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Imagine a series of remotely controlled laboratories strung out like
space stations along the seabed and equipped with sensors to monitor physical,
chemical and biological activity of the Earth’s crust over a period of
months, or even years. Far-fetched though this may seem, it is already technically
feasible, and firmly on the agenda of marine scientists who are exploring
the mid-ocean ridges. These long, narrow mountain chains, between two and
four kilometres below the sea’s surface, are regions of intense geological
activity. It is here that erupting magma cools into the new ocean crust
that covers two-thirds of the Earth’s surface. Compared with the slowly
forming continental crust, which makes up the other third and is on average
about 2000 million years old, the ocean crust is recycled rapidly in geological
terms, and has a mean age of only 100 million years.

Over the past 12 months, Europe and the US have sent reconnaissance
expeditions to three of the most accessible and contrasting of the Earth’s
12 or so mid-ocean ridges: the Reykjanes Ridge, which lies southwest of
Iceland, the Mid-Atlantic Ridge and the East Pacific Rise. Japanese scientists
are exploring the remote ridges of the Indian Ocean, and more expeditions
are planned over the coming months by researchers from other countries.
But, while these snapshots are useful, the explorers now want to monitor
the mid-ocean ridges continuously – hence their call for a string of permanent
laboratories on the seafloor.

Continuous crust creation

The latest projects build on the three decades of research since the
great revolution in earth sciences. In September 1963, two geophysicists
at the University of Cambridge, Fred Vine and Drummond Matthews, took the
first steps in turning the idea of continental drift – at the time still
on the wilder fringes of geology – into a widely accepted theory . They
brought together evidence that had recently been discovered to show that
not only is ocean crust being created continuously at mid-ocean ridges,
and moving outwards on each side, but that the rate of crust creation can
be measured.

Over the following years, other researchers helped to transform this
concept of the ocean floor spreading into the theory of plate tectonics,
which postulates that the Earth’s surface is divided into large plates that
are moving relative to one another at a speed of between 1 and 20 centimetres
a year. The final step in this transformation came in September 1968, when
three geophy-sicists – Bryan Isacks, Jack Oliver and Lynn Sykes – at the
Lamont-Doherty Geological Observatory in New York State, added evidence
supporting the theory that they had gathered from observing earthquakes.
By then, it was no longer intellectually respectable to disregard the fundamental
revolution brought about by plate tectonics – the first quantitative explanation
of the geological behaviour of the outer part of the Earth.

In the static world before the theory of plate tectonics was developed,
the oceans were seen as dim, dark receptacles. From time to time, the ocean
crust rose to allow animals and plants to troop from one continent to another,
but generally it was submerged, inert and unexciting. Plate tectonics showed
that the ocean crust was being continuously renewed, was much younger than
the continents and completely different in origin. It also revealed the
significance of the mid-ocean ridges. As Vine and Matthews had predicted,
the youngest ocean crust was found at the mid-ocean ridges in a band only
a few kilometres wide. Geologists now believe that all ocean crust is constructed
in these narrow zones, at the rate of 3 square kilometres of ocean floor
every year, renewing the whole ocean floor every 100 million years.

The first significant attempt to understand crustal construction was
mounted in the 1970s, when a French-American programme known as FAMOUS focused
on a section of the Mid-Atlantic Ridge a few hundred kilometres southwest
of the Azores. The US Navy allowed geologists, for the first time, to use
the results from its then secret multibeam echo-sounding system, from which
they were able to make the most detailed contour maps of the seafloor ever.
Deep-diving submersibles, the American Alvin and the French Cyana, also
took geologists down to the mid-ocean ridge to examine seafloor volcanoes,
lava flows, fissures and faults for the first time. At the slow-spreading
Mid-Atlantic Ridge, these features lie in a deep rift valley running along
the line of the ridge, whose two sides are moving apart at a rate of 3 centimetres
a year.

In the mid-1970s, attention turned to the fast-spreading East Pacific
Rise, where crust is created at the rate of 10 to 20 centimetres a year.
Instead of a deep, axial valley, there is a smooth rise at the axis. Towards
the end of the decade, geologists were astonished to discover areas of hot
springs close to the chain of volcanoes strung out along the axis. These
hot spring fields, each up to 200 metres across, emit water at temperatures
of between 350 and 400 °C, which precipitates sulphides of copper,
iron and zinc as it mixes with the cold sea water. Because the pressure
is so great at these depths – between 200 and 400 atmospheres – the emission
remains liquid. (Incidentally, these fluids are so toxic that the European
Community would not allow a factory to discharge anything like them into
the sea.) But the real surprise is that this hostile environment supports
a wide range of specialised organisms, which are nourished by bacteria that
live on the sulphide in the hot spring water. Suddenly, biologists, too,
were interested in mid-ocean ridges.

