November 12, 1995
Location: Approximately 59 51' South Latitude x 57 50' West Longitude
Sea Beaming in the Southwest Scotia Sea looking for plate boundaries.
Update: As we start our second day surveying in the Drake Passage, the
people on board are getting their sea legs again. It is a welcome sight
to see smiling faces at the mess hall table again. We celebrated Veterans
Day a bit late with spice cake sp ecially prepared by the ship's cooks.
There are a number of veterans on board, but this celebration was made at
the request of the OSU science group in recognition of Gary Klinkhammer's
service in Vietnam. Gary is the project director for the Oregon State
science group. The cooks also made fresh ice cream today, so those of us
that made it to mess today had two special desert treats.
Stepping out on the deck you see only sea. Water in all directions,
waves, and white caps galore. It is in sharp contrast to the calm sunny
"icescapes" that we had become accustomed to in the Bransfield Strait.
The evening was a time for cards and practical jokes for the members the
crew and science staff that were not on watch. Ben Sloan found his cruise
mascot Barbie doll had been ceremoniously removed from on watch next to
the Sun computer used to analyze Sea Beam data and make maps. A few
members of the crew had moved her to a new more precarious watch position,
dangling from the ceiling above the computer. Everyone had a good laugh,
and Ben took the joke in stride, posting a note on the watch bulletin
board from the doll expressing her displeasure with her treatment, and a
vow never to appear on watch again.
Sitting watch during survey times is a tedious task. On watch we are
responsible for monitoring all equipment and keeping a written log. Data
about location, depth, speed, and a number of other types of information
are constantly being recorded to disks on the ships computers. The
purpose of the written log is to provide a quick overview of the cruise
that can be used as a reference to locate data that has been recorded by
the computer. When on watch, we have five computer monitors and five TV
monitors that we have to watch to make sure that data is being stored by
the computers. In the event of a problem, we have to make a notation in
the log, and then take the necessary steps to make sure that the problem
is corrected. We make written notations of the monitor data every five
minutes when we are on survey. We must also monitor the plotter which
produces a plot of the sea floor from the rough data being collected by
the Sea Beam system. Because of the constant repetition, it is very
difficult to concentrate especially when we have rough seas. It takes an
extra effort to make sure that numbers are not being transposed in the
logs. There are two of us that sit on watch at all times. We frequently
spell one another with breaks so that we can try to stay relatively fresh
and alert.
The reason for this survey in the Scotia Sea is to locate prominent
features on the sea floor that would be consistent with crustal plates
moving against one another. The area that we are surveying in analogous
to the San Andreas Fault in California. At the San Andreas, there are a
number of tectonic plates which are moving relative to one another. The
moving plates create stress in the crust. This stress can be resolved in
a variety of ways. The most violent way to relieve tectonic stress is
thro ugh an earthquake. Prior to an earthquake, the crust bends and folds
producing hills and ultimately mountains. This happens very slowly, and
on the surface of the earth, the forces of wind and weather erode away
these features very quickly often before permanent features are formed.
On the deep sea floor, there is very little motion to cause erosion of
these features. The folding of the crust appears as parallel bands of
mountains on the sea floor. These stress relieving mountain systems are
very steep and distinctive. We have been able to locate some of these
previously unidentified mountain formations during this survey on the Sea
Beam maps that we have produced. This is an exciting first step in
collecting data in this area by the University of Texas team.
We were able to locate four parallel ranges of mountains during our
first day os surveying. Each range was about 6 miles long. Their typical
elevation was over 3000 feet, and from base to base across the mountain,
the distance was about one mile. The slopes on these mountains is 45
degrees or greater. On either side of these parallel ranges we find long
flat plains. The plains are the parts of the crustal plates which are
pushing against one another. The area that we have surveyed is about 75
miles by 75 miles. The mountain ranges, or fold in the crust, appear as
small regions of closely packed contour lines on the Sea Beam maps.
To give you an example of how the plate system works you can make a
simple model using just a piece of paper. Take a piece of paper and place
it face down on a table. Imagine that the paper is a plate under stress.
Place your right hand palm down with your thumb and fore finger just on
the right edge of the paper, pointed toward the middle of the paper. Do
the same with your left hand on the left side. Keeping your hands palm
down, slowly slide your thumbs toward one another. This motion of your
hands is like the plates moving toward one another. You can see that the
paper folds and bends to permit the motion of your hands. Try this with a
large piece of paper. (24 inches or more) and you will see that different
patterns of parallel folds occur. Try moving your hands at angles to one
another with a piece of paper in between. Think of this as an experiment
in plate dynamics.
The folding and bending in the earth's crust actually takes millions of
years to form mountain ranges, but the stresses that produce earthquakes
can take just hundreds of years. For an earthquake to occur, the stress
in the plates is moved by the plates sliding quickly past one another
because the crust being bent and folded breaks or tears, releasing the
built up stress in seconds. This produces an earthquake.
In the Scotia Sea we are looking at a plate system that was active
about three million years ago. Because the features produced by the
moving plates have been preserved on the sea floor, this gives us an ideal
record of how plates interact with one another. By looking at these old
plate systems, we can better understand how the newer earthquake producing
systems work.
