Download The Plate Tectonic Model

The Plate Tectonic Model
What to Look For:
• Plate margins can be places where the plates diverge, converge, or slip
laterally past each other.
• Most divergent margins have oceanic crust on either side (oceanic ridges),
but very young ones might involve a continent. These quickly evolve into
oceanic ridges.
• Transform margins form as a consequence of diverging plates. Therefore
most are found crossing oceanic ridges. Occasionally a continent drifts
onto one and becomes involved.
• Divergent margins are the most diverse and complex. They can involve
either oceanic or continental crust or both and are the root cause for the
formation of many different types of rocks and structures.
• The continents were formed by the long and repetitive activity of
convergent margins.
Though it should be pretty obvious by now we should formally outline the mechanics of
plates movement.
This will allow us to summarize the consequences of those movements that we’ve used as
“evidence” for plate movements.
Brittle Mantle
Asthenosphere
Low Velocity Zone – Mantle is plastic
from here to some uncertain depth.
Plastic Mantle
Remember the
basics of Earth
structure that
we studied in
Geol. I when we
talked about
earthquakes.
We’re only
worried about
shallow structure
now.
Lithosphere
Crust -- ~ 5km thick under oceans, variable, but averaging 20-25km thick under continents.
??
Brittle Mantle
Lithosphere
Crust -- ~ 5km thick under oceans, variable, but averaging 20-25km thick under continents.
Low Velocity Zone – Mantle is plastic
from here to some uncertain depth.
Asthenosphere
Plastic Mantle
??
??
Though we sometimes refer
to them as “crustal plates” it
is not technically the crust
that moves. It is, instead, the
lithosphere, and we should
properly call them
“lithospheric plates”.
These brittle pieces of rock
are pretty consistently about
100km (60 miles) thick –
roughly the distance from
Americus to Columbus. The
plate then comprises both
the crust (in its entirety) and
the upper part of the mantle.
Indeed, the plates are mostly
mantle.
Below this is a zone that
slows earthquake waves
because the mantle here is
plastic rather than brittle.
This marks the top of the
asthenosphere which is also
an important player in the
plate tectonic system. How
deep the mantle behaves
plastically is a matter of
considerable debate.
A 100 km brittle shell on a 12,756 km Earth means that its thickness is only about 0.78%
the diameter of Earth.
The average thickness of a chicken eggshell is about 0.3mm, and the average diameter of
a large egg is about 40mm (across the smaller circumference). This means that the shell
is about 0.75% as thick as the egg is big, or roughly the same proportion as the
lithosphere is to the Earth’s diameter.
Instead of being reinforced by a tough organic membrane attached to its base, the
lithosphere sits directly on the asthenosphere -- a substance that is capable of slow flow.
Breaking into an egg is therefore theoretically somewhat harder than it would be to break
into an Earth scaled down to the same size!
Not surprisingly then, with a mobile asthenosphere beneath it, the lithosphere is broken
into pieces. These are what we call plates, though, unlike dinner plates, photographic
plates, or plate glass, they are not flat. They are curved, like the pieces of a broken
eggshell are curved.
For reasons we will get into next, these pieces are prone to movement on the
asthenosphere beneath them, and there is no reason for them to move in the same
direction. As they move in different directions, their edges (or margins) can interact in
some predictable ways. (Remember that they do not “slide” across a flat surface, they
“rotate” on a spherical one.)
Divergent Margins
The three basic types of margins are these:
The Sub-Types of Plate Margins Refer to the
Types of Crust Involved
• Ocean/Ocean margins have oceanic crust on either side.
• Ocean/Continental margins have an oceanic plate on one side
and a continental piece on the other.
• Continental/Continental margins have continent on both
sides.
Almost all divergent margins are Oceanic/Oceanic – i.e., they are ridges. Any ridge
will serve as an example: M.A.R., E.P.R., etc.
Ridge
Rift
EXPECT:
- shallow focus earthquakes
- normal faults
- mafic igneous rocks
- no metamorphism (certainly
not regional)
- no sedimentary rocks, or thin
pelagic ones
Some few continental rift zones exist on Earth, but most are “extinct” – not actively rifting any more.
The East African Rift is an exception. Its walls are normal fault scarps and it has abundant mafic rocks
and volcanoes, such as Mts Kenya and Kilimanjaro.
Later …
Ridge
Rift
The reason that active continental examples are uncommon is that they evolve into oceanic ridges in
pretty short order, if they remain active. The Red Sea and Gulf of Aden Rifts have already done so. The
M.A.R. began as a continental rift of western Pangaea.
In addition to the two biggies you should be able to spot many other small mafic
volcanoes in the photograph. Lake Victoria is to the left.
