Category Archives: Science in Context

Marie Tharp and the mid-Atlantic rift; a prelude to plate tectonics


Map of North Atlantic mid-ocean ridge

The history of science is littered with the misplaced contributions by women, contributions that at best were pushed aside or ignored, and at worst thought of as shrill outbursts. Witness Rosalind Franklin’s fraught journey to DNA’s double helix, the recent unveiling of Eunice Foote’s experimental discovery of the greenhouse effect of CO2, and Bell Bernell’s discovery of pulsars, as corrections to a history where women found it difficult to escape the status of ‘footnote’. We can add Marie Tharp (1920-2006) to the growing list of corrections. In 1952 Tharp discovered the central rift system in the mid-Atlantic Ocean ridge (that later would become a critical component of sea floor spreading and plate tectonics) but for many years was regarded as a minor player in the burgeoning, post-war field of oceanography.

During the War, Tharp in her early twenties took advantage of opportunities to engage in university study, openings in science and engineering left by men who had gone to battle. She completed a Master’s degree in geology, but given that geology is a field-based discipline, and that women weren’t supposed to go into the field, she extended her studies to a Master’s in mathematics. In 1948 Lamont Geological Laboratory (now Lamont Doherty Earth Observatory) hired 28 year-old Tharp to draft maps of the Atlantic ocean floor, based on the growing database from SONAR and historical soundings. She worked with well-known geologist-oceanographer Bruce Heezen, who spent much of his time at sea. It must have been tedious work, but she counted herself lucky to have a position at all. This was a time when very few American universities (or anywhere else for that matter) offered science and engineering positions to women; a time of patriarchal condescension – “Mad Men” versus “Hidden Figures”.


description of SONAR
Tharp poured over depth and positional data for years, constructing 2-dimensional profiles of the Atlantic Ocean floor. She was aware, as other oceanographers were that an elevated region of sea floor apparently separated east and west Atlantic. This was initially mapped in 1854 by US Navy oceanographer, geologist and cartographer Matthew Maury, and later confirmed with depth soundings taken during the HMS Challenger expeditions (1873-1876 – Challenger had 291 km of hemp onboard to do this kind of thing; the ridge is generally deeper than 2000m). Tharp wasn’t surprised to find the Atlantic ridge on her profiles. What did catch her attention was the rift-like valley in the central part of the ridge; a geomorphic structure that was consistent through all her profiles. She immediately recognized the importance of this, because it implied significant extension, a pulling apart of Earth’s crust in the middle of the ocean. At the time, the general consensus was that ocean floors were relatively benign, featureless expanses. Her discovery indicated otherwise.


bathymetry profiles mid Atlantic

According to Tharp’s bio the response by Heezen and his colleagues was that she was being a typical woman – you know, “girl talk”. One can imagine the coffee room banter; ‘she’s great at drafting cross-sections but should leave the interpretation to the more learned’.

However, after some months and more profiles all showing the same rift- like structure, Heezen gradually accepted that this was real. A turning point for Heezen was the coincidence of several mid-ocean earthquake epicenters along the ridge. This was mid 1953. He understood its potential significance, particularly for those who thought that the hypothesis of continental drift had some credence (Heezen was not initially one of those people).

Ocean bathymetry studies in other basins in the early 1950s (Indian Ocean, Red Sea) revealed similar mid-ocean rifts. Tharp had by this time surmised that a rift valley coursed its way almost continuously the entire length of North and South Atlantic, a distance of 16,000 km; it was the largest continuous structure on Earth. The Lamont Doherty group gradually realized that the Atlantic structure, together with those discovered in other ocean basins, represented a gigantic Earth-encircling system of mid-ocean rifts, more than 64,000 km long.

Heezen presented their research to a 1956 American Geophysical Union conference in Toronto. Marie Tharp barely received a mention. She did co-author a few subsequent publications as an ‘et al.’, but it was a kind of ‘also ran’; the accolades and approbation went Heezen’s way.

Tharp was fired by the Laboratory, the victim of a spat between Heezen and Lamont boss Maurice Ewing, but she continued to develop the maps at home. Marie continued to work in the background, as the humble and grateful recipient of a job she considered to be fascinating; “I worked in the background for most of my career as a scientist, but I have absolutely no resentments. I thought I was lucky to have a job that was so interesting”.

Marie Tharp and Bruce Heezen

Marie Tharp was named one of the four great 20th century cartographers by the Library of Congress in 1997, was presented with the Woods Hole Oceanographic Institution Women Pioneer in oceanography Award in 1999, and the Lamont-Doherty Heritage Award in 2001.

There is no question that Tharp’s discovery influenced the promotion of Continental Drift in the geoscience community. Alfred Wegener’s bold hypothesis (1915) had one major problem – there was no known mechanism that could move oceanic crust and continents around, like some precursor shuffle to a jigsaw puzzle. In 1929 Arthur Holmes posited a mechanism that involved large convection cells in the mantle, but this too lacked an important degree of empirical verification. Discovery of the mid-Atlantic rift provided a solution to this vexing problem, and in 1962 Harry Hess proposed that new magma, via mantle convection cells, was erupted from mid-ocean rifts allowing oceanic crust to spread outwards. This was Sea Floor Spreading, a precursor to the grand theory of Plate Tectonics – the tectonic shift in geological thinking wherein oceanic crust is created at mid-ocean rifts and consumed down subduction zones, with the continents playing tag.

Marie Tharp’s doggedness in her belief and understanding of mid-ocean rifting is now celebrated. It’s taken a few decades, but she is no longer a footnote.


