This comes from the U.S. Geological Survey – a fantastic site!!! also likely to be a good reference point for accurate information since there seems to be a bit of uncertainty about some things (like when looking for a definition for plate tectonics I found a site that said plate tectonics is the process of sea floor spreading… think it should be more like sea floor spreading occurs due to plate tectonics maybe… )

from http://pubs.usgs.gov/gip/dynamic/historical.html#anchor9464740

In geologic terms, a plate is a large, rigid slab of solid rock. The word tectonics comes from the Greek root “to build.” Putting these two words together, we get the term plate tectonics, which refers to how the Earth’s surface is built of plates. The theory of plate tectonics states that the Earth’s outermost layer is fragmented into a dozen or more large and small plates that are moving relative to one another as they ride atop hotter, more mobile material.

When this text mentions outer layer, it is important to note that they are talking about the lithosphere rather than just the crust – when they say earth’s surface to me I think of the crust – no doubt middle years students would too if they have just learned about the egg thing.  

So the theory is the earth’s lithosphere is made up of plates that fit in or join together.  There are 7 main plates and 7 smaller plates (and some itty bitty ones we wont go into…) – most, aside from the pacific plate, contain some land and ocean.  The plates are sitting on top of the very hot semi-solid asthenosphere – a part of the earth’s mantle that lies directly underneath the lithosphere.  Because of the semi solid state of the asthenosphere, plates can move/ float slowly over its surface.   

Here-in lies another discrepancy in on-line info… the asthenospere has been described as anything from solid to liquid, depending on what source you read…

In looking for piccies of tectonic plates I came across loads of maps… but almost none of them had NZ in them, and if they did they didn’t have the plate boundaries lying over the top of the south island… and I KNOW they do lie over the south island.

Good old wikipedia has come good though

So the plates:

Main ones: Australian plate, Pacific plate, Antarctic plate (largest), African plate, South American plate, North American plate, Eurasian plate

Little ones: Nazca plate, Scotia plate, Indian plate, Caribbean plate, Arabian plate, Philippine plate, Juan de Fuca plate

Notable things for Australia – we sit smack bang in the middle of a plate (no lines running through us…) and NZ sits in the middle of the joins of the Australian plate and the Pacific plate – with the joins running through the middle of the south island.

Where the plates join we have “fault lines”.  These are the regions where the majority of earthquakes, volcanoes and growing mountains are, and they occur because of the way the plates edges interact with each other (more on this in a minute).

Tectonic plates are constantly in motion because of convection currents that occur within the mantle.  The convection current in the mantle is where extremely hot molten rock from the mantle sitting close to the core rises because it is less dense that the cooler more solid layers above (liquids are less dense than solids so they rise).  In return, the cooler denser semisolid mantle close to the lithosphere falls down to the core due to gravity (more dense things sink due to gravity…).  This heat cycling occurs on a constant basis.

Wow – I know about convection from physics!!! how clever… So… there is a possibility students have done global warming stuff or have talked about convection in other classes.  They would likely be familiar with the concept but may need help remembering and translating it to the mantle…

It would be good to look at a picture explaining convection currents… it is easy to pick up from this picture…

from http://www.ucmp.berkeley.edu/education/dynamic/session1/sess1_earthcurrents.html

So the plates, as you can see by the upper layer in the picture (denoting the lithosphere), get pulled a little across the surface of the earth depending on the direction of the convection currents.  Plates get pulled an average of 2-10cm per year and this is why continents are moving.

Because the convection current causes movement in different directions, plate edges interact with each other.  There are 3 ways the plates may interact with each other – by being squished together, by being pulled away from each other, or by being pushed sideways across each other.  This causes boundaries that are CONVERGENT (pushed together), DIVERGENT (pulled apart) or TRANSFORM (pushed sideways).

Convergent boundaries (Con comes from a Latin word meaning together.  Think about Convergence Collides) – these are also called destructive boundaries.

