Transcript from Epic of Evolution: Life, the Earth and the Cosmos (BEP 210A)
February 23, 2000 - Lecture by Michael Wysession
“Conclusions and hypotheses concerning the earth’s interior are in a state of flux . . .” (B.
Gutenberg, 1959, p. v)
Today I am going to talk a little bit more about the forces moving the tectonic plates, as well as
the nature of the plate boundaries where I said that the action of geology mostly occurs. I’m
going to talk about the opening and closing of oceans, and the cycle that is involved there,
leading eventually to a history of our plates over the past 700 million years.
This map shows the best understanding we currently have of the large scale plate structure of our
planet, and I want to reiterate that there are three types of plate boundaries: 1) convergent plate
boundaries, where the plates are coming together and colliding; 2) divergent plate boundaries,
where the plates are moving apart; and 3) transform plate boundaries, where plates are sliding
past each other. As I previously mentioned, there are also there are some uncertain boundaries.
This map shows a little better the uncertainty in the boundary between North America and
Eurasia, which runs somewhere down through eastern Siberia. But the motions between North
America and Eurasia are so slow at these locations, that we can’t really tell where one stops and
the other starts. The distribution of subduction zones is what drives the motions of the plates. If
we look at the speeds of plates, we see that the plates that have the largest percentage of their
boundary subducting back into the mantle move the fastest. The Pacific Plate has a lot of its
boundary subducting, so it moves fast. There are a lot of places where the Pacific is sinking back
into the mantle: beneath the Aleutian Islands of Alaska, eastern Asia, Indonesia, and New
Zealand. The Pacific Plate is one of the fastest moving plates, at about 10 to 15 centimeters a
year. Other little plates such as the Philippine Plate, the Cocos Plate and Nazca Plate are also
moving very, very quickly. There are actually two things that set these plates apart from other
plates. They all have a lot of their edge that is subducting, but they also contain no continents.
Plates with continents tend to move slowly. Plates such as North America, South America,
Eurasia, and Antarctica are moving on the order of 0 to 2 or 3 centimeters a year: about a tenth of
the speed at which the oceanic plates are moving. The reason for this is that continents act like
big, heavy river barges. They have very deep roots, and those continental roots extend a long
ways down into the rock of the mantle. Think of a boat dragging its keel along the bottom of a
shallow stream. We have to move these plates through solid rock, which makes for a sluggish
stream. The deep roots of continents sometimes go down as far as 200 or 300 kilometers, acting
like very deep keels, and they provide a tremendous amount of resistance to plate movements.
So here is the basic idea. Once a piece of oceanic plate begins to subduct into the mantle, a large
force of slab pull (gravity) acts on the sinking slab, making it easier to continue subducting.
Think of a tablecloth falling off the edge of a table. Once you pull the tablecloth half way off the
table, gravity will take over and pull the rest of it off. The speed of the sinking plates actually
increases until the gravitational force of slab pull is balanced by the frictional resistance of the
mantle to the slab being pulled through it. This is the plate’s terminal velocity. It is just like your
terminal velocity if you jump out of an airplane. You don’t speed up in definitely. Your speed
increases until the frictional force of air resistance equals the force of gravity, and then you fall
the rest of the way down at this speed. However, if there are continents embedded in the plate,
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the plate will move more slowly, because of the continental resistance of the deep roots of the
continents.
Let me return to the situation involved with subduction, and in particular, the case of an oceanic
crust subducting beneath another oceanic crust. The result is an arc of volcanoes that I
previously described. As you bring sediments and the water down, rock melts, rises up as
magma, and erupts as volcanoes. But I want to point out another interesting feature, and this is
that is you get earthquakes occurring all within the zone of subduction. The earthquakes occur
for at least three different reasons. One is that we actually are cracking open the stiff oceanic
plate, and this cracking occurs in the form of earthquakes. Imagine that you took a piece of
plywood and began bending it. That act of bending causes a tremendous amount of cracking.
