Geography 140
Introduction to Physical Geography
Lecture: Tectonic Processes at Plate Boundaries
E. Because of the differing absolute rotations of lithospheric plates
over the surface of the earth, adjacent plates can experience three
RELATIVE motions with respect to one another: Plates can be moving
apart or diverging from one another; plates can be colliding or
converging; and plates can be shearing alongside one another. I'll
discuss some of the features of each of these three kinds of boundary
zones.
1. Constructive zones are defined by two adjacent plates diverging or
moving away from one another. They are also known as divergent
zones or zones of spreading.
a. Constructive zones are usually located in oceans, though there
are a couple in land areas (e.g., East Africa's Rift Zone, the
one filled with all those lakes, such as Lake Turkana, Lake
Victoria, Lake Tanganyika, and Lake Nyasa).
b. In oceans, they are marked by the great "mid oceanic ridges,"
which are sort of like mountain ranges under the sea, emerging
from the abyssal plains.
i. These can be found running down the middle of the Atlantic
Ocean, the eastern and southern Pacific Ocean, and the
Indian Ocean. You can see them in this figure. The light
blue areas are relatively shallow ocean water, and the mid-
oceanic ridges show up as slender areas of shallower water:
![[ mid-oceanic ridges, GLOBE DEM + Smith & Sandwell bathymetry, NOAA, NAtional Geophysical Data Center
]](http://ftp.ngdc.noaa.gov/seg/topo/img/globsmi3.jpg)
ii. This feature is produced by the lithospheric plates pulling
apart along a line. The resulting fractures and faults
allow material, often melted into magma, to well up from
the æsthenosphere below, in what's thought to be a
convection plume carrying heat up from the planet's core.
iii. Upwelling leads to vulcanism along the ridge, with great
numbers of volcanoes under the sea.
a. These volcanoes produce basaltic, high temperature lava
(remember the Bowen Reaction Series in the lecture on
the composition of the earth's crust?).
b. Basaltic magmas tend to be runny lavas, so these are not
explosive volcanoes. Rather, it produces "pillow
basalts," smooth blobs of volcanic rock.
c. So, you have a lot of vulcanism in constructive zones,
but it isn't particularly violent.
iv. The convection plume, being made of hotter material
upwelling from the interior, elevates the thin ocean floor
in this area, creating a gradient or slope, down which the
lithospheric plates can slide away from the mid-oceanic
ridge, making it that much easier for new material to
upwell into the rift.
v. The magma coming up from below solidifies on either end of
the two plate edges, thus creating new lithospheric
material "glued" onto the edge of the plates. That is why
this area of divergent movement is called a "constructive
zone": New lithospheric material is being "constructed" in
this area. The new material slides away from the central
rift with the rest of the slab.
vi. One of the lines of evidence supporting this mechanism of
sea-floor spreading and continual renewal of the
lithosphere in a constructive zone is the progressively
older age of lab-dated ocean floor rocks collected from
areas farther away from the mid-oceanic ridge, as you can
see in this map, where reds and browns are younger rocks
and greens and dark blue are old materials.
![[ world crustal age, NOAA, National Geophysical Data Center ]](https://home.csulb.edu/~rodrigue/geog140/worldcrustalage.gif)
vii. As mentioned above in the discussion of the Euler
Principle, this motion isn't equal all along a rift, so
shear stress arises in the lithospheric plates. Also
adding to the shear stress is different rates of upwelling
of æsthenospheric materials along the ridge. The
result of this shear stress in rigid lithospheric material
is the formation of and motion along transform faults (or
strike-slip faults) to equalize the stress.
viii. Creation of or movement along such transform faults
produces frequent, shallow, low magnitude earthquakes in
the region.
c. This mechanism of sea-floor spreading, then, builds up the
lithosphere (i.e., adds new material to it).
d. Such a process cannot go long unopposed: Divergence implies
convergence. The creation or addition of new lithospheric
material must be balanced by the destruction or subtraction of
old lithospheric material somewhere else.
