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Naturally Speaking

Wednesday, May 11th, 2011

Community Contributed

By G.T. Larson

What we see of Molokai today is but a portion of its former size. At its largest, Molokai was probably at least a third larger in area than today, mainly on its north coast. Molokai, like the rest of the Hawaiian Islands, is a shield volcano. As has been discussed in an earlier series, most of Earth’s volcanoes are strato volcanoes, also called composite volcanoes.

This type of volcano have tapped reservoir of relatively cooler more viscous magma. Composite volcanoes usually have narrower bases and steeper sides than shield volcanoes. Some familiar examples are Mt. Hood in Oregon and Japan’s Mt. Fujiyama. These volcanoes have more of a tendency to “clog up,” resulting, if enough pressure builds up, in a violent explosion such as Mt. St. Helens in Washington State. Shield volcanoes are a type of volcano that has tapped a very hot, fluid supply of magma, usually basalt. They are much less likely to have explosive events; though, if the rising magma hits enough water if can have explosive steam related events.

When Molokai was forming, layer upon layer of lava built up a large curving dome, which at its highest may have been over 10,000 feet high. The north side of the dome was the same profile as the south side, a gradual rise in elevation. This gradual dome shape is characteristic of shield volcanoes. The south walls of Pelekunu and Wailau valleys are remnants of the original crater, or more accurately called caldera. Simply put, a caldera is a very large crater; craters can be inside a caldera, but not vice versa. Shield volcanoes are also known for their rift zones. These are areas along the flanks of the main volcano that vents form allowing the release of volcanic material.

These rift zones radiate out from the main caldera, usually in two or three spoke like zones. If you look carefully at a map of Molokai?s west end, you see two arms or spokes radiating out to the northwest and southwest from Maunaloa, the remnants of the West Molokai volcano. The arm going southwest heads toward La`au Point; this rift zone created La`au Point. It continues beyond La`au over 20 miles in what is called Penguin Banks, a shallow land mass below the ocean’s surface. The northwest rift zone created Ilio Point. The many hills one can see to the north of the road to Kepuhi Beach including Ka`eo, the hill where the ancient adze quarry is located, are all volcanic vents of the northwest rift zone.

So what happened to the other half of Molokai?s north shore? We will examine this in our next installment. Aloha Ke Akua.