Beach microcosms and river analogues


We are regular visitors to the beach; walks with the kids-grandkids, the dog, swimming, fishing, or just sitting and cogitating. It’s easy to get lost in the timeless rush of waves, their impatient foam. My mind reels at the thought that the sea has been doing this for more than 4 billion years. It’s a bit like getting lost in the night sky. There’s so much to discover.

Beaches are geological domains – part of a continuum that extends to the deep ocean, but a part that is easily accessed.  Geological stuff happens there. My attention is always grabbed by the small streams that drain across beaches at low tide. Whenever we came across one of these my kids would scatter, lest they be regaled yet again about the fascination of miniature worlds. I admit it was a bit over the top, so it goes…

Some beach outflows come and go with the tides, others are more permanent leakage from inland drainage. Some trickle, others rush. They are all fascinating, as microcosms of grander floodplain or massive deltas. Project this microcosm to the real world of geological process, of cause and effect. In doing this, you are engaging in the scientific process of creating your own analogy, an insight into a larger universe.

The streams usually start afresh with each tidal cycle. As tides recede, stream flow begins to erode its channel, deepest at the top of the beach. The channels may be straight and narrow, or broad networks of braided sand. Continue reading


Witness to an impact


The dinosaurs were snuffed out in a geological instant (well not exactly, but that is a popular image).  The Chicxulub bolide, its girth 10-15 kilometres, collided with Earth 65 million years ago, leaving a 150 kilometre-wide crater and enough dust and aerosols in the upper atmosphere to darken latest Cretaceous skies for decades.

Like all planetary bodies in our Solar System, Earth has received its share of meteorite and comet impacts. We still bear the scars of some. Every day, bits of space dust and rock plummet towards us – most burn up on entering the atmosphere, but a few make it to the surface. Occasionally they even startle us with air-bursts – Tunguska in 1908,  Chelyabinsk (2013), both in Russia. But humanity has never witnessed a decent sized impact, at least in recorded history. It’s all theoretical. Continue reading


Crossing the harbour bar


A safe harbour offers a place of refuge. Those in peril (or evading taxes), running before a storm, crossing a figurative bar to welcome respite. Non-figurative harbours, the coastal kind, have traditionally provided safe haven for mariners escaping inclement weather or foes.

Harbours fill and are emptied of seawater on the tide. Sea water that enters or exits is commonly focussed through narrow inlets. Here, powerful currents are generated that carry fish, sediment, flotsam, and unwary boats. Filling on an incoming tide is like a cleansing, a renewal; outgoing tides reveal channel arteries that keep alive the bars and broad flats of mud and sand, textured Kandinsky-like.

Northern New Zealand’s west coast has 6 harbours distributed along a 300 km stretch of coast. Each is protected by large sand barriers that have built over the last 2-3 million years with sand moved inshore by successive rises and falls of sea level.

Many of New Zealand’s harbours are drowned valleys, where rising sea level (following the last glaciation) has inundated dissected landscapes. Rising seas have crept up valleys, leaving the exposed high ground to front an intricately embayed coastline, islands, and estuaries that extend their marine fingers far inland.

New Zealand’s west coast is open to large swells, generated by westerly winds across a 2000 km expanse of Tasman Sea. Sea conditions along this coast are often rough. Access to the open sea via harbour inlets, requires sailors to ‘cross the bar’ – the zone of shallow, constantly moving sand. Strong tidal currents, particularly out-going tides can increase wave heights even further, as well as making wave conditions in general very choppy. The sea condition can change rapidly. Many a boat has come to grief across these west coast bars, a mix of bad luck and poor judgement (NIWA has real-time images of current bar conditions at several locations).

The oceanographic and geological term for sand bars at the entrance to harbours and lagoons is tidal delta. Tidal deltas can form on the seaward margin, in which case they are called ebb tidal deltas (because they are downstream of the outgoing tide). Those that form inside harbours and lagoons are flood tidal deltas where sand is deposited by incoming tides.

Raglan Harbour is small but it sports a very nice example of a symmetrical ebb tidal delta. The delta extends 1.5 – 2 km from the harbour mouth. Darker hues (image below) that mark the main channel contrast nicely the shallower sand bars on either side over which waves tend to break. These marginal sand deposits are called swash bars.

Westerly swells approach the coast with relatively straight crests. As they pass over the shallow delta platform, they move at a slower speed because they interact (friction) with the sea floor. Some of the wave energy is transferred to the sea floor such that sediment is moved as ripples and dunes. Slowing waves also build in amplitude (height); this is the region where waves break. However, the same waves in the adjacent, deeper water are moving at a faster pace – trace the crests of each wave and you will see it ‘bending’ around the delta.

Most of the tidal delta remains submerged even at low tide. Parts of the swash bar that are exposed during low tide show evidence for sand movement, mostly as ripples, large and small. Sand is moved during flood and ebb tides. The shape of these sand bars changes from one tide to the next, demonstrating that this is a dynamic environment.


The Raglan tidal delta consists almost entirely of sand. In contrast, Raglan Harbour and its estuaries contain a high proportion of mud. So where does all that sand come from?

The tidal delta is part of a much larger system of sand transfer – supply and demand from the adjacent continental shelf to the adjacent beaches, shallow sand bars (commonly formed by rip currents) and sand dunes. Sand in the inshore region is also moved along the coast by long-shore currents and it is this sand that continually feeds the delta. The delta in turn, via its main channel, moves sand back onto the shelf, completing the cycle.

