So much for another very
attractive and convincing theory. Again
we are back to knowing nothing about what is actually happening around hot
spots and have to put our imaginations on hold.
It was fun while it lasted and I certainly bought into it to the extent
of struggling to work around the problem when I considered the evidence for
crustal migration.
Suddenly the idea of a very deep
plume is plausibly replaced by a large zone of hotter rock that may have
developed as a concentration point in the center of the pacific plate. The question then is this feeding heat both
horizontally and vertically to the island chain which certainly shows a
migrating plume of some sort at work there.
Fitting this all together is
going to provide headaches for some time.
That is the nature of seriously contradictory new evidence.
Where did the heat come from or
are we looking at the artifact of a chemical separation that we have yet to
figure out. After all this is a basaltic
event.
800-Mile-Wide Hot Anomaly Found Under Seafloor off Hawaii
findings contradict long-held theory that a plume directly fuels Hawaii
Hot lava spills into the sea from under a hardened lava crust on the Big Island
of Hawaii
Photograph by Patrick McFeeley, National Geographic
Dave Mosher
Updated May 27, 2011 (First posted May 26, 2011)
Hawaii's
traditional birth story—that the volcanic islands
were, and are, fueled by a hot-rock plume running directly to Earth's scorching
core—could be toast, a new study hints.
Scientists say they've found solid evidence of a giant mass of hot rock
under the seafloor in the region. But it's not a plume running straight from
the core to the surface—and it's hundreds of miles west of the nearest Hawaiian
island.
Until now, the researchers say, good seismic data on the region has
been scarce, so it was tough to question the traditional explanation: that a
stream of hot rock directly from around Earth's core formed the 3,100-mile-long
(5,000-kilometer-long) chain of islands and undersea mountains in the Pacific Ocean .
As Earth's crust slid over the plume, as if on a conveyor belt, the
erupting seafloor built mounts, mountains, and islands out of layers of cooled
lava over tens of millions of years—or so the conventional wisdom goes.
But after analyzing 20 years' worth of earthquake data,
geophysicists say they've found an 800-mile-wide (1,300-kilometer-wide) region
of hot rock in the Hawaiian region—but nothing beneath the Big Island of Hawaii.
The island, the youngest in the chain, is traditionally thought to be above the
purported plume.
Although the new evidence flies in the face of the giant-plume theory,
"we can't rule out a narrow plume below the island, but the main source
comes from a different place. It can't be linked directly below," said
geophysicist Robert van der Hilst of MIT, co-author of the new study, led by
his colleague Qin Cao and appearing online today in the journal Science.
Volcano formation starts where Earth's mantle—the planet's thickest
rock layer—meets the molten outer core, some 1,800 miles (2,900 kilometers)
below the surface.
The outer core heats the mantle's bottom rocks into buoyant putty,
which rises toward the crust, as if in a lava lamp. Within a few miles of the
crust, the rock decompresses, melts, and often oozes—or erupts—out of the
Earth's surface.
A technique called seismic tomography uses the sounds of earthquakes
rippling through the planet and bouncing around to detect such plumes, or hot
spots. But this kind of data has been limited for Hawaii
"It's been very difficult to image the mantle below Hawaii
By contrast, Van der Hilst said, the new study analyzed two decades'
worth of seismic data and extracted subtle but clear signals.
Those signals point to the giant anomaly, about 410 miles (660
kilometers) down: a relatively disklike segment of rock between 540 and 720
degrees Fahrenheit (300 and 400 degrees Celsius) hotter than its surroundings
and between 370 miles (600 kilometers) and 1,000 miles (1,600 kilometers) west
of the Big Island
The team suspects the plume is pooling at the boundary between the
upper and lower mantle, then snaking its way to the crust below the archipelago
before rising to feed Hawaii
More evidence is needed before it's known for sure what's stoking the
volatile island chain, but at the least, the new study "shows the process
of how our planet loses heat is more complicated than we thought," van der
Hilst said. It may also help explain how other Pacific seamounts came to be, he
added.
Magma Chamber Surprisingly Close to Hawaii
Lava source found within two miles of surface, research suggests.
Lava glows through a hole in the crust of Kilauea volcano on the Big Island
of Hawaii
Photograph from Photolibrary
Richard A. Lovett in San
Francisco
Published December 23, 2010
A giant magma chamber burning beneath the Hawaiian Islands is closer to
the surface than any other magma chamber yet measured—as little as 1.9 to 2.5
miles (3 to 4 kilometers) below the surface, scientists say.
But Hawaiians don't need to worry about plunging into the magma below,
say, during an earthquake—two miles of solid rock is more than enough to keep
that from happening, according to geology undergraduate Julie Ditkof, who
presented the findings December 14 at a meeting of the American Geophysical
Union in San Francisco.
Ditkof, of Ohio State University, determined the chemical composition
of crystals in a thousand samples of volcanic rock taken from Hawaii's
most volcanically active regions, the Big
Island (map) and an adjacent undersea volcano called
Loihi.
Using a method developed in Iceland—another volcanic island—by Michael
Barton, Ditkof's advisor and research partner at Ohio State, she paid
particular attention to olivine, the first mineral in Earth's crust to
crystallize as temperatures and pressures drop.
The ratios of certain elements in each olivine sample told her how deep
the lava had been when it left the magma chamber and began crystallizing—and
therefore the depth of the magma itself, said Ditkof, whose findings have not
been published in a peer-reviewed journal.
Heated Debate
The magma-depth discovery may settle a longstanding debate.
In 2008 workers doing exploratory drilling for a geothermal plant
reported that they had accidentally
broken through to a shallow magma chamber near the Big Island's Kilauea volcano.
Even after the incident, though, scientists were unsure as to the
general depth of the magma chamber, or chambers, that produce most of the Big Island
Some prior studies of other minerals in Hawaiian volcanic rock, for
example, had suggested that the local lava comes from as deep as perhaps 11 to
25 miles (18 to 40 kilometers) down.
But seismic
studies appeared to show magma chambers at much shallower depths—about 1.2
to 3.7 miles (2 to 6 kilometers).
"We wanted to prove one right or one wrong," Ditkof said.
Magma to Power Hawaii
The discovery of magma so close to the surface suggests that Hawaii
That could be good news for the state, which generates 90 percent of
its power from imported oil, "resulting in electricity that costs five
times more than the national average," according
to Scientific American.
Ditkof said, "Very shallow magma chambers mean there is a very
shallowgeothermal heat
source that could be tapped," for example to generate steam to power
turbines in electrical plants, she said.
"If someone could utilize that, you have amazing geothermal
potentials."
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