As the slab then goes down into the Earth, it is then straightened back up, and this rebending
causes more earthquakes to occur. As with the initial bending, these earthquakes occur within the
subducting lithosphere itself. We also, however, get earthquakes due to friction between the two
plates. These tend to be the very largest, most destructive earthquakes. Think of two colliding
plates as being initially stuck to each other by friction. As the subducting plate continues to try to
move down into the mantle, it actually begins to drag the overriding plate down a little bit with
it. This creates a tremendous amount of stress, as both plates are now being bent. All of a sudden
the stress will exceed the force of friction, the bottom plate slides down into the mantle, and the
overriding plate snaps back up. The process of the two plates sliding past each other is the
earthquake, and this can move hundreds of kilometers of rock a large distance in a matter of
seconds, and it releases a tremendous amount of energy.
[Q: What was continental resistance?]
Continental resistance was due to the deep roots of the continents dragging through the rock of
the mantle. Think of them as deep keels of a boat.
Here is a plot of actual earthquake locations across a cross-section of the earth beneath Japan.
The biggest packet of earthquakes occurs at the boundary between Asia and the Pacific Ocean
floor. These are the big destructive earthquakes. They’re also not far from Japan, and can be
tremendously destructive. The earthquake in Kobe in 1995 caused more than 100 billion dollars’
worth of damage in less than a minute. And there was an earthquake in Tokyo in 1923 that
caused the city to go up in flames and about 143,000 people burned to death in firestorms.
Because of this constant risk, Japan has a very active earthquake monitoring system. That is why
maps like this exist. But you can notice other places in this cross-section where earthquakes
occur besides the boundary between the sinking Pacific plate and the Eurasian Plate. Where the
Pacific plate begins to bend, east of Japan, and where it straightens out, deep beneath Japan,
there are also many earthquakes. There are actually other mechanisms for generating earthquakes
here, they are all essentially due to the fact that giant 60-mile thick sheets of rock are being
moved around the surface of the Earth, and then being stuffed into it.
[Q: Are those earthquakes causing tsunamis?]
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Earthquakes that break into the ocean seafloor cause tsunamis. If the earthquake ruptures the
bottom sediment layer, and the water column is moved during the earthquake, the result can be a
large water wave called a tsunami.
Let’s go back to the situation of an ocean plate sinking beneath a continent. Imagine that this is
the Juan de Fuca plate (a small fragment of the Pacific Ocean sea floor) sinking beneath the
states of Washington and Oregon. That would mean that the resulting volcanoes over the
subduction zone are Mt. St. Helens, Mt. Rainier, Crater Lake, Mt. Baker, and so on. Anyone
who has been to Seattle knows that this is no joke. Parts of Seattle are built on ancient mud flows
from a previously catastrophic eruption of Mt. Rainier. These volcanoes occur because the water
allows the rock to melt at a lower temperature, but the presence of water also does something
else. The eruptions of these volcanoes are filled with the water that’s brought down, in the form
of steam. The water comes back up, and the steam causes the eruptions to be extremely violent.
This is very different from mid-ocean ridge volcanoes. You can go to a volcanic eruption in
Iceland, and you can stand there and you can just watch the lava bubble out. Or you can do that
in Hawaii as well. I have been to Hawaii while the lava is erupting, and I’ve actually walked on
moving lava flows. The lava forms a hardened crust on top, but you don’t want to stand on it too
long because it’s over 1,000 degrees C, and your feet start to get pretty. But the lava comes out
as a very calm, mild eruption. People rarely die. This is very different from subduction zone
volcanoes. I’m sure you have seen pictures of Vesuvius, Krakatoa, Pinatubo, and Mt. St. Helens.
There is a tremendous amount of gas that is trapped within the magma, and when it gets near the
surface it expands outward, and that often blows the volcano open. With the 1980 eruption of
Mount St. Helens, the top few kilometers of the mountain were literally blown away. This can
obviously be very dangerous and destructive.
[Q: Could you clarify the reason that these volcanoes occur?]
I will talk more in three weeks about how the chemistry of this works, but essentially the water
causes the rock to melt. If you take almost any material, it will melt at a much lower temperature
with water present than if it’s totally dry. So you initially have rock down here that’s 1,5002,000 degrees, but it’s solid because the pressure is keeping it in a solid form. Add a little bit of
water to it, and voila - it’s almost like magic. You’ve now changed the balance between
temperature and pressure. You allow that material to melt and become liquid, and once it
becomes melt, it takes up more volume, is more buoyant, and is more easy to move. So, the
liquid squirts up, and erupts up at the top as a volcano.
