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Challenger Deep got its name from the
British survey ship Challenger II, which pinpointed the deep water
off the Marianas Islands in 1951.
Then in 1960, the US Navy sent the Trieste (a submersible - a mini-submarine
designed to go really deep) down into the depths of the Marianas
trench to see just how far they would go. They touched bottom at
35,813 feet.
That means, while they were parked on the bottom in the bathyscaphe,
there were almost seven miles of water over their heads!
If you cut Mount Everest off at sea level and put it on the ocean
bottom in the Challenger Deep, there would still be over a mile
of water over the top of it!
Hydrostatic Pressure
When you get into the ocean (or any body of water) and you start
diving down from the surface, the deeper you dive the more water
is over the top of you. The more gallons of water you put between
you and the surface of the ocean, the greater the pressure is on
your body because of the weight of the water over the top of you.
This pressure is called hydrostatic pressure. You can really get
a sense of hydrostatic pressure when you go into a swimming pool
and dive all the way to the bottom of the deep end. You'll feel
the hydrostatic pressure against your ear drums, like they're being
squeezed or pushed in. Well, you can imagine how incredible the
pressure must be in the Challenger Deep with almost seven miles
of water overhead - it's 16,000 pounds per square inch!
Plate Tectonics and the Subduction Zone
So how come the Challenger Deep is so deep? Well, the earth's crust
isn't one solid piece of rock, it's really pretty thin, like the
shell of an egg is compared to the size of the egg. In fact, it's
made up of huge plates of thin crust that "float" on the molten
rock of the earth's mantle. While floating around on the mantle
the edges of these plates slide past each other, bump into each
other, and sometimes even crash. The oceanic crust is much heavier
than the continental crust so when the plates crash into each other,
the oceanic plate plunges downward toward the molten mantle, while
the lighter, continental plate rides up over the top. The forces
driving the two plates together are really intense so the underlying
oceanic plate (the subducted plate) creates a trench where it drags
the edge of the continental crust down as it descends underneath
(check out the picture at left).
This is what's happening on the bottom of the Pacific Ocean off
the Marianas islands. The really deep part of the ocean is in the
bottom of the trench created by the subducting ocean crust.
So, How Do They Know?
In 1984 the Japanese sent a highly specialized survey vessel out
the the Marianas Trench and collected some data using a piece of
equipment called a narrow, multi-beam echo sounder.
What an echo sounder does is send high frequency sound waves (that
the human ear can't hear) through the water down to the ocean bottom.
Sound waves will travel through water, even faster than they travel
through the air, and bounce off solid objects, such as the ocean
bottom. The echo sounder measures precisely how long it takes for
the sound waves to be returned to the surface and determines the
depth based on the rate of return. These soundings are plotted on
a graph by a computer to make an "echo map" of the ocean bottom.
The deepest measurement of the Challenger Deep currently available
was taken by the Japanese and was found to be 35,838 feet.
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