Kahua A‘o: Plate Tectonics
Plate Tectonics has revolutionized geological thinking since the early 1960s. In essence Plate
Tectonics says 2 things:
1. THE EARTH’S SURFACE IS DIVIDED INTO A NUMBER OF PIECES (THE PLATES), AND...
2. THE PLATES MOVE
--First, a little history---
Since geology started being serious, geologists have been looking for ways to explain a number of
things, including:
1. The bi-modal distribution of the Earth’s surface (oceans and continents).
2. The spatial and depth distribution of Earthquakes.
3. The distribution of Volcanoes.
4. Large-scale features such as mountain ranges and rift valleys.
5. The linearity of many mountain ranges.
6. The distribution of similar fossils, rocks, and geological structures in places that are far apart.
In the 19th century, geologists noticed crumpled mountain ranges such as the Appalachians and
figured that as the Earth has cooled it has contracted, and its skin is getting wrinkled. BUT…if
the Earth is contracting, how could there also be areas of obvious tension such as the East
African Rift and the Basin & Range in SW N. America??
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Then, when radioactivity was discovered,
geologists decided that this internal heat source
meant that the Earth wasn’t simply radiating away
its primordial heat, it was also producing heat.
The calculated cooling rate (incorrect, as it turned
out) was lower than internal heating rate if
radioactivity-produced heat was factored in. So
rather than shrinking, maybe the Earth is actually
expanding! Perhaps Earth started out as all one
continent. As the planet expanded, that one
continent would have to rift apart (to produce rift
valleys), and perhaps basalt could well up into the
cracks (to produce the oceanic crust). This would
explain how the pieces can be fit back together
like a puzzle. However, this makes explaining the
crumpled-up mountain ranges, which were
obviously produced by collision, kind of difficult.
The problem, obviously, is that there are areas
Fig. 1, top, showing how a shrinking planet could
produce crumpled zones where two thick
segments collide during shrinkage. Middle image
shows a side view of a graben. Bottom shows
how an expanding planet that started out as one
single continent, would eventually develop multiple
continents.
that show clear evidence of previous collisions and other areas that show clear evidence of having
been pulled a part. Any explanation that produces only contraction or only expansion isn’t going to
solve the problem.
Along came ALFRED VEGENER, a German meteorologist, who, in 1912, suggested that the
continents drift slowly across the Earth’s surface, sometimes breaking apart, other times colliding.
He likened them to Ships of Stone sailing through an ocean of stone. The main selling point of his
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theory was that there would be both tension and compression: compression at the leading edge of a
plate, tension at the trailing edge. Wegener was mainly trying to explain the fit between South
America and Africa. He was not the first to puzzle over this, however: Francis Bacon noted the fit in
1620, and Antonio Snider, in 1858, published maps depicting continental drift and likened the
process to breaking up a sheet of ice on a pond. If all this were true, THE PIECES SHOULD FIT
BACK TOGETHER AGAIN. We’ll come back to this in a bit, but first, let’s take a break to see what
these “plates” are, actually.
The Earth can be divided into layers, sort of like a great big onion. There are two main ways in
which the layers are distinguished: 1) by their chemical properties; and 2) by their physical properties.
Both schemes are equally valid, but if the terms are used carelessly (which is often the case), things
can get confusing. Neither of the schemes for describing the Earth’s layers is better than the other –
which one you use depends on what you are trying to discuss. An analogy might be a classroom.
The teacher can
divide the students
by gender, hair
color, age,
hometown, etc. All
of these
classifications will
operate on the same
set of students, and
none are inherently
better than the
others.
Figure 2. The compositional (left) and physical (right) layers of the Earth.
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THE CHEMICAL LAYERS
CRUST: comes in 2 flavors, continental and oceanic.
Continental crust is thick (30-60 km), has a lower density (about 2000 kg/m3), and is rich in minerals
that are silicates of aluminum, calcium, potassium, and sodium, such as feldspars and quartz.
