3

Across the spreading ridge

The winds, the sea, and the moving tides are what they are. If there is wonder and beauty and majesty in them, science will discover these qualities … If there is poetry in my book about the sea, it is not because I deliberately put it there, but because no one could write truthfully about the sea and leave out the poetry.

Rachel Carson, Acceptance speech of the National Book Award for Nonfiction (1952).

The diary

Wednesday 28 December 1972

The last few days on board have been quiet. We have been moving steadily south at eight and a half knots on grey to blue seas. Last night the rolling became excessive and we had to change course because a hatch broke and we were shipping waves—but generally it’s been pretty smooth—or am I just becoming acclimatised to the roll? Yesterday I put some pollen slides out on the front of the pipe rack. I collected them today and replaced them with fresh ones. I’m hoping to pick up any wind-blown pollen, and will be surprised if I catch anything but rust and rainwater, but it’s worth a try. They are very exposed to the prevailing westerlies, and as far from the salt spray as I can safely get them.¹

The report on the first hole progresses—my part in it was really finished with description of the rock sequence. Peter Barrett is doing the sedimentology. I feel I should be contributing more at this stage but my turn will come. In the lab I have been trying to process nannofossil oozes—limy muds with algal remains—for dinoflagellates. These common single-celled organisms possess characteristics of either plants or animals; they may photosynthesise or they may ingest prey as a nutrition source. Many are a key part of the marine ecology although freshwater forms are known, but I have had no recovery so far.

Ship life is physically tiring—the propping against the roll and heave of the ship is exhausting and the lab floor is hard and cold. And a good deal of the night seems to be spent just hanging on to the bunk! Tonight I will fix myself a firmer nest … I count myself very lucky never to suffer seasickness. Poor Chen, one of the palaeontologists, gets very sick when the ship slows to a stop over a new drillsite.

No further sea life is visible—we have today the same albatrosses and shearwaters.

It’s too early to say yet who are the major workers or the contributors to life on board. Denny H as co-chief scientist rarely gets up from his bunk so bad is his sea-sickness; the other co-chief, Larry Frakes, a sedimentologist, is an enthusiast; Peter Webb, a palaeontologist who works with foraminifera—tiny organisms of calcium carbonate—is good value; Derek’s [Burns] English humour is welcome … the American Art Ford, who will take on the study of the crystalline rock of the ocean basement is quiet and hard-working; the rest are still unknown. But there’s a general eagerness to make a contribution as part of what is very much a team effort.

Friday 30 December 1972

We are slow in approaching our next hole (Site 265; Site 2) because of heavy seas and winds, at least that’s the explanation—for this part of the world it surprises me how mild the conditions are—it’s sunny, but the wind is cold. I gave the Antarctic parka its first airing today. Mine, dug rather randomly out of a big trunk full of gear, is so huge I can hardly stand up in it, but it was welcome. The bird life has increased—yesterday we were joined by three or four big dark giant petrels; they are smaller than albatross but fly in the same way and have large heavy pale hooked beaks. Today a flock of about a hundred white-faced storm petrels—brown of back and white of chest—follows erratically in our wake. It’s intriguing to see such tiny birds so far from land.

I talked home last night via the ham radio. The call was unexpected and gave me a start when I was called to run up three decks to the radio operator; I felt a bit rushed and nervous. There is a sense of filling in time today—I hope tomorrow will be busier. Late night film marathons are dreary and there are too many bad films. I would rather sit in the lounge and talk.

The Southeast Indian Ridge

We drilled the next three sites, labelled officially Sites 265, 266 and 267, on the southern flank of the Southeast Indian Ridge. This feature is a low rise—a wide, subsea mountain range that lies midway between Australia and Antarctica. Its topography is smoother than many of the world’s mid-ocean ridges, and it consists of a series of east–west trending ranges. Its name comes from the fact that it can be traced into the Indian Ocean where it forms the mid-Indian Ridge, and links to another major ridge that encircles Africa. It is thus part of the network of mid-ocean ridges that encircles the globe, extending through all the world’s oceans, lying in most cases (although there are exceptions) midway between continental masses. The ridges are key to understanding the processes of sea-floor spreading and plate tectonics. At their junction, two pieces of oceanic crust—the tectonic plates into which the Earth’s surface is divided—are moving away from each other. At these mid-ocean ridges new sea floor is constantly pushed apart by magma rising up from the Earth’s mantle below. The descriptive term ‘divergent tectonic boundary’ is apt. The system of ridges and the processes occurring there have been described evocatively as ‘the wound that never heals’.

The Southeast Indian Ridge separates the Australian plate from the Antarctic plate. It’s an active spreading centre, which means that Australia is moving in a northeasterly direction at a rate of 69–75 millimetres a year. The rate varies a little along its length, as does the shape of the ridge, but the average rate of movement makes the Australian plate one of the fastest moving on Earth. Earthquakes are frequent—though usually small—on or close to the crest of the ridge, showing it to be an active volcanic centre.