At convergent boundaries the plates literally collide with each other – this can be a bit destructive so this is also sometimes called a destructive boundary.  If you think about pushing two sheets of paper together quickly what happens? usually you will have some push up on one side and then one layer will slip underneath the other – this is also what happens with plates so makes sense… I think a visual representation would certainly be needed to establish that in some student brains…

picture from http://www.plate-tectonics.org/plate-tectonics/types-of-plate-boundaries.html

The denser plate of the two coming together is pushed under the other in a process called SUBDUCTION.  Active volcanoes often arise in subduction zones.
The 3 common scenarios  of convergent boundaries:
1. Oceanic plate meets oceanic plate – the denser plate is subducted resulting in a deep sea trench.  There is one of these off the coast of Japan – where the eurasian plate meets the pacific plate –  movement with this plate is the cause of most of japans earthquakes and tsunamis.  The trench is shown by the dark region next to this map of Japan (from http://godzilla.wikia.com/wiki/Japan_Trench).
Japan as actually at the junction of 3 plates – the Eurasian plate, Philippine plate and the Pacific plate.

2. Oceanic plate meets continental plate – the oceanic plate boundary subducts under the continental plate boundary and the edge of the plate is lost into the molten mantle.

3. Continental plate meats continental plate – in this case the denser of the two plates subducts under the less dense plate and mountains are often formed at the boundary of the top plate.  An example of two continental plates converging is in the Himalayas – caused by convergence of the Indian plate and the Eurasian plate.  As the Indian plate is pushing into the Eurasian plate the Himalayas are growing in size by up to 1 cm every year!!

Divergent boundaries

At divergent boundaries the plates are being dragged away from each other – this causes cracks in the lithosphere which are then closed up by molten rock from the mantle to form new crust.  Divergent boundaries mainly occur in oceanic ridges but can occur on land – an example is this divergent fault in Iceland

from http://www.sciencephoto.com/images/download_lo_res.html?id=693650090

because new crust is created at divergent faults, this type of fault will make a continent or ocean slowly grow in size (unlike convergent boundaries which tend to involve loss of crust so make continents shrink).  Divergent faults in oceans cause the formation of giant ridges which act as spreading zones – where sea floor spreading occurs.  Sea floor spreading is thought to be the cause of continental drift. (but in thinking about this more logically… it is actually the convection current that is ultimately responsible for continental drift… 

http://www.wwnorton.com/college/geo/animations/sea_floor_spreading.htm

this is a cool animation to help visualise how sea floor spreading causes continental drift

Transform boundaries

Transform plate boundaries occur where two plates slide past one another.  Unlike convergent and divergent boundaries, crust is not created or destroyed.  Most transform boundaries occur in the ocean and are formed as a result of offsets in mid-ocean ridges.  Here are two pictures that explain this from http://users.indigo.net.au/don/nonsense/transoffsets.html – basically the plate boundaries do not occur in smooth straight lines – they have offsets – so when you have divergent activity and sea floor spreading from ridges, you will have transform boundaries occurring along the plate boundaries).


http://www.iris.edu/hq/programs/education_and_outreach/animations/2 is an animation to describe this.

Transform faults also occur through some continents – the most familiar to us is the one that runs through the south island of New Zealand.  There are also a transform fault running through California http://geology.com/plate-tectonics.shtml has an interactive map that shows all the major faults and what kind they are.

Transform faults are areas of earthquake activity – the great amount of friction bought about by the rocks in the lithosphere causes a resistance in transverse movement.  Despite the convection currents pushing the transform faults, they will catch and quite often will not move immediately.  When the pressure builds up to a high enough level the transformation will occur, the crust at the boundaries will slide and the sudden release in energy will trigger an earthquake.  Transform activity has been responsible for major earthquakes in Christchurch and San Fransisco.   New Zealand, and many other places, have a mixture of transform and convergent boundaries.  A picture of one of these faults can be seen here – from http://www.sciencelearn.org.nz/Contexts/Earthquakes/Sci-Media/Images/Ostler-Fault-Mackenzie-Basinalong with the result of one of the transform slips!!!


Google Earth file http://bbs.keyhole.com/ubb/ubbthreads.php?ubb=download&Number=138517&filename=286222-PB2002.kmz  OMG GOOGLE EARTH has all of the faults on the 3D globe showing a different colour line for boundary types!!!  Google Earth would have to be a FANTASTIC teaching aid in this section.  I think the info about the plate boundary types needs demonstrations – or at least the animation clips I have listed – Think this would be hands-on stuff best suited to the Explore phase… 

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