larger than the density of the other side of the plate.
This may cause a plate to move.
Oceanography 10, T. James Noyes, El Camino College
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Oceanography 10, T. James Noyes, El Camino College
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The Observations and the Basic Ideas of Plate Tectonics
A vast amount of evidence from both the continents and the ocean floor supports the idea that the
continents move and oceans grow and shrink.
If you examine the coasts of South America and Africa, you can see that they match rather nicely
together; the match is even better between their continental shelves. The east coast of North
America and northern Africa also match, as do other parts of the world. Not only do the shapes
match, but fossils, mineral deposits (e.g., coal, diamonds), and mountain ranges 2 match along the
edges of the continents as well. The matching of these features suggests that the continents were
once connected to one another and therefore they must have moved to reach their present
positions. If the continents were not connected in the past, there would be no reason for the
features to match: it would simply be a coincidence. This is possible – unlikely things do happen
(e.g., someone wins the lottery) – but when determining what we believe, we typically favor
ideas that are more probable, not less likely. In other words, arguing that these features of the
continents are likely to match by chance would be something like saying that if you choose 7
pieces out of jigsaw puzzle box, they are likely to fit together AND produce a nice picture. This
is possible, but certainly not something that most of us would expect to happen.
The “Fit” of the Continents.
Alfred Wegner.
Just a few examples of fossils that “match” on different
continents. Alfred Wegner and the USGS.
The fossils, in particular, are interesting. Currently, the plants and animals on different continents
are different from one another. They have evolved to adapt to the present environment (e.g.,
climate, ecosystem). However, land animal and plant fossils are the same on some continents
(e.g., South America and Africa) until a particular point in time, when they begin to start
showing evolution in different directions. This suggests that their environment began to change
and that they were no long able to interbreed, presumably due the separation of the continents.
We also find remains of tropical plants and animal like coral reefs near the Poles, and signs of
erosion by glaciers 3 in the tropics. These observations suggest that continents that are now close
to the Equator were once at the Poles and vice versa 4.
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and more!
Huge, slowly-moving sheets of ice.
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These observations might be explained by climate change since there were extreme ice ages and warm periods
except for the fact that both were true at the same time.
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Oceanography 10, T. James Noyes, El Camino College
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Oceanography 10, T. James Noyes, El Camino College
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Observations from the ocean floor are crucial to understanding how and why the continents
move. As we discussed way back in topic 1A, most ocean trenches lie close to land or islands.
In addition, earthquakes (often starting deep inside the Earth) and volcanoes occur near trenches.
If the trench occurs next to a continent, the continent has volcanic mountains near the coast (like
Mt. St. Helens in Washington, Mt. Fuji in Japan, the Andes in South America, Popocatepetl near
Mexico City, Krakatoa near the Indonesian islands of Java and Sumatra, etc.), and if a trench
occurs next to an island chain, then the islands are giant volcanoes (e.g., Mariana Islands,
Aleutian Islands in Alaska). The mid-ocean ridge is also a site of volcanic activity and
earthquakes. In addition, several very different lines of evidence (sediment thickness, radiometric
dating, seafloor magnetism) all show that the ocean floor is very young 5 close to the mid-ocean
ridge and gets older and older the farther it is from the mid-ocean ridge.
Earthquake Locations. Notice how
earthquakes tend to occur near the locations
of the mid-ocean ridge and trenches (shown
in the pictures below). NASA.
The Mid-Ocean Ridge is the dark, red line
in the picture on the left. It indicates very
young ocean floor (recently cooled lava).
Yellow and green colors indicate old ocean
floor (that cooled from lava long ago). The
oldest ocean floor is shown in blue.
National Geophysical Data Center, National
Oceanic and Atmospheric Administration,
Department of Commerce.
Most of the world’s trenches (blue lines)
are found around the edge of the Pacific
Ocean, and volcanic mountains and
islands are next to the trenches, so we call
this region the “Ring of Fire.” USGS.
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A “young” rock is made of material that “recently” became solid rock. In the case of the volcanic rocks
of the sea floor, the sea floor is “young” if it cooled from lava into solid rock “recently.”
(When geologists use the word “recently,” they could be discussing that last few million of years.)