2. Destructive zones are where plates converge (or collide) with one
another. These are also known as subduction zones, because one of
the converging pair of plates will be carried under the other:
"Subduction" means carrying (duction) under (sub).
a. As plates converge, the thinner, denser, more mafic layer slides
under the thicker, lighter, less mafic layer: subduction.
b. The subducted layer eventually softens and becomes
indistinguishable from the æsthenosphere. In other words,
lithospheric material is subtracted from the lithosphere and
added to the æsthenosphere. That's why this is called a
destructive zone: Lithosphere is destroyed here.
c. Earthquakes are frequent and some can be very strong in a
destructive zone (another reason to call them destructive
zones?). This is because the lithospheric slab is rigid and
does not just bend easily into the æsthenosphere (plastic
or elastic deformation): It fails and it is rock failure that
produces earthquakes.
i. There is an interesting spatial pattern to earthquakes in a
destructive zone. There are a lot of shallow earthquakes
right near the line of contact between the converging
plates, and some of these earthquakes are quite high in
magnitude. There are also some very deep earthquakes in
the region, but they have epicenters farther from the line
of convergence and on just one side of it.
a. The shallow (sometimes strong) quakes are being
generated in the two plates as they converge. The parts
of the lithospheric plates responsible for the
earthquakes are still on the surface, above the
æsthenosphere, so they are cool and rigid and fail
readily, and so the foci (latitude, longitude, and
depth) of the earthquakes are shallow (under 60 km from
the surface).
b. The earthquake with very deep foci (deeper than 60 km,
and some have been recorded as far down as 700 km!) are
coming from the subducted lithospheric slab. These deep
quakes are rarer and are responsible for only 15 percent
of the earthquake energy released globally. The
reduction in frequency and overall energy release
reflects the warming and softening of the subducted slab
as it slides down into the æsthenosphere.
c. By recording the X-Y-Z coördinates (latitude,
longitude, and focal depth) of earthquakes around the
world, it is possible to plot them. This creates images
of the subducted plates, allowing us to "see" the
structure of the mantle in the region around destructive
zones!
1. The average angle of descent is about 45°.
2. There is a lot of variation, though: Some slabs are
descending at very gentle angles and some are sinking
nearly vertically.
d. The classic subduction zone formed by the oceanic edges
of two converging plates (e.g., the Philippine and
Pacific boundary) or of one oceanic edge and one bearing
a continent (e.g., the South America and Nazca boundary)
is called a Wadati-Benioff Zone (or B-zone).
e. When subduction brings two continental masses together,
subduction slows and the plates are thickened by the
peeling off and accretion of buckled up ocean floor
sediments and other rocks and their complex thrusting
over one another, the subduction zone is called an
Amferer Zone (or A-zone). An example would be the
Tibetan Plateau and the Himalaya and Hindu Kush
mountains marking the collision of the Indian Plate with
the Eurasian Plate, which wiped out the old Tethys Sea
that once existed there and created the world's tallest
mountains.
d. Subduction also produces volcanic activity.
i. Pressures of convergence and subduction generate great
heat, and chemical metamorphoses tend to lower the melting
point of lithospheric rocks.
ii. Local pockets of rock, thus, are melted into magma.
iii. Hot magma rises through overlying lithosphere.
iv. Some of it never makes it to the surface. It will rise up
through cracks in the overlying rock, melting some of it as
it goes (a process called anatexis), and then stop moving
to the surface. The magma, trapped in warm crust, cools
very slowly and turns into intrusive igneous rocks (which
ones depends on the minerals in the magma and the progress
through the Bowen Reaction Series). These trapped magma
bodies are called plutons (for "way down there in Pluto's
realm").
a. The largest plutons are called batholiths. These are
humongous: The Sierra Nevada is largely one big
batholith, tilted up on its east side!
b. Small plutons oriented in the same direction as the
country rock are called sills (they may be really small,
like a cm), and particularly thick sills are called
laccoliths.
c. Small plutons cutting across any layering in the country
rock are called dykes. They may be following faults or
cracks and, they, too, can be very thin.
v. Some of the magma almost makes it to the surface in a
volcano but the eruption ends and the magma solidifies in
the throat of the volcano, forming plugs. You can see one
in a pleasant weekend drive up to Morro Bay: Morro Rock in
the harbor is a volcanic plug, and you can make out several
others in a line from Morro Bay to San Luis Obispo.
