First, click on this, and download and print the document.* It contains the beginnings of each plate boundary you will need to know how to draw.
* - I'm not kidding about this. Print out this document and practice the drawings, or you will regret it when you take the exam.
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Note:
this image is flipped from the image in the document you
downloaded. That is DELIBERATE, to encourage you
to learn the processes and characteristics of an
Ocean-Continent Convergent Boundary. You will draw
your own as a mirror image of this one. As you
draw it, carefully observe the characteristics in this
kind of boundary. |
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Consider
the characteristics of Continental Crust vs. Oceanic Crust:
Avg. Silica Composition: very low Avg. Density: 3.3 g/cm3 (Note that I've stretched this upwards just a bit, compared to your downloaded drawing, to be able to write in the densities and thicknesses of the crusts. |
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So in a
Convergent boundary between thick, lower density
Continental Crust, and thinner, higher density Oceanic
Crust which will go down and which will go up? The
thinner, high density material bends down and gets
pushed into the Earth in a process called "Subduction".
What happens as a result of this? |
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First of
all, note the Marine Sediment
in green. Most of the world's ocean
crust is covered by a layer of marine sediment, composed
of microfossils from planktonic organisms, but also fish
teeth, sea urchin spines, and many other
organisms. Also present is wind-born dust often
from deserts, volcanic ash, ice rafted debris,
river-born clay, and even micro-meteorites. The
layer is usually not much more than a kilometer thick. As the ocean crust gets subducted, much of this sediment gets scraped off and plastered on the leading edge of the Continental Crust to form a huge accumulation known as an "Accretionary Wedge" or "Accretionary Prism". Some of the sediment makes it down into the mantle, carrying water and minerals down there. The other thing we see happening here is that the Oceanic Crust literally bends down, and you can see that because of that, the depth of the ocean gets deeper in a long, narrow channel known as a "Deep Ocean Trench". This is where the deepest depths of the ocean are found, as deep as 11 km (remember the average is only ~4 km). |
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As the
crust (and the rest of the plate below it) gets
subducted, two things occur: 1. the motion is not smooth. The crust sticks, stresses rise, and eventually earthquakes occur above the downgoing crust. This creates a zone of earthquakes dipping (generally at about 45o) down away from the Deep Ocean Trench, known as a Benioff Zone after its discoverer. This Benioff Zone often goes as deep as 700 km. 2. As the plate is pushed deeper into the Earth, temperatures rise, and coupled with water from above (which lowers melting temperatures), partial melting of the plate and the overlying mantle occurs, forming a magma chamber. |
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The magma
formed by the partial melting of the Oceanic Crust and
Mantle is less dense than the surrounding rock, so it
rises to the surface through the continent,... |
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...
forming a row of volcanoes called a "Continental Volcanic Arc". This is what I want you to be able to draw and understand about an Ocean-Continent Convergent Boundary: 1. Subduction Zone (the entire region where a plate is subducted) 2. Accretionary Prism/Wedge formed from Marine Sediment 3. Deep Ocean Trench 4. Benioff Zone (downward dipping zone of earthquakes) 5. Magma Chamber leading to... 6. Continental Volcanic Arc Where would you find features like this? Anywhere that you find volcanoes near a coast, generally with a trench offshore, and lots of earthquakes, that is evidence that you are at an ocean-continent convergent boundary. Look at a map of earthquake locations and a phyisiographic map that shows coastal mountains and trenches and see if you can figure out where to find that kind of tectonic boundary. Now click your "back" button to go back to the Plate Tectonics page. |