Introduction to Plate tectonics
So much material on this is common to the NCEA geology courses, I have decided to put them all in one place.

This assumes you have already had some introduction to ther rock cycle. If not, please review it here

Please note that this is still a work in progress as at the beginning of Term 4 2009.

1. Tectonic Plates

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click for original (slightly modified)

The outer layers of the earth (the crust and upper mantle, or the lithosphere) is divided into a number of more-or-less rigid structures called tectonic plates. There are about 15 of these plates and they move in various directions as indicated on the map on the right (slightly modified from Wikipedia).

Plates incude the crust and an upper part of the mantle. Together these parts of the Earth are called the lithosphere. They differ from the lower parts of the Earth because they are solid.
Under the lithosphere is the asthenosphere, which is 'plastic' enough to allow plate movement (although it is not a true liquid like magma). It is convection in the asthenosphere (see diagram below) which drives tectonic movement.

Crust
There are two sorts of crust:
Continental crust is about 30 km thick, and if you melted it all and then re-solidified it it would have a composition between a granite and a diorite (or a rhyolite and an andesite). Indeed, a lot of continental crust is granite or sedimentary rocks formed from eroded granite.
peridotite.jpg
Peridotite (green) in basalt
Oceanic crust is much thinner, about 5 km. It is made almost entirely of basalt and gabbro, so is denser than continental crust.
Mantle: The mantle is different from the crust because of its mineral makeup. The mantle is made of a dense green rock called peridotite. Sometimes bits of mantle come up to the surface. For example, near Nelson a mountain called Mt Dun is made of such rock, and it has been given a special name - dunite (this is a world-famous rock). Both the St Heliers and Taylor Hill volcanoes near Sacred Heart contain bits of mantle that were brought up by the basalt magma on its way up - you will see them as olive-green inclusions in the rock, similar to that on the right.
Basalt and andesite are both formed by melting of mantle material. Basalt is formed by dry melting when an increase in temperature or a decrease in pressure force the mantle to partly melt. Andesite is formed when water is mixed with the mantle; the water comes from subducted oceanic crust where it is released when the increase in heat and pressure cause minerals to change.

Convection Currents and sea-floor spreading

Oceanic_spreading.png
Wikipedia: click to link to original
Below the lithosphere, the part of the mantle called the asthenosphere is subject to a very slow movement ( a few cm per year) called plastic flow. Because it is hotter at the bottom than at the top, it sets up convection currents as shown in the diagram to the left.
(In three dimensions, the convection cells actually form approximate hexagons and pentagons. A visualisation of that is here .)

Above the rising asthenosphere, the lithosphere is pulled apart. This causes deep faults and pressure release. Both processes trigger partial melting in the mantle, forming basalt magma. This rises, some of it is erupted to form basalt pillow lavas and related structures. More solidifies below the surface as gabbro and similar rocks. Together, this material becomes oceanic crust.
The newly formed crust, and its attached lithospheric mantle, is pulled away from the place of eruption by drag from the asthenospheric convection (called 'slab pull'). As it moves away, it cools and shrinks. This means that the centre, above the rising convection, is raised because of heating and expansion. It forms a mid-ocean ridge. In the centre of the ridge is a rift valley, formed by forces pulling apart the crust.

Sea-floor striping

Oceanic_magnetic_striping_example.gifThe oceanic crust is younger closer to the ridge and older further away, in a symmetrical pattern on either side of the rift. As it cools down, it records the direction of the Earth's magnetic field at the time of formatio. The Earth's field reverses from time to time, resultng in a pattern of magnetic 'stripes' either side of the ridge as shown in the diagram to the right. This pattern greatly surprised geologists when it was first discovered, and when the hypothesis about how it was formed was put forward it formed some of the most powerful evidence for the then-new theory of plate tectonics. Geologists could carefully count the stripes all around the world, and the stripe counts and age of the basalts as determined by radiometric dating (potassium-argon or K-Ar dating) matched in many different places. This is something that would be very difficult to explain in any other way. The ability of a theory to make predictions like this is strong evidence that the theory is correct (something the Young Earth Creationists conveniently ignore).



How a new plate gets started: a new zone of rising asthenosphere can form below a continent (this is because the continental crust acts like a blanket, holding in heat and warming the mantle below it). When this happens, the continent starts to break up. The pull apart forces create a rift valley, such as the East Africa Rift. After a few million years, this widens into a narrow sea such as the Red Sea. As the sea further widens, a mid ocean ridge forms. This is how New Zealand broke off from Gondwana about 80 million years ago.
Places where the rising asthenosphere is pulling apart tectonic plates are called divergent plate boundaries.

