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L
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O
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18a
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18.a. Describe the major geologic events of formation
and weathering which have taken place on the island of O'ahu.
View Figure below showing changes to O'ahu from origin to present.
Drawing 1. About 3 million years ago, the Wai'anae volcano arises
above the surface of the ocean. It is building out of one of the two hot
spot "vents," or openings, on the ocean floor. Black triangular
areas represent fresh lava flows.
Drawing 2. A million years later, the Wai'anae volcano has greatly
enlarged. At about 2 million years ago, the Ko'olau volcano arises above
the ocean surface. It comes from the other branch of the "hot spot"
vent. The two vents are about 50 miles apart. Now, there are two islands,
side by side.
Drawing 3. As the Ko'olau mountain range develops, its lava overflows
the older Wai'anae slopes, and one island is formed. The overlap flows of
the two volcanoes form the Leilehua (Schofield) plateau. (Heiaus, like Kukaniloko
(just outside Wahiawa), the birthing site of ali'i, found at this overlap
site, may indicate Hawaiian awareness of mana or
the spirit of the region where the two volcanoes which formed the islands
meet.) The Wai'anae range is no longer active volcanically and erosional
forces begin to wear it down.
Drawing 4. As the Ko'olau matures, it also begins to erode. Both
the Ko'olau and Wai'anae mountain chains develop ridges and valleys due
to erosional forces. Mount Ka'ala, the prominent "end cone" and
the present-day summit of the Wai'anae, develops. It is resistant to erosion
because of its composition and, even today, it is 1,000 ft. above any other
formation on the island of O'ahu.
Drawing 5. The island sinks and the sea level rises. (At Kaena Point
one can see evidence of this by the upper beach shelves, now above the coastline.
They are like stairsteps where erosion took place at different sea levels.)
Erosion shapes the two mountain ranges into their present-day form. About
1/2 of the Ko'olau range on the windward side is gone. Much of this loss
probably is due to massive landslides into the ocean as well as erosion
by water.
Drawing 6. Water levels recede primarily due to the Pleistocene ice
age.
Valleys, worn away by aboveground erosion, will later be "drowned"
valleys under water. Pearl Harbor is being created by this process as it
erodes above sea level.
On the windward side of the Ko'olau, in the center of the range at the site
of an old caldera, Kawainui marsh starts first as an ocean bay. Later it
closes off to become a marsh, the largest fresh-water marsh on the islands.
About 20 to 30 thousand years ago, the last major post erosional eruptions occur on O'ahu:
a. Salt Lake Crater and Red Hill
b. Punchbowl and Diamond Head. (Diamond Head may have been formed in several days, with
massive deposition of airborne ash which filled in Manoa Valley making
it an unusually broad and flat-floored valley).
c. Koko Head, Koko Crater, Sea Life Park area, last set to erupt.
Drawing 7. Water level rises again due to melting of ice caps. Pearl
Harbor is formed by water filling the valleys created between the mountain
ranges. Waikiki is a marshy lagoon. (Its present state is altered considerably
by the draining-activities of humans.)
Volcanoes on O'ahu are not extinct (as on Kaua'i) but are dormant. Theoretically
there still could be an eruption on O'ahu though it is not highly likely.
About one half of the original land mass of O'ahu has slipped into the sea
through erosion and landslides. In a few million years, O'ahu will vanish,
much like the islands of the North-West chain above Kaua'i.
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