The beach south of the tidal delta continually changes its profile. At times the profile is an uninterrupted swath of black sand along most of its length (about 3 km). At other times a significant volume of sand has been removed exposing ancient boulder deposits from nearby Karioi volcano; sand removal frequently occurs during stormy weather. The sand dunes also participate in this budgeting exercise. Sand transfer from the beach (and dunes) is probably a combination of movement directly offshore by rip currents and wave undertow, and long-shore movement towards the delta. Sand replenishment and removal from the beach, and addition to the tidal delta, is part of a much larger system of sand supply and demand – nature’s sand budget.

Sand moved onto the swash bars helps to replace sand that is removed by the deep, fast-moving channel. Channel flows in narrow inlets like the one at Raglan are commonly 4-6 km/hr (1-2 m/second), which may not sound fast (try swimming against it) but is sufficient to move large volumes of sediment during each tidal cycle. There are some small sand bars in the harbour itself, but the channel is an effective flushing mechanism that prevents the estuaries and tidal flats from clogging up.

Changes in sea level have a profound impact on coastal sand systems. If sea level falls, the beach and dunes would follow the retreating shoreline, the harbour would eventually become the domain of non-tidal rivers and swamps, and the main channel would be free to meander over a broad expanse of exposed continental shelf. Tidal deltas might be more ephemeral structures, constantly on the move. This was probably the scenario during the last glaciation, when sea level was more than 100m below its present position.

Perhaps of more immediate concern is a rise in sea level (the present situation) which would erode older foreshore beach and dune deposits, and destabilise some cliff areas south of the Harbour. The Surf Club at the south end of Ngarunui Beach would need to move – yet again. The Harbour area flooded at high tide would increase, resulting in a greater volume of seawater entering and exiting the narrow inlet. To accommodate this, the inlet would need to expand, or the speed of current flow would need to increase. Changes such as these would have an immediate effect on the size and shape of the tidal delta.


Bluebottle entanglements; or how to ruin your day at the beach


The southern hemisphere is coming into summer; it’s done this every year for as long as I can remember. For New Zealanders, and pretty well anyone else in this ocean-locked world there is an exodus, a migration as the population ups-sticks and heads to the beach. Unlike our nearest neighbour, we are not thwarted by crocs, sea snakes, Stone Fish or Box Jellyfish; Great Whites mostly ignore us. From the point of view of dastardly critters, these shores would be considered benign. Except for Bluebottles.

Bluebottles galore; entanglements that can ruin a perfect day at the beach (soft, squishy and potentially dangerous).  There are thousands of Bluebottle stings reported every year in New Zealand and Australia. Bluebottles are related to jellyfish, a very pretty blue, puffed up balloon-like, stranded along the high tide line, bedraggled. These creatures, delicately laced, frequently litter NZ beaches (and elsewhere), blown ashore on the tide.

Bluebottles belong to a group of marine animals (a phylum) called Cnidarians, a group that includes corals, sea anemones, true jellyfish, and siphonophores. They all have stinging cells (nematocysts), although corals, sea anenomes and many jellyfish tend to be relatively benign – except to the small critters they like to eat.

Bluebottles are not Jellyfish, they are siphonophores. A true Jellyfish is a single organism, a medusa that possesses a central gut and nervous system; they are all free swimming (Sea Anemones also are single organisms, consisting of a polyp attached to rock, shell or sediment).  Bluebottles are colonial organisms containing a myriad, microscopic, multicellular animals, or zooids, that find solace in community living. Despite being individuals, zooids are attached to and dependent on each other. Zooids tend to have specialized functions; some are attuned to digestion, others to swimming or carrying nematocysts in the tentacles .

The two most common species are Bluebottles that inhabit the Pacific and Indian oceans (the species Physalia utriculus), and the Atlantic (Physalia physalis), the latter more commonly known as the Portuguese Man o’ War (see image at the top of this post).  Both have an easily identifiable gas-filled bladder (pneumatophore) in an attractive blue with hints of mauve, from which dangle tentacles – the things do the damage to passing small fish and people. The bladders provide the only means for movement by catching wind and waves (again, unlike Jellyfish that propel themselves).

Portuguese Men o’ War tend to be larger than their Pacific cousins, with tentacles extending 10m, and even 30m below the sea surface.  Bluebottles have smaller pneumatophores, and fewer and shorter tentacles. The tentacles contain many stinging cells called nematocysts; their sole function is to catch and stun prey. Nematocysts on Bluebottles and Portuguese Men of War can penetrate skin to inject venom. A single stinging cell will do little damage. Unfortunately, tentacles tend to wrap their prey (including arms and legs), in an act of evolutionary hubris that inflicts multiple stings manifested in a nicely symmetrical, cork-screw like pattern of welts.

Bluebottle stings are painful- I can attest to this. In most people, this is as far as it goes, but if you are unfortunate to have tentacles wrapped around large areas of your semi-naked body, the venom can induce nausea and headaches, and in more serious cases, difficulty breathing or cardiovascular failure (happily the latter are rare).

There is plenty of advice on how to deal with Bluebottle and Portuguese Man o’ War stings. First and foremost, don’t try to rub or scrape off the tentacles; this will only exacerbate envenomation. Use seawater to wash thoroughly the affected area. Some authorities recommend dabbing vinegar on the welts to help ease the pain; others suggest this only makes matters worse (this link is an Open Access document). I must admit, a bottle of vinegar is not usually on my list of things to take to the beach, unless I’m planning to cook shellfish.