Oceanography 10, T. James Noyes, El Camino College
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Oceanography 10, T. James Noyes, El Camino College
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The theory of plate tectonics provides the following explanation for these features. At the midocean ridges, the plates are separating, pulled apart by the motion of convections cells of semisolid mantle rock beneath the plates. Lava then comes up to fill in the resulting gap and cools
into solid rock when it comes into contact with cold ocean water, becoming new, young ocean
floor. The ocean floor cools more as it ages (making it contract) and more and more sediments
pile on top of it, so it becomes more and more dense with time. As the ocean floor moves away
from the mid-ocean ridge (“sea-floor spreading”), it either pushes a continent (“continental
drift”) or runs into another plate, leading to earthquakes. Both plates cannot occupy the same
location, so the plate with the higher density dives into the interior of the Earth (We say that it
“subducts” = “dives.”), making the sea-floor deeper (i.e., a trench). The diving plate begins to
melt, creating hot, low-density magma which works its way back towards the surface, creating
volcanoes.
Volcanoes Near Trenches
The mantle rock above the diving ocean crust undergoes what is called “partial melting.”
The diving ocean crust is heated by the surrounding mantle (friction between the plate
and mantle rock is an important factor) and the increased pressure. Water and sediments
are also carried down by the plate. As the water and carbon dioxide (from the breakdown
of calcium carbonate sediments) mix with mantle material, they lower its melting point
and its density, allowing the resulting magma to rise upwards until it hits the nonsubducting plate above. The magma begins melting the bottom of the plate and mixing
with the plate’s material, which alters the composition of the magma. Wherever a crack
in the plate permits, the magma works its way toward the surface of the Earth until it
comes out as lava. The magma itself can force its way up: (1) it opens cracks by
expanding – remember: heating an object makes it larger – and (2) it melts the
surrounding material.
Oceanography 10, T. James Noyes, El Camino College
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Oceanography 10, T. James Noyes, El Camino College
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Countless studies over the last century have verified the motion and the boundaries of the plates
suggested by plate tectonics. Scientists currently use the GPS (global positioning system) to
monitor plate motion, but it is simple enough to do without advanced technology: measure the
distance between two objects (houses, trees, fences, etc.) on two different plates, and then do so
again after each earthquake. Most plates move at least few centimeters per year (an inch or a bit
more per year) and no faster than about 15 centimeters per year (about 6 inches per year).
The Plates of the Earth’s Lithosphere. USGS.
Notice that a plate can contain both continental and oceanic lithosphere.
For example, the North American plate includes both North America
(continental lithosphere) and the bottom of the western Atlantic Ocean
(oceanic lithosphere). Lava coming out of the mid-ocean ridge cools and
solidifies onto whatever plate is nearby and becomes part of that plate,
whether it is continental or oceanic lithosphere.
Oceanography 10, T. James Noyes, El Camino College
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Oceanography 10, T. James Noyes, El Camino College
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Why do the plates move?
What drives plate tectonics?
The ultimate cause of motion inside the
Earth is the heat of the Earth’s core 6.
The core heats the bottom of the
mantle 7, lowering its density and
causing it to rise. Cooler mantle
material slides in to replace it, and the
process repeats in giant, slow-motion
convection cells. In addition, the end of
a subducting (“diving”) plate can be
more dense than the mantle, because it
has cooled down and has thick, heavy
layers of sediments piled on top of it. It
Motion (“Convection Cells”) within the Earth. USGS.
dives down owing to its higher density,
and can pull the rest of the plate with it. (This is called “slab pull.”) The diving plate also drags
the nearby mantle rock with it, helping to push the convection cell in the mantle. (This is called
“slab suction.”)
Earthquakes
Earthquakes occur when the Earth moves suddenly. The magma of the mantle flows in
convection cells which steadily rub against the bottom of the plates, pushing them slowly away
from mid-ocean ridges and towards trenches. However, the plates run into one another, so they
resist being moved. Instead of moving, the plates are “squeezed” and “twisted” where they
collide. Eventually, they cannot deform any more, and the pressure to move builds up until it
overcomes the resistance 8: then the plates move (“slip”) all at once, and snap back into shape.
This is an earthquake. Thus, instead of moving slowly and gently with the convection cells
over time, the plates “give” (deform) until they cannot “give” any more, and move all at once.
Most earthquakes occur near the edges of plates, because most of the resistance to motion is
exerted at the edges, so this is where most of the deformation (bending, squeezing) takes place
and the tension builds up 9. Plates can resist moving for several reasons. A plate can rub against
the side or bottom of another plate, and the friction between them keeps them from moving. In
addition, the rock in front of a moving plate (another plate or the semi-solid asthenosphere) can
block it, possibly forcing it to bend.