Many intro-geology textbooks say that the average composition of the continental crust is
granodiorite (a rock sort of like granite), but this often confuses students into thinking that the
continental crust is granodiorite. It is probably better to say that continental crust is very diverse
and includes pretty much every rock type that exists, from basalt to granite to limestone to
sandstone to metamorphic rocks, and so on.
Oceanic crust is thin (2-5 km), has a higher density (about 2500 kg/m3), and is rich in minerals that
are silicates of magnesium and iron, such as olivine and pyroxene. The average composition of
oceanic crust is basalt. This is much more realistic because much of the oceanic crust really is
basalt or its intrusive equivalent, gabbro.
MANTLE: The mantle has af higher density than the crust (about 3300 kg/m3), and is composed
mostly of olivine and pyroxene (or their high-pressure equivalents). A rock consisting of olivine
and pyroxene is called peridotite. No drill holes have ever reached the mantle, although people
have tried. There are a few slivers of the uppermost mantle that have been slapped up onto the
surface where plate collisions occurred and thereby exposed on the surface. Such rocks are
called ophiolites. Additionally, some volcanoes occasionally bring up pieces of the mantle as
inclusions in their erupting magma. Such inclusions, whether they come from the mantle or
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crust, are called xenoliths (foreign rocks). Most of what we think we know about the mantle
comes from indirect study (earthquakes). A very prominent change in the propagation speed of
earthquake waves (from slower to faster) called the Mohorovicic Discontinuity (usually called
the Moho) defines the crust-mantle boundary.
CORE: The core is the densest layer, and it consists of an iron-nickel alloy, perhaps with a little bit of
sulfur mixed in. We cannot sample it, and no magma sources are that deep (so no volcanoes
bring up core xenoliths), so all evidence is indirect:
1. Many meteorites consist of iron-nickel alloys. Most meteorites are thought to be fragments
of a broken-up planet that was once like Earth. We don’t find lumps of iron-nickel alloy lying
around on the sEarth’s urface so if Earth is indeed like that broken-up planet, our Fe and Ni must
be down in the core. Additonally, we generally know the elemental abundance of the inner solar
system, and because the crust and mantle combined do not contain sufficient Fe to match the
solar system abundance, Earth’s Fe (and Ni) must be in the core.
2. From studies of orbital mechanics involving the moon and sun we know that the average
density of the Earth is about 5500 kg/m 3. Crust and mantle rocks are both less dense than this
so there must be something denser to make up the difference:
densities of...
Earth = 5500 kg/m3
crust = 2000-2500 kg/m3
mantle = 3000-3500 kg/m3
FeNi meteorites = 7000 kg/m3 but under core pressures = 10,000-12,000 kg/m3
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People sometimes wonder why mantle convection can’t drag up pieces of the core from the coremantle boundary. What they don’t realize is that the density difference between the core and
mantle is 3x greater than the density difference between the crust and atmosphere. How often
does convection in the atmosphere drag up pieces of the crust?
Thus the Earth is chemically zoned. 90% of the whole Earth is Fe, O, Si, and Mg, but 90% of the
crust is O, Si, Al, and Fe. The elements get layered but because they form compounds they are
not layered simply by their atomic weights. For example, uranium, lead, and thorium, which are
dense elements, readily form low-density compounds with oxygen and are thus concentrated in
the continental crust.
THE PHYSICAL LAYERS
Based on evidence from earthquakes, gravity surveys, and lab experiments, we can also divide the
Earth into physical layers:
LITHOSPHERE: “rock layer”: This layer includes the crust and the uppermost mantle. It is cold,
rigid, and brittle. It extends down to about 100 km. The “plates” of plate tectonics are pieces of
lithosphere. Therefore we should avoid the term “crustal plates”, and instead use “lithospheric
plates”.