Split ridges

Meanwhile, geologists were intent on finding out how mid-ocean ridges
vary along their length. Originally, they had thought of a ridge as being
more or less uniform along its length, and set about investigating what
they considered to be a typical section. But as Jeff Fox of the University
of Rhode Island and Ken Macdonald of the University of California Santa
Barbara discovered in the 1980s, the ridge is actually split into segments.
The variety of segments, and the nature of the boundaries between them,
cast new light on the dynamics of ocean floor spreading, showing that the
formation of crust is not as simple as geologists had first assumed.

By the late 1980s, geologists generally agreed about the structure of
the simpler fast-spreading mid-ocean ridges and the processes by which ocean
crust is created. They pictured the plates as moving apart, with the soft
mantle rising between them, not as a continuous sheet but as a series of
plumes tens of kilometres apart. Each plume feeds a separate segment of
the spreading axis. As the hot mantle rises, it starts to melt. This molten
rock is less dense than the rest of the mantle that is left behind, and
so rises through it and continues to rise until it reaches a thin magma
chamber between 1 and 2 kilometres below the seabed. Magma is extruded from
here through narrow, vertical dykes, emerging as lava flows. The resulting
lava pile forms the upper ocean crust. At the same time, crystals of silicate
minerals pile up on the floor of the magma chamber and sink under their
own weight. They form thick layers of coarse-grained gabbro, which makes
up the lower crust.

At times, the crust is fractured, creating faults that penetrate through
the upper crust. These faults, and other cracks and pores in the lava, allow
cold sea water to penetrate downwards close to the magma chamber. There
the water becomes heated, reacts chemically with the rock and is transformed
into a hot, acidic hydrothermal solution, rich in sulphides and metals.
Its density is much reduced by heating, and it rises through the crust to
form the hot springs on the ocean floor. The animal communities that colonise
these vent sites vary in different parts of the world. In the Atlantic,
the vents are covered by swarms of small shrimps, whereas in the East Pacific
Rise the springs are dominated by thickets of immense tube worms up to 3
metres long and 10 centimetres in diameter. In the southwest Pacific, the
characteristic animal is a hairy snail.

While geologists agree about the basis of this refined model of mid-ocean
ridges, the research that led to it posed more questions than it answered.
For instance, how is melt produced in the mantle, how does it rise through
the overlying rocks and how does it mix in the magma chamber? How do creatures
survive in the hot springs, and how do they colonise newly formed hot springs,
some of which are thousands of kilometres away?

Some of these unanswered questions have led geologists far beyond the
confines of their own discipline to look at, for example, what causes the
crust sometimes to be deformed by faults and at other times to be split
by dykes. They also want to know what controls the flow of water in hot
spring systems; why the water temperature is limited to 400 °C, and
why is it as high as this; and why the spreading rate has such a dramatic
effect on the ridge profile. Answering the many questions arising from this
complex system will require the combined efforts of researchers from fields
as diverse as theoretical physics, geology and experimental physiology.

In 1987, an interdisciplinary group of scientists met in Oregon to
discuss the way forward, and three major projects emerged: the US Ridge
Interdisciplinary Global Experiments (RIDGE), funded by the National Science
Foundation; the British Mid-Ocean Ridge programme (BRIDGE), funded by the
Natural Environment Research Council; and the French-American Ridge Agreement
(FARA), supported jointly by the two governments. The projects are now in
full swing. A powerful international network of expertise and interests
is focused through a parallel venture called InterRidge. At present, 16
countries have formal or informal links with InterRidge, including the US,
Britain, France, Germany, Japan, Canada, Australia, Iceland, Italy, South
Korea, Mexico, Norway, Portugal, Russia, Spain and Sweden.

The expeditions to the Reykjanes Ridge, the Mid-Atlantic Ridge and the
East Pacific Rise over the past 12 months were part of the RIDGE, FARA and
BRlDGE projects. Their aim was to discover more about how mid-ocean ridges
varied and to select critical areas for more detailed study towards the
end of the programmes. The results exceeded all expectations. Explorers
mapped the contours of the ocean floor in new areas of the ridge system,
located young basalt lavas along the axes of the mid-ocean ridge, and discovered
two new hot spring fields in the Atlantic.