To reach the ridge and the sites on its southern slopes we had sailed across the Diamantina Fracture Zone, then across the South Australian Abyssal Plain, an essentially flat feature of the Southern Ocean floor, which in places reaches depths of around 6,000 metres.

The abyssal plain; sediments of the sea floor

Abyssal plains are features of the deep ocean floor—the abyss—and they are known to be the ‘flattest, smoothest and least explored regions on Earth’ (Wikipedia). The smooth surface is usually interrupted by abyssal hills—low mounds, rarely more than a few hundred metres high. These are sometimes elongated, with the long axis parallel to the spreading centre, a shape that possibly results from stretching of the sea floor associated with spreading at the mid-ocean ridges. Abyssal plains are usually underlain by oceanic basalts of the sea floor, dark-coloured rocks rich in iron and magnesium-silicate minerals.

A steady rain of sediments that blankets and smooths any uneven surfaces causes the flatness of the world’s abyssal plains. There are clays and silts that come from the eroding of the continents. This detritus from the land is washed out to sea by turbidity currents—short-lived, often violent events that carry a mix of water and sediment from the shallow edges of the ocean basins into the deep sea where they accumulate as layered deposits, or turbidites. Slumping or sliding from the steeper basin edges sweeps these sediments out to sea. Characteristically, turbidite beds are graded, with the larger and heavier fragments settling first at the base and the finer, slower-settling clays on top. The nearer the land, the coarser the beds are overall.

Other deposits come from continental dust blown out to sea, and from the fine, airborne debris of volcanoes. But much of the sediment that rains onto the sea floor from above is of biological origin, coming from the shells or skeletons of tiny organisms living at the sea’s surface. These particles—pelagic biogenic sediments—sink from the upper layers of the ocean to accumulate at depth. Mostly, the temperature of the sea surface controls the kinds of microorganisms that thrive in the upper zone of the ocean.

Generally, the warmer waters of the tropical oceans support more microorganisms with calcareous shells—those made of calcium carbonate. Foraminifera are prominent among these (Figure 3.1). These single-celled animals have chalky, chambered shells perforated with tiny holes through which the internal cell plasma protrudes. Of equal importance are marine algae with carbonate-rich cells. These are the coccoliths, striking in their architecture, with calcareous disks covering a spherical body or coccosphere (Figure 3.2). Their remains, when they accumulate on the sea floor, are usually called nannofossils, in reference to their small size.

Figure 3.1. Foraminifera; Globigerina bulloides. Scale bar 100 microns.

Source: Courtesy of Katsunori Kimoto; image database of planktonic forams. JAMSTEC (Japan Agency for Marine-Earth Science and Technology).

Their accumulated deposits, when solidified, often form the limy deposits we know as chalk—the dramatic White Cliffs of Dover are a prime example, their soft white chalk being made up mostly of coccoliths.

These tiny fossils—microfossils—rich in calcium carbonate, are frequently used as ‘palaeo thermometers’, giving a measure of the temperature of the oceans in which they lived, and of the amount of glacial ice on the globe at that time. These measures are based on the chemistry of oxygen in the calcium carbonate (CaCO₃) in their walls.

Figure 3.2. Coccosphere with coccoliths; Gephyrocapsa oceanica. Scale bar 1.0 microns.

Source: Wikipedia Commons.

Oxygen is made up of differing isotopes, reflecting the different numbers of protons and neutrons in its different forms. Its most common form has 8 protons and 8 neutrons, giving an atomic weight of 16, written as ¹⁶O. This is the ‘light’ form of oxygen. Heavy oxygen, a rarer form, has two extra neutrons, and an atomic weight of 18 (¹⁸O). The ratio of these isotopes reflects the composition of the seawater in which they lived, and has been changing through time in response to past climates. The ratios are affected both by temperature and by glacial ice; in a most general way, a colder world, and a world with glaciers, would be evident in a greater concentration of ¹⁸O in a sample of fossil walls.

In the deep ocean depths, factors other than the ecology of the organisms dwelling at the sea surface come into play and influence the nature of the bottom sediments. As the calcareous shells reach certain depths, they begin to dissolve. Waters at these depths are undersaturated in calcium carbonate because of cooler temperatures and higher pressures there. This results in a pattern of sea bottom sediments where only the higher areas, such as the tops of mid- ocean ridges, are covered with carbonate-rich sediments. As the sea floor on the ridge flanks descends to greater depths in the processes of sea-floor spreading, the carbonates are dissolved out and only clays, usually red or grey in colour, remain.

The cooler seas of high southern latitudes lie south of a boundary within the ocean—the Antarctic Convergence—sometimes referred to as ‘the Polar Front’. The Antarctic Convergence is closely linked with the Antarctic Circumpolar Current, discussed more fully in Chapter 7. Upwelling of deep, nutrient-laden waters make the zone a region of high organic productivity. North of this boundary, microorganisms mostly have shells of calcium carbonate; to its south lies the domain of organisms with shells of silica. Deposits on the sea floor, dominated by either carbonate or silica, should reflect the changing position of this boundary through geological time.