ASTHENOSPHERE: “weak layer”: This layer is part of the upper mantle. Although solid, is hot,
weak, and ductile (able to deform). Deformation of the asthenosphere allows the tectonic plates
to move.
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MESOSPHERE: “middle layer” This is most of the mantle. It is hotter than the asthenosphere, but
because the pressure is higher, it is a much less ductile solid.
OUTER CORE: Seismic evidence shows that this layer of iron-nickel alloy is liquid. Thermal
convection and rotation of the Earth cause the outer core to move. It is therefore a moving
electrical conductor, and this movement produces a magnetic field, namely the Earth’s magnetic
field.
INNER CORE: This layer is also iron-nickel alloy, and although even hotter than the outer core, the
high pressure keeps it solid.
Where is all this heat coming from??? What drives plate tectonics? Where does the heat for
producing volcanoes come from? The two main sources of heat are decay of naturally-occurring
radioactive elements, and heat leftover from Earth’s formation. Even though Earth formed 4.6
billion years ago, rock is a very good insulator so it isn’t unreasonable for there to still be some
heat left. A tiny amount of heat energy comes from meteorites striking the Earth, but it is so tiny
that it can be ignored.
OK, back to plate tectonics...
Some pieces fit together very easily, such as the east side of South America fitting into the west
coast of Africa, especially when you fit the continental shelves rather than the coastlines. If you
put these pieces back together you can explain a bunch of things:
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1. Identical LAND fossils on both sides of the Atlantic (animals that didn’t appear to have been
swimmers based on fossil evidence would otherwise have had to swim from one side to another).
2. Identical TREE fossils in many places (trees are even worse swimmers than big animals). The
fossils of one particular tree, Glossopteris flora, can be found in India, S. America, and Australia.
Glossopteris fossils older than ~225 million years are pretty much the same in all three places,
but from that age foreword they are more and more different. If, prior to ~225 million years ago
(a geologist would say ~225 Ma), India, S. America, and Australia were connected, it would make
sense that the same trees might be found on all three. However, if around 225 Ma the continents
started to drift apart into different latitudes, it also makes sense that the Glossopteris trees would
start to evolve differently.
3. Coal forms from what were once tropical rain forests and swamps, but there are coal deposits in
what are now cold places (Antarctica, N. America, Europe). If these continents were once closer
to the equator where the climate is warmer, warm swamps could have existed.
4. There is evidence that some 300 million years ago a single glacial ice sheet covered parts of S.
Africa, S. America, India, and Australia. By looking at scrape marks in the rocks caused by the
moving ice (called glacial striations), the directions of the ice movement can be determined. If a
single ice sheet covered all these places in their present positions it would have been huge.
Additionally, in their present locations, most of Africa, S. America, India, and Australia are too
warm to be glaciated, even during ice ages. Finally, there is no evidence that there were huge
glaciers in the N. hemisphere at that time. But…if all the southern continents were joined
together and in a location much farther south than they are today, all these problems can
be solved.
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The problem was (and still is, to some
extent)...HOW DO THEY MOVE?
Wegener said they slide or “sail” through
the oceanic crust, but there are no
earthquakes to indicate that
continents are plowing through solid
rock.
Holmes (1928) said that there might be
convection in the asthenosphere and
like a conveyor belt, the continents
can get dragged/carried along, break
up, or collide.
Figure 3. Top: The southern continents in their present
positions, showing the extent of ~300 million year-old
glacial deposits. Bottom: Reconfiguration of the
southern continents, producing a much more reasonable
ice sheet, with glacial striations indicating radiallyoutward motion.
By the 1930s (Wegener died in 1930), the debate had become a stalemate with the “drifters” looking
for a way to move the continents and the “anti-drifters” looking for ways to explain many geologic
observations without moving the continents.
THEN, in the 1950s, largely due to oceanographic work begun during World War 2, scientists started
making great strides into two relatively-uncharted areas: The OCEANS and
PALEOMAGNETISM, the study of the Earth’s magnetic field.
