The rig superintendent on JOIDES Resolution is Pepe Estevez, a Spaniard who hails from the Canary Islands. Pepe was here for the original drilling at Hole 735B during Leg 118. In the meantime, Pepe retired, worked for awhile in real estate, and then came back, which might be likened to going to heaven with the option to return. In any case, he is a very skilled driller, and we are glad to have him.
Pepe is as interested in the eventual outcome of this drilling as anyone. Just after midnight each morning when I start my day, Pepe and I have the usual exchange about the latest core of gabbro, always ending with Pepe asking, "Where is the peridotite?" All I can do is point my finger straight down toward the floor, twisting it a bit in simulation of the action of the drill. Peridotites have been dredged from all over the nearby fracture zone, named after the Woods Hole vessel, Atlantis II, and from many others along the Indian Ocean ridges. The rocks are always fragments, usually intensely altered. Their relationships to other rocks in the same and nearby dredge hauls, such as gabbros and basalts, are extremely difficult to work out. Our erstwhile other co-chief scientist, Henry Dick, has made most of a career studying them. They have to be down there somewhere.
As for gabbro, we are now 1350 meters into the stuff, which is a lot of rock, a lot of the same sort of rock. There is a great deal to be made of the fact that we have obtained so much gabbro, but there is definitely an itch to see a real fundamental change in the rocks before we stop drilling in less than two-weeks time. The change everyone would like to see is peridotite.
What is the fascination with peridotite? Well, first of all, peridotite would represent the mantle, the next deeper "layer" of the earth below the crust. On the continents, where most of us walk around, the mantle is about 18 miles, or 30-odd kilometers, down. It is detectable only seismically - as something that produces a kink in the travel paths of seismic waves generated by earthquakes or explosions - and is certainly way out of reach of contemporary drilling technology. Beneath the ocean crust, however, the mantle is only about 7 km below the sea floor, pretty generally everywhere. It shoals a bit in some places, and is a little deeper in others, but 7 km is a fair average.
The kink in the travel paths of seismic waves is the clue to identification of the mantle, beneath both the continents and ocean basins, as peridotite. The kink represents a change in the rate the sound waves propagate through the rocks, increasing from the crust to the mantle from about 6.5 km/sec to something like 8 km/sec. The one rock type which we have in any abundance on the surface of the earth through which sound can move this fast is peridotite, primarily because it consists mostly of a fairly dense mineral, a silicate of magnesium and iron, called olivine. That sonic velocities can only be matched by peridotites has been certified by some of the best labs in the business. Nothing else but a rock with a lot of olivine will do. Some very pure olivines, pea-green in color, can be found in the dark beaches of Hawaii, where sea cliffs made of lavas containing olivine are ground to sand by waves. Rarely, the individual crystals are large enough and pure enough to approach gem quality, in which case the semi-precious mineral is given the name peridot, whence peridotite.
However, we are not in the business of drilling for gem stones. Back in the 1950's, there was a man who liked to study rocks named Harry Hess. He was a professor at Princeton, and also during World War II the commander of a naval vessel, USNS Cape Johnson. The ship was used to land troops and supplies during some of the most famous, and arduous, amphibious operations of the Pacific war. In the course of these duties, Hess became acquainted with echo-sounding records of the sea floor obtained by the Navy, which remained classified until after the War. Hess wrote one of the first post-war scientific papers using de-classified echo-sounding data, about the discovery of flat-topped volcanoes deep under water, which he rightly surmised represented the wave-truncated surfaces of former islands, now deeply subsided. He named these "guyots", after the building in which the Princeton geology department was housed, which itself was named for Arnold Guyot, a rather well known 19th century geologist. It is not known whether the flat top of the building contributed conceptually to Hess's decision to call the seamounts "guyots".
Hess's first love, however, was serpentinite, a rock type on which he had written his doctoral dissertation. Serpentinite is altered peridotite, basically having the same chemical composition, but with a lot of water added. In one of his other projects after the war, Hess organized a research program in the Caribbean. The Caribbean contains large islands such as Puerto Rico and Cuba, and bordering nations such as Venezuela, which are rife with serpentinite. The rocks usually occur in what geologists call "ophiolites", which is a fancy term with a Greek root that stands for a slice of ocean crust faulted into place on land. Hess surmised that the only way to get dense peridotite all the way to the surface of the Earth even in a fold mountain belt was to make it lighter, that is, to hydrate it to serpentinite. Adding water to peridotite involves as much as a 30% expansion of the mineral crystal lattices within it, which of course reduces the density of the rock. Hydrated peridotite is thus buoyant, even with respect to our gabbros on Leg 176. Hess went so far at one point as to propose that there is such a thing as a serpentinite magma, although this was later disproven experimentally.