vi. Magma that makes it to the surface produces volcanoes, and
magma extruded onto the surface is called lava.
vii. As the process of subduction continues, volcanoes multiply:
They tend to occur in groups or lines.
viii. At sea, they produce seamounts, some of which break the
surface to become volcanic islands, then arcs of volcanic
islands (e.g., the Lesser Antilles in the eastern
Caribbean), then archipelagos of fused volcanic islands
(e.g., the Greater Antilles of Cuba, Jamaica, Santo
Domingo, and Puerto Rico).
a. In the tropics, volcanoes often support coral, which
produces a reef around a volcanic island.
b. Sometimes, there's no central volcanic island. Instead,
there's an atoll of coral-based islands ringed around a
central lagoon where a seamount never broke the surface
or where it was eroded below the surface of the sea or
was covered by rising sea levels. You see these all over
the South Pacific, such as Enewetok (where the first
hydrogen bomb was tested ....).
ix. Volcanoes will also develop on continental crust where
convergence has brought continental crust to the
destructive zone. The magmas that produce these volcanoes
often incorporate a lot of continental granite-related
rocks through anatexis. With their lower melting point and
greater viscosity and gassiness, such magmas can produce
explosive eruptions.
x. Volcano types reflect the different types of magma
supplying them and their locations on more granitic
continental crust or more basaltic oceanic crust.
a. Shield volcanoes are those derived from the hottest,
least viscous, runniest magmas, which are relatively
enriched in ferro-magnesian minerals and relatively
impoverished in silica. Such magmas produce runny,
dribbly eruptions, not the violent sorts like Pompeii or
Thera or Mt. St. Helens. The low viscosity lava
produces a low angle, broad-based volcano, usually out
at sea. Seen in cross-section, they would sprawl out in
a shape resembling an ancient warrior's shield, hence
the name. The low angle makes them look low, but they
can attain amazing heights. Think of Hawai'i's Mauna
Kea: It's over 4 km above sea level (13,000 ft.), but
it rises from the sea floor 8 km (26,000 ft.) below sea
level!!!! It is taller than Everest, if you consider
its base on the sea floor!
![[ Mauna Kea seen from Mauna Loa, Hawaiian
Center for Volcanology ]](http://www.soest.hawaii.edu/GG/HCV/mkea-cr.gif)
b. Cinder cones are those formed from more felsic magmas,
which are silica enriched, viscous, gassy, and ...
explosive. They shoot lava into the air, where it cools
pretty instantly to form tephra, and falls around the
volcano's vent, building up a steep cone of ash and
tephra. Nasty little affairs. These are the classic
volcanoes most people think of when they do think about
them at all.
![[ Red Cones, Mammoth, C.D. Miller, 1982, USGS
]](http://volcanoes.usgs.gov/Imgs/Jpg/LongValley/03714277-098_med.JPG)
c. Stratocones or composite volcanoes are "undecided"
volcanoes: They alternate between explosive rhyolitic
phases and runny basaltic phases. A cinder cone forms
in an explosive era and is then cemented by a runny
eruption which consolidates the cinder materials and
protects them from rapid erosion. It may revert from
runny to explosive several times, which allows the
mountain to grow to truly impressive and scenic heights.
The most beautiful volcanoes on Earth are stratocones:
Shasta, Fuji, Kilimanjaro.
![[ Mt. Shasta, USGS, Lyn Topinka, 1984 ]](http://vulcan.wr.usgs.gov/Imgs/Jpg/Shasta/shasta.jpg)
e. Tsunamis are often produced by subduction-related earthquakes
and volcanic activity.
i. These are seismic shock waves in water.
ii. They are sometimes popularly called "tidal waves," but they
have nothing to do with tides, so that's a misleading name.
iii. These are long period, fast-traveling waves: They can hit
700 km/hr!!!
iv. When they encounter the shoals, where the seabed becomes
shallow as it approaches land (especially if it's a long,
gradual rise), the wave is forced to slow by interacting
with the sea bed. This increases its amplitude (because
it's still delivering the same amount of energy), and the
wave suddenly changes from a nearly imperceptible blip out
at sea (high speed, low amplitude) to a huge crest crashing
on the shore. The height of the wave at the shore can be
10-20 times its height out at sea. So, a 1 m wave out on
the open sea can be 10-20 m (say, 30-65 ft. or so) when it
crests.