Oceanographic studies , which were started during the war in an effort to find places to look for,
and/or hide, submarines, came up with the following findings:
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1. There is a 65,000 km-long, 2000 km-wide, 3 km-high mountain range snaking through the central
parts of most of the oceans, kind of like the stitches on a baseball. This was named the midocean ridge.
2. There is a graben at the summit of this ridge indicating that it is a zone of tension.
3. Ocean sediments get thicker the farther away from the crest of the ridge you are.
4. There is a high amount of heat escaping to the surface along this ridge.
5. There are lots of shallow earthquakes along this ridge.
6. There are lots of deep earthquakes along many of the oceanic margins where the deep trenches
are.
7. Rocks dredged from the summits of the mid-ocean ridges are volcanic.
8. The oldest sediments anywhere in the oceans are only ~200 million years old.
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9. There are areas of anomalously high gravity over the
ridges and of anomalously low gravity over the
trenches.
Paleomagnetic studies came up with interesting
discoveries as well. Rocks have weak magnetic
fields. When a volcanic rock containing the mineral
magnetite cools, the magnetic field of all the
magnetite grains line up in a direction parallel to the
direction of the Earth’s magnetic field. This means
that a stack of rocks, accumulated over time, acts like
a tape recorder showing the direction from the rocks
in each layer to the magnetic pole.
If you go to a place and measure the orientation of the
rocks’ magnetic fields and find that they are different
from the present magnetic field, either the magnetic
poles have moved relative to the rock or the rock has
moved relative to the magnetic poles. Or both. How
do you decide which happened? Plot the apparent
pole positions of two different continents through time.
If the paths are the same (i.e., the magnetic field in
the 150 million year-old rocks of Continent A points to
the same location as the magnetic field in the 150
million year-old rocks of Continent B, then the pole
Figure 4. Diagrams showing the different
results for true polar wander vs. apparent
polar wander (caused by plate wander).
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has been moving and the continents have remained fixed. If the paths are different, however,
then the pole has remained fixed and the continents have moved.
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The magnetic field does drift around a little (called secular variation), but most of what is sometimes
called “apparent polar wander” is actually due
to the continents moving, not the magnetic
poles.
In 1962, Harry Hess proposed SEA FLOOR
SPREADING, and subtitled his paper “An
Essay on Geo-poetry”. This idea said that
magma rises from depth beneath a mid-ocean
ridge and pushes its way to the surface. In so
doing it pushes the existing rocks to the side.
The next batch of magma then pushes itself to
the surface, and so on. This would mean that
the farther you were away from the ridge crest,
the older the crust would be and the longer it
would have had to sit around and collect
sediments. One problem with this idea is that if
the plates are pushed apart, there should be
indications of compression acting on the two
plates. Instead, there is evidence of just the
opposite, namely tension, indicated by the
graben running along the crest of every midocean ridge.
Figure 5. Diagrams showing how sediments are
thicker the farther you go from the central graben
of a mid-ocean ridge (top), how a stationary
magma source and moving plate can produce a
chain of ever-older volcanoes (middle), and how
bands of normally- and reversely- magnetized
rocks produce symmetric stripes centered on a
mid-ocean ridge.
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J. Tuzo Wilson suggested that if you can find a long-lived magma source that didn’t move (maybe
because it was below the moving plates), but produced volcanic islands high enough to break
sea level then as the crust moves over it a linear chain of volcanoes will be produced. These
volcanoes will be young at the “upstream” end and old at the “downstream” end. Such an
unmoving magma source is now called a “hotspot” although “hot column” would probably be
more appropriate.
Vine, Matthews, and Morley said that the lava that is erupted at a mid-ocean ridge should display
thermo-remanent magnetism (TRM), and will record past magnetic directions (just like the onland rocks). The magnetic time scale was known from on-land studies and thus you should be
able to figure out the ages of the oceanic crust rocks using this sideways version of the
paleomagnetic time scale.