In September 1992, a FARA expedition led by Charlie Langmuir, a geologist
from Lamont-Doherty, came across a new hot spring field only a day’s voyage
southwest of the Azores. While dredging for basalt lava, they brought up
large lumps of iron sulphide with clumps of mussels still attached. The
new field was named, appropriately, Lucky Strike. Six months later, an expedition
led by Bramley Murton, a geologist from the NERC’s Institute of Oceanographic
Sciences Deacon Laboratory, at Wormley, Surrey, discovered the Broken Spur
field in a segment of the Mid-Atlantic Ridge, at 29 degrees North. The discovery
followed an intensive survey of the water column, seeking clouds of dispersed
sul-phide particles. Both segments look promising for future research.

Before these expeditions, only three hydrothermal vent sites were known
along the entire Mid-Atlantic Ridge: Snakepit at 23 degrees North, Trans
Atlantic Geotraverse (TAG) at 25 degrees North and Steinaholl at 63 degrees
North. Initially, BRIDGE researchers assumed it would take some time to
find a suitable segment for study, and even longer to find the rare vents.
So they had planned to move gradually from large-scale reconnaissance studies,
involving mapping over hundreds of kilometres, to a scale below 50 kilometres
and, later, to studies of individual volcanoes and hot spring vents. These
recent discoveries have allowed the more fine-scale work planned by BRIDGE
researchers, such as the biological studies, to be brought forward: biological
studies under the RIDGE programme are already well under way.

Several other BRIDGE expeditions are in prospect. Last month, Murton
led another party to the Reykjanes Ridge, to collect basalt samples from
which he intended to discover how the magmatic processes vary along the
ridge as it comes closer to the sea level towards Iceland where, exceptionally,
the mid-ocean ridge crosses land. This month, Martin Sinha of the University
of Cambridge and Christine Pierce of the University of Durham are visiting
the same area to study the crust’s structure and to hunt for magma chambers,
using seismic and electromagnetic exploration techniques.

Dynamic observations

The work to date reveals, through a series of snapshots, a dynamic
environment, geologically, geophysically and biologically. Spreading segments
do not produce a fixed amount of new crust each year, nor do they erupt
lava all the time. Periods of intense activity seem to be triggered every
hundred years or so, after movement has stretched the crust by a few metres.
Chang-ing activity at hot spring fields may be synchronised with this periodicity,
and biological communities must change in sympathy. Such complex, dynamic
systems are hard to understand if they are observed only at intervals. If
they can be observed through a period of change, their internal dynamics
are more clearly revealed. ¿ìè¶ÌÊÓƵs are now seeking ways to achieve this.

One way is to be ready to make observations during one of the short
periods of activity, when faults are moving, volcanoes are erupting, or
a new hydrothermal system is emerging. Much was learnt from observing a
swarm of earthquakes on the Reykjanes Ridge in November 1990, and a greater
advance has since come from the unexpected ‘greening’ of the US Navy. Since
June, the navy’s once-classified arrays of hydrophones, the sensitive underwater
microphones devised to track submarines, have been directed towards finding
volcanic eruptions and small earthquakes on the ocean floor. Chris Fox of
the National Oceanic and Atmospheric Administration organised the first
listening programme, which was focused at the Juan de Fuca mid-ocean ridge
in the Pacific, a section that runs 300 to 400 kilometres off the coast
of Oregon and Washington in the northwestern US. Just four days after listening
started, an eruption was heard. ¿ìè¶ÌÊÓƵs on a Canadian research ship in
the area pinpointed the source of the sounds, and found new lava flows overlain
by large plumes of warm water. Clearly, new ocean crust was being created
below the ship. The eruption lasted about ten days.

Another approach is to install major automatic observatories at carefully
chosen sites, to operate for several years in continuous contact with scientists
on land. This formidable task, for which international coordination would
be essential, has been discussed by several groups in the US and in Europe.