The seas to the south of the Convergence are rich in diatoms (Figure 3.3). These beautifully sculptured single-celled algae are sometimes arranged in colonies; glassy walls of opaline silica surround their cells. The shells, or frustules, are finely perforated and commonly form two valves, which sit one inside the other, like two petrie dishes, often forming chain-like structures. These most common phytoplankton are for the most part photosynthesisers, getting their energy from sunlight, thriving in waters rich in silica and other nutrients. They are abundant not only in the cooler waters of the Arctic and Antarctic or Subantarctic seas, but also in lower latitudes where deep waters are brought to the surface by the vagaries of the ocean circulation. Confined to the upper layers of the ocean—the photic zone—where sunlight enables photosynthesis to happen, diatoms are of supreme importance as the base of the food chain. Around Antarctica, population explosions or blooms of these algae occur in the extended daylight of summer. In winter, when sea-ice spreads north, away from the continent’s edge, they continue to grow, either trapped within the ice, or on its undersides, and are released into the open ocean with the spring melt. In these seas the phytoplankton sustain the shrimp-like krill, which in turn provide food for the larger marine animals—fish and penguins and the large baleen whales.

Silica also forms the perforated, glassy walls of radiolarians, minute protozoans that are another important part of the microscopic plankton in the upper layers of the ocean. These are zooplankton, feeding on other plankton, rather than photosynthesising like the diatoms.

Figure 3.3. Diatoms from the Southern Ocean. Scale bar 20 microns.

Source: Courtesy of Julien Crespin, formerly of Weizmann Institute, Rehovot, Israel.

The steady rain of detritus from the ocean surface accumulates as sediment on the sea floor. At first the sediments are soft and watery, ‘soupy’ is an apt term; later they become sludgy, and consolidate into firmer strata. The soft muds are referred to, appropriately, as oozes. Their composition further defines them—those rich in diatoms are diatomaceous ooze, or more generally, because of their chemical composition, they are simply siliceous ooze. There are radiolarian oozes too, and carbonate-rich mixes including Globigerina oozes (named for a particular genus of foraminifera) and nannofossil oozes, all of which are broadly contained within the chemical term calcareous ooze. Oozes can be of mixed composition—some of those drilled at Site 266 were described as predominantly rich in diatoms, but with traces of radiolarians, foraminifera and nannofossils.

Ernst Haeckel; science, art and the sea floor

The beauty and startling complexity of radiolarians and diatoms was brought to the attention of both the scientific and the popular public when microscopic fossils in material dredged by HMS Challenger’s expedition were illustrated by the flamboyant German biologist, artist and atheist Ernst Haeckel. Haeckel described some 4,000 species in sea floor samples that HMS Challenger collected in the course of its four-year, globe-encircling voyage of 1872–76. These were lavishly illustrated in 140 colour plates in a series of reports published after the expedition.

Haeckel, born in Prussia in 1834, was one of the first to encourage the teaching of evolution. He first visited Charles Darwin in Down House in 1866. Though an ardent admirer of Darwin, he was prone to misinterpret his ideas, or at least to add his own idiosyncratic slant on them. Among his prodigious output of books, popular lectures, papers and journal articles, he promulgated a view of evolution that was more in line with the views of Jean-Baptiste Lamarck—that is, that features acquired by organisms through interaction with their environment during their lifetime could be passed to future generations. Haeckel is also remembered for his contention that embryos in their development reflect stages in the developmental history of their species—‘ontogeny reflects phylogeny’.

But it was his views on human evolution that sullied his reputation as a biologist. His contention was that the human races had evolved separately, and in hierarchical form, with Mediterranean peoples as the highest level ‘at the head of all the races of men, as the most highly developed and perfect’ (Haeckel 1914). Unsurprisingly, this view has been interpreted as contributing to the rise of Nazism. The biologist Stephen J. Gould linked what he called Haeckel’s evolutionary racism to ‘his call to the German people for racial purity and unflinching devotion to a just state’ (Gould 1977, p.77). Others, however, have found little historical evidence to link Haeckel’s views on evolution with the somewhat later rise of Nazi nationalism.

Yet in spite of his tendency to put forward rather wild and untested ideas—his ‘irrational mysticism’ according to some critics—Haeckel has left an indelible mark on biology, both through the terms he coined, words such as phylum, phylogeny, ecology, protist, and by his portrayal of nature as art. How passionately he felt about this is clear not only in the Challenger reports, but also in his Artforms in Nature, a series of coloured prints issued in 10 sets between 1899 and 1904. His subject matter included radiolarians, foraminifera, corals, sponges, anemones and other marine life forms. From these it is clear how much Haeckel’s art is stylised; intricately detailed, yet accurate in its portrayals; often highly coloured, even psychedelic in effect (see Figure 3.4). So much has its popularity continued to grow that it is now a commercial flood, reproduced as posters, coffee mugs and coasters—even printed on fashion accessories and shoes. It has inspired artists in jewellery, glass and iron. Surely there is no other artist who has set life, often microscopic life, so vividly before so wide an audience.