Hess also followed up his Navy observations by paying attention to the post- war development of marine geophysical research. One preoccupation of the major oceanographic institutions in the 1950's and 1960's seemed to be the spending down of much of the left-over ordnance from the War, in an odd sort of weapons disposal, by carrying out seismic refraction experiments in as many places as possible. Such experiments usually involved two ships, one the "receiver" ship, which stayed in one place and listened, the other the "shooting" ship, which traveled in pre-arranged patterns with someone on deck regularly throwing fuse- lit explosives of different sizes over the side. The ensuing explosions sent shock waves through the water and into the rocks. Much of the energy bounced off the seafloor directly, and was quickly recorded by acoustic sensors on the receiving ship. Other parts of the sound wave entered the underlying rock, and picked up speed in proportion to the rock density, thence to bend and curve back up toward the receiving ship, where the diminished pulse could still be recorded. Some of the energy from the bigger charges, set off at the larger distances from the receiving ship, actually reached the mantle, picked up speed again, and returned to the receiving ship.
The amount of effort expended on this work is hard to imagine. A single station could take 36 hours, with everyone up and attending to sometimes balky equipment for the duration. Special training and licensing was required to handle the explosives, to become a "shooter". More than once there were accidents involving premature detonation. One time in my recollection, an air- gun, which is a heavy steel contraption with fins, and which periodically provides a low-energy pop of sound from where it trails just behind the ship, was blown out of the water by an explosion barely seconds after an ignited charge was thrown into the water. The gun, which must have weighed about 200 pounds, flew back onto the deck, just missing the shooter. The shooter was a graduate-student colleague of mine, Jim Yount, brother of Robin Yount, a standout baseball player over many years with the Milwaukee Brewers.
In any case, at almost every place, over hundreds of refraction stations throughout all the ocean basins, the same picture emerged. There was an abrupt kick in seismic velocities at about 7 km depth. A nearly universal seismic layer existed that formally became defined as the transition between the crust and the mantle in the ocean basins. The transition was called the Moho, an abbreviation of the Croatian name of the seismologist who first discovered the discontinuity beneath the continents in the early part of the century.
For a long time, marine geologists did not know what to make of the ocean crust. For one thing, there wasn't much sediment over most of it, as revealed by recordings made using air guns for sound sources. Then again, the crust was uniformly thin, only 7 km. But how old was it? A standing doctrine in geology, dating from the middle of the 19th century, was the permanence of continents and ocean basins, that the two have occupied their respective places on the face of the Earth essentially from the dawn of geologic time. The continents themselves are very old. By the 1950's geologists understood that the continental interiors dated from the Precambrian, thus were billions of years old, and had experienced numerous cycles of mountain building and erosion. There was speculation that the oldest materials in the ocean basins might be even older than the oldest rocks on continents. But why should the ocean crust be old, but thin and relatively unsedimented?
In the early 1950's, though, came a great surprise. Hess's guyots were dredged, including one of them named after his ship, Cape Johnson. Some of them still had classified Navy positions. A geologist named Ed Hamilton, who wrote the principal report for this work, carried out during Scripps Institution's MidPac Expedition, only briefly conveys what early marine geology must have been like. Wholesale dredging had not been done before. A dredge station, called a "haul", involves dragging a heavy chain bag with a gaping steel mouth over the bottom, in hopes of snagging and ripping off rock. A heavy steel wire is attached to the dredge. This is passed over a massive grooved wheel, essentially a large pulley, suspended from a movable A-frame out past the end of the vessel, to a strong cable reel, or winch, on the aft working deck, which is called the fantail. The wire is payed out, or pulled back, using the winch. But the wire has its limits. Hamilton's report, a Memoir of the Geological Society of America, records a desperate learning curve, with something like nineteen unsuccessful attempts, many of them involving loss of the dredge, before the first rocks came on board.
A dredge station is typically carried out with a tensiometer recording snags on rock. The fantail is cordoned off in case the wire should snap and whip back. At each snag the tension mounts, both on the wire and among the observers of the tensiometer. At times, the tension reaches nearly to the breaking point along the wire, which is supposed to be a weak link either at the dredge bale, or at the shackle between the wire and the dredge itself. However, the idea is to get the dredge back with some rocks, which usually doesn't happen if the weak links fail. Sometimes, the situation provides an end-run around the weak links, such as when the wire coils itself around a rock. Then, in order to get free, all you can do is pull and wait for the snap. One time on one of my early cruises, the main block holding the pulley on the A-frame was ripped from its mountings, in what was later called a "fatigue failure", with the wire then parting as it was dragged violently over the steel transom at the end of the deck. The ship itself jumped. There is nothing quite like watching a tensiometer going from nearly 20,000 pounds to zero instantaneously, and feeling the ship jump at the same time. Getting free of hang-ups during a dredge station is one of the great arts of doing geology at sea.