There is also the mistaken belief that peeing on the affected area will help. Urinating on oneself might be awkward, so you would probably need a willing accomplice.  But the real kicker here is that pee makes the nematocysts release more venom. So, if anyone suggests this remedy, do let them know it is nonsensical, notwithstanding the public spectacle. Tentacles can also release venom long after they have been blown ashore.  So it’s best to admire them from a distance.

Enjoy summer.


The Pink and White Terraces, Magic Lanterns, and 19th Century Narratives



A collection of late 19th century lantern slides made by Arthur Whinfield, a native of Worcester, England, has recently been restored and digitized.  I was alerted to the Whinfield collection by an old friend Justin Hughes, an archaeologist working in Worcester, UK, who indicated that the collection contained several slides of the famed Pink and White Terraces near Rotorua, New Zealand. These iconic geothermal wonders were destroyed by the eruption of Tarawera, on June 10, 1886.

Lake Rotomahana, in the shadow of Tarawera volcano, looks peaceful enough. Its waters are ruffled only by wind and the wakes of small boats. No hint of impending doom. No hint of the destructive explosions 132 years ago, eruptions that completely changed the local landscape, destroyed a village and its inhabitants, and obliterated a geological icon – the Pink and White Terraces.

New Zealanders look upon the Pink and White Terraces with a kind of fondness, even though no one alive has seen them. Mineral-rich waters spurting from geothermal springs and erupted from geysers above the former Lake Rotomahana, deposited silica in a cascade of rimmed terraces and pools; ever shrouded in steam. As their name indicates, there were two sets of terraces. The larger White Terraces descended 25m, stair-like, into the lake.  Their pink counterparts were terraced through 22m. Mineral content was more pronounced in the pink variety, with precipitation of arsenic and antimony minerals, and gold.

The terraces disappeared on June 10, 1886. Eruption of Tarawera was focused along a 17 km rift that extended from the volcano summit, through the terraces, and into Lake Rotomahana. Whether the terraces were obliterated, buried, or partly submerged in modern Lake Rotomahana is still debated.

By all accounts the terraces were spectacular; witness the written testimonies of geologists (like Ferdinand von Hochstetter in 1859) and Victorian gentlefolk, renditions in oil and water-colour (like the Blomfield painting above), and photographs – grainy, black and white, tinted amber with age. This was the late 1800s, and photography was in its infancy, the act of recording an image a laborious process.

Enter Arthur Whinfield. Whinfield was a peripatetic photographer who in the 1880s captured the magic of cities and landscapes in the Americas, Asia, Africa, Europe, Australia, and New Zealand. Part of his legacy resides in a collection of lantern slides (more than 2100 of them) that were donated to the Worcester Diocesan Church House Trust by his wife in 1918. A century later, in partnership with the Worcestershire Archive and Archaeology Service, the slides have been restored, digitized, and made available for public display. Included in the collection are 11 glass slides of the Pink and White Terraces, and the aftermath of the Tarawera eruption.

Whinfield took many of the photographs he used in his slides, but also borrowed from other photographers, and this seems to have been the case for several Pink and White Terraces images.  Te Papa Tongarewa  (the New Zealand National Museum) has an extensive, publicly accessible collection of photographs, paintings and prints related to the Terraces. I was able to identify individual photographers in some of the slides from the Te Papa collection.

Click on each image below for a larger format, then use the back-click arrow to return to the article.

The classic image of the Pink Terrace with Maori guides (or is this a family scene?) and a canoe in the foreground is shown below. This iconic photograph was taken by Burton Brothers Studio in 1885, and later used by Muir and Moodie Studio in a popular postcard (early 1900s; one penny postage required). Whinfield’s slide (left) is a copy of this scene (acknowledging the photographer on the lower left corner).

Whinfield slide left; Muir & Moodie postcard right


Moodie and Muir also produced a postcard from the White Terraces image below (left); the colour would have been added by hand to the printed photo. The Whinfield slide (right) is an uncoloured version of this image (compare the shape of the steam clouds at the top).  The terrace flights are nicely portrayed in this slide.

Muir & Moodie postcard left; Whinfield slide right


One of the more panoramic views of the White Terraces shown by Whinfield (left) is similar to a photograph taken by Burton Brothers in January-May 1886 (in the Te Papa collection) but is viewed from a lower elevation and records a different steam profile; the original photographer may have taken more than one shot from this location (I have not been able to determine who the photographer was for Whinfield’s slide). Here the terraces clearly dip their toes in Lake Rotomahana.  The terraces were a popular tourist attraction, in part because bathing was possible in the lower pools.

Whinfield slide left; Burton Brothers photo right


The original black and white photograph for the three slides below (Pink Terraces), was taken by Charles Spencer.  The three Whinfield slides are identical, with the right image slightly enlarged and a mirror of the other two. The view provides some detail of several small pools that appear to have been filled completely by silica.


The two slides below show detail of White Terrace pools and the intricate patterns wrought by precipitation of silica.  Dark stains daubed on the pool walls may have been algae. I have not been able to determine the attribution of either slide. The slide on the right is labelled ‘White Terrace Crater’ and may have been taken close to one of the active geysers near the top of the terraces.


Whinfield’s slide (below, left) showing the aftermath of the eruption at McRae’s Hotel is slightly different to one taken by photographer George Valentine (1886, McRae’s Hotel and Sophia’s whare) – the man on the right in Valentine’s image has his arms folded; in Whinfield’s slide they are not. The viewing angle is also slightly different – the ladder (foreground) is more oblique in the Whinfield slide.  The hotel probably collapsed under the weight of volcanic ash. Other photographs (not in the Whinfield collection) show the back of the hotel to be demolished completely. The trees were also stripped of foliage.