6
The Earth’s core is warm, because of heat produced during the initial collisions between the rocky asteroids that
produced the earth and the decay of dense, radioactive materials.
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There is a debate within the geophysics community about whether convection cells reach all the way down to the
core. Some scientists think that there 2 layers within the mantle, each with its own convections cells. There is some
data which support this view, but if this is true, then it is not clear why large deep earthquakes occur: stay tuned!
8
Have you ever tried moving a couch on carpeting by yourself? It does not move at all until you provide enough
pressure, but then it moves easily. When you let up the pressure, the friction with the carpet brings it to a stop.
9
There are other mechanisms that can cause earthquakes. For example, as a plate dives into the Earth, some parts
get warmer than others, so they get larger and the plate cracks where warm rock meets cold rock. This is why glass
trays can break when you take them out of the oven or glass pitchers break when you pour hot water into them: one
part cools more quickly than another part.
Oceanography 10, T. James Noyes, El Camino College
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Oceanography 10, T. James Noyes, El Camino College
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Rock Magnetism
Evidence from rock magnetism provided
some of the crucial evidence that made the
majority of scientists switch from earlier
theories about the Earth to the modern
theory of the plate tectonics in the 1960s.
When lava cools into solid rock, magnetite
particles (little iron-rich minerals) in the
lava orient themselves with the Earth’s
magnetic field. (They behave like little
compass needles.) Thus, the rock records
information about the direction and
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strength of the Earth’s magnetic field at the
time when the rock “formed.” Studies of volcanic lava flows on land showed scientists that the
Earth’s magnetic “north pole” changes location with time 10. In addition, sometimes the
magnetic north pole “flips,” going to South Pole 11! By comparing results from volcanoes on
different continents, scientists found that the continents must have shifted location 12 in
agreement with the fossil evidence and other kinds of evidence. The “straw that broke the
camel’s back” came from observations of the sea floor. The rocks of the ocean floor are
magnetized pointing north and south in long “stripes” that run parallel to the mid-ocean ridge
and are a mirror images of one another across the ridge (their magnetism “points” in the same
direction and they have the same thickness). This suggests that rocks on either side of the ridge
formed at the same time (because they were influenced by the same magnetic field) and then
moved outward from the ridge together 13. It is hard to think of another logical explanation.
10
Scientists monitor it today. It shifts a couple hundred feet each year.
This happens every million years or so. In fact, the Earth’s magnetic field has been weakening for the last few
centuries and we are due for a “flip:” We may actually experience a “flip” over the next few centuries. The Earth’s
magnetic field helps protect us from the charged particles (“radiation”) of the “solar wind” from the Sun which not
only affect background radiation, but disrupt electromagnetic networks (e.g., radio, cell phones, even electric power
grids), so this could cause some surmountable – but expensive – problems for future generations.
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This was the only way to get their data from different continents to make sense: either there were multiple
magnetic north poles or the continents were moving.
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In addition, the thickness of the “stripes” on the ocean floor is proportional to the time between the “flips” of the
Earth’s magnetic field determined from volcanoes on land. In other words, the more time that has passes, the
thicker the stripe is, just what you would expect if rock is slowly moving away from the mid-ocean ridge and more
more rock is being added at the mid-ocean ridge.
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Oceanography 10, T. James Noyes, El Camino College
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Oceanography 10, T. James Noyes, El Camino College
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How do we know what the interior of the Earth is like?
Like everyone else 14, scientists believe that the interior
of the Earth is hot, because lava comes out of
volcanoes. We cannot drill all the way through the
lithosphere to reach the mantle (the lithosphere is about
60 miles thick!), so we must use other methods to learn
about the inside of the Earth. Every earthquake that
occurs ripples outward, giving us clues about the
Earth’s interior. By measuring how long it takes the
earthquake to reach each monitoring station of our
USGS
worldwide network 15, scientists have built up a “picture”
of the interior of the Earth using the same principles as
those of “ultrasound” (e.g., used to visualize a baby in the
womb 16). Earthquake waves travel at different speeds
depending upon the kind of rock and the rock’s
temperature and pressure, something that can be measured
in laboratory experiments. By combining travel times
from many different earthquakes and earthquake speeds
for different rocks, scientists can work out the thickness of
the layers of the Earth, their composition, temperature,
Ultrasound. Courtesy of Sam Pullara,
(Creative Commons 2.0 Generic)
pressure, and more. In addition to providing information
about the Earth’s crust, lithosphere, and mantle, these results suggest that the Earth’s iron core
has two parts, a solid inner core and a fluid outer core. The motion of the fluid part of the core is
presumably responsible for the Earth’s magnetic field, since iron is a magnetic substance. It
makes sense that the core is iron, since iron is the densest substance made by stars in large
quantities before they “die” in a supernova. If the early Earth formed from more and more
asteroids being pulled together by gravity, then the collisions would have produced vast amounts
of heat, making the early Earth a molten ball of rock. The higher-density elements like iron
would have tended to sink towards the center, while the lower-density substances at the surface
would have cooled and become solid by radiating their heat into space. The cool, outer later then
served as an insulator, trapping the heat inside except where it leaks out, mainly at places like
volcanoes. High density radioactive materials would have sunk towards the center, and their
decay continues to warm the Earth’s core to this day.