Hess originally thought that the guyots might be as old as Precambrian. The rocks brought on board during MidPac Expedition, were a scientific bonanza. The guyots proved to be Cretaceous in age, on the order of 100 million years old, and they were capped with sunken limestone reefs comprised of the fossils of organisms which had occupied the tidal zone in the time of the dinosaurs. The summits of the seamounts were also indeed eroded volcanoes, since basaltic lava rock, much like that of Hawaii or Tahiti, was obtained. Some hauls even contained cemented sands made up of grains of basalt and altered olivine - perhaps once peridot. Although the guyots were surely old, they were barely more than crust-rust compared with the bulk of the continents. Tall features deep under water in the ocean basins, at least, were young, and the seafloor on which they were built was restless. Since the guyots stretch from Hawaii to Japan, a huge tract of ocean crust in the western Pacific had subsided some 700 fathoms (about 1300 m) in a fairly short period of geological time. The sedimentary record on the continents, in fact much of the geological record familiar to most geologists, marks fluctuations in sea level, but these comings and goings of the sea, especially in the middle of continents, are mere lappings compared with the subsidence of the guyots. This result gave thoughtful geologists, like Hess, something to ponder.
Harry Hess also believed in direct solutions. Following a panel meeting at the National Science Foundation in 1957, one of the sort which reviews scientific proposals, Hess and Walter Munk, a geophysicist at Scripps, came to the conclusion that they hadn't seen very much in the group of proposals being reviewed that really came to terms with the main problems of geology. The idea they hatched was, Why not answer the main problems of earth science, which concerned the age and characteristics of the deep rocks of the ocean basins, and their contrast to the continents, all at once, by drilling?
Hess had one other bee in his bonnet. Contemporary interpretations of the seismic result, the 7-km-deep universal seismic layer, made little sense to him. Many seismologists were now proposing that the seismic transition to the mantle in the ocean basins marked the change from gabbros, like those we are drilling on Leg 176, to peridotites, with the gabbros representing slowly cooled basaltic magma. Dredging by this time had shown that the top of the ocean crust consisted of basaltic lava at places like the Mid-Atlantic Ridge. The basalts were draped with only a few hundred meters of sediment near the continents. Below that was terra incognita, and the speculation that the underlying rocks were gabbros was simply that - speculation, although some gabbros had also been dredged from fault scarps in the Atlantic. However, Hess knew that neither sedimentary basins nor piles of basalt, like Hawaii, can accumulate to such a uniform thickness everywhere. Not even major gabbro intrusions on land, like the ones Hess knew best, the Stillwater complex in Montana, and the Bushveld in South Africa, have a uniform thickness. Therefore, in the same paper in which he came up with sea-floor spreading, and almost as an aside, he offered the postulate that Moho represents a chemical transition, with some thinner and variable accumulation of basalts and gabbros lying above serpentinized peridotite. Moho thus represents the limit, either thermally or physically, to which water can exist in significant quantities in the upper mantle. Hess thought that the water might actually come from the Earth's interior. This was in effect a holdover of his serpentinite magma idea. Now, we know from the bulk compositions of fresh basalts that their source in the mantle is very dry. Therefore the current variant of Hess's idea is that there is a limit to the depth to which water can penetrate along cracks through the crust and into the upper mantle. The Moho is at the end of a cracking front, the maximum depth that water from the deep oceans can work its way downward.
Nevertheless, none of this was known for sure. It all remained untested, and remains so to this day. Hess, the man who believed in direct solutions, saw drilling as the answer to this problem as well. The drilling concept spawned from his discussion with Walter Munk was taken on by a group with the odd name of American Miscellaneous Society, or AMSOC, and formalized as the Mohole project. Hess, being a well-connected admiral in the Navy by this time, knew how to get things funded. This is all amply recorded in a book entitled A Hole in the Bottom of the Sea by the Mohole director, Willard Bascom, which is in our ship's library, and in several memoirs by participating scientists. Mohole had many objectives, some of them already mentioned. Nevertheless, the ultimate target was Hess's, and it stemmed from his career-long fascination with peridotite and serpentinite. The idea of drilling a hole all the way to the mantle in the ocean basins, solving most of the problems in one bold, daring act, made sense to the man who had landed troops in the Pacific. Besides, on the scale of the lunar program, then looming, the cost would be peanuts, and it would apply directly to the planet we inhabit, rather than to a lifeless stone.
Now we, on Leg 176, in effect inherit Hess's mantle. In thirty years of scientific ocean drilling, this leg is the very first time that there has been an opportunity, even a ghost of a chance, to drill through the crust-mantle transition, and to see whether Hess was right or wrong. There are lots of complicated scientific issues at stake, more than in Hess's day. These can be explained in their proper place. But what gives our venture its most serious emotional resonance is this almost legendary heritage of bold dreams and strong action, with a pang of regret that it has taken so long to get this far. However else we consider it, our drilling does get to the very nub of major scientific problems that have not been solved since Hess's day.
To reach the mantle, even in a place like Hole 735B where nature has brought it close the surface and removed maybe as much as half of the overlying rock, is still not easy. As I said, we are 1350 m down, and continue to plow assiduously through gabbro. The mantle is down there, in the direction I point each morning in my conversations with Pepe. Some day, maybe tomorrow, we will get there.