Whinfield slide right; George Valentine photo right


The title of the slide below Rotomahana Looking to Site of Pink Terrace, indicates a view towards the former terrace, or perhaps close to it, in the aftermath of the eruption. If the location is correct, the image is important because it shows that destruction of the Pink Terraces was complete. The Mounds of volcanic ash cover almost everything. Characteristic erosional rills suggest rain soon after the eruption, where surface water run-off redistributed the ash (probably towards the lake).  I could not determine who the photographer was.

Whinfield’s slides, recently brought to life, are delighting and informing audiences today, just as they must have done when he presented them to an eager 19th century public. These days we never think twice about the projection media at our fingertips. It seems almost to be part of our subconscious, but to Whinfield’s audiences there must have been a sense of excitement, awe, and puzzlement, not just in the images they were seeing, but the fact they were seeing them at all. The havoc wreaked by distant volcanic eruptions, was delivered to their living rooms by a rapidly developing technology.

The Whinfield Terrace collection may not contain photographs of his own taking, but this is not important. An iconic landscape was taking shape in people’s minds, a narrative in images. Folk who may never have left their own village became informed; witnessing the real world shaped by unimaginably ferocious forces – a kind of 19th century Scicomm.


I would like to hear from anyone who has additional information on the images in Whinfield’s slides, particularly information relating to the original photographers.


Credits: The Whinfield collection is owned by the Worcester Diocesan Church House Trust. I am grateful to the Trust for permission to use the digital images of the slides. Thanks also to Justin Hughes of the Worcestershire Archive and Archaeology Service for bringing the slide collection to my attention, for arranging to forward the images, and for his patience with my incessant questions.


Crème brûlée, jelly sandwich, and banana split; the manger a trois of layered earth models


Some things in science are just too difficult to comprehend: the temperature at the center of the sun (15,000,000oC), the age of the earth (4.6 billion years), the size of a nano-particle (1-100 nanometres, or billionths of a metre). We can include in this list of imponderables, the skinny outer layers of the earth: the one we are in daily contact with (the crust), and other layers beneath it. Our familiarity with the crust is usually in terms of the dirt, rock, and water we work with. But what is it like 30km down? And, beneath the crust, the upper mantle is beyond reach of our senses. What does this layer look like? How does it respond to being pushed around?

Some scientists (geologists, geophysicists) spend a great deal of time pondering questions like these. The crust and upper mantle layers are collectively referred to as lithosphere. Beneath the continents it averages 150 km thick; beneath the oceans, it is as thin as 10 km beneath the mid-ocean ridges. Given we spend our entire lives on the uppermost veneer, a reasonable person might ask ‘why is it important?’.

A few common answers include: Most earthquakes are generated in the lithosphere; Magmas erupted at volcanoes melt at these depths. But the overarching reason is that all tectonic plates are born and destroyed as lithosphere. Plate tectonics governs pretty well everything that happens on earth over geologically short and long time-scales. So, what appears arcane at first sight, does have practical applications.

Enter the dessert trolley. There are three choices: a crème brûlée, a jelly sandwich, and a banana split. Proposed as models of the layered earth, they serve a dual purpose: they provide visual descriptions of how the lithosphere might be structured and, after evaluating the merits of each, they can be consumed.

The crème brûlée is a two-layered model.  A viscous fluid base (custard) is capped by a thin crust of caramelized sugar. The crust behaves in two ways. Poke it gently in the centre, and it will bend slightly – release the pressure and it will return to its original shape.  This represents elastic behaviour (think also of wire springs, or rubber bands). Press it too hard and it will break into several ragged pieces; in this instance, you have exceeded the elastic limit, or strength, and induced brittle failure. Earthquakes represent brittle failure where earth’s crust fractures, is displaced, and in the process causes mayhem. The crème brûlée model is probably the simplest of the dessert trio in terms of its relevance to the lithosphere.

The jelly sandwich is potentially the more variable of the three analogues. It is a three-layered model where two pieces of bread are separated by a layer of jelly.  Here, the upper bread layer represents a strong upper crust, and the jelly a weak lower crust. The bottom bread layer is compared with a strong upper mantle – in contrast to the weak custard (mantle) layer in the crème brûlée. The upper and lower bread layers are both quite bendy (unless you have toasted the bread). If you use plain white bread, then bending will be uniform. But if you prefer whole-grain slices there will be lots of lumps and greater heterogeneity, and hence a less predictable response to the application of pressure, or stress. The jelly is much less fluid than custard. It can behave elastically – witness the wobbling, that represents deformation from which it recovers, but at a certain point it too will fail.  Bread is less rigid than a crème brûlée crust; any kind of twisting or bending will probably result in some permanent deformation (i.e. it doesn’t bounce back to its original shape). Unlike the crème brûlée crust, bread is less prone to brittle failure.

The banana split adds another level of complication to models of the lithosphere. The rationale for this model is that the lithosphere contains zones of weakness, particularly near the boundaries of tectonic plates – imagine these plates colliding or sliding past one another, where the forces are large enough to create mountain belts and consume oceans. Here, scoops of ice-cream represent blocks of crust and mantle that are separated by large, very deep faults. This is a very temperature-dependant model. As the ice-cream melts there is a zone of weakness between it and the adjacent scoop (block). The presence of fluid, particularly water, exacerbates this weakness. In this dessert, we need to translate the fluid boundary between scoops of ice-cream, to structures 10s of kilometres deep. Modern examples include the Alpine fault in New Zealand, and San Andreas Fault in California. Some of these large structures can last for very long periods of geological time (100s of millions of years), and potentially influence events in the crust-upper mantle long after they first formed.

