(National
Geographic) |
Everybody knows Hawaii has volcanoes. In fact, the Hawaiian islands are completely built up from the
ocean floor. A
volcano-making machine seethes deep beneath the island
of Hawaii that geologists call a hot spot. Over the past
44 million years it has churned out 82 volcanoes that
today form a vast archipelago that stretches across the
Pacific. As if on a colossal assembly line the islands
were built in succession over the hot spot then borne
away to the northwest by movement of the Pacific tectonic
plate. Hot spot lava oozes slowly and builds huge, rounded
mountains by slow accretion. Hawaii, the youngest island,
is still under construction. Two of the world’s most active
volcanoes, Mauna Loa and Kilauea, continue to pave over
existing island real estate and create additional acreage.
Lo'ihi, a new seamount 20 miles (32km) offshore about
3,000 ft. (915m) below sea level, and fired by the same
hot-spot, is giving birth to the chain’s next island due
to rise above the waves in another 60,000 years.
The
source of raw material for this age-old construction project
lies more than 50 miles (80km) underground within the
earth’s mantle. Here, rock kept in a plastic state by
heat and pressure rises, cools, and sinks. Rising currents
bring up heat from deep within the earth melting the mantle
into magma. Lighter than the surrounding material the
magma oozes toward the surface. As the volcanoes are pulled away from the
hot spot, the rate of eruption diminishes until finally
they are extinct. As
they age, the islands erode and sink, until finally they
disappear under the ocean.
Island
of Hawaii
(USGS) |
The
Big Island of Hawaii is a geologic microcosm of the entire
archipelago. It
has five subaerial (above the ocean) volcanoes. Like the chain, the volcanoes are oldest
to the northwest and youngest to southeast.
Kohala
Volcano from the South
(USGS) |
Kohala
is the oldest of five volcanoes that make up the surface
of Hawaii Island. Geologists
believe Kohala Volcano last erupted 60,000 thousand years
ago, its oldest lava is dated at 460,000 years old. It is extinct and in the erosional stage
of its life cycle. Its eastern windward flank is characterized
by great erosional valleys, spectacular waterfalls, and
dramatic sea cliffs. A dramatic landslide whose rubble extends
fifty miles out on the ocean floor formed the sea cliffs.
The summit elevation is 5480 feet or 1670 meters.
Mauna
Kea
(USGS) |
Mauna
Kea is the tallest mountain in Hawaii at 13,796 feet or
4205 meters. Indeed,
measured from its base off the ocean floor Mauna Kea is
the tallest mountain in the world.
It is dormant and last erupted around 4,500 years
before present. Mauna Kea is in the post shield stage and
is characterized by numerous cinder cones, two glacial
valleys and an alpine glacier lake near the summit.
The oldest exposed lava on Mauna Kea is about 250,000
years old. The
summit is often capped with snow and is home to the greatest
astronomical complex on earth.
Hualalai
Summit
(USGS) |
Rising
to 8,271 feet (2521 meters), Hualalai last erupted in
1801. The lava
at the Kona International Airport and at Kekaha Kai State
Park is from that flow.
The oldest exposed lava on the mountain is the
impressive Pu'uwa'awa'a and Pu'uanahula eruption dated
at 120,000 years before present.
Hualalai has great well balanced features: wonderfully
diverse native forest, rugged and hostile dry lowlands,
incredible pit craters, surreal eruptive vents, awesome
lava tubes, stark cinder highlands, an astonishing collection
of archeological sites, and thousands of acres of remote
inaccessible backcountry.
Mauna
Loa,
Southwest Rift
(USGS) |
Mauna
Loa is the world’s largest mountain. It is also one of the most active volcanoes
on earth, last erupting in 1984.
Mauna Loa makes up over half the surface area of
the Big Island. It
is in the late shield-building stage of its lifecycle.
Its summit rises 13,570 (4136 meters) above sea level.
New imaging technology has allowed us to see another
aspect of Mauna Loa. As the volcano has built itself up off the ocean floor, it has
also pushed down a mirror image into the earth’s crust. This depression extends 26,000 feet from
the ocean floor. From
the deepest point to its summit, Mauna Loa rises 56,000
feet!
Kilauea
Summit
(USGS) |
Kilauea
is the youngest and most active volcano on the island. Indeed, it is often touted as the world’s
most active volcano.
Over 90% of its surface area is covered with lava
less than 1500 years old. Though the summit rises to only 4093 feet
(1248 meters), Kilauea continues to grow and someday may
completely cover its giant neighbor Mauna Loa.
The summit caldera measures two by three miles. As of this writing Kilauea enters its 21st
year of near continuous eruption—the longest sustained
eruption recorded historically.
The
dramatic lava fountain of Kilauea’s Pu’u O’o vent is shown
in it’s early eruptive history. Pu’u' O’o’s lava fountains
attained heights as high as 1500 ft. (457m) above the
crater rim. Lava fountains are propelled upward by intense
expanding gases in the magma. Three of the most common
volcanic features people see are pahoehoe lava, a’a lava,
and cinder cones.
Pahoehoe
lava
(USGS)
|
A lobe of pahoehoe slowly oozes forward. Pahoehoe
is the smooth unbroken lava that looks like the top of
a brownie pan. Pahoehoe is generally hotter than a’a,
and contains more dissolved gases which results in a more
fluid flow.
A'a
lava
(USGS) |
An advancing flow of a’a lava crushes a deserted
pick-up truck. A’a is the jagged, broken lava that looks
like it was bulldozed. It is more viscous than pahoehoe.
Similar to a river freezing the cooling chunks on the
surface are ‘rafted’ along with the molten mass.
Cinder
Cones
(Carl
Waldbauer) |
Cinder cones of Mauna Kea on the Pohakuloa saddle. Cones
are characteristic of late stage eruptions within the
long life cycle of a volcano. Each one is an eruptive
event in the volcano’s distant past. High gas content
propels lava skyward, which then falls back to the ground
in small ‘frozen’ pieces; like lava popcorn.
Steam plumes arise as molten lava enters the ocean.
Transported down slope in underground tubes the lava hits
the water at nearly 2000 degrees F. forming new benches
of lava. The steam plume, called laze, is a form of hydrochloric
acid and contains microscopic particles of glass. Extreme
heat and violent water, plus the threat of lava bench
collapses, make these lava entry areas very dangerous.
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