THEN...knowing the ages and the distance from the ridge crest, you could figure out the spreading
RATE. People did this exercise, and it turns out that there are variable spreading rates. The
East Pacific Rise (EPR), which is producing the pacific plate, is spreading at 16-22 cm/year. This
is what is called the “full spreading rate”, meaning that if you were standing on one side of the
EPR, it would appear that the other side was moving away from you at 16-22 cm/year. The
velocity of one plate, the half-spreading rate would be half of the full spreading rate, namely 8-11
cm/year. The EPR is has the highest spreading rates of all mid-ocean ridges.
The spreading rates are easy to determine from the ages of rocks (determined via paleomagnetism
or by dating fossils in sediments on the plate), and comparing the ages to the distances from the
mid-ocean ridge crest. However, this does not determine the absolute motion of either plate of a
spreading system if the ridge crest itself is moving as well. The problem is to find something that
isn’t moving so that you can have an absolute reference point. For a long time, hotspots such as
Hawai‘i have been considered “fixed”, meaning they don’t move relative to the center of the Earth
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nor to each other. They therefore have been used as a reference frame against which to
determine the motion of the plates. Some recent evidence, however, is starting to cast some
doubt on whether hotspots really are fixed.
Spreading centers only explain the places where plates are created. We know that the Earth isn’t
expanding, so what compensates for the creation of oceanic crust at mid-ocean ridges (and at
continental rifts)?
For a long time it had been recognized that earthquakes are not scattered randomly over the globe.
Instead, there are places on Earth where earthquakes are very common and other places where
they almost never occur. In a particular type of place where earthquakes are common,
specifically at the edges of some oceans, it was also noted that the farther you were from the
ocean, the deeper the earthquakes got. For example, there are shallow earthquakes just
offshore of the W coast of S. America. As you move East (i.e., inland), the earthquakes get
deeper. These sloping bands of earthquakes were known as “Wadati-Benioff zones”, after Kiyoo
Wadati and Hugo Benioff, the seismologists who first described them, but prior to the early
1960s they hadn’t been explained. Not
long after Harry Hess’ seafloor spreading
idea for producing oceanic lithosphere
came along, people started looking for
ways to get rid of oceanic lithosphere
because we know that the Earth is not
getting bigger. One idea to “consume”
oceanic lithosphere and simultaneously
explain the Wadati-Benioff zones is that of
Figurae 6. Cross-section diagrams showing how
earthquakes dip away from an ocean basin in a
Wadati-Benioff zone. At left is the situation without a
plate tectonics reference. At right, a down-going (i.e.,
subducting) plate is shown relative to the earthquakes.
Note that in the shallower part of the subduction zone
the earthquakes are at the top of the downgoing plate,
and are due to binding between the downgoing and
overlying plate. Deeper earthquakes, however, are
within the downgoing plate.
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“subduction zones.” These are places where oceanic plates are consumed by diving under
other plates. The dipping zones of earthquakes are essentially traces of the down-going plates.
With all these new ideas and concepts in mind, Hess and Wilson figured out all the plate boundaries
in general (except in the SW Pacific), the types of boundaries (except in the SW Pacific), and
what happens at each (except in the SW Pacific). By the late 1960s, geology textbooks had to
start being re-written.
WHAT REALLY MOVES??
The cool, rigid, brittle LITHOSPHERE.
WHAT DOES IT MOVE ON??
The weak, mushy ASTHENOSPHERE. This explains how movement can take place without
having to generate earthquakes (other than at plate boundaries). This was the mechanism that
eluded Wegener and made his farsighted idea difficult to swallow at the time.
The plates themselves are rigid - gentle warping is possible in the interiors and intense
deformation takes place only at the boundaries with other plates.
WHAT CAUSES THEM TO MOVE??