Geologically, we now have a cycle of rock that I will talk more about when I talk about
sediments and erosion. However, let’s recap what is going on here. Let’s start with the
mountains. We have erosion (primarily through the agents of rain and ice) that wears away the
mountains, dissolves the rock, breaks it up into fine particles, and washes them down into rivers,
and the rivers dump the sediments out into the ocean. Sometimes that sediment floats thousands
of miles away, but it still settles on the seafloor eventually. By the time the ocean plate comes
back to the location of this trench you can have several miles’ worth of sediment on top. In the
case of the Caribbean Plate, you’ve got the Orinoco River and the Amazon River dumping lots
of sediment onto the oceanic part of the South American plate. These sediments are the remains
of the eroded Andes mountains. We end up with almost 10 kilometers’ worth of sediments on
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top of the oceanic plate as it is trying to get stuffed into the Caribbean Trench. Most of this
sediment gets scraped off into a large accretionary wedge. The island of Barbados is totally
comprised of scraped-off sediments from South American rivers. So two things happen to those
sediments. Some of them get scraped off as if with a big spatula, and become added on to the
edge of the continent, and cause the continent to grow a little bit. North America is filled with
little slivers of accretionary wedge sediments that got stuck on to our growing continent during
different past collisions. The rest of the sediments get carried back down with the ocean crust,
but those sediments melt with the water and come back up again to form new mountains. So we
can’t get rid of the continents. It’s not possible for us to take that rock and stuff it back down
into the mantle, at least, not for long. If we erode them away and dump them on the ocean
seafloor, they don’t go back down into the mantle with the rest of the oceanic plate. And
remember, ocean sea floor doesn’t last long (at most 200 million years) before it subducts back
into the mantle. The lighter rock that makes up the continental crust either gets scraped back onto
the edge of the continents, or it comes back up with the lava to form new rock. In either case, it is
then ready to be eroded again, giving us a continuous cycle of rock. So we’ve really got two
cycles of rock at work here. We have the mantle rock, which is a very heavy rock, and the
continental rock, which is much lighter. The mantle rock comes up at mid-ocean ridges, becomes
the ocean plate, sinks back into the mantle, maybe goes all the way to the core-mantle boundary,
where it may heat up and come back up again as a hot spot plume, or form new ocean crust. And
then we have the continental rock that is tied in with this cycle of erosion, where sediments get
dumped on the ocean floor, but are then either scraped off or re-melted and added back onto the
continent again.
While we’re talking about cycles, let’s talk a little bit about the life cycles of oceans. Our
Atlantic Ocean is 200 million years old. As I said, this is little more than 4 percent of the age of
the Earth. It has not been around very long. Oceans are transient features. Continents are here
to stay, albeit in constantly changing form, but oceans come and go. They close up and open
again. I mentioned that two hundred million years ago you could have walked from New York
to Africa. They were connected to each other, and in fact, large portions of New England are
actually African crust. Almost all of Rhode Island is former Morocco. The present Atlantic
Ocean began to open up with a process called continental rifting. We can see some places where
this is going on now. The Red Sea is a good example of the recent rifting of a continent. The
Arabian Peninsula used to be very nicely attached to Africa, but is now moving away from it, so
the Red Sea is opening up and becoming an ocean. That is what happened at some point with the
Atlantic Ocean. We would have had a continuous piece of continent that contained both North
America and Africa (as well as the other continents). Then they began to get pulled apart. Rifting
occurred, as well as a stretching and thinning of the continental crust near the rock. Magma
came up, erupting as volcanoes. We see evidence of this along what is now the eastern side of
the United States. The Palisades Cliffs of New Jersey are an example of this rift volcanism. Go
to New York City and look across the Hudson River at New Jersey, and you will see a 1000 foot
high wall of rock there. That rock, the Palisades Cliffs, was formed from lava that came out 200
million years ago when the other side of the Palisades Cliff was Africa, and the lava poured out
over large areas. Lot’s earthquakes occurred, creating smaller rift basins. One of these, the
Newark basin, lies beneath parts of northern New Jersey. Sometimes continental rifts don’t
make it into oceans, and the spreading stops. We happen to live right near one of those
examples. The Reelfoot Rift, south of here where the New Madrid earthquakes occurred, is a
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failed rift. The crust of North America is thinned here, causing it to have a lower topography.