v. Tsunamis are major killers, because they travel so fast and
it's hard to get warnings out to everyone in their paths
and get them evacuated in time (and in the 1964 Alaskan
earthquake tsunami, Californians actually ran down to the
beach to see "the big wave" -- natural selection in
action?).
vi. If you have a fast computer and a fast connection to the
Internet, you may enjoy opening an animation by Professor
Nobuo Shuto of the Disaster Control Research Center, Tohoku
University, Japan, which shows the the 1960 tsunami
generated by the 9.5 1960 Chile earthquake across the
Pacific. You can view this animation with the Quick Time
Movie Player by clicking here.
f. Subduction also creates oceanic trenches, the deepest places on
Earth.
i. These are the notches between the edge of one lithospheric
plate and the bent back of the subducted plate.
ii. The deepest such trench is the Marianas Trench (over 11 km
deep! It separates the Philippines Plate from the Pacific.
You could cut Everest (8.8 km tall) off at the base, turn
it upside down, and drop it in the Marianas Trench -- and
you would lose it!!!
iii. A few of the other trenches:
a. The Peru-Chile Trench is the longest one on Earth,
nearly 6,000 km long, running along the west coast of
South America (where the Nazca Plate is being
subducted).
b. The Puerto Rican Trench separates the Caribbean Plate
from the North American.
c. The Aleutian Trench is off the coast of Alaska,
separating the northern (Arctic) part of the North
American Plate from the Pacific Plate.
d. The Philippines Trench is between the Philippines Plate
and the Eurasian.
g. Subduction is also responsible for just the opposite of
trenching: Orogeny or mountain building.
i. Orogeny can be accomplished through vulcanism, but I
covered that earlier in discussing volcanic hazards.
ii. Orogeny can also be brought about by folding and faulting.
a. As long as oceans exist, their floors are coated with
pelagic sediments (dust and the remains of sea creatures
great and small) and with sediments derived from the
erosion of the continents. These sediments build up
great beds of marine sedimentary rock.
b. These materials do not subduct into the
æsthenosphere along with the basaltic lithospheric
slab, because they are less dense and more buoyant than
the basalt: They bunch up and eventually accrete onto
the edges of continental landmasses sliding down towards
the destructive zone.
c. Too, when plates converge and one or both has
continental crust on it, that material will also not be
subducted, again because it is too buoyant to be
subducted. It, too, bunches up and slivers and thrusts
up to build the thickness of the continental crust.
d. Rock can resist a certain amount of compression,
tension, and shear stress up to a point.
1. If it bends but then rebounds when the stress is
relieved, it is said to have undergone elastic
deformation.
2. If it bends but then no longer is capable of
straightening out, it is said to have undergone
plastic deformation. Plastic deformation can fold
rock in all kinds of ways and thereby build up
mountain ranges.
A. Rock layers bent upward form an anticline.
B. Rock layers bent downward form a syncline.
C. Anticlines and synclines normally alternate.
D. If the folds become truly extreme and the rock
bends back on itself, you have a recumbent fold.
![[ diagram of anticline and syncline, Devil's Punchbowl educational site ]](http://www.devils-punchbowl.com/images/synclineanticline2.gif)
![[ photograph of anticlines and synclines near Calico, California, Allen Glazner, Geosciences, University of North Carolina, Chapel Hill ]](http://www.geosci.unc.edu/faculty/glazner/Images/Structure/CalicoFolds.jpg)
E. You can even get situations where the anticlines
and synclines themselves dip into the ground in
the direction of their central axis or strike:
These are called plunging anticlines and synclines
![[ plunging anticlines and synclines, E. Mantei, SMSU ]](http://www.emporia.edu/earthsci/amber/students/almanza/plunge2.gif)
3. If rock fails under the stresses, there will be an
earthquake, and the result will be the production of
a fault or its extension or readjustment. This
photograph shows both folding (a syncline, an
anticline, AND a fault):
![[ anticlines and synclines and fault photograph, Geology and Geophysics, University of Wisconsin, Madison ]](http://www.geology.wisc.edu/courses/g112/Images/anti_syn.axes.gif)
A. Faults have dips and strikes and rakes.
I. The dip is the angle the fault makes with the
ground, dipping below the surface at
thus-and-such an angle.
