ShareThisBy DAVID PERLMAN
Thursday, November 02, 2006
Slowly, slowly, the Big Island of Hawaii is sinking toward its doom. From its palm-fringed beaches to the summit of Mauna Kea, nothing will remain of that volcanic island but a small, stony lump on the bottom of the Pacific Ocean in the far northwest, thousands of miles from where it stands today.
And then it will disappear completely, swallowed into the Earth's heaving crust.
Scientists say Hawaii's fate _ some 80 million years into the future _ is a certainty not just for the Big Island, but for all the major Hawaiian islands.
As those islands vanish one by one, new volcanoes will rise from the seabed where Hawaii is now; and ultimately, those too will move northwestward and shrink beneath the waves, just as if a huge undersea conveyor belt were carrying them along from birth to oblivion.
This sad end to the tropical islands as we know them is being revealed in a geologic saga playing out deep on the ocean floor. It is there that one huge moving slab of the Earth's crust, called the Pacific plate, moves the islands along toward their fate a few inches each century.
And it is there that a great "hot spot" of magma, created by the fiery heat of the Earth's core 1,800 miles beneath the seabed, is fueling the replacement islands with magma, while the massive plate itself is twisting slowly toward the west.
Berkeley scientists and their many colleagues have chronicled these island and plate movements with data painstakingly acquired by researchers at the Berkeley Geochronology Center, aboard research vessels roaming the far Pacific, at the U.S. Geological Survey's paleomagnetism lab in Menlo Park, Calif., and at Stanford and many other research centers.
"The islands," said Warren Sharp, a geochemist at the geochronology lab in Berkeley, "are all just passively riding on that Pacific plate. The moving hot spot formed them, the plate carries them, and over time they will all disappear."
Sharp and David Clague, a marine geologist at the Monterey Bay Aquarium Research Institute, reported on new clues to the process in a recent issue of the journal Science.
And earlier, geophysicist David Scholl of Stanford and the USGS led a research group concluding that the great "Hawaiian Hot Spot" is not fixed in one place, as scientists previously believed, but actually moved southward between 47 million and 81 million years ago.
"This finding," Scholl said, "will break across a cherished idea about how things work in the innards of the Earth."
The reports add fresh insights into the dynamic behavior of the entire Earth's crust, where vast tectonic plates are constantly moving against each other to cause volcanoes, earthquakes and the formation of new land masses.
When the Hawaiian Islands disappear in the distant future, they won't be the first to go, for they are just one part of a long chain of volcanoes that rose from the Pacific floor and vanished, one after another, over millions of years.
For example, Midway Island, part of the Hawaiian chain, was once a volcano, too. Now it is only a coral atoll barely above sea level, with two small, flat islands that held Navy runways during World War II. They, too, will vanish one day.
From Hawaii, the islands of the Hawaiian chain stretch northwestward for more than 1,500 miles, with each island eroding and shrinking into the sea floor in turn until they become merely low rocky reefs, and more flat atolls, barely above the surface.
Beyond the atolls, at the end of what's called the Hawaiian Ridge, the chain becomes completely submerged and then bends in a mysterious kink to continue due north for 2,200 miles, forming an even longer string of totally submerged islands known as the Emperor Seamounts.
When and how that kink in the seamounts developed has long been a major puzzle to scientists who study the dynamic movements of the Earth's crust, but answers are now appearing.
Sharp and Clague reported in Science recently that the mysterious bend in the Emperor Seamount chain is about 50 million years old, and probably took at least 8 million years to develop.
The entire chain of Emperor Seamounts and Hawaiian islands, tens of millions of years old, consists of at least 129 volcanoes, according to Sharp and Clague; together, they stretch for more than 3,700 miles north toward the deep-sea region between Siberia and Alaska known as the Aleutian Trench.
And there the chain ends. The islands and seamounts vanish forever as the Pacific plate that has carried them north dives 25,000 feet deep into the trench's great arc in an earthquake-generating process called subduction. There the Pacific plate is butting up against the western end of the North American plate, and that is where Hawaii, and Loihi too, will ultimately die, after riding the great conveyor belt around the seamounts' bend for 80 million years.
E-mail David Perlman at dperlman(at)sfchronicle.com. For more stories visit scrippsnews.com)