That is why the Mississippi River runs down it, because it is a depression in our crust. But the
rifting could have separated America into two different continents but it ran out of steam.
There’s another mid-continental rift that was geologically active about 1.2 billion years ago. The
rift stretches from Kansas all the way through to Michigan, and again, a tremendous amount of
volcanism occurred. The continent almost broke apart, but it didn’t. Sometimes these do break a
continent in two, however, and at some point become an ocean. The current Atlantic Ocean is a
young ocean, meaning that it hasn’t yet begun to close up again. The Pacific Ocean is a mature
ocean. It is spreading (opening) in some places, but subducting (closing up) in others. The
Mediterranean Sea is an example of a very old ocean that was once much larger and is now
almost closed up. Africa is twisting, rotating into Europe, closing up the Mediterranean Sea.
Eventually Africa and Europe will be continuous continent straight across from each other.
[Q: What is the definition of rifting?]
To create a rift is to separate, so rifting simply means a separation of the continent into two
pieces. And these rifts that occur in continents can eventually become large oceans.
[Q: When you say that the Mediterranean is closing up, what kind of timeframe are we talking
about?]
Roughly 25-50 million years, perhaps. A fairly short time.
[Q: Maybe you said, but where does all the water come from?]
In the case of the Red Sea, it would have rushed in from the Indian Ocean suddenly at some
point in time. In other words, if you have a developing continental rift, you have a large, deep
valley. Think of Death Valley. This will end up being below sea level. And at some point when
the rift opens up to the edge of an adjacent ocean, you will have ocean water rush in to fill it.
Now, this was a long time before Moses. We’re going back 15-30 million years ago, but there
would have been a point when ocean water would have rushed into the Red Sea.
The shape of ocean sea floors is very important. When new rock comes up to fill the gap left by
the plates moving apart, the rock is very hot and it sits more buoyantly, and the depth of the
ocean water is thinned. Imagine if you were suddenly to stop this picture right now; stop the
plate motion, stop the spreading. As the ocean plates continued to get old and cold, they would
all slowly begin to sink down, and our global sea level would drop. Now, let’s imagine a time
when the plates are moving very fast, so we have a lot of ridges spreading open, and most of the
ocean plates are very young and hot. The sea floors will not be as deep, and water will be
pushed up onto the continents. So we can change the sea level, and therefore the location of
continental shorelines, without changing the amount of water, without changing the size of
continents, and without changing the amount of ice that exists as polar ice caps. The sea level
can change as a result of the speed at which the plates are moving. At the beginning of
Monday’s class I asked how it was that St. Louis was covered with water 450 million years old,
the time when the fossil I showed you had formed. A large factor involved with this was that
that was a time of very active plate motions, and those plate motions caused the whole seafloor
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to rise higher, meaning that the water has less room in the oceans and had to spill out over onto
continents.
At the eastern edge of the North American continent, from Newfoundland to Florida, is a
continent/ocean boundary, but it is not a plate boundary. This is called a passive margin. It’s a
boundary between a continent and an ocean, but with no plate boundary, there is no current
geological activity. However, remember that that this boundary formed quite dramatically as
part of continental rifting, and a lot of faulting was occurring when the North American and
African continents rifted apart. Those faults are still here, right beneath New York, Virginia, and
so on. As I will show later, this is where subduction is going to occur again at some point.
One other important aspect of mid-ocean ridges: 2 weeks ago, Ursula talked about the origin of
life, and she said she needed a nice body of water somewhere to start life going. Well, one of the
top candidates for a place on Earth where you could first have life start is right down here at
these mid-ocean ridges. This is so for a number of reasons. You had a nice warm location. The
new lava keeps the area very warm, and also causes a vigorous circulation of seawater. Cold
water gets sucked into the crust, where it heats up and comes back out at the ridge through
chimneys that are called black smokers. They look like smoke stacks. The water that comes out
of them is about 300 degrees C, and it is black because it is filled with all sorts of nutrients and
minerals. Mid-ocean ridges had all the materials needed to make life: salt water, energy (heat),
and nutrients. Most importantly, there was significant protection from ultraviolet radiation.
