![](PlateTectonicsDrawingsDivWrong1.jpg)
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When
students think about a Divergent Boundary, they often
think about a continent splitting (which DOES happen,
and we'll discuss this when we discuss Ocean
Basins). This leads to some problems, though.
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![](PlateTectonicsDrawingsDivWrong2.jpg)
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First,
students will often envision a continent (the average
thickness of which is 30-35 km, or roughly 20 miles)
splitting apart, leaving an enormous crevasse. But
NOWHERE on Earth is there a crevasse anywhere near that
depth.
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![](PlateTectonicsDrawingsDivWrong3.jpg)
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Second,
almost ALL divergent boundaries on Earth are in the
ocean, not on land. So this view of a Divergent
Boundary is WRONG. DO NOT USE THIS.
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![](PlateTectonicsDrawings2Div.jpg)
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So
instead of a continent, let's look at the ocean.
The image to the left is what you should see in the
document you downloaded and printed.
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![](PlateTectonicsDrawings2Div2.jpg)
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The first
factor you should be aware of is that a Divergent
Boundary is ALWAYS a place where there is a heat source
deep underground. Remember that "temperature"
refers to the rate at which atoms and molecules
move. Higher temperature means that atoms and
molecules are vibrating faster, which means that they
move farther apart, the material expands, its volume
increases, its density (mass/volume) decreases, and what
happens to low density materials? Like a hot air
balloon, they move up.
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![](PlateTectonicsDrawings2Div3.jpg) |
Because
of that, the crust over a heat and magma source is
uplifted. This creates a bump, and since the
Divergent Boundary is not a point but a line between
plates, this bump is an elongated, elevated feature: a "ridge"
(or sometimes a "rise").
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![](PlateTectonicsDrawings2Div4.jpg) |
Magma
rises from the magma chamber to form new oceanic crust
where the crust separates at the Divergent
Boundary. As the plates move apart, more magma
rises to the surface, creating more crust, in a
continual* process. This is how new seafloor is
created.
*Eruptions occur every few years to every few thousand
years. On million-year timescales, the process is
continuous and in conveyor belt fashion, seafloor is
created at the Divergent boundary and moves away.
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![](PlateTectonicsDrawings2Div5.jpg) |
At the
very top, tension causes a narrow channel right on top.
Why does the uplifted area drop as you move away from
boundary on either side? Remember that it was
uplifted in the first place because it was warm.
As it cools off, it gets denser, and subsides. The
surrounding seafloor averages about 4 km deep, while the
top of the ridge is about 2.5 km in average depth.
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![](PlateTectonicsDrawings2Div6.jpg) |
This is
what I want you to be able to draw - and label.
Important features:
1. Mid-Ocean Ridge
2. Mid-Ocean Rift
3. Magma chamber below the ridge.
4. Magma rising to surface from magma chamber
5. Subsidence away from ridge. |
![](color_etopo1_ice_lowDiv.jpg)
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This is a
world physiographic map, on which I've marked the
divergent boundaries in red. Some of them may
overlap with Transform Boundaries (which will be
discussed shortly) and there are gaps which represent
Transform Boundaries as well. To see it, click on
it to go to a new web page. It is a very large
image.
Where can you "see" divergent boundaries? Mostly,
they are in the ocean, as mid-ocean ridges and
mid-ocean rises. At this point in geologic
history, the only divergent boundary on land is the East
Africa Rift System. But as Pangaea and as
Gondwana began to break up roughly 200-100 million years
ago, there were divergent boundaries (as rift systems)
between where the continents were breaking up.
Now click your
"back" button to go back to the Plate Tectonics
page. |