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The Ocean Floor
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Before we delve into plate tectonics let's take a look at the discoveries in the oceans that led to the Plate Tectonics theory. In the 1960's scientists had research data that indicated that the Atlantic Ocean was growing. This brought about a renewed interest in Wegener's Continental Drift theory. During Wegener's time scientists didn't have the tools to study the ocean floors but in the 1940's new instruments were developed that allowed scientists to map the ocean floor and record earthquakes in the ocean's crust.
Mid-Ocean
Ridges, trenches, and Rifts
This new
mapping made is possible to determine the depth of ocean trenches. These
trenches are long and narrow in shape. The trenches around the borders of the
pacific ocean are almost 10 kilometers deep in some places.
The Atlantic
Ocean floor was found to have a huge underwater mountain chain named the
mid-Atlantic ridge. A ridge is a long, narrow chain of hills or mountains.
This ridge is part of an underwater mountain chain the winds 65,000 kilometers
around the earth. While the ridges in the Pacific ocean tend to be flat-topped
mountains the ridges in the Atlantic are two parallel chains of mountains.
There is a 50 kilometer rift, or valley between these chains.
Sea-Floor
Spreading
In the early
1960's scientist began to toy with the theory that new crust is forming at the
ocean ridges. Evidence for this idea was found on the ocean's floors. Cracks
were found along the the middle of mid-ocean ridges where the ocean floor was
splitting. Magma was rising out of these cracks, hardening and forming new
crust. As new magma is pushed up, the new harden crust is pushed away at both
sides carrying older crust along with it. This forming of new crust on the
ocean floor is called sea-floor spreading. This new crust suggests that the
whole crust of the earth is moving not just the continents as Wegener had
speculated years earlier. Between 2 and 20 centimeters of new crust forma a
year in oceans throughout the world.
Rocks Have
Magnetic Records!
It's a little
difficult to understand so read closely. When the magma pushes up and away the
newly formed crust grains of iron and other magnetic substances in the magma
tend to line up in the direction of the earth's magnetic field. This field
extends from the North pole to the South pole. When the magma hardens the
position of the rock is preserved. Scientist have found that the location of
North and the location of South have reversed many times in the earth's
geological history. The is to say a North seeking compass would point south
rather than North. Ships tow instruments that measure the magnetic property of
rocks. Their records indicate that the magnetic properties of the rocks
switches as one travels father away, in either direction, from one of the
mid-oceanic ridges.
Figure it
Out!
For each
metric unit below find the equivalent in the specified units. You'll find
conversion tables in
Science Tools. After you
do the math, you'll have to show your work, go to the online length conversion
table and check your answers. Next describe something that is approximately
that length. Be sure to use at least 3 adjectives to describe your object. Be
sure not to tell what it is...the idea is your description should tell the
reader what the object is. Share your description with someone near you...if
they correctly identify your object than you've written a good description, if
they can't, rewrite it using more descriptive language and try again with
someone else.
10
kilometers to yards
65,000
kilometers to miles
50
kilometer to yards
2 and 20
centimeters to inches
Consider
why scientists all use the metric measuring system. What do you think would
happen if they didn't?
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