Right now, the reason we don’t die from solar radiation is because we have an ozone layer in our
atmosphere that filters out most of the ultraviolet radiation. Four billion years ago the Earth did
not have that ozone layer. Ozone is a gas made of molecules that consist of three atoms of
oxygen. There was no free oxygen in the atmosphere back then, however, and so the surface of
the Earth would have been a tremendously deadly place to live because of the very high levels of
radiation. However, that radiation doesn’t pass through water very well, so at the bottom of the
sea at mid-ocean ridges you wouldn’t have had to worry about that. Submarine trips to midocean ridges have found that they are teeming with life -- huge colonies of clams and worms and
crabs. There are tube worms that are 10 feet long, and even many forms of life that are not found
any where else on Earth. There is no sunlight, but that’s all right. Now, that doesn’t mean that is
the actual place that life started – we may never know that for sure - but it’s certainly a good
candidate.
[Q: You mentioned earlier, I didn’t quite catch the black smokers.]
They are chimneys. If you go down and travel along the mid-ocean ridges, you see these tall
chimneys with huge amounts of hot water rushing out of them, and life teeming all around.
Water is drawn into the crust away from the ridge, which heats ups and then rises back out along
the ridge.
I have not yet spoken about the third type of plate boundary. I mentioned that there are three
types of boundaries: convergent, divergent, and conservative. The convergent boundaries are
subduction zones. The divergent boundaries are continental rifts and mid-ocean ridges.
Conservative boundaries are also called transform faults. Transform faults are places where the
plates slide past each other, but they don’t crash in or move apart. In our country, the best
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example is the San Andreas Fault, where the western part of California is actually part of the
Pacific Plate, and is sliding northward relative to the North American Plate. This causes a
tremendous amount of earthquake activity, but no volcanic activity to speak of. What it does do
is move rock quite dramatically. At some point Southern California will be off the coast of
Portland or Seattle. This is about 50 million years down the line, but it will happen eventually.
As I mentioned before, the plate boundaries are not simple, and the Pacific/North American plate
boundary is actually a complex set of faults, of which the San Andreas is a part. All of the Los
Angeles region is riddled with faults and earthquakes. In fact, none of the big earthquakes that
have occurred recently have occurred on the San Andreas Fault. Many of the big earthquakes in
the LA region have been on faults we didn’t even know about
Let me mention one other interesting example of a transform fault: the Dead Sea Fault. This is a
fault that goes up through the Gulf of Aqaba (also called the Gulf of Elat), and heads on through
the Dead Sea up the eastern Mediterranean coast, all the way to Turkey. It is interesting that the
plate boundary in this region used to come up between Egypt and Sinai. The Sinai Peninsula
used to be part of the Arabian Plate, and the Gulf of Suez was opening up along with the Red
Sea. At some point, however, that plate boundary died out, and the Sinai got stuck onto Egypt,
and the Dead Sea transform fault was created so that the Arabian Plate is sliding northeast
relative to the Sinai Peninsula. At some point the Red Sea would have been a huge, deep, dry
valley. The perfect example now is in the form of the large rift valleys in Kenya. It turns out
that the eastern part of the African Plate is actually moving away from the rest of Africa, and you
get very deep rift valleys occurring. Eventually one of those rift valleys will open to the ocean,
and the water will flood in, and it will become just like the Red Sea. At some point the ocean
would have flowed in through the Gulf of Aden and into the Red Sea. The Gulf of Suez was
opening up along with the Red Sea, and this would have connected the Mediterranean with the
Indian Ocean, but that stopped. Sinai is now part of Africa, and the Dead Sea Fault allows the
Arabian Plate to slide up past the Mediterranean and Africa.
There aren’t that many places on continents where you have transform faults. The San Andreas is
one example. There is also a transform fault called the Queen Charlotte Fault, where the Pacific
Plate slides past western Canada. The Anatolian Fault in Turkey is the other big continental
transform. This fault just had a set of magnitude 7 earthquakes last year that killed tens of
thousands of people. Most plate boundaries, however, are either subduction zones or ridges.