Figure 3.4. Radiolaria. Pl. 99 from Ernst Haeckel, Report on the Radiolaria collected by H.M.S. Challenger during the years 1873–76, 1887.

Source: HMS Challenger online. Wikisource Creative Commons.

Continental drift; towards the unifying theory of plate tectonics

The idea of the continents in motion is an ancient one. But only since the 1960s have ideas been brought together in a theory that draws on information from so many sources—from structural geology, seismology, palaeontology, geophysics, Earth magnetism, oceanography and a range of other Earth-related disciplines. Plate tectonics as a concept is as important to the Earth sciences as evolution is to the biological sciences. Indeed, even within the sphere of biology, plate tectonics has made its own major contribution, clarifying the paths by which plants and animals might have moved around on the Earth’s surface. It has answered fundamental questions such as how great mountain ranges have formed, and why earthquakes and volcanoes are found in particular regions of the globe.

There were a number of phases in the development of this unifying concept. The idea that the continents we know might not have always occupied their present positions goes back as far as the Flemish cartographer and dealer in antiquities Abraham Ortelius, who has claims to have produced the first atlas. In 1587 he published his Thesaurus geographicus. In a revised and expanded version of this in 1596, he put forward his ideas relating to the drift of continents, suggesting that the Americas were ‘torn away from Europe and Africa … by earthquakes and floods’ and further that: ‘The vestiges of the rupture reveal themselves if someone brings forward a map of the world and considers carefully the coasts of the three (continents)’ (USGS 2012).

Francis Bacon in 1620 might also have had an early inkling of this concept in his Novum Organum. But Bacon, while drawing attention to the presence of similarities in fragments of the coastlines of Africa and South America, made no suggestions that these had formerly fitted together as parts of a jigsaw.

It was the German Alfred Wegener who first seriously proposed the idea that the apparently excellent fit of the continents on both sides of the Atlantic was the result of their being once united and subsequently disrupted. Wegener was primarily a meteorologist and geophysicist; most of his publications concerned meteorology, atmospheric physics and polar exploration. His standard textbook on meteorology, published in 1911 and which dealt with the thermodynamics of the atmosphere, was inspired largely by his extensive fieldwork in Greenland, whose icecap was eventually to claim him. He saw service in the First World War, and it was allegedly while recovering from war injuries that he worked on his new theory of continental drift. He expressed the origin of his ideas thus:

The first concept of continental drift came to me … as far back as 1910, when considering the map of the world, under the direct impression produced by the congruence of the coastlines on either side of the Atlantic. At first I did not pay attention to the idea because I thought it improbable. (Wegener 1966, p.1)

The apparent fit of the coastlines on both sides of the Atlantic intrigued Wegener, but he also felt that he would need more evidence of their former union from the geology of each landmass. Accordingly, he undertook what he referred to as ‘a cursory examination of the relevant research in the fields of geology and palaeontology’. In a study obviously much deeper than cursory, he showed fossil plants and animals to be similar on both sides of that ocean. These were life forms that could not have been transported across a wide ocean gap. The strongest evidence came from the presence of identical fossil species in matching coastal sequences in Africa and southern South America. Further, he was able to trace geological structures; mountain belts, geological fracture zones and other structures in the continents on both sides of the Atlantic. To Wegener, these examples were compelling evidence that the now widely separated continents had once been joined.

Wegener brought his ideas together in The Origin of Continents and Oceans, first published in German in 1915. Subsequent and expanded editions followed in 1920, 1922 and 1929, and the first English translation appeared in 1924. In these, he postulated that the present continents had once been united into a single enormous landmass that he called Pangaea, from the Greek ‘all the earth’. Subsequent splitting of Pangaea and the drifting apart of the fragments of continent resulted eventually in the positions of the present continents.

The geological community as a whole responded negatively, sometimes vituperatively, to Wegener’s ideas of continental drift. The widespread opposition was based on the perceived lack of any convincing mechanism whereby continents might plough their way through an apparently solid ocean floor without breaking up in the process. He visualised them as icebreakers ploughing through ice sheets and driven by centrifugal and tidal forces.

Much of the opposition to Wegener’s theory came close to ridicule—particularly in North America, where in 1926 the American Association of Petroleum Geologists organised a symposium specifically in opposition to his theories, and most authors, according to science historian Henry Frankel, ‘had a field day attacking Wegener’. The main objections raised related to the proposed mechanisms for propelling the continental blocks. The tidal forces Wegener envisioned were considered too weak to move continental masses. In the light of contemporary geological and geophysical understanding ‘the conception is improbable in the highest degree’ (Frankel 2012, vol. 1, p.169).