14
For example, what is hell suppose to be like? e.g., “A snowball’s chance in hell.”
The network was built to monitor nuclear test-ban treaties. Nuclear explosions set off vibrations that can be
detected by the network, so the network is used to make sure that nations live up to their agreements. (In other
words, the network is used to detect nuclear “cheaters.”). Nuclear explosions are different from natural earthquakes,
so the network can easily tell them apart. Nuclear explosions create very small earthquakes, so the network has no
difficulty detecting and measuring even small earthquakes.
16
I believe that the technique was developed by earth scientists and borrowed by the medical community.
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Oceanography 10, T. James Noyes, El Camino College
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Oceanography 10, T. James Noyes, El Camino College
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CASE STUDIES IN PLATE TECTONICS
We will now discuss the different possible situations that can occur along the edges of plates,
and look at specific places in the world where these events are happening.
USGS
Divergent Boundaries
Divergent boundaries are places where plates are moving apart or “diverging.” There are two
kinds of divergent boundaries: where oceanic lithosphere is separating and where continental
lithosphere is separating.
Oceanic lithosphere is moving apart at the mid-ocean ridge. The motion of magma beneath the
plates rubs against the bottom of the plates, pulling them apart (this is called “rifting”), and then
magma rises up in between the plates to fill the gap. This magma (lava) meets the cool ocean
water and cools into new, solid lithosphere on each side of the
At the top of the mid-ocean ridge
ridge. Initially, the crust at the ridge has a low density, so it
there is a “rift valley” where the
floats higher than the neighboring lithosphere. The crust ages
plates are pulling apart. It is not
as it moves away from the ridge, and its density increases
very deep though, not nearly as
(because it cools), so it sinks down (a bit) into the mantle.
deep as a true ocean trench.
The motion of magma deep in the earth can also begin to pull continental lithosphere apart.
In this case, a continent can be split apart. The magma which rises up to fill in the resulting gap,
though, is just like magma which rises up at the mid-ocean ridge, so when it cools, it forms new
ocean floor (oceanic lithosphere), not continental lithosphere. As the continents get farther and
farther apart, a new ocean grows in between them. This is happening right now at places where
the mid-ocean ridge “runs into” land like Gulf of California (between Baja California and
mainland Mexico). There are some places in the world where a continent is just beginning to
split (e.g., the East African Rift Valley in Africa).
Oceanography 10, T. James Noyes, El Camino College
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Oceanography 10, T. James Noyes, El Camino College
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The motion of the semi-solid mantle can pull continental lithosphere apart. Magma rises up at the mid-ocean
ridge, cools, and becomes new ocean lithosphere. This is happening now in the Gulf of California.
Growing and Shrinking Plates
Plates always “move away” from the mid-ocean ridge.
However, some plates have the mid-ocean ridge on
both sides (e.g., the African plate). How can the plate
be moving in opposite directions on opposite sides?
How can the plate be moving both east and west?
The answer is that the plate is growing (getting bigger);
new material is being added along both boundaries.
It is not entirely clear whether it is the plate that is
moving or the location of the mid-ocean ridges relative
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to the plate, or both. The red arrows really show the
direction from newer rock to older rock. They cannot show the “absolute” motion of the plate
relative to some fixed, permanent reference point because the entire surface of the Earth is in
motion, including the mid-ocean ridges and trenches.
Similarly, plates always “move towards” a trench. At the trench, one plate is subducting (diving
down into the Earth) and melting. Some plates are completely surrounded by trenches (e.g.,
Filipino plate), so they are slowly being pushed down into the earth. They will shrink and shrink
until they are completely destroyed.