II. The strike is the direction of a line the
fault plane makes by intersecting with the
ground. The fault may well be visible on the
surface as some sort of irregularity (a ridge,
a trench, a scarp or small cliff, or a distortion
in the path of streams that cross it), but it may
make absolutely no evident trace on the
ground if it's buried by stream deposition or
other such processes.
![[ strike, slip, and rake of a fault, ]](http://quakeinfo.ucsd.edu/~gabi/erth15/supps/fault.gif)
B. Faults largely responding to tensional (pulling)
stress typically take the form of normal faults,
where one block, the hanging wall, moves down the
dip, sliding down over the other block below, the
foot wall. If this is common in a region, it can
produce a landscape alternating between fault
block mountains (horsts) and grabens, or sunken
valleys between them. A good example of a horst
and graben system is the Death Valley graben and
the Funeral horst to its east and the Panamint
horst to its west.
![[ normal fault, USGS ]](http://geology.wr.usgs.gov/docs/parks/deform/normfaultLABEL.gif)
C. Faults largely responding to compressional
(squeezing) stresses often form reverse faults,
where the hanging wall (the upper block) moves UP
the dip. In extreme cases, you can get a thrust
fault, where the hanging wall is completely
squeezed out on top of the footwall.
![[ reverse fault, USGS ]](http://geology.wr.usgs.gov/docs/parks/deform/reversefaultLABEL.gif)
D. Shear stress produces strike slip faults (like the
transform faults that cross the mid-oceanic ridges
in constructive zones and the San Andreas Fault,
about which more later). Movement is horizontal
along the trace of the fault plane with the
surface.
![[ strike-slip fault, USGS ]](http://geology.wr.usgs.gov/docs/parks/deform/strikeslip.gif)
I. If you are standing on one side of a strike-
slip fault and see the landscape on the other
side has been displaced to your right after
an earthquake, you are looking at a "right-
lateral" fault.
II. If the landscape on the other side is shifted
to your left, you're at a "left-lateral"
fault.
III. California is dominated by right-lateral
faults, including the San Andreas; one rare
example of a left-lateral fault is the
Garlock Fault, which forms the southernmost
boundary of the Sierra Nevada (the part
called the Tehachapi Mountains) and the
Antelope Valley up in the Mojave Desert.
E. Of course, in the "real world," what we usually
see is faults and earthquakes that show some
aspects of more than one "pure" fault type: You
might see lateral motion along a strike-slip fault
that also has some vertical displacement, too.
F. The dominant fault type in a destructive plate
zone boundary, though, is the reverse fault (and
its thrust variants) because of the extreme
compressional forces associated with subduction.
G. There can be normal faulting in the lithospheric
plate above a subducted slab, though, as a
subducted plate can actually create tensional
forces by bulging a wide area of the overlying
plate. For example, the basin-and-range
topography of the American West between the
Sierras/Cascades and the Rocky Mountains
(alternating horsts and grabens) is being produced
by the wide tensional forces associated with the
burial of the Farallon Plate under the American
West (remember that buried major plate that
remains on the surface only in the Juan de Fuca
platelets and the Cocos minor plate and the Rivera
platelet?)
3. Conservative zones are still other boundaries between adjacent
plates in which the dominant relative motion of the two plates is
lateral and the dominant stress exerting force on it is shear.
a. Averaged out over the entire boundary, the lithosphere is
neither created nor destroyed here, hence it is "conservative"
of matter, leading to the catchy name.
b. A transform fault or strike-slip system divides the two plates
along a conservative contact. So motion is mostly lateral,
similar to the motion seen along the transform faults that cut
perpendicularly across a constructive zone's rift zone.
c. Conservative zones are nearly as much "fun" as a destructive
zone, because earthquakes are frequent and can be quite strong.
i. In the section on destructive zones, I explained fault
morphology (normal and reverse dip-slip faults and strike-
slip faults) and I mentioned epicenters and foci. Since
earthquakes are so common in conservative zones, I might as
well elaborate a bit more on quakes.
ii. The focus of an earthquake is the actual area where the
rock fails, producing or moving along a fault plane and
creating displacement vertically or horizontally or both.