However, it turns out that ridges are not continuous ridge systems. Mid-ocean ridges actually
consist of alternating segments of ridges and transforms. It takes a lot of work for the plate to
tear open, so the plate finds an easier way to do it. It turns out it’s really easy to slide plates past
each on transform faults. If a plate spreading ridge is oriented diagonally relative to the direction
of spreading, the plate boundary will reorient itself into a set of ridges that are perpendicular to
the direction of spreading, connected with small transform faults. This minimizes the total length
of spreading ridge. The whole mid-ocean ridge system is covered with these little transforms
separating segments of ridges.
Let me go off in a different direction for a little bit before I come back to this. Edward Albee
said in Zoo Story, “Sometimes you have to go a long ways out of your way to come back a short
ways the right way.” We have GPS sensors deployed around the world that tell us how the
plates are currently moving, so I know that the Pacific Plate is moving northwest relative to
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North America at 5½ centimeters a year. But what about the history of these plates? Suppose I
want to know how they’ve moved, going way back in time. It turns out that there is a very
curious set of circumstances that has allowed us to determine how the plates have moved, and
what their history is. The root of this is in the Earth’s core. I’ve been dealing primarily with the
mantle, and its convection and plate tectonics. All this time, however, the liquid iron in the outer
core has been bubbling away. The liquid iron is very fluid. It has the viscosity of water, so it is
very actively convecting. The fact that the liquid is iron is very important, because of the theory
of electricity and magnetism that says whenever you have an electric current moving, you create
a magnetic field. Well, we don’t have a big wire down there with electrons traveling through it,
but do have a big mass of iron with a lot of electrons in it. If we move that iron through
convection, it’s just like moving a current through a wire, and a magnetic field is created. The
convection of the liquid iron creates Earth’s magnetic field.
Now, the convection pattern has two parts to it. It has a thermal part, and involves motions that
are up and down, just like the boiling pot of soup on the stove. Liquid iron in the outer core rises
up to the core-mantle boundary, loses its heat into the mantle, cools off, and sinks back down, in
a big cycle. However, the Earth is also spinning on its axis, and that creates a rotational force
called the Coriolis Force. This is a twisting motion. The Coriolis Force is why weather patterns
usually move from the southwest to the northeast across the United States. It is also why
hurricanes spiral counter-clockwise in the Northern Hemisphere, and clockwise in the Southern
Hemisphere. As a result, instead of our liquid iron simply going up and then down, as it goes up,
it does all sorts of loopy twists. I can’t draw what the convection patterns look like, because we
really don’t know what they look like. But we know that they are fairly complicated, and when
they form these twisting loops, they generate a magnetic field that comes out of one pole of the
Earth and into the other. The remarkable thing about the Earth’s magnetic field is that it looks
just like there was a giant bar magnet sitting in the inside of the Earth. It would give you the
same magnetic field. Now, we certainly don’t have a giant bar magnet inside the Earth. We
have a very complex set of convection patterns, but remarkably enough, they give us this same
sort of magnetic field. And if you’re a stickler for details, technically the north pole is a
magnetic south pole, which is why if you have a little compass, your north in your compass
points to the south magnetic pole at the geographical North pole. In electricity and magnetism, as
Claude has already said, opposites attract.
But here’s the strange thing. The polarity of the Earth’s magnetic field reverses randomly over
time. In other words, your compass is currently pointing with the end on your compass pointing
north. But if you were here 700,000 years ago, it would have said the opposite. The “N” on your
compass would have pointed to the South Pole. The magnetic field will occasionally shut down,
and then power itself back up, and it seems that sometimes when it powers itself back up, it
comes back up with the same polarity, but sometimes when it powers itself back up it comes
back with the polarity reversed. So if you are part of a migrating flock of geese who fly north
every year because they have little bits of magnetite in they brains, you are going to be confused
for a couple years. The field is going to point in the wrong direction, and you’re going to fly
north in the wintertime.
[Q: Why does it randomly shut down?]