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Oceanography 10, T. James Noyes, El Camino College
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Convergent Boundaries
Convergent boundaries are places where plates are coming together
(“colliding”) or “converging.” There are three kinds of convergent
boundaries: where oceanic lithosphere meets continental
lithosphere, where oceanic lithosphere meets oceanic lithosphere,
and where continental lithosphere meets continental lithosphere.
The upper part (“crust”) of continental lithosphere is made of
granite, whereas the upper part of oceanic lithosphere is made of
basalt, so continental lithosphere has a lower density than oceanic
lithosphere. When they meet, the higher-density oceanic
lithosphere is forced to dive down into the Earth (“subduct”).
To dive, the oceanic plate must “bend”
downwards, so this makes the ocean floor deeper
where the two plates meet: a trench. As the
oceanic plate dives down into the Earth, it melts,
creating low-density magma which rises up
beneath the other plate. It melts its way through
the continental lithosphere. More and more lava
comes up, cools, and piles up, forming volcanic
mountains at the edge of the continent. This is
happening along the coast of South America,
where it created the Andes Mountains.
It also is happening along the coast of
much of Central America and the
Pacific Northwest of the United
States, where it produced the Cascades
mountain range and the famous
volcano Mt. St. Helens.
Both shallow and deep earthquakes
are common along this kind of plate
boundary, because the plates grind
against one another near the surface
and down deep, stopping their motion.
Eventually the pressure to move
overcomes the friction between the
plates and the diving plate slips
forward. In other words, an
earthquake occurs. The continental
lithosphere is compressed (squeezed)
by the oceanic lithosphere pushing
into it; it “snaps back” into shape like
a bent pencil will become straight
once you release it.
Oceanography 10, T. James Noyes, El Camino College
Mt. St. Helens, 1980. USGS
USGS
Above: Cascades Mountains, in
Washington, Oregon, and
California. USGS.
Left: Andes Mountains (brown &
red) and nearby trench (dark blue)
National Geophysical Data
Center, National Oceanic and
Atmospheric Administration,
Department of Commerce.
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Oceanography 10, T. James Noyes, El Camino College
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When two pieces of oceanic lithosphere meet,
the plate with the higher density subducts
(dives down into the Earth). However, in this
case, both plates are made of basalt, so neither
is more dense than the other simply by virtue
of their composition. Instead, it is the older
plate that subducts. The older plate has cooled
down more, because it has been a longer time
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since it formed along the mid-ocean ridge. In
addition, there has been more time for
sediments to pile on top of it. Both the cooling of the plate (reducing its size) and the weight of
the sediments make the older plate more dense than the younger plate. It dives down into the
Earth, making the ocean floor deeper (a trench), and partially melts, producing magma that rises,
melts its way through the other plate, and bursts out at the surface. The molten material cools
into solid rock on the surface and piles up higher and higher as more and more magma comes up.
If these volcanoes grow high enough, they produce a chain of islands along the trench on the
other plate. This is happening at the Mariana Trench and Mariana Islands, and along the south
side of Alaska at the Aleutian Islands.
Trench
Above: Mt. Cleveland in the Aleutians, NASA.
Left: Many of the Aleutian Islands of Alaska
are active volcanoes, USGS.
Once oceanic lithosphere gets old enough, it must start diving into the Earth: it is
simply too dense to float at the surface. If the oceanic lithosphere is pushing
continental lithosphere, the plate can fracture along the boundary. A trench forms
between the continental lithosphere and oceanic lithosphere, splitting the plate.
Oceanography 10, T. James Noyes, El Camino College
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Oceanography 10, T. James Noyes, El Camino College
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The upper layer (“crust”) of continental lithosphere is made of low-density granite, so
continental lithosphere is never dense enough to sink down into the mantle. Thus, when two
pieces of continental lithosphere collide, the continental crust 17 does not dive down deep into the
Earth. Instead, the rock is squeezed upwards (and downwards), building tall mountains. Each
time the plates slip forward towards one another, the resulting earthquake lifts the mountains
higher and higher, and pushes down some continental crust a bit lower 18. For example, the
subcontinent of India is being pushed northeast into Asia, producing the Himalayas, the
mountain range with the tallest mountain in the world, Mt. Everest.