The epicenter is the area on the surface directly above the
focus.
![[ epicenter and focus, USGS ]](http://geology.wr.usgs.gov/docs/parks/deform/eqepifoc297x164.gif)
iii. Earthquake strength can be measured either as magnitude or
intensity.
a. Magnitude measures the actual energy released released
during an earthquake. There are several ways of
measuring magnitude, but the two most commonly
encountered are the local magnitude (or Richter scale)
and the moment magnitude (a more comprehensive scale
that roughly parallels the Richter).
1. The local magnitude (ML) scale was devised
by Charles Richter of Caltech back in 1935. It
involves measuring the amplitude of earthquake waves
recorded on a seismograph. Amplitude is the height of
a wave crest or the depth of a wave trough, oh, heck,
here's a picture:
![[ transverse wave characteristics ]](http://astro.uchicago.edu/cara/outreach/se/ysi/1999/diagram.jpg)
Anyhow, Richter measured the largest amplitude wave
on a Wood-Anderson seismograph in millimeters, took
its logarithm, and then factored in distance from the
focus (which has to do with the difference in arrival
times between the primary and the secondary wave
fronts) to create his scale. Each whole number
difference reflects a difference in amplitude of 10
times the amplitude of the next lower number, and the
difference in actual energy release is about 31 or 32
times!
![[ Richter scale calculation, J. Louie,
University of Nevada Reno, 1996 ]](http://www.seismo.unr.edu/ftp/pub/louie/class/100/richter-scale.GIF)
2. There are several problems with this quick 'n' dirty
approach, though, which makes the Richter system max
out around 8 or so, when there is quite a variation
in actual energy release among great earthquakes that
is not reflected in the ML scale. So, all
sorts of magnitude scales have been devised for
particular situations, but the most comprehensive and
intellectually satisfying one is the Moment Magnitude
scale (Mw), which measures magnitude as a
function of the rigidity or shear strength of the
rocks involved, the area of the fault plane that
moved (reflected in the pattern of aftershocks), and
the average displacement along that plane. While
this system of measuring magnitude correlates with
the Richter scale and other ground-motion based
magnitude scales up to about M=7 or 8, it has no
upper limit and can accurately represent truly
monster quakes.
b. Earthquake strength can also be expressed as intensity,
which measures an earthquake's severity in terms of its
effect on people and the built and physical
environments. The measurement system used is the
Modified Mercalli Intensity Scale.
1. It varies from I (generally not felt) to XII (total
destruction). For the whole scale, you can visit
this link.
2. Variations in it can be mapped for a single
earthquake (magnitude is aspatial: It describes the
energy released at the focus not its effects away
from the epicenter, so there are no variations for a
given quake to map). You can see one created for the
Loma Prieta quake of 1989 by visiting this
link.
iv. Earthquakes generate several types of wave motion.
a. Body waves move through the body of the earth. There are
two types:
1. Primary waves are comPressional waves, those in which
the motion of individual rock molecules is back-and-
forth along the direction the wave is traveling.
These are the fastest-moving waves and, so, they
arrive at any seismometer first (primary).
2. Secondary waves are Shear waves. Molecular motion is
back-and-forth at right angles to the path the waves
are moving. These waves travel more slowly than
primary waves and, so, they arrive at a seismometer
later than the primary waves do.
A. The difference in arrival times tells you how far
the focus of the earthquake is. Remember that was
how Richter corrected his local magnitude scale
for distance in the graph above.
B. These are the waves that cannot pass through
liquids and the seismic shadow on the other side
of the earth from a quake, where no secondary
waves are recorded, tells us the outer core of the
earth is liquid (see lecture 28).
![[ primary and secondary waves, Exploratorium ]](http://www.exploratorium.edu/faultline/basics/images/pswaves_lrg.gif)
b. Surface waves travel along surfaces, including the
surface of the earth and some discontinuities in the
crust. They travel more slowly than either body wave
and their amplitude is such that they do a lot of damage
on the surface. There are a couple of these, too:
1. Love waves are horizontal along the surface, the rock
molecules moving back-and-forth along the surface at
right angles to the path the wave is travelling.