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We have no idea, to be honest with you. There are some very complex theories. One theory is
that it doesn’t actually shut down, but transfers the power into a form that we can’t see. There
are two types of magnetic field lines. One has field lines that that comes out of the Earth. This
is called a poloidal field. The other form has the field lines wrapped entirely within the core.
This is called a toroidal field. The idea is that during the time of reversals, the convection occurs
such that all motions give no field lines that come out of the Earth, but only field lines that are
coiled horizontally, and therefore aren’t visible at the surface. The some of this energy goes
back to a poloidal field, which we see at the surface again. What triggers this, we’re not sure. It
happens too frequently to be due to geological events. For instance, if you go back .7 million
years (or 700,000 years ago), the polarity of the magnetic field was flipped. Go back a little bit
more, and it flips again. As you go back in time, it’s reversed, normal, reversed, normal,
reversed, normal, and so on. After only 3 million years, the magnetic field has flipped its
direction quite a few times. Plate tectonics and geological events just don’t do much over short
time scales like this. Think back to the example of the moving blackboard. Over a million
years, not much motion has occurred within the Earth’s mantle. The reversals seem to take about
1,000 years to occur. Geologically, that’s pretty fast. Here’s the interesting thing. Imagine that
you had a volcano that erupted continuously over the period of a couple million years. Are there
any volcanoes that do this? Well, it happens on islands like Hawaii. We’ve had continuous
eruptions on Hawaii and the other islands of the Hawaiian hot spot chain. The Hawaiian island
chain is about 100 million years old, so we’ve had volcanism on the Pacific plate from the
Hawaiian hot spot for a long time. When the lava comes out and cools, it freezes within the rock
the direction of the magnetic field at that time of formation. So the rock that is coming out right
now at Hawaii and cooling, freezes in a magnetic field that points to north. However, if we go
back 700,000 years ago, the rock that formed at that time that froze in a magnetic field that
pointed south. We can drill down into the rock of Hawaii, or other volcanoes along the
Hawaiian Island chain, and we can determine the ages of those rocks through radiometric dating.
This means that we can create a time scale for when the magnetic reversals occurred. We can
create a geologic record of these geomagnetic reversals. Now, take the time scale and turn it
sideways, and put it in the oceans, and this is what we have in the ocean crust. As lava comes up
and cools at mid-ocean ridges, it is really forming long strips of rock that are constantly
becoming part of the new plate. All of the rock forming at a certain time along the mid-ocean
ridge is going to freeze in the magnetic field of that time. So all the rock that is cooling right
now at the ridges has a magnetic field that’s pointing north. Go back 700 million years, and the
stripe of rock forming then at the mid-ocean ridges was freezing in a magnetic field that pointed
south. The result is an ocean crust that consists of these alternating stripes of magnetic field. We
can sail across the oceans, constantly measuring the magnetic field given off by the rock of the
ocean sea floor, and get a time scale for the ages of the ocean sea floor. The pattern of normal
and reverse polarities in the ocean floor is exactly the pattern that we saw in the lava in Hawaii.
We got a time scale from the Hawaiian lava, and now we can go back and apply that time scale
to the oceans. Ships have since gone across most areas of the oceans, measuring the magnetic
field of seafloor. The result is a map of the ages of the ocean seafloor. The youngest rock, age 0,
is currently being formed at mid-ocean ridges. The oldest ocean ages, about 200 million years
old, exists at the edges of passive margins, like the East Coast of America. The motions of the
plates are perpendicular to the bands of magnetic stripes that represent regions of the seafloor of
the same age. We call these bands of equal-aged rock, isochrons. Looking at the inferred plate
motions, we can now see that Africa indeed fits in really nicely right here against North America
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and that has spread apart, rifted away, from North America at a fairly continuous rate over this
time. And again, the northeast corner of South America does indeed fit right into the notch in
western Africa, and that’s where it was 150 million years ago. Notice that India has rifted away
from Antarctica. I will talk more about this in the next class. Notice, also, that the width of these
magnetic stripes tells how fast the spreading has occurred. In the Pacific, we have very wide
bands. That’s because over the same period of time, these plates have moved a greater distance
apart than in the Atlantic, where the plates have moved a relatively short distance. So the
magnetic stripes in the ocean crust give us both the direction and the speed that the plates have
moved over the last 200 million years.
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