All models in science are simplifications of the things we try to explain. It may be the case that some consider the dessert trio to be trivial, even silly, providing little useful scientific information for the representation of the crust and mantle.  But the utility of models and analogues is not only in scientific explanation, but to present a complex world in visually interesting, and yes even amusing ways. Models and analogues need to stir the imagination of folk who are not directly involved in this kind of research but have a vested interest in it. In this regard, the dessert trio works, even if folk can relate to them only via our taste buds.


A chance encounter with James Ussher, circa 1650


A chance encounter of a different kind. The 9th century Book of Kells is exhibited in the Trinity College library (Dublin). The book contains 340 sumptuously illustrated folios written on vellum (calf skin), that make up the four Gospels (the entire book can be viewed online). It was written and illustrated around 800AD, in either Iona (Argyllshire, Scotland) or Kells (County Meath, Ireland), and possibly both. Monks from the St. Colum Cille Monastery in Iona, fled to Kells in 806 to evade further slaughter by marauding Vikings (who seemed to have a penchant for religious orders). Hence the uncertainty surrounding the location of the Book’s compilation.

Two or three folios are exhibited at any one time in the library.  There is something special about seeing this, regardless of one’s beliefs. That a document this ancient has survived pestilence, religious and political purges, plundering and the general ravages of time, is a testament to the enduring belief that human history is worth preserving.

A visit to the Book of Kells exhibit eventually leads to the Long Room, a cathedral-like hall with high barrel-vaulted ceiling dedicated to preserving old and ancient manuscripts, tomes, letters and assorted documents. It was first built in 1712) but enlarged in 1860 to accommodate a burgeoning collection.

One of the College’s 17th century benefactors was James Ussher, Archbishop of Armagh (although he never lived to see the actual Long Room). He was instrumental in purchasing books for the then nascent College library. Ussher became famous, or infamous, for his dating of Genesis creation at October 23rd, 4004 BC. He based this calculation (published in 1650) on biblical genealogies and historical events. Given what we now know about the age of the earth, Ussher’s calculation is generally viewed with a mix of ridicule and begrudging respect.

Acting on a hunch, I asked one of the library attendants if any of Ussher’s books were available for viewing. He suggested I check with the folk in charge of ‘reading room’ passes. This took a while but I was eventually issued a pass and ushered (sic) to the reading room. The first task was to search the catalogues.  One index of books had an entry annotated with scribbled comments on “creation dates”. Bingo. This particular volume appeared shortly thereafter in its own protective box. A rather plain unassuming book was placed carefully on a special reading rack. Instructions were given on how to turn the thin, brittle, dog-eared pages.

The journey through 400 year old documents like these is more one of personal discovery than some momentous event of universal import. The book was a collection of Ussher’s notes, annotated texts, and the odd piece of correspondence. Flicking through these pages gave, for a moment, a view into Ussher’s thinking. Here were his working notes, in a mix of Latin and English. Marginal scribblings on printed text and fragments of scripture, ecclesiastical compositions, calculations of cumulative dates gleaned from the scriptural register of births and deaths, crossings-out and corrections. Here was a person, a Bishop of repute, who tried genealogical combinations, recognized errors, approximated, and perhaps even fudged the odd date. Maybe there were even doubts about the progression of time.

The opportunity to look at Ussher’s notes came out of the blue. It was worth the multiple forms I was required to sign, acknowledging Trinity College’s copyright. I took photos of the text, but cannot display them. What a shame. A 400 year-old document originally intended to be read by all and sundry, now (figuratively) locked away by some arcane law (unless I payed a fee). I don’t decry all copyright conditions, just this one, that applies to a document written by a person who probably never intended such conditions of ownership to apply.

We now know that Ussher’s date for the beginning of earth history has a rather large margin of error. As absurd as his date may sound, it was derived from serious scholarship, in a manner we would now call ‘multidisciplinary’. This in itself is something to celebrate.


Bishop James Ussher, and the beginning of everything


In 1650 AD/CE, James Ussher, Bishop of Amargh and Primate of all Ireland, published the scholarly “Annals of the Old Testament, deduced from the first origins of the world” where he concluded that the universe, and everything in it, began at noon, October 23, 4004 BC. To many modern scholars of the earth, this seems outlandish, even ridiculous. Creationists though still attest to its veracity.

It is easy to fall into the trap of judging old ideas with modern concepts and proofs, rather than judging them according to the precepts and traditions of their day; of course, the real kicker here is that one needs to have some knowledge of those cultural contexts and traditions. I was reminded of this during a re-read of Stephen J. Gould’s Fall in the House of Ussher (Natural History, 1991). In an early paragraph Gould writes “To this day, one can scarcely find a textbook in introductory geology that does not take a swipe at Ussher’s date as the opening comment in an obligatory page or two on older concepts of the earth’s age (before radioactive dating allowed us to get it right). Other worthies are praised for good tries in a scientific spirit (even if their ages are way off), but Ussher is excoriated for biblical idolatry and just plain foolishness. How could anyone look at a hill, a lake, or a rock pile and not know that the earth must be ancient?” (Gould often used diverse ‘lead-ins’ to his essays, the pithy import of which don’t become apparent until one is well into the reading).  It is basically a statement about making judgements.