However, Wegener’s ‘mobilist’ ideas, that the present continents had shifted in the past, did have some supporters. Some scientists, at least, were prepared to put his ideas to the test in an unbiased way. The eminent English geologist Arthur Holmes, for instance, suggested that convection currents generated within Earth’s mantle might be responsible for moving continental blocks. He appealed to radioactivity deep within the crust as a heat source for this motion; ascending currents could rupture the crust and move its fragments sideways. The continental fragments would thus be carried along with the sea floor, rather than ploughing their way through it (see Holmes 1965). Even with such a believable mechanism proposed, Holmes’s ideas didn’t receive universal support, reflecting perhaps a stubborn resistance to change from a variety of geologists and even biologists, who held to firm fixist ideologies, embracing the idea of fixed continents.

Reception to the ideas of continental drift was more sympathetic in the southern continents. Wegener’s ideas found support in the work of the eminent South African geologist Alexander du Toit, whose extensive fieldwork in Africa and South America convinced him that the present continents of the southern hemisphere had formerly been united to form Gondwana Land. Du Toit published his ideas in 1937 in Our Wandering Continents: An Hypothesis of Continental Drifting. He dedicated this volume to the memory of Wegener, who had perished on his Greenland glacier in 1930. Du Toit’s focus on Gondwana Land, the southern mass of Pangaea, encompassing the continents of Africa, southern South America, Australia and India, was based on his deep knowledge of parallels in the stratigraphy and palaeontology of these continents, reflecting their former union.

The sequence of Permo-Carboniferous sedimentary rocks (about 300 million years old) on all these continents begins at its base with tillites—rocks with clasts or fragments ranging in size from boulders to sand grains, all jumbled together in a manner showing that they originated from glacial activity. He would have been most familiar with the Dwyka Tillite of southern Africa, which is imprecisely dated at 290–300 million years old. The glacial sediments are everywhere overlain by coal-bearing strata containing the fossil seed fern Glossopteris; this colonised swampy habitats when the southern continents warmed after the glaciation.

Within scientific circles Glossopteris has achieved iconic status for its impact on the early developing ideas of continental drift. Part of its fame is also linked to the history of its discovery, and to its relationship to the heroic age of Antarctic exploration. The discovery of fossilised leaf fragments of Glossopteris collected by Edward Wilson, artist, physician and scientist on Scott’s ill-fated polar expedition in 1911, has already been told, with their description by palaeobotanist A.C. Seward in 1914. When Seward published these fossil plants he is unlikely to have been aware of Wegener’s proposal of continental drift, the English translation of which did not appear until 1924. Seward, however, did compare the Antarctic specimens with others from India, and commented on the climatic implications of the find, suggesting too that the Glossopteris flora might have originated in Antarctica. Du Toit picked up Seward’s comments. While not agreeing with the suggestion of an Antarctic origin for the flora, he made clear that the spread of the flora, across the now widely separated continents, suggested ‘the absence of geographical hindrances to spreading, such as mountain ranges or seas’ (Du Toit 1937, p.83).

From South Africa came another voice in support of mobilism. This was that of another palaeobotanist, Edna P. Plumstead. Her enduring reputation rests on her discovery of the subtle reproductive structures of Glossopteris—structures that are intimately linked with the large tongue-shaped leaves—and also on her recognition of the similarity of the flora from all Gondwana continents, including Antarctica. She considered that almost identical fossil floras found now on widely separated areas could only be explained by invoking former movement, or splitting apart of continents that were once united (Plumstead 1962).

A revival of interest in continental drift began in the 1950s, when scientists studying the past history of Earth’s magnetism breathed new life into the controversy. Earth’s magnetic field can be thought of as a giant magnetic dipole thrust through the Earth’s centre, at a slight angle to its rotation. At the poles, a compass needle points vertically downwards; near the equator it is horizontal. The magnetic poles differ, just a little, from the geographic poles. In the South, the magnetic pole now lies in the Southern Ocean, and is said to be moving north at a rate of 10–15 kilometres a year.

Tracking the position of the magnetic poles through geological time shows that they appear to have ‘wandered’ in relation to their present position, and that their so-called ‘polar wander curves’ vary from continent to continent. This suggests that different continental blocks have moved independently of each other with respect to the present position of the magnetic poles, a situation that can best be reconciled with the ideas of continental drift.

It was particularly in Britain that there was a concentration of interest in palaeomagnetism, although testing the continental drift theory was not the initial driver of this research. Again, science historian Henry Frankel has provided a very detailed account of the development of this discipline primarily in Britain, and the way in which its focus shifted from a general understanding of palaeomagnetic properties to its value in the drift controversy.