Himalayas
USGS
Himalayas, NASA
USGS
Himalayas, NASA
17
The part of the lithosphere beneath the continental crust can still dive down into the Earth. This can actually
cause neighboring continental and oceanic lithosphere to break into separate plates. The oceanic lithosphere will
then begin subducting beneath the continental lithosphere, creating a trench along the coast.
18
Some continental crust goes down (it has to go somewhere), but it does not sink down into the mantle.
In other words, it does not sink down very far.
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Oceanography 10, T. James Noyes, El Camino College
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Transform Boundaries
The mid-ocean ridge is not one,
continuous ridge, but instead is
broken into segments or “pieces.”
There are “jumps” (gaps) between
the segments, and these jumps or
gaps are called “transform faults.”
A transform fault is a place where
two plates slide next to one another
in opposite directions horizontally.
The lithosphere on either side of the
fault moves away from segment of
the mid-ocean ridge where it was
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“born.” The plates on either side of
the fault rub against one another, so friction between them keeps them from moving until the
pressure becomes too much and they “slip” (an earthquake). We live near one large transform
fault called the “San Andreas Fault” that runs between a segment of the mid-ocean ridge in the
Gulf of California and a segment off the coast of Northern California.
San Andreas Fault, USGS
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Hotspots
Molten rock rises at certain, special spots beneath the lithosphere, not just
at the mid-ocean ridge 19. The magma heats the bottom of the lithosphere;
the lithosphere expands more in some places than others, which opens up
cracks through which magma flows to the surface of the lithosphere. Lava
breaks through again and again, slowly building an underwater volcano 20.
The volcano is a “seamount,” an underwater mountain, until in reaches the
surface and becomes an “island.”
The volcano grows on the lithosphere of a plate. Since plates move, the
island is slowly carried off the hotspot by the motion of the plate 21,
causing the volcano to become extinct (it loses its source of lava). As
time passes, the island (and the lithosphere it rides on) cool down more
and more, causing them to contract (shrink). This makes the island
smaller, so it sinks, and this can carry it beneath the waves, making it into
a seamount. In addition, as the lithosphere and volcano cool, they become
more dense, so they sink down a bit into the mantle. Oceanographers
often find “flat-topped” seamounts called tablemounts or “guyots” (geeohs). They were once islands at the surface and had their top eroded away
by waves before sinking deeper into the ocean. Corals are able to survive
when their island sinks by growing straight upwards towards the surface.
Coral Reef Development.
They form coral atolls, islands whose tops are made of coral reef, not
Coral grows upwards as the
volcanic lava.
island sinks. USGS.
A volcano begins to grow on the “hotspot,” but is carried off the hotspot by the plate upon which the volcano grows.
Without lava, the volcano cools and shrinks, but a new volcano grows on the hotspot.
19
Hotspots can exist near the mid-ocean ridge (e.g., Iceland), and cause even more volcanism than normal.
Hotspots can also be located under a continent. For example, the geysers of Yellowstone National Park are
caused by the heat of a “hotspot.” These hotspots can also cause volcanoes.
21
Hotspots can and do move as well, but observations suggest they move slower than the plates move above them,
so it is primarily the motion of the plates that produce hotspot island chains.
20
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Oceanography 10, T. James Noyes, El Camino College
11A-16
After one volcano leaves the hotspot, another volcano
grows on the hotspot. This volcano also leaves, and
another volcano grows. This happens again and again,
creating an “island-seamount chain.” The classic
example is the Hawaiian Islands which are one end of
the much larger Hawaii Islands-Emperor Seamount
Chain. Currently the southeast corner of the “big”
island 22 is above hotspot; this is where you can see
active volcanoes at “Hawaiian Volcanoes National
Park.” The New England Seamounts along the “Great
Meteor hot-spot track” near the east coast of the United
States also formed in this manner.
Hawaiian Islands. The red dot shows the
location of active volcanoes. NASA.
Part of the Hawaiian Island-Emperor Seamount Chain, USGS.
Steam Vent, HVNP. T. James Noyes
Steam emerging from Iki Crater, HVNP. T. James Noyes
Recent research has shown that there is not one big magma chamber directly under Hawaii.
Its head may have cooled and been dragged away by the plate, or there may be lots of little
“fingers” of magma instead of one big chamber. If this is the case, lookout Maui, because
this would make it harder to predict where volcanoes will occur! Stay tuned: there are new
things to be discovered!
22
The big island of Hawaii is actually 2 smaller islands linked together by the lava that flowed from the volcanoes.
Oceanography 10, T. James Noyes, El Camino College
11A-16