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By David F.
Salisbury
January 31, 2002
Scattered around the earth are approximately 40 specific fixed areas of isolated
volcanic activity known as hot spots. As the crustal plate moves over a hot spot,
successive eruptions can produce a linear series of peaks or seamounts with
the youngest peak above the hot spot.
-Fundamentals of Oceanography
These days, nearly every introductory
textbook in geology and oceanography includes a discussion of hot
spots - hot plumes of molten mantle material that sit beneath the
rigid plates that form the Earthıs crust and periodically punch
through them to produce outbursts of volcanic activity. Iceland,
Yellowstone, the Hawaiian and Galapagos Islands were all created
by hot spots. But a deep-sea drilling expedition this summer has
produced new evidence that will force the textbook authors to revise
their descriptions because it indicates that one of the largest
hotspots - the one that created the Hawaiian Islands - has not remained
fixed through the last 80 million years, but spent the first half
of its life in motion.
"This forces us to re-examine the geologic
evidence we have on all the hot spots to see if any others show
evidence of similar motion," says William G. Siesser, professor
of geology and a member of the international team that made the
discovery.
The hot spot theory - first proposed
in the 1960s - has provided one of the most dramatic proofs of plate
tectonics, which holds that the Earthıs crust is broken up into
a system of rigid "tectonic" plates that are in continual motion
at rates of a few centimeters per year.
In the hot spot theory, a plume of
hot mantle material sits just below a crustal plate and, as the
plate moves past, the molten material periodically punches through
the crust to create a chain of volcanic mountains.Hot
spots tend to be located where the crust is thinnest, which means
they are mostly found on the seafloor, where they create strings
of seamounts and volcanic islands. If you drained the Pacific Ocean,
you would see a striking 5,000-mile string of seamounts and islands
that runs in a straight line from the Kamchatka Peninsula in Siberia
south to a point just west of Midway Island and then turns abruptly
to the southeast to form the Hawaiian Island chain. This is the
trail left by the hot spot that is currently under the island of
Hawaii and is responsible for its volcanic activity.
While the Hawaiian Island chain provided
the basic geological data that confirmed the hot spot theory, geophysicists
have had trouble explaining why the hotspotıs trail should have
abruptly changed direction some 43 million years ago. Scientists
have tried to explain this abrupt, geologically speaking, change
in direction by arguing that the Pacific plate must have changed
direction at this point. Unfortunately, this doesnıt fit well with
other geological evidence. Officials of the Ocean Drilling Program
- an international effort with headquarters at Texas A&M and funded
by the National Science Foundation in the United States and comparable
government agencies in 18 other countries - decided to schedule
its specially equipped deep-sea drilling ship, Joides Resolution,
to spend two months in the northern Pacific in hopes of resolving
this long-standing mystery.
Joides Resolution is a high-tech, seagoing
research center with the ability to drill and core into rock for
several thousand feet below the ocean floor in areas where the ocean
floor itself lies up to 27,000 feet below the surface.
"The sleeping cabins are very small,
but the laboratories are quite spacious and filled with state-of-the-art
equipment, as good as anything you can find on a university campus,"
says Siesser, who has participated in six, two-month cruises aboard
the ship.
That is appropriate because the scientists
work in two 12-hour shifts. "On this cruise I was lucky, I was assigned
the noon to midnight shift," he said. That meant he would usually
get up at about 11 a.m., have breakfast in the galley with a number
of his comrades and then go to work.
The ship travels to carefully selected
locations - in this case sites in six of the early seamounts that
the Hawaii hot spot created - to drill out and extract long, cylindrical
cores of rock that are brought on board. The cores contain a series
of thin layers of material that were deposited on the sea floor
at specific periods in the past.
The task of the scientists is to read
the record of events in the history of the ocean recorded in the
cores at each site. Each of the scientists on the 24-member team
specializes in areas such as sedimentology, paleontology, petrology,
volcanology, tectonics, geochemistry, geophysics and other areas.
Siesserıs specialty is the study of
micro-organisms fossilized in the rocks. Because different species
of micro-organisms lived at different periods, Siesser can date
a particular layer in a core to within a million years or fewer
by examining the collection of microfossils found there. His work
provides the "basic calendar" that the other scientists rely upon
in their analyses.
He is one of only about 10 people in
the United States who studies a group of microfossils called calcareous
nanoplankton that provides one of the best dating methods of the
kind for the last 200 million years. These tiny marine organisms
are closely related to diatoms, forams and amoebas.
When the ship reaches the spot where
it is going to drill, it takes about 24 hours to lower the drill
train to the seafloor. In the relatively soft material that it encounters
as it begins to drill, it can produce a new nine-meter core every
hour.
Each time a new core came up on his
watch, Siesser took a small sample from the bottom of the core,
broke it up and mixed it with a small amount of water in a test
tube and spread the mixture on a microscope slide, leaving a thin
residue that revealed hundreds of tiny fossils when viewed under
a microscope. In this fashion, Siesser was able to identify the
types of nanoplankton in the bottom layer of the core and to use
the information to come up with a rough estimate of the layerıs
age. He would then give that estimate to his fellow researchers
before doing a more thorough analysis of the core.
The number of samples he must analyze
to date the different layers in a given core depends on the length
of time that it spans. The less time between the dates of the top
and bottom layers, the fewer intermediate samples he must analyze.
During the periods when the new cores were coming hourly, Siesser
didnıt have much time to waste. But, as the drill penetrated deeper
into the seafloor, the rock becomes harder and, as a result, the
rate at which new cores were brought on board dropped to one every
three to four hours.
"Normally, there is plenty of time
to take a break, go get a cup of coffee," he said. During the voyage,
the scientists took cores from six of the seamounts in the Emperor
Seamount Chain extending from Kamchatka to the Midway Island. The
first of these were fairly far north so, although it was summer,
the weather was cold and chilly. "There wasnıt much incentive to
go for walks on deck," Siesser says.
These cores provided the scientists
with the detailed information they needed to reconstruct the hot
spotıs early movements. Because of the structure of Earthıs magnetic
field, a compass needle is horizontal at the equator, but points
straight down at the North Pole. Thus, the angle of the needle is
an indication of the latitude. Lava contains magnetic particles
that act like compass needles and the orientation of these particles
becomes fixed when the lava cools. So, by measuring the orientation
of these particles in the old seamount cores, the onboard experts
were able to determine the latitude at which they formed. Combining
this information with Siesserıs dates and relevant information about
the plateıs motion allowed the scientists to determine what the
hot spot has been doing for the last 80 million years.
Their reconstruction indicates that,
for its first 37 million years, the hot spot was moving at about
the same rate as the Pacific plate, but in a different direction.
The southerly direction of the Emperor Seamount Chain is the result
of the sum of the motions of the hot spot and the plate. Forty-three
million years ago, however, the hot spot became stationary while
the plate continued to move. That is when it began producing the
Hawaiian Island chain.
Last summer may be Siesserıs final
deep-sea drilling voyage: "After the last few trips, Iıve said,
'Thatıs my last!' I've already spent an entire year at sea on these
voyages. But then, a few years later when I'm asked, I've decided
to go again!"
Professor Siesser's home page:
http://geo.cas.vanderbilt.edu/siesser.htm
Ocean Drilling Program web site:
http://www.oceandrilling.org/
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