In addition to the apparent wandering of continents in relation to the magnetic poles, Earth’s magnetism undergoes a complete reversal from time to time; the positions of magnetic north and magnetic south changing relatively quickly. A magnetic field whose direction is the same as the present field of the Earth is referred to as ‘normal polarity’; the opposite as ‘reversed polarity’. The record of such polarity switches is preserved in the newly formed crust and sediments of the Earth. More recently, the dating of these polarity switches using known sequences of volcanic rocks with a time scale based on radiometric ages has proved crucial to identifying the timing and mechanisms underlying sea-floor spreading. In the 1960s, patterns of magnetic reversals were recognised as ‘magnetic stripes’ on the sea floor. As magma is extruded from mid-ocean ridges onto the sea floor it bears the magnetic signal of the Earth at that time. This record, be it normal or reversed, is thus preserved. The next extrusion may bear the opposite signal; the pattern of magnetic ‘stripes’ is symmetrical about the crest of the ocean ridge.

I was introduced early in my career to these magnetic sea floor stripes. The significance of these patterns on the sea floor was recognised in 1963, when Fred Vine, then a PhD student in Cambridge, and his supervisor Drummond Matthews published a paper in the journal Nature entitled ‘Magnetic Anomalies over Oceanic Ridges’. This was the year that I began my own PhD in Cambridge. I had heard the idea presented there in student seminars, such as given in the Sedgwick Club, that venerable society of geology students. However, not being a geophysicist, I was impressed by the artistry of the patterns, but struggled then to grasp the idea and couldn’t predict its ultimate significance!

One of the aims of our own Leg 28 cruise was to test the ages of the sea floor on the southern flank of the Southeast Indian Ridge. Because new sea floor is being generated at the ridge, and moving away from the ridge as the magma cools, it would be expected that the sea floor would become systematically older towards the south—the spreading occurring in a north–south direction. The ages of the sea floor had already been estimated by previous marine geophysical surveys in the region, using reference to the palaeomagnetic time scale. So we were able to check the accuracy of the estimated ages either by getting an age—from radiometric or geochemical data—on the basalts that underlie the sea floor, or else we could use the fossil content of the sediments immediately overlying them. In all cases the basalts proved too weathered to be dated by radiometric techniques. The ages obtained by palaeontology—by using the fossils—however, were consistent with the expected magnetic ages. So, in ‘our’ Leg 28, Site 265, closest to the ridge crest, gave an age of 12–14 million years based on nannofossil oozes and chalks; this is close to what might have been expected from magnetic data. At Site 266, further from the ridge crest, the fossils gave a date of around 22 million years, close to the expected 23–24 million from magnetism. For Site 267, the deepest of the three sites on the ridge, the fossils declared an age of 38–42 million years—within the range of magnetic ages.

Mapping the sea floor

A fresh understanding of the motion of continents came in the 1950s as new knowledge was gleaned of the structure of the ocean floors. The key to this was recognition of the role of the mid-ocean ridges as sites where new sea floor is being generated. The presence of some kind of plateau or ridge-like structure in the middle of the Atlantic was then not new but could be traced back to early oceanographers. A chart of the Atlantic sea floor published in 1854 by Matthew Maury, director of the US Navy Depot of Charts and Instruments, showed a plateau in the mid-Atlantic that he named the Dolphin Rise. The voyage of HMS Challenger confirmed the existence of such a rise in the northern sector of the mid-Atlantic. Then, surmising that such might be continued into the south, the Challenger expeditioners obtained depth soundings on their homeward voyage in 1876 that confirmed the presence of a narrower mid-ocean ridge in latitudes as far south as southern Africa.

It was not until World War II that there was a substantial increase in understanding the structures and dynamics of the sea floor relating to the mid-ocean ridges. Anti-submarine warfare demanded a better understanding of the behaviour of sound through water, and this led to an intensified study of the sea floor itself. Marine geology developed rapidly in the postwar environment. This was exemplified by the development and expansion of marine geology in oceanographic institutions in the United States. Academic organisations such as the Scripps Institution of Oceanography in California, linked to the University of California, the privately funded Woods Hole Oceanographic Institution in Massachusetts, and the Lamont-Doherty Geological Observatory of Columbia University, all underwent significant expansion. The US National Science Foundation was established in 1950 and further facilitated marine geological research. Governments elsewhere, in Britain, the USSR, France, Canada and Germany followed suit.

Prior to World War II the physiography of the ocean floor could only be understood through closely spaced records of ocean depths made by physical soundings—by dropping measuring devices overboard as on HMS Challenger, which only gave depths for single points. It was during World War II that echo-sounders were developed that gave a continuous record of sea floor depth below the passage of the recording vessel. Improvements in the resolution of these followed, with higher precision depth recorders being developed in the 1950s. One who was able to take advantage of his naval command, and of this advance in wartime surveying technology was Captain, later Rear Admiral Harry Hess. Using his ship’s echo sounder, he was able to collect ocean floor profiles and gravity data cross the North Pacific.