Mt. Kenya,
Kenya
Kilimanjaro,
Tanzania
Image from Google Maps
Oceanic/Continental divergent margins are rare, if they exist at all.
There is probably no theoretical reason a rift could not initiate immediately beside a
continent, though it should be easier to accumulate the necessary heat in the mantle
beneath the continent, where the insulating ability of the continent comes into play.
This heat is what uplifts the rift area and initiates the crustal fractures that become
the bounding faults.
There are no modern examples of this type of margin. Furthermore, no convincing
ancient ones exist either. In any event, if a rift was ever initiated at a continental
margin its future would be exactly the same as if it originated within the continent – it
would rapidly evolve into a ridge by creating new basaltic crust on either side.
Transform Margins
Because the transform margins are simply a mechanism for managing differential spreading rates, they are
always associated with spreading centers – that is, oceanic ridges. They cross the ridges at roughly right
angles.
They only occur in continental crust during the initial stages of continental rifting or if a continent has
drifted onto one. The one remaining example on Earth is the San Andreas Fault in California and Mexico,
where the continent has drifted over the Pacific rift system, now called the East Pacific Rise and the
Gorda/Juan de Fuca Ridge system.
EXPECT:
- shallow focus earthquakes
- strike-slip faults
- no igneous rocks (except that
the crust is basalt)
- dynamic metamorphism
- no sedimentary rocks (or thin
pelagic ones)
The trace of the San Andreas Fault is easy to spot from near Hollister southeastwards.
There are many additional related faults in the area too.
20 km
Image from Google Maps
Convergent Margins
Oceanic/Oceanic convergent margins are common, but they are not the dominant type by any means. They
invariably produce a trench where one plate subducts beneath the other (and pulls its edge downward),
and a volcanic arc, fed by the melting of the lower plate in the hot mantle.
Volcanic arc
Trench
EXPECT:
- earthquake foci in a Benioff zone
- reverse faults
- folds
- intermediate and felsic igneous rocks
- regional, contact, and dynamic
metamorphism
- lots of sedimentary rocks of complex
composition (e.g., lithic sands) but
little limestone.
If there is another island on the subducting plate it
will approach the trench as time goes by.
When it reaches the trench it will not be able to
subduct because it is too thick and its density is too
low.
That trench will no longer be functional because both
sides will be “continental” crust, so something has to
give somewhere else.
SPOILER ALERT: We will come back to this process
later in the term. For now, think about the
consequences.
Larger, compressionally
deformed island arc.
We now have a bigger landmass. Each time this
happens that landmass grows even bigger.
At what point does it stop being an “island” and
start being a “continent”?
NEW TRENCH!
Ophiolites
Back-Arc
Basin?
The present continents were built up in exactly this
way during the Archaean Eon of Earth History.
Initially there were not even island arcs – these
were constructed by subduction even earlier, then
aggregated until, by the end of the Archaean, the
cores of the modern continental cratons were in
existence.
We will look at the evidence for these statements
in the last part of the course. Remember, science
never buys into anything without evidence, no
matter how much sense it makes a priori.
Something similar would occur if an island-arc
collided with a continent. Much of the Georgia
Piedmont is metamorphosed island-arc volcanics
and sediments.
An Oceanic/Continental plate
margin has the volcanic arc
immediately on the edge of the
continent. Remember that this
is called an Andean continental
margin.
A Continent/Continent convergent margin builds
the largest of all mountains. Thrust faulting and
folding continues until the mountains can go no
higher, then a new trench has to form “behind”
one or both of the continents.
Ophiolites
The Himalaya rise abruptly from the Indian Plains (to the south in the image) to as much as 8,848m
(29,029‘) above sea level at Mt Everest. Most or all of the lineations to the north of the highest
(snowy) peaks are probably fault scarps (or fault-line scarps). What kind of faults should they be?
Everest
50 km
The Himalaya are very young mountains – still rising. If you look on Google Maps at the Urals or the Appalachians, both much older
continental-collision mountains the compressional structure should be very obvious as you zoom in on them.
Take-Home Message:
• Plate margins can be places where the plates diverge, converge, or slip
laterally past each other.
• Most divergent margins have oceanic crust on either side (oceanic ridges),
but very young ones might involve a continent. These quickly evolve into
oceanic ridges.
• Transform margins form as a consequence of diverging plates. Therefore
most are found crossing oceanic ridges. Occasionally a continent drifts
onto one and becomes involved.
• Divergent margins are the most diverse and complex. They can involve
either oceanic or continental crust or both and are the root cause for the
formation of many different types of rocks and structures.
• The continents were formed by the long and repetitive activity of
convergent margins.