There are 4 ideas that have been proposed. The third is the most plausible, although the answer
may include some contribution from one or two others:
1. “ridge-push” The plates are pushed apart by magma pressure at mid-ocean ridges. The big
problem with this idea is that the ridges themselves appear to be zones of tension, not
compression. Another problem is that compared to their lengths and widths, the plates are very
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very thin. It is difficult to imagine that compressive forces could act across such wide, long, but
think plates without causing them to buckle appreciably.
2. “trench-pull” Once the leading edge of a plate is pushed under at a subduction zone, gravity will
pull it under and the rest of the plate will move along with it. This idea also suffers from the thinplate problem, namely it is hard to imagine how the plate could transfer the tensional stress
across its length and width without pulling apart.
3. “drag” Convective motion of the underlying asthenosphere drags the plates along. The beauty of
this idea is that it overcomes the thin-plate problem. It also explains why the crests of mid-ocean
ridges are zones of tension – if two convective “rolls” are moving in opposite directions, they will
pull their overlying plates apart. This would provide the perfect scenario for mantle rock to well
up to take its place, melt a little bit, and thereby provide material to build new oceanic lithosphere.
4. “sliding downhill” There is a little bit of mantle-upwelling at mid-ocean ridges so it is “downhill”
away from them. Calculations show that only a 1:3000 slope is required. This idea also doesn’t
have the thin-plate problem because gravity would work along the entire area of the plate, not
just one edge.
WHAT ARE PLATE MARGINS LIKE AND WHAT DO THEY HAVE TO DO WITH VOLCANOES?
There are 3 types of plate margins: divergent, convergent, and transform.
1. DIVERGENT MARGINS. Here new lithosphere is being formed. Most divergent margins are
oceanic (= mid-ocean ridges) but there are also continental rifts, the most famous of which is the
East African Rift. Of course all the present oceans started out as continental rifts at one time.
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Divergent margins are characterized
by tensional forces - graben, rift
valleys, and normal faults are
common. Here the new
lithosphere is thin and hot,
making it ductile; large stresses
cannot build up so earthquakes,
although common, are shallow
and not very big. The shape of a
mid-ocean ridge depends on how
much material is upwelling from
Figure 7. Diagrams showing the different profiles of fast and slow
spreading ridges.
below, which also controls the spreading rate. A high spreading rate is associated with high
supply from depth, namely more rapid upwelling of the upper mantle. This rapid upwelling
partially balances the subsidence that takes place as the two plates pull apart. On the other
hand, if the supply rate is low, there is ample chance for collapse along the summit of the
spreading ridge to take place; this collapse is much less balanced by by low supply rates,
leading to impressive, well-formed axial graben.
Volcanic activity is almost always basaltic. The most common volcanic rock on Earth is Mid Ocean
Ridge Basalt, also known as MORB. MORB has an extremely monotonous composition and only
geochemists and petrologists get very excited about it. Importantly, geochemistry has shown that
MORB derives from very shallow mantle. This suggests that the upwelling at mid-ocean ridges
does not involve material moving up from great depths. If the entire mantle is convecting (which
seems likely) then the upwelling at mid-ocean ridges is a consequence of plate tectonics, not a
driving force.
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2. CONVERGENT MARGINS. Here old lithosphere is consumed and/or mountain ranges are built.
The exact nature of a convergent margin depends on what is colliding with what.
A) When two continental plates collide, because both are thick and buoyant, neither wants to dive
under the other. Instead the just crumple up, producing compressional mountain ranges. The
most obvious present example of this is the collision of the Indian continental plate against the
southern margin of the Eurasian continental plate, forming the Himalayan mountains. Many
other mountain ranges, including the Appalacians, Rockies, Alps, Urals, etc. are the result of
continent-continent collisions in the geologic past. There are lots of big earthquakes associated
with these continent-continent collisions, but there is no volcanism.
B) Whenever oceanic lithosphere is involved a convergent margin will consist of a subduction zone.