But much of the credit for drawing the accumulating information on the sea floor together in the form of detailed and comprehensive maps rests with geologist Bruce C. Heezen of the Lamont-Doherty Geological Observatory in New York, and his partnership with the map-maker and geologist Marie Tharp. Together, using depth and seismic data that Hess had accumulated, the pair was able to produce maps that have been considered to be the most important pieces of scientific art of the twentieth century.

Marie Tharp is often described as Bruce Heezen’s assistant, or as the cartographer he hired to organise the data he collected at sea, and eventually to illustrate his maps of the deep ocean floor. But hers was a much more significant contribution than a mere cartographic assistant. For many years she was unable to go to sea herself, limited by naval regulations that denied women this privilege. But it was her acute assessment of the reliability of available depth measurements and her interpretation of the sea floor landscapes that led to her discovery that a narrow V-shaped groove seemed to be consistently present along the mid-ocean ridge crests. This suggested a similarity to the rift valley of East Africa, a region where the Earth’s crust is known to be splitting apart. Marie Tharp allegedly pointed this out to Bruce Heezen, but his initial reaction was to deny the implications of this—it looked too much like continental drift and confessing that would have been severely career limiting at that time.

Producing maps of the sea floor from this data had its own problems, particularly in its early phases. The pair—who were by then partners in life as well as science—were unable to produce regular contour maps for security reasons. These were considered to provide information that could potentially be used by enemy submarines. So they fell back on a known method—physiographic diagrams—that could produce images of what the terrain would look like from a low-flying aeroplane. These gave realistic images of how the sea floor might appear if the ocean basins were to be drained of water.

To enhance the images of the sea floor they were able to enlist the help of a landscape painter, the Austrian Heinrich Berann. The National Geographic Society of the United States assisted them in this. Berann, born in Innsbruck, developed his unique approach by combining the traditions of European landscape painting with modern cartography; his work had already found application to the tourist industry in Austria and in the United States, particularly through his popular maps of national parks for the National Geographic Society. In 1967 the National Geographic magazine produced a map of the floor of the Indian Ocean. This was based on the results of an earlier Indian Ocean expedition, a massive international and interdisciplinary study of the northeastern sector of that ocean. In the planning stages of that project, Marie Tharp was approached and asked if she would produce a map, and quickly, of the Indian Ocean floor, to show where the gaps in information might be.

After a frenzy of activity, the map of the sea floor was completed with the help of graduate students and was presented at an international conference in New Delhi, with Bruce presenting the map and Marie answering questions about it. Although the pair had to weather criticism from the director of their institution over budgetary and other issues, the map was finalised with Heinrich Berann’s artistic input. It was published in 1967 by National Geographic in the form of a foldout supplement to the regular journal, thus reaching a potential audience of some 6 million souls, something of a standout among science communication efforts.

The map of the floor of the North Atlantic followed (Figure 3.5). Marie Tharp’s accurate plotting of Heezen’s data showed the North American continental shelf dropping abruptly to the abyssal plain, then the sea floor rising to the summit of the mid-ocean ridge, then descending again to the flatness of the abyssal plain as the European and African coasts are approached.

The careful art of Heinrich Berann is worthy of a close look. He used a range of blue-grey tones, with skilful lighting from a source on the right of the map illuminating the highs of the Mid-Atlantic Ridge and the continental slope offshore from the North American coast. The groove of the central ridge, and of transform faults that cut at right angles across the ridge are almost black. The fine detail shown thus creates an almost rippling effect. The sediments that smooth the abyssal plain are depicted in light browns. All is in stark contrast to the ochre-yellows of adjacent landmasses. A grossly exaggerated scale (some 40 times) intensifies the dynamics of the sea floor structures.

Bruce Heezen died suddenly in 1977, at the age of 53. At the time he was undertaking research in a submarine off Iceland. Marie was then working on a research vessel at the surface; they were due to meet up in Reykjavik to discuss the details of the soon-to-be-published World Oceanographic Seafloor map. Instead of the planned meeting, Bruce Heezen’s body was taken to Reykjavik to be shipped back to the United States.

Figure 3.5. Sea floor map of the North Atlantic. Artwork by Heinrich Berann.

Source: National Geographic Image Collection 1968.

After his death, Marie carried on on her own, completing what work she could. Of prime importance to her was first to complete the World Oceanographic Seafloor Panorama. This was funded, not by the National Geographic, but by the Office of Naval Research, and was published in 1977. Subsequently, she went on to carefully curate all of their joint work and correspondence, and to contribute much of this to the Library of Congress. To support herself financially she established a map distribution company and lived in her house in New York until she died of cancer at the age of 86.

Figure 3.6. Marie Tharp and Bruce Heezen.

Source: Courtesy of Estate of Marie Tharp and Lamont-Doherty Earth Observatory.

