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. 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.

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.

Brittle Mantle

Plastic Mantle

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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.

Brittle Mantle

Plastic Mantle

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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.

Ridge

Rift

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.

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.

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.

Later …

Image from Google Maps

Mt. Kenya, Kenya Kilimanjaro, Tanzania

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.

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.

Image from Google Maps

20 km

Convergent Margins

Volcanic arc

Trench

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.

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.

NEW TRENCH!

Larger, compressionally deformed island arc.

Ophiolites

Back-Arc Basin?

SPOILER ALERT: We will come back to this process later in the term. For now, think about the consequences. 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”? 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.

Ophiolites

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.

Everest

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?

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.