DEEP SEA HYDROTHERMAL ACTIVITY
PROFESSOR ADAM SCHULTZ
Apart from pictures of volcanic eruptions the sight of "smoking
chimneys" and pillow lavas forming on the ocean floor has captured the
imagination of the public for several years now. We were very fortunate
to have the opportunity to hear from Professor Adam Schultz what it is
like to be so close to the "Plexiglas" portholes in the submersible and
watch the action.
"Alvin" is occupied by two scientists and a pilot, all inside the 2M
diameter titanium sphere to reach the depths and cope with the pressure
there. Its maximum speed of about 4Km/hr and with lights penetrating
only about 15M means that pre-marked targets laid by the mother ship are
needed to avoid delays reaching sites of interest.
Finding the sites we now visit has involved a vast amount of exploration
from many sources, some specific research trips, but just as much
information has come as a spin-off from everything from transatlantic
cable laying to sonar mapping from redundant military equipment.
The matching of earthquake data to mountain ranges found to divide the
oceans, magnetic strips of sea floor with "mirror image" patterns have
gradually indicated the targets to aim for.
It was the results of measuring of heat loss through the ocean floor and
trying to link the results to the ridges where new crust was forming
that attracted attention. If the spreading ridges had new crust and were
higher than most of the ocean floor, they should be losing heat at a
greater rate than was being recorded. Also deep sea drill cores showed
alterations to the rocks suggesting reactions with sea water. It was
even possible to estimate that the water temperature must have been
around 350 degrees C. by measuring the silica content of the
hydrothermal fluids.
As the new crust moves away from the ridge it cools and cracks which
allows the sea water to penetrate to the hot basalt. Spreading rates
vary between 3cm/y to 7cm/y in parts of the Pacific and the axial zones
up to 1KM wide.
The interaction of the sea water with hot rock in the system becomes
very complicated with both leaching of some minerals and deposition of
others such as magnesium and sulphate into the crust. Release of
hydrogen sulphide, hydrochloric acid and silicic acid makes the water
more corrosive and manganese zinc and copper are transported up the
vents.
Samples of some of the active chimneys showed that the composition
changes as the walls build and the hot water deposits minerals on the
inside and the outer skin cools against the normal sea water. Material
falling around the columns from the "black smokers" also adds to the
structures and surrounding surface. The black particles, mainly iron
sulphides, look very spectacular but they are really only the tail end
of the whole recycling procedure.
Most chimney sites are along parts of the Mid Atlantic ridges where
spreading rates are slower and the areas where sea water can penetrate
are narrow and more restricted. Some areas above depressions where magma
has been removed have found to have deposits left by dense fluids
sinking and not mixing with the sea water.
Much of the results of exploration along these active ridges has helped
to explain long-standing questions about the regulation of the
composition of sea water and the associations of various mineral
deposits with suites of rock identified as oceanic in origin. These
ophiolites in Cyprus, Oman and Canada among other places have provided a
source of metals since early times.
The discovery of the "Extremophiles" living so close to
the hot springs and smoking vents is a topic on its own account.
Collecting these specimens and keeping them alive in our "alien"
environment is difficult but some bacteria have already been harnessed
into industrial action and surely more will follow into their
equivalence of outer space!.
Our thanks go to Professor Adam Schultz for a very informative lecture.
For once the usual request "can we have a Field Trip please?" is
definitely not appropriate.
Our good wishes go to him and all his colleagues we hope all their
voyages will be completed safely in the future.
Antarctic Diatoms
Dr Jenny Pike
Jenny gave us a "blow by blow" account of her recent
visit to Antarctic Waters to sample the distribution of modern diatoms
in the sea. They combined readings of temperature, salinity and plots of
the different species to achieve a reference base to use when fossil
cores and samples are being examined. This is done quite frequently
especially when rates of climate change are being assessed.
Fossil assemblages can be very unrepresentative because many factors
such as seasonal patterns, preservation conditions, sedimentary regimes
can severely restrict the picture we draw of the environment of the
time. Micro-biota also present difficulties as often only certain body
parts survive and estimates of numbers have to be revised as Jenny
explained when some samples proved to be remnants of only part of a life
cycle.
The costs of making such a long trip are considerable and so "hitching a
hammock" on one of the B A S survey ships meant that she had to cope
with not having some equipment, such as a suitable microscope, with out
being able to complain too much! We saw some spectacular pictures of the
ice sheets around the West Antarctic Peninsula and the difference in
spring sea temperatures between the east and west sides were very plain
just by counting the numbers of ice floes on the surface. They had to
revise part of the tour as the collapse of the Larsen B ice shelf
blocked access to one of the bays they were due to visit.
Back home, processing the samples began and then the problem of
analysing the results to display the findings began. With so many
different variables:- sea water temperature, depth and salinity etc.
several types of mathematical processes were employed to screen the
results. It was found that the types of diatoms found were in four basic
groups from the near shore, inner-shelf, outer-shelf and slope areas.
The seasonal effect also showed up when some of the samples were clearly
only part of the total life cycle of the creature.
The effort involved served to emphasise how much care is needed when
using the fossil record to draw conclusions about the past environment
or make predictions for the future. We enjoyed the lecture and it
complimented the previous one as another example of up-to-the-minute
research-its problems and successes.
OUGS in Norway
Linda Fowler
For the last session of the day Linda Fowler gave us a
talk about a recent O.U.G.S. excursion to Southern Norway. They took a
clockwise route from Bergen up the Western coast to see the Devonian
sediments in these fault controlled basins in the much older quartzites,
schist and gneiss. The Hornelen Basin is the largest and was subject to
strike-slip movement during deposition. Added to this down faulting
along the northern side has produced shorter overlapping fan deposits
while the south has larger fans spreading further into the basin with
finer longer toes trending westwards as the basin filled from the east.
The basement rocks have been through many phases of metamorphism and
tectonic alteration and it is possible to see all these effects within
very short distances, sometimes producing mylonites now close to
relatively undeformed suites of rocks as the complicated thrusting has
mixed and matched the blocks.
Towards the end of the trip a visit to Larvik to see the home of the
ornamental stone we all recognise was a "must". Buildings in the main
street had names made up of letters of the stone and looked very
impressive. This intrusive igneous rock is also found occasionally in
Greenland and Canada in small amounts.
The visit was quite strenuous with a lot of miles covered and the usual
mixed bag of weather but was appreciated by all the participants.
Once again the whole day was worth the trip and it was
good to renew acquaintances and meet some new faces.
We do hope that next year some more people will be able to attend as
visitors are always welcome. Remember a lot of their members are new to
geology as well so we all learn something along the way!. Our thanks to
The Severnside Branch Committee for a good session.
References:- O.U.G.S. COURSE BOOK S238 The Earth's Physical Resources. |