2. Rayleigh waves are like waves in the ocean: Rock
molecules move in a circular pattern like a Ferris
wheel, the motion at the top of the cycle pointing in
the direction the wave itself is coming from.
![[ Love and Rayleigh waves, Exploratorium ]](http://www.exploratorium.edu/faultline/basics/images/rayleighlove_lrg.gif)
c. These waves travel at constant speed ratios to one
another, with primary waves always being the fastest
ones, but all waves are affected by the materials
through which they travel and change their speeds (while
preserving the ratios among their speeds).
1. They all go faster in more rigid, well-consolidated
materials (e.g., solid granite) and slower in less
consolidated materials (e.g., loose sand or clay).
2. They go faster in uniform materials than they do in
mixed materials.
3. They all go faster in denser materials.
d. There is a nasty trade-off between speed and amplitude,
however. As with all waves, they are carrying a given
amount of energy, which sets their speed, frequency, and
amplitude. Slowing down necessarily means an increase
in amplitude to carry the same amount of energy. So,
that is why earthquakes aren't as violent in solid,
dense, uniform rock materials as they are in
unconsolidated, mixed, low-density alluvium (river
deposited materials) in valleys. The alluvial materials
are subject to liquefaction in a violent quake
(vibrating so much that the soil grains lose all bonds
with one another -- and all material strength -- so that
they can't support buildings). On the other hand,
though, homes on hills are vulnerable to landslides
during a quake.
d. Now, while earthquakes are a very significant hazard in a
conservative zone, much as in a destructive zone, vulcanism is
not as common nor is it as violent in conservative zones as it
is in destructive zones. While there may be local small areas
of subduction and divergence here and there in a conservative
zone because of irregularities in the transform fault system
that defines it, this is never of such a large scale as to
produce a serious volcanic hazard. So much for that movie,
"Volcano," set in Los Angeles!
e. Our San Andreas system is an example of such a zone. The San
Andreas Fault separates the North American Plate (southeast
motion in this area) from the Pacific Plate (northwest motion in
this area). So, despite California's trendy culture, we really
are in a conservative zone (geologically)!
4. At the average speed of continental drift (global positioning
satellite (GPS) measurements show speeds of 1-15 cm/year), the
entire lithosphere should get recycled roughly every 200,000,000
years or so.
a. This is supported by the fact that ocean floors have nowhere
been dated much older than 200,000,000 years, showing mafic
ocean crust does indeed recycle. You can see this for yourself
by revisiting the map of ocean floor ages above. See the blue
area on the map farthest from the mid-oceanic rifts? Check out
its age on the map's legend.
b. But the continental crust has been securely dated in some areas
as old as 3.96 BILLION years old (in eastern Canada) and there
are some zircons in younger rock that have been dated back to
4.1-4.2 billion years. This at first seems paradoxical, until
you remember the light, felsic nature of the topmost continental
crustal layer. Its lightness keeps a continents' rock matter on
top, not subducted and renewed.
c. In this manner, a relatively stable, thick craton has been built
as the core or skeleton of each continent. Cratons are ancient,
vast areas of metamorphosed rock.
i. At the surface, a craton that has been eroded smooth in the
course of hundreds of millions or billions of years of
relative stability is referred to as a shield.
ii. A platform is a part of a craton, which is buried by
sedimentary rocks, so you can't see the crystalline
basement rocks right on the surface.
d. The edges of the craton accumulate terranes, the flotsam and
jetsam of the lithosphere, and, thus, the continents are much
larger than the cratons at their cores.
e. A particular episode of subduction comes to an end whenever two
continental landmasses are brought together at a convergent
zone, because neither can be subducted. This is probably how
the cratons were built up and why they're relatively free of
earthquakes and vulcanism now. India isn't going to get much
farther into Eurasia, and so we may be seeing the beginning of
what might be a stable craton a couple hundred million years
from now!