There are ocean-ocean subduction zones where one oceanic plate dives under another and
there are ocean-continent subduction zones where an oceanic plate dives under a continental
plate. The oceanic plate always does the diving because it is thinner and denser. Volcanism is
very intimately associated with subduction zones.
Features of subduction zones include:
1. Andesitic “arc” volcanism
2. high-pressure, low-temperature metamorphism
3. deep, large-magnitude earthquakes
4. a deep trench, sometimes filled with sediments
5. chaotic “melange” of rocks
The volcanism does not come from friction of the two plates rubbing together. It is a little more
complicated. The down-going plate includes oceanic lithosphere created at a spreading center
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plus all the wet ocean-floor sediments that have accumulated. Those sediments that are not
scraped off at the trench get subducted and along with the plate they heat up as they get deeper.
Pretty soon the water is driven out of the sediments by the heat. This water (and other fluids)
migrates upward into the wedge of mantle between the downgoing plate and the overriding plate.
The melting point of rocks can be lowered - sometimes considerably - by the addition of water,
and sure enough the water driven off the downgoing plate causes melting of the mantle wedge.
This melt migrates to the surface to erupt at volcanoes.
Subduction-related volcanoes form lines parallel to the plate boundaries and 150-300 km “inland”
from the actual boundary. The actual boundary is the deep trench. The volcanoes form “arcs”
because of the relationship between a downgoing plate and the curved Earth. If the dip is
shallow, the curvature is strong (it has a small radius), and if the dip is steep the curvature is less
pronounced (a large radius). A vertical dip would give a straight line of volcanoes.
The relationship is as follows:
-young subducting plate, small , small radius
-old subducting plate, large , large radius
This points out the importance of density differences between the subducting plate and the mantle
into which it is subducting, and also shows that “trench pull”, although it may not be actually
driving the plate motion, cannot be ignored altogether.
Calculations show that the lithosphere loses its brittle properties (its ability to break and thus generate
earthquakes) by the time it has subducted to about 700 km blow the surface. That is why there
are no earthquakes deeper than this. Until the advent of seismic tomography, 700 km was
therefore the deepest we could “see” the downgoing plates. Tomography looks at the
attenuation or accentuation of seismic waves as they pass through the Earth - hot places
attenuate seismic waves, cool places speed them up. Even though below 700 km, downgoing
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plates can’t generate earthquakes, they are still different (cooler) than the surrounding mantle
and tomography can “see” them down to about 1400 km.
3. TRANSFORM MARGINS.
Transform margins are those where two plates are moving past each other. The most common are
found at transform faults near mid-ocean spreading centers. The spreading centers can’t bend
very well so to turn corners, segments of offset are needed. Between two segments of spreading
the two pieces of plate are moving in opposite directions and strike-slip motion is taking place.
Earthquakes here are relatively shallow and small. Outside the two spreading segments the
plates are moving in the same direction, never-the-less there is still a scar left over from earlier,
and these “fracture zones” are weaknesses that remain in the plate. They help to explain some
of the non-volcanic earthquakes that we sometimes have here in Hawai‘i.
There are also continent-continent transform margins and the most famous of these is the San
Andreas fault (which is actually a system of many faults). The Pacific plate is almost all ocean
but it carries a small sliver of continental crust. This sliver is rubbing against the N. American
plate (all continent) at a transform fault. Earthquakes are not very deep, but they can be pretty
large. Some parts of the fault system are tightly locked; earthquakes are infrequent but very
large. Other parts are poorly locked; earthquakes are frequent (almost regular), but not very
large. Still other places are not locked at all and constant creep occurs rather than earthquakes.
Los Angeles is on the Pacific plate, which is moving to the NNW. In perhaps 60 million years L.A. will
have moved up past San Francisco, most of which sits on the N. American plate. L.A. may be an
offshore island but it will not fall into the sea!