2. Subduction

2. Subduction

When plate boundaries push together (converge), one will slide under the other in a process called subduction. The place where this occurs is called a convergent margin. Either the oceanic plate can subduct below the continent as below, or it can subduct below another oceanic plate.
convergent_margin.png
Plate boundary beneath Central North Island


  • oceanic crust will always subduct beneath continental crust (because it is denser). This forms a continental arc.
  • if oceanic crust meets oceanic, one will subduct if it is denser and heaver. This forms an island arc (below left)

islandarc.png
Island arc (Kermadecs)


n New Zealand, there is a continental arc from Mt Ruapehu to White Island, and an island arc from there to Tonga. There are some transitional effects off the Bay of Plenty.

In a subduction zone
  • There is always a trench, where the oceanic lithosphere starts to sink.
  • Near land, the trench fills up with sediment e.g. Hikurangi trench off the North Island East Coast.
  • Most of the sediment in the trench can't be subducted. It gets pushed back up onto land by plate motion.
    This forms an accretionary prism. Since more is pushed onto land all the time, sediment is older further inland from the trench.
  • Above some particular places where the subducting crust releases water, you get volcanoes, which are usually composed of andesite or similar rocks.

In New Zealand, there is evidence of some 'rifting' behind the arc ('back arc rifting') as seen in the first subduction diagram. Out at sea, this has produced the Havre Trough. On land, it has produce the Taupo Volcanic Zone. However, most of the rift valley is full up of volcanic material and only in places is it easy to see (e.g. Waikite Valley).

For some reason, this rifting has produced large caldera volcanoes such as Lake Taupo and Lake Rotorua. Yellowstone Park in the USA is another place where this is happening.
Out at sea, you can clearly see the effects of the rifting but there is much less volcanism in the Havre Trough than in the Taupo Volcanic Zone, possibly due to differences in rock type. in the crust

Secondary convection:
Sometimes the subducting lithosphere can set up a small secondary convection cell above it. This can cause rifting, and can trigger melting to form basalt magma. The Taupo Volcanic Zone is thought to be such a rift (back-arc rift), and the rhyolite volcanism there is thought to be the result of huge volumes of hot basalt magma piling up at the bottom of the crust and triggering crustal melting. Not all subduction zones have back-arc rifts, and not all such rifts have associated volcanism.


The diagram above shows the different parts of the plate tectonic model. Plate tectonics are responsible for the formation and growth of continents, for orogenies and marine transgressions.

1. Convergent boundary: At a convergent plate boundary, material is erupted (to form volcanic rocks), intruded (to form plutonic rocks) or deformed by faults and folds, and buried. Such material is too low in density to be subducted again, and so tends to accumulate at these plate boundaries and is later uplifted. You can see in both of the trenches shown above that sediment accumulates. .If the convergent boundary is along the edge of a continent material eroded and carried into the sea gets pushed up onto land, forming successively younger zones as one approaches the plate boundary. This is termed accretion, and the place where it happens is an accretionary prism. The East Coast of the North Island is an example.
2. Continental Rift Zone: On the far right of the diagram above is a continental rift zone. This is how the split between NZ and Gondwana would have begun (about 80 my ago). The great East Aftrican Rift Valley is a modern example. Sometimes, as when we split from Gondwana, they widen to become an ocean spreading ridge, shown in the middle of the diagram. Some of the volcanic activity between Rotorua and Taupo is probably due to rifting, but this rift is unlikely to develop further (see Level 3 notes for more detail on this).
3. Continental passive margin: this is not shown on the picture. This is where a continent gives way to an ocean, with no convergent margin. These are often slowly wearing away and subsiding, and are characterized by extensive river deltas, plains, swamps and coral reefs. The south-east coast of the USA and much of the coast of Australia good examples. Later in the tectonic cycle, these will form siltstones, coal measures and limestones (this sequence is characteristic of a marine transgression). Such deposits are found in NZ from 40-22 million years ago, so we can infer that the tectonic environment was like this. For example, on the Gold Coast of Australia, the rivers and swamps will form siltstones and coals, and the Barrier Reef will someday become limestone. As the area sinks beneath the sea, the reef will move landward therefore the limestone will overly the silt and coal.
4. Island arc: this is another type of convergent boundary. Island arcs can eventually be caught up by plate movement and pushed against a continent. There are old volcanic rocks in Southland and Nelson which are island arc volcanics, and the Dun Mountain belt shown in the map below was probably caused when this collided with the developing continent. This is why these volcanics and intrusives lie between the eastern and western blocks of the Rangitata depositional phase. The Aleutian Islands off Alaska represent a modern example of an island arc that lies between a continent and some of its depositional area.