Confronted by a “hill, lake, or rock pile”, you try to imagine how long it took to get there. Your cogitation will go no further, from the point of view of actual explanation or some empirical value, unless you also happen to know about geological and geomorphic processes, their short-livedness or longevity. Without this knowledge, a priori, the longevity of this pile of rock will remain a mystery. For Ussher, this kind of knowledge was not part of his paradigm, his view of the universe.  Quite the opposite; his view was imbued with a theological tradition that was still trying to deal with the monstrous notion that man and earth were not the centre of the universe. Galileo’s excommunication had taken place a mere 17 years earlier.

Ussher lived and rationalised the universe in a tradition of theological doctrine, a tradition imbued with a belief in divine intervention. He was admired at the time as a scholar, who impressed the nobility so much that on his death in 1656, Oliver Cromwell ordered him buried at Westminster. The starting point for Ussher’s Old Testament genealogy was the death of King Nebuchadnezzar II (634-562 BC/BCE) of Babylon; from there he worked back through the generations to Adam, using a combination of biblical patriarchy and secular history.

Ussher was by no means the first to calculate a creation time-line. Luminaries like the Venerable Bede (672-735), who counted all the begets and begots in Genesis, traced creation to 3952 BC. Even Isaac Newton and Johannes Kepler, two giants in the history of science who we tend to associate with the rise of empiricism, concurred that 4000 BC was not just a nice round number but had meaning as the beginning of everything, based on biblical and secular histories. However, it was Dr. John Lightfoot, Chancellor of the University of Cambridge no less, who in 1642 used biblical genealogies and classical history to calculate the beginning at 9am, October 23, 4004 BC. October the 23rd was a Sunday (appropriate) and close to the autumnal equinox. Ussher must have been aware of Lightfoot’s work, and some historians have suggested that there was a degree of one-upmanship between the two academics. One historical footnote has even suggested that Lightfoot’s addition of “9 am” to the October 23rd date, was inserted after Ussher’s 1650 publication in a fit of academic pique (I guess some things haven’t changed).    Lightfoot’s work is now little more than a historical footnote to Ussher’s opus.

Following Ussher’s publication, The King James Bible (first completed in 1611) added dates to the margins of Old Testament scripture. Thus, “In the beginning…” (Genesis 1:1) has the margin note ‘4004 BC’, and the date Noah’s ark became stranded on Mt Ararat “May 5, 1491 BC. The annotations have been attributed to Ussher rather than Lightfoot, perhaps because he was in court-favour at the time. Use of Ussher’s dates in marginal notation continued in some published Bibles into the 20th century.  Perhaps the whole idea of a creation time-line needed a personality, and James Ussher was it.  Ussher’s legacy is that he still bears the brunt of the creationist claims of a ‘young’ earth, and a tendency to denigration from the scientific community who know how old earth really is.

And this is the central theme of Stephen Gould’s complaint. We now know earth is about 4.6 billion years old; this knowledge derives from radiometric dating of the most ancient earth rock, meteorites, and moon rocks.  But this knowledge was established only in the last half of the 20th century. Earlier estimates of the age of the earth (and solar system) through the 19th century, ranged from a few 100,000 to 300 million years (e.g. Charles Darwin, Charles Lyell, Lord Kelvin). All these early calculations by learned folk are taken (as they should) to be the best estimates at that time, given what was known about earth processes. The calculations were scholarly, and for the most part, well considered. We don’t ridicule them, simply because we now know them to be incorrect (radiometric dating methods were not part of the geologists’ toolbox until the early 1900s).

Likewise, Ussher and Lightfoot used, in a considered fashion, the information available at that juncture of the 17th century – the genealogies in biblical scripture and classical history. Galileo and Copernicus had certainly rocked the boat in terms of humanity’s place in the universe, but not its longevity. In the 17th century, the only way to decipher time past was through written history, or a vivid imagination.

According to the Old Testament, there were 75 generations between Adam and Jesus, and if we assume that each lived 50 years (with overlap of first-born sons – sorry, but it was all about the patriarchy back then), some quick, back-of-the-envelope arithmetic gives us about 3000-4000 years from creation. And again, if we treat the 6 days of creation as a kind of metaphor, where ” For a thousand years in thy sight are but as yesterday when it is past, and as a watch in the night” (Psalm 90, King James Bible), then 6000 years gives an upper limit for Day 1. These dates became ingrained in Renaissance and Enlightenment Christian views of the universe. For some in the 21st century they are still ingrained.

By all accounts, neither Ussher or Lightfoot were charlatans; they did not take all those BC genealogies through arcane twists and turns to prove a point (something modern Creationists are quite good at).  They were not trying to disprove science. No doubt there were errors of interpretation, and perhaps errors of omission.  There were certainly difficulties in aligning events according to the Julian, Gregorian, and Hebrew calendars, as Gould and other scholars have pointed out.

We often think of the Enlightenment as the beginning of modern science, but in the 17th century it was still nascent, and in a state of almost continuous tension with approved church doctrine. Ussher and Lightfoot may have been sympathetic with the emerging science (I’m not sure historians really know this), but they were also immersed in theological tradition. They took the information that was available and used it in a scholarly fashion.

So, next time you hear someone disparage Ussher and his colleagues, remind them that cultural, philosophical, political, and religious contexts are important when judging historical events. It is also worth keeping in mind that, 100 years hence, theories and hypotheses that today have scientific value, may send future generations into paroxysms of laughter.