5. Quickie history of plate tectonics and continental drift.
a. Back around 600 million years ago, all the landmasses were
together near the South Pole region, forming a huge
supercontinent called "Rodinia." There was one great ocean, the
Panthalassic Ocean, to its north.
b. The supercontinent began breaking up into a number of pieces,
largely concentrated in the Southern Hemisphere, about 530
million years ago.
c. These had sort of clumped into two large groups by 430 million
years ago, with a new ocean between them, the "Caledonian
Ocean."
d. By 300 million years ago, these two had clumped up together
again, to form another supercontinent, "Pangæa,"
concentrated longitudinally roughly about the Prime Meridian,
its bulk in the Southern Hemisphere but with a significant
portion well into the Northern Hemisphere. A new ocean, the
Tethys Ocean, was opening up like a pie slice on the eastern
side.
e. Around 150 million years ago, Pangæa had begun breaking
into two huge continents, as the mouth of the Tethys Sea opened
up westward.
i. The northern great continent was "Laurasia" (named for the
Laurentian Shield, the exposed part of the North
American craton regions, and Eurasia). It included
the land that would become North America, Eurasia north of
the Alpine system (e.g., Alps and Himalayas), and
Greenland.
ii. The southern great continent was Gondwana (named for
ancient rock formations in the Gondwana District of India,
where an Austrian geologist named Edward Suess began to
suspect that Africa, India, South America, Antarctica, and
Australia once belonged to a single supercontinent he
called Gondwanaland after these rock formations that
resembled those on the other continents).
f. By 100 million years ago, Laurasia and Gondwana began to break
apart with the formation of the Mid-Atlantic Ridge system that
would eventually be covered by the young Atlantic Ocean. India
broke off around this time and started moving toward Eurasia,
pushed by the rifts opened in the young Indian Ocean.
Antarctica, Africa, and Australia were pushing apart by then,
too.
g. By 20 million years ago, India had smacked into Eurasia, wiping
out the Tethys Sea in between and forming the Himalayas and
Tibetan Plateau, and the other continents were fully
recognizable as the modern continents.
i. You can read about this in more detail and view world maps of
the process by clicking here.
6. From looking at this history and perhaps reviewing the maps on the
linked page (5.i), you may suspect that, on a grand enough scale,
we're looking at a cyclical process, and you would be right.
Continents diverge and converge, oceans form in rift zones and
expand as the continents diverge and then shrink out of existence
as the continents converge. This idea is called the "Wilson
Cycle." In ideal form, it looks like the image below, and you can
learn about it in more detail if you like by clicking here, and this
page can lead you to an even more detailed explanation.
![[ Wilson Cycle, L. Fichter, Geol & Env Sci, James Madison U ]](http://csmres.jmu.edu/geollab/Fichter/Wilson/rockcycl.gif)
And that's about it for tectonic plate boundary types. Make sure you know the
three boundary types (constructive, destructive, and conservative) and the
relative motion of adjacent plates in each (divergent, convergent, and
shearing, respectively). Be sure of the prominent features of each of these
three zones, including the nature, location, frequency, and magnitude of
associated hazards (volcanoes, earthquakes, tsunami).
Know the different shapes of volcanoes and the kinds of magmas that typically
produce them. Remember the landforms that these volcanoes can produce out at
sea.
Make sure you recognize the different types of deformation that rocks can
undergo. Remember the names of the folded structures that plastic deformation
can produce in sedimentary rock, as well as the different types of fault that
rock failure can produce.
Know the difference between epicenter and focus (sometimes called
"hypocenter") and between magnitude and intensity. Know the difference betwee
local magnitude (Richter scale) and moment magnitude and their relative
advantages.
Be able to describe the four main types of seismic waves in terms of their
media (body or surface), their relative speeds, and how a slow-down in their
speeds affects their amplitude and why that is important in understanding the
distribution of damages from a given earthquake.
Be able to explain why ocean floor rocks are generally younger than
200,000,000 years, while continental rocks in cratons can be billions of years
old. Have a general idea of the history of Earth's continents and oceans over
the last 600,000,000 years and how that relates to the Wilson Cycle.
Document and © maintained by Dr.
Rodrigue
First placed on web: 11/06/00
Last revised: 07/05/07