I was jolted into a reminder of this pair and their productive but sometimes feisty relationship, when I visited Iceland in 2015. In Reykjavik Harbour, puttering out on a fishing trip, my attention was drawn to a chunky white vessel bearing the American flag and tied up at the dock. The name on its side showed it to be the Bruce C. Heezen; the array of instruments on its stern deck showed that it was one of the US Navy’s oceanographic vessels. It was the first US Navy vessel to be named by civilians. A little research showed that Grade 9 students entering a competition chose the name. Further investigation showed that its launch was attended by the appropriate admirals, but also, as a Matron of Honour, by Bruce Heezen’s mother. The citation, or press release of the launch, in 1999, notes Heezen’s role as a marine geologist in coming to an understanding of the Mid-Atlantic Ridge with its central groove, in plate tectonics; it also makes a passing reference to his contribution to the ‘famous Tharp physiographic maps’. It gave no further elaboration of Marie Tharp’s role in their partnership.

The young American writer Hali Felt published a biography of Marie Tharp in 2012. In this biography, replete with imagined conversations and described in The New York Times as ‘a testament both to Marie Tharp and the author’s imagination’, Felt documents the battles that Marie had for recognition of her work. First were the US naval regulations prohibiting women going to sea; then the ongoing battle for funding to keep the map-making going, a battle that resulted in her doing a great deal of the work from her New York home. Nevertheless, towards the end of her life Marie Tharp enjoyed a great sense of achievement, commenting:

I worked in the background for most of my career as a scientist, but have absolutely no resentments. I was lucky to have a job that was so interesting. Establishing the rift valley and the mid ocean ridge that went all the way round the world for 40,000 miles—that was something important. You could only do that once. You can’t find anything bigger than that, at least on this planet. (Woods Hole Oceanographic Institute 1999)

Perhaps the pinnacle of recognition for her achievements came with the 2016 publication of a book for children about her life; Robert Burleigh’s Solving the Puzzle Under the Sea.

In reality, the new understanding that these maps made of the dynamics of a mobile rather than a fixed picture of the Earth is considerable. Bruce Heezen’s initial reluctance to admit that the groove running down the centre of the mid-ocean ridges was linked to the spreading of the sea floor was overcome when it was discovered that the epicentres of shallow earthquakes appeared, in the Atlantic and Indian oceans, to follow the line of the rift. This, and the fact that the profiles of the Great Rift Valley in Africa made by Marie showed that there, too, shallow earthquakes were confined within the walls of the valley. This seemed to confirm that these ridges were the zones where new sea floor was being generated.

The observations, made possible essentially by Marie Tharp’s very precise mapping, provided a very visual and aesthetic view of the Earth. This was an important peg in the subsequent development of plate tectonics. But that is what it was—one major contribution that enabled the rapid development of the theory, particularly during the 1960s. It contributed to the reintroduction of a mobilist view of the Earth, one that had been fiercely rejected by the American geological community. But it was just that—a contribution, though a vital one. Bruce Heezen initially interpreted what they saw in the maps as due perhaps to an expanding Earth; the symmetry of the Atlantic mid-ocean ridge he felt supported such a view, a view that has subsequently been rejected.

What was still to come was an understanding of the way in which ocean crust, generated at the mid-ocean ridges, was subsequently drawn back—or subducted, into the deep Earth—in the major belts of deep earthquakes like that of the Andes or the broader ‘ring of fire’ around the Pacific. The contribution made by Harry Hess was significant in this respect. In 1950, back at his alma mater Princeton University, he produced an informal report to the Office of Naval Research, advancing the theory that the Earth’s crust could move laterally away from long volcanically active ridges in the ocean. It was only after the description of the world-encircling ridges and their grooves by Marie Tharp and Bruce Heezen that he was able to fully understand what his profiles across the North Pacific Ocean meant. He published his theory in 1962 as a paper, ‘History of Ocean Basins’, in a volume produced by the Geological Society of America.

In an early expedition—Leg 3, in December 1968 to January 1969—Glomar Challenger herself played an important role in confirming the generation of new sea floor at the mid- ocean ridges. Drill sites positioned on the Mid-Atlantic Ridge between South America and Africa showed that sediments immediately above the oceanic basement—dated by their contained microfossils—increased in age with distance from the ridge crest.

A number of other scientists are often quoted as being influential in the development of plate tectonic and sea-floor spreading theory. The Canadian Lawrence Morley independently developed an explanation of the sea floor magnetic stripes. The hypothesis of sea floor spreading and the significance of the magnetic stripes is often referred to as the Vine-Matthews–Morley hypothesis. Then fellow Canadian J. Tuzo Wilson in 1965 contributed ideas on the nature of sea floor faults, and the way in which tectonic plates move against each other; Dan McKenzie, a fellow PhD student at Cambridge, suggested in 1969 how processes in the Earth’s mantle—the layer between the Earth’s crust and its core—were responsible for movements of tectonic plates.

It has been said that great scientific discoveries seldom arise from a single thought, but they do seem to emerge at particular times and places. This is the case with Heezen and Tharp from the 1950s to the 1970s at Columbia University, with Harry Hess at Princeton, and with Vine, Matthews, McKenzie and others in the 1960s at Cambridge.

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