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Hawaii's Oahu Used To Be a Bigger Island
sciencehabit (1205606) writes "The two volcanoes long thought to have formed the Hawaiian island of Oahu had a head start: They grew on top of an older volcano that's now submerged northwest of the island and partially covered by it, new research suggests. Tests indicate that the long-lost peak—now dubbed Kaena volcano—grew from the sea floor and broke through the ocean's surface about 3.5 million years ago, eventually reaching a height of about 1000 meters above sea level before it began sinking back into the sea. At its largest, ancient Oahu would have measured about 1900 square kilometers (about 20% larger than modern-day Oahu) or larger. Over the course of its lifetime, Kaena volcano spilled between 20,000 and 27,000 cubic kilometers of molten rock, the researchers estimate. When Kaena volcano became largely extinct isn't clear."
27,000 cubic kilometers is 56608 gigabuttloads.
So in exactly what sense are these "new" volcanoes not just new vents in the old volcano? Seems to me if they weren't using the old magma channels they wouldn't have opened near the top of an existing volcano - instead they would have opened someplace around the base (underwater) instead. Perhaps more to the point - in exactly what sense is an old volcano which is still spewing lava from it's slopes extinct?
As I understand it it's not exactly uncommon for a volcano to go dormant for a while and then sprout new calderas on it's slopes when it gets active again, particularly if a magma "cork" formed in the old caldera, or the old magma channels collapsed while it was dormant. Seems to me the actual summary should be something along the lines of "what was long thought to be two volcanoes forming Oahu has been discovered to actually be a pair of relatively recent new calderas on a single older, larger volcano."
--- Most topics have many sides worth arguing, allow me to take one opposite you.
Of course it used to be larger. Hawaii is growing, all the rest of the islands are shrinking (because they are no longer growing).
I knew a marine geologist in Honolulu when I lived out there who was trying to predict the size of the tsunami that would result the next time a huge chunk of Oahu breaks off and falls into the sea. Apparently some pretty large chunks have fallen off in the past.
My first reaction was "Um.. Duh?"
I learned this in middle school, I believe. The Hawaiian Islands are a chain that's formed by the movement of the Pacific Plate across a hot spot. As it passes over, a volcano builds up and builds up, eventually forming an island that keeps growing as long as the vent stays over the hot spot. Eventually, it will move on and start eroding, while another volcano starts up at the ocean floor. All of the Hawaiian Islands except for Hawai'i (The Big Island) are currently shrinking, while Hawai'i is still growing. If you look at charts of the Pacific Ocean's floor, you can see a long chain going all the way to the subduction zone at the Aleutian Trench.
End of line..
http://maps.ngdc.noaa.gov/view...
Zoom in on the Hawaiian archipelago; de-selecting multibeam bathymetry surveys and switching the base map to "Shaded Relief (GEBCO_08) will give
you a nice image of what the chain looks like under water. Kaena Pt is the westermost tip of the island of Oahu. Also note that Maui Nui was once a much
larger island, encompassing Maui, Molokai, Lanai and Kahoolawe. Loihi is visible to the the southeast of the island of Hawaii.
Breathe continuously
The headline is not news. Everyone knows that the islands are all eroding away, except where new material is being added through volcanic action. But the story is about the ancient volcano that predated the volcano that we knew built the island. That's news, just poorly presented, in slashdot editorial tradition.
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For years and years the bathametry off the islands was classified. The reason was that our submarines would hide in the oddly rugged land forms of the subsurface. The lay of the sea floor was declassified when it was felt that our enemies had already mapped it. When the data were made available they unleashed a bonanza of geologic investigation for it became clear that there was a very active process taking place. Much of the strange forms around the islands, all of them, are due to very large slumps, where entire sides of volcanos slumped beneath the ocean suddenly, in a day or less.
I remember being at Wyanii on the west, dry leeward, coast of Oahu and seeing hundreds of individual lava flows in the rugged mountain side. Later I learned that one of the huge blocks that subsided from the coast sat right off shore and realized that the landscape could have been created in an instant as the west edge of the pile slipped off into the deep sea to its west. The ocean off the Islands is more than 3,000 feet deep.
On the east coast of the Big Island is a very active volcano that has been putting out floods of low viscosity basalt to its east. There is also a huge gravity fault that has opened up a miles long rift that faces east. One of these days the whole cubic miles of rock could side off to the east into the abyss. I fear that the wave created will destroy everything on the east edge of the Pacific Basin. I have thought that one might look at places on the West Coast of the U.S. for evidence of the mega wave that could result, because it has happened before many times.