In July, the idea was aired more fully at a BRIDGE workshop at the IOSDL
at Wormley. Workshop members envisaged a small number of undersea laboratories
equipped with seismometers, strain gauges and tilt meters, to monitor earthquakes,
volcanic tremors and quieter events, such as crustal stretching. Scanning
sonar devices would monitor hydrothermal vent fields, while video cameras
would concentrate on individual hot spring chimneys. Chemical sensors would
measure changes in the chemistry of the fluids emerging from hot springs,
while grids of temperature sensors measured the heat flowing from them.
Biological life on the ocean floor could be monitored by free-swimming vehicles
mounted with video cameras, and seafloor incubators would conduct experiments
with organisms under near-natural conditions. To reveal more about how these
areas are colonised, artificial springs, already being developed at the
University of Leeds, could be used to dispense sulphide into the water to
encourage bacteria and larvae to settle.

Some of the instruments needed to make this possible are not yet adapted
to work in the deep ocean, while others are not up to running continuously
in such a hostile environment for periods of between one and five years.
But similar instruments do already exist, and have been used under real
conditions for months at a time, either singly or together. In research
centres in the US, Britain and France, scientists are developing them and
working on the systems that will link them to land.

In the absence of natural activity, researchers can always create it.
Next year the Ocean Drilling Program’s drill ship, JOIDES Resolution, will
drill a series of holes in a major hot spring field, the TAG field, on the
Mid-Atlantic Ridge at 25 degrees North. In Britain and the US, BRIDGE and
RIDGE scientists and technologists are planning to send instruments to examine
the disturbance such drilling will cause to fluid flow, temperature and
chemistry, and to the biological community of shrimps and anemones. It will
be similar in many ways to the disturbance caused by a new fissure breaking
through the vent field.

One of the driving forces of the work is the recognition that we know
more about the surface of remote objects in the Solar System than about
the dark seafloor of our own planet. Vine and Matthews’s paper on continental
drift triggered far more than they could possibly have imagined. A whole
new area of scientific activity has emerged, built firmly on foundations
laid more than a quarter of a century ago.

Johnson R. Cann is professor of earth sciences at the University of
Leeds and chief scientist of the BRIDGE project. Cherry Walker is scientific
coordinator of the BRIDGE project.

* * *

Bringing it all together

In September 1963, the paper by Fred Vine and Drummond Matthews wove
three different strands of thought, all current at the time, into a single
elegant hypothesis.

First, measurement of the direction of magnetisation of young lava flows
had shown that the Earth’s magnetic field has reversed in direction periodically
in the past, so that the north magnetic pole became the south magnetic pole,
and vice versa. Careful dating of the lava flows was allowing the construction
of a reversal timescale, showing a characteristic pattern of switches –
22 in all over the past 4.5 million years.

Secondly, proponents of continental drift had begun to argue, following
the lead of Arthur Holmes of the University of Durham and Harry Hess of
Princeton University, New Jersey, that continents drift apart as oceans
expand, and that the expansion might be happening at mid-ocean ridges. Thirdly,
magnetic surveys off the Pacific coast of North America showed that the
Earth’s magnetic field is distorted by striped anomalies running parallel
to sections of mid-ocean ridges.

Vine and Matthews suggested that if new ocean crust was being created,
it would be magnetised alternately, in one direction and then in the opposite
direction, as the Earth’s field reversed; they claimed that the strips of
crust of alternative magnetisation would produce the observed anomalies.
If so, then the pattern of anomalies should be a mirror image on each side
of a mid-ocean ridge, and that pattern should correspond to the pattern
of the reversal timescale. If, in turn, that could be demonstrated, then
not only would the creation of new crust be certain, but measurement of
the rate of creation would be made possible, because the dates of individual
magnetic reversals had been established on land.

Using a magnetic survey of the Juan de Fuca Ridge, off western North
America, Vine and Tuzo Wilson of the University of Toronto demonstrated
that all of these conditions could be met and, in 1965, they were able to
make the first estimates of how fast the crust is created. Within a few
years, the principle had been extended to other oceans and to older crust,
and ocean floor spreading was established.

In 1967, this theory was expanded into the broader concept of plate
tectonics. Dan McKenzie of the University of Cambridge and Bob Parker of
the Scripps Institution of Oceanography in California showed how the northeast
Pacific mid-ocean ridge could be seen as the result of the movement of rigid
plates of the solid outer layer of the Earth, which is known as the lithosphere,
over the softer interior. This idea was extended to the whole Earth over
the next year. In September 1968, Bryan Isacks, Jack Oliver and Lynn Sykes
of what is now the Lamont-Doherty Geological Observatory in New York state
completed the picture by adding the latest information from earthquakes
to the global story.

In the course of these five years, a new quantitative, predictive model
of earth sciences had been developed.