Footnote: The historical annotations CE (Common Era) and AD (Ano Domini) refer to dates since the first year of the Gregorian calendar. Thus, BCE is before the common era, and refers to the same dates as BC (before Christ). The difference between the older BC/AD and recent BCE/CE notations is that the latter has no Christian connotations.


Archeomagnetic Jerks; our decaying magnetic field


“Archeomagnetic Jerks”. This interesting phrase refers, not to people, but to our global magnetic field; the one that protects us from incoming solar radiation and protects all those electrical devices we’ve come to rely on, including satellites. The magnetic field is generated by earth’s solid core; it envelopes our earth. The magnetic poles (not the same as the geographic poles), move around a bit. Measurements of the field over several decades indicate that the north magnetic pole is migrating south, towards Siberia and has moved about 1000 km since it was first pin-pointed in 1831. Geological investigations of ‘fossil’ magnetic fields also demonstrates that the magnetic field has flipped hundreds of times over past millennia, where north becomes south (see an earlier post for details). Disconcerting as this sounds, we can take some comfort in the fact that these polarity reversals do not coincide with any extinctions.  Homo sapiens was around during the last reversal (780,000 years ago) and, I’m happy to report, survived intact. We will survive the next reversal, although some of the electrical accoutrements we have amassed, might not.

Earth’s magnetic field is generated by rotation, or convection of a liquid nickel-iron layer that surrounds the solid iron core; it is referred to as the liquid outer-core. The heat necessary to drive convection comes from the solid inner core; temperatures for the outer core range from about 2700C to 7700C. Movement of the liquid iron is also driven by forces generated by earth’s rotation, called coriolis forces. Convection in the outer core is not uniform, and variations in rotation, perhaps analogous to eddies, produce variations in the magnetic field.  One region of significant variation in the magnetic field is the South Atlantic Anomaly (SAA), a relatively narrow band where magnetic field strength is much lower than expected; this region extends from central South America to central Africa.

The SAA is thought to evolve from complex interactions in earth’s liquid outer core beneath Africa and central South America.  And although the SAA is considered by some as a possible harbinger of wholesale magnetic pole reversal, the extent of the anomaly has a more immediate impact because of the interaction between the magnetic field and the Van Allen radiation belts (these radiation belts were one of the first discoveries made by an orbiting satellite).  The radiation belts (there are usually two concentric belts) are doughnut-shaped regions in space where charged particles from the sun are trapped as they interact with the magnetic field. In doing so, they protect us from incoming solar radiation. However, the radiation ‘doughnut’ is not oriented symmetrically with earth’s axis of rotation but is slightly off-kilter. This means that one part of the radiation belt comes very close to earth – in fact about 200-300 km, and this low region is what defines the shape of the SAA.  An important consequence of the SAA is that solar radiation is significantly more intense over the extent of the anomaly; orbiting satellites that transit the region of the anomaly are fitted with protective shields to prevent failure of electrical systems.  For example, Hubble Space Telescope passes through the anomaly 15 times a day.

Globally, the strength of the magnetic field has decreased about 15% in the last 200 years. The current scientific dilemma with the SAA is that it seems to be expanding as the magnetic field weakens. This observation, given voice by several media outlets, has led to some predicting dire consequences during an imminent magnetic field reversal. The problem here is that scientists do not know whether this weakening is an unusual event, or one that anomalies like the SAA cycle through from time to time.  It is also not well understood whether the SAA is a relatively recent phenomenon that has been around for a few hundred years, or has persisted over much longer periods of time, perhaps waxing and waning in its extent.

In a recent study, Jay Shah and other geophysicists looked at the magnetic signatures in 46,000 to 90,000 year-old volcanic rocks from Tristan da Cunha.  These isolated volcanic islands in the South Atlantic lie within the SAA and may provide records of older magnetic anomalies. They discovered at least 4 periods of significantly reduce magnetic intensity, and concluded that the SAA could be a persistent anomaly, or at least one that recurs from time to time.  Although the results are preliminary, they suggest that decreasing field strength in the SAA may have happened before, but without wholesale field reversal (there have been no reversals in the last 90,000 years).

The idea that the SAA is a long-lived phenomenon has received an additional boost in a study of archeological materials by Vincent Hare and colleagues, who measured the preserved magnetic signatures in Iron Age mud from southern Africa.  The archeomagnetic materials used in this study were burnt, or baked mud from various Iron Age facilities such as grain storage and hut floors (perhaps baked by cooking and heating fires).  Mud baked above a certain temperature (known as the Currie Point) will retain the magnetic signatures present at the time, in much the same way as solidified volcanic rocks.  Measurements on these materials show significant changes in the magnetic field intensity, between 1225AD and 1550AD, and an earlier period around 500 to 600 AD.  Abrupt changes in field intensity like these are commonly referred to as archeomagnetic jerks.

Despite the ‘End is nigh’ approach taken by tabloids and other popular media to this scientific phenomenon, the actual science is equivocal.  It appears that the South Atlantic (magnetic field) Anomaly is long lived – at least many 10s of thousands of years, and that the magnetic field intensity of the anomaly has waxed and waned several times.  In this context, the current state of decay of the magnetic field both globally and in the SAA, may be nothing more than a repeat of other historical and prehistorical events.  However, on a more sobering note, we are overdue a complete magnetic pole reversal.  No doubt the geophysicists will keep us posted. In the meantime, if a pole reversal takes place tomorrow, you may have to get used to subtracting (or adding) 180o from your compass bearing to ensure you end up where you want to go.