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Biggest
Mountain ....
MAUNA LOA
Slumbering Giant
Bet
you thought that the biggest mountain was Mount Everest! If you haven't
already checked it out here at "Extreme Science", Mount
Everest does hold a world record, but not as the biggest mountain.
Everest is the highest elevation on land, but it is only a single peak
in an entire mountain range. Mauna Loa is a single mountain on the island
of Hawaii. The island of Hawaii is actually an island made up of five
volcanoes which "blend" together because of their closeness to each
other, making a single island.
Only about 13,448
ft/4100m of Mauna Loa are above sea level, so it may not seem
like a very tall mountain. But, when you start measuring Mauna
Loa from its true base on the bottom of the ocean, in the Hawaiian
Trough, the total height exceeds that of Everest by over 3/4 of
a mile. Mauna Loa is Hawaiian for "Long Mountain", probably because
of its long, gently sloping shape. If you want to get really technical,
Mauna Kea, a neighbor of Mauna Loa on the same island of Hawaii,
is actually the tallest mountain in the world. Mauna Kea
is about 350 ft/107m taller than Mauna Loa, but its mass
doesn't compare to that of Mauna Loa. Mauna Loa takes up a lot
of space because its mass is 9,700 cubic miles/40,000 cu km of
mountain.
Mountain
Born of Fire
What makes Mauna
Loa such a big mountain is the way it was formed. As part of
the Hawaiian islands, Mauna Loa is a volcano, just like many
others found on the island chain. The volcanoes in the Hawaiian
islands are different from the cone-shaped, explosive, "fire
breathing" dynamos that most of us think of when volcanoes come
to mind. Mauna Loa is one of many
shield
volcanoes that make up the Hawaiian
Islands. These are volcanoes that, compared to their more violent
companions, erupt slowly and quietly. What really distinguishes
a shield volcano is its shape - they are usually much wider
than they are tall. Shield volcanoes are created when red hot
lava oozes out from cracks, or
fissures
in the earth's crust.
The
biggest mountain in the world, Mauna Loa, is a shield
volcano with gently sloping sides.
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In the case of
the Hawaiian islands, the fissures were in the ocean floor.
The lava cools as it comes in contact with the ocean water.
The newly deposited lava raises the level of the ocean floor
just in the area around the fissure (see Figure 1). Over time,
and many oozing eruptions, a gently sloping sea
mount forms
(see Figure 2). A sea mount is, quite simply, a mountain on
the floor of the ocean, only the top doesn't reach above the
surface. Over a million years, or more, lava slowly builds up
the sea mount until the top reaches above the surface of the
sea. When this happens, it finally earns the title of island
(see Figure 3).
Hot
Spots
About 30 years
ago a Geophysicist named J. Tuzo Wilson came up with an idea
to explain why there was volcanic activity out in the middle
of the Pacific Ocean, in the middle of the huge Pacific Plate.
At the time, scientists thought that volcanoes only happened
at plate boundaries, but nobody could explain why they were
happening out in the middle of a
tectonic plate.
The idea of plate tectonics said that
the earth's crust is made up
of plates that are constantly moving around. The Pacific
Plate is a hunk of the earth's crust that is covered by the
entire Pacific Ocean.
Dr.
Wilson said that there are "hot spots", under the earth's crust
in some places. These are called
"hot spots" because they are places where a lot of heat is concentrated
in a small area. The heat causes the overlying rock to melt, forming
magma
(what lava is called before it is expelled from the crust - see
Figure 3). Since the magma is liquid and is lighter than the surrounding
rock it "floats" to the surface and forces its way out of fissures
in the crust. Over time, the continual outpouring of magma can
form a sea mount or island volcano if the hostpot is under the
ocean floor, as in the case of the Hawaiian Islands. There is
just one hot spot that never moves. But the Pacific Plate continually
(and slowly) moves north over the hot spot, forming a new volcano
on the overlying plate each time.
Doing the Science
The
idea of hot spots under the earth's crust is a scientific
theory.
Scientists had a lot of questions about why there were volcanic
islands way out in the middle of the Pacific plate. It just didn't
seem to fit in with their theory of plate tectonics. Dr. Wilson's
idea of hot spots helped the island volcanoes to fit into the
theory of plate tectonics. If the Pacific plate was moving over
a hot spot, then that would explain why a chain of sea mounts
and volcanoes had formed as the plate moved. If this was true,
then the volcanoes should be of different ages, from oldest to
youngest in a single direction.
In
order to test his theory, Dr. Wilson took samples of volcanic
rock from each of the volcanic islands in the Hawaiian chain and
tested them to see how old they were on a geologic time scale.
He found that the oldest rocks were from the northernmost island
of Kauai, which also had the most weathering of rock. He also
found that progressively younger rocks were found on the Hawaiian
islands the further south he went (see
map). The youngest rocks of all
were found on the big island of Hawaii, the southernmost island.
In fact, new "rocks" are still forming on the island of Hawaii,
making it the youngest volcano in the island chain. There is even
more evidence to support his theory because there is a new volcano
forming on the sea floor south of Hawaii, called Loihi. Right
now it's just a sea mount, but if the lava continues to build
up on its slopes, someday it will be a new island.
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Photo courtesy of
J. Griggs, USGS (1991)
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