"What a confusion for Geologists" - Geologizing with Darwin

The first stop of the voyage of the Beagle (1831-1836) was “Quail Island” (today Island of Santa Maria) – a small island located in the bay of Praia of the larger island of St.Jago (today Santiago, Cape Verde Islands). This visit is especially interesting as it provides some glimpses in Darwin’s geological background at the beginning of his adventure and his later “evolution” as geologist.
 

Darwin collected basic experience as geologist during a field trip across Wales and surely know the geological theories of the time, especially regarding the formation and age of the earth. The notion of a 6.000 year old earth was already dismissed by scholars and even the interpretation of gravel and sand deposits as the remains of the biblical flood (the “Diluvium“) was questioned. However the notion of earth’s history of a succession of catastrophic events was still fiercely discussed.

Many geologists at the time proposed that geologic processes in the past differed significantly from recent processes; even certain types of rocks (and the formation of these rocks) were limited to certain time periods, when today unknown geological processes were shaping the earth. The lawyer Charles Lyell challenged this interpretation of earth’s history, arguing that common and slow processes still observable today also acted long time ago.

Captain FitzRoy offered Charles Lyell’s recently published and controversial “Principles of Geology” as welcoming gift, but Darwin probably didn’t find time to read the book in the first weeks of the expedition. His former mentor, botanist John S. Henslow, even “advised me to get and study the first volume of Principles, which had then just been published, but on no account to accept the views therein advocated.“

It’s therefore even more surprising to read in Darwin’s autobiography (1876-1881) the following phrase:

“The very first place which I examined, namely St. Jago, in the Cape de Verde islands, showed me clearly the wonderful superiority of Lyell’s manner of treating geology.”

Darwin also emphasises how the visit of St.Jago converted him to Lyell’s geology:

“The geology of St. Jago is very striking, yet simple: a stream of lava formerly flowed over the bed of the sea, formed of triturated recent shells and corals, which it has baked into a hard white rock. Since then the whole island has been upheaved. But the line of white rock revealed to me a new and important fact, namely that there had been afterwards subsidence around the craters, which had since been in action, and had poured forth lava. It then first dawned on me that I might perhaps write a book on the geology of the various countries visited, and this made me thrill with delight. That was a memorable hour to me, and how distinctly I can call to mind the low cliff of lava beneath which I rested, with the sun glaring hot, a few strange desert plants growing near, and with living corals in the tidal pools at my feet.“

Fig.1. Profile of the island of St. Jago as seen by Darwin in 1832. Darwin was the first to study the geology of the Cape Verde Islands (from DARWIN 1876). Darwin recognized three distinct layers of rocks, a lower series with volcanic rocks composed of volcanic breccias and magma dikes (deposited under water - identified with "A"), a limestone with fossils ("B") and finally a cover of basaltic lava ("C"). Darwin, trained by Sedgwick, noted also the contact metamorphism between the former hot molten lava and the earlier cool limestone.
It is curious to note that Darwin adopted the geological terms used by German (not British) geologists to describe the rocks observed in the field, here the strong influence of Alexander von Humboldt works, read by the young Charles, is recognizable

Darwin uses in later publications the similarity of the fossils found in the carbonate sediments (Darwin’s line of white rock) and the still living animals on the shore as evidence that no substantial change in the geologic processes forming these rocks occurred over time.  
However from the geological notes he made during the field trip on St. Jago it emerges that young geologist Darwin was still struggling to accept this idea. More important, accepting slow geological processes made it necessary also to accept a very old earth.

During one of his daily excursions on St.Jago Darwin discovered a mature baobab-tree (gen. Adansonia) growing on the bottom of one of the large valleys carved into the hard basaltic rocks of the volcanic island.

“In this [one of the valleys north of Praya] grows the celebrated Baobab or Adansonia; this tree only 45 feet high, measured two feet from the ground round the solid trunk. 35.-Some of the same species in Africa were supposed by Adanson to reach the enormous age of 6000 years.-The very appearance of the tree strikes the beholder that it has lived during a large fraction of the time that this world has existed.“

Darwin notes that a 6.000 year old tree would have experienced a significant period of earth’s history, implying that earth, despite older than proposed by scrupulous clergymen, would be not much older. However the eroded valleys in the thick lava shields, characterizing the landscape on St. Jago, need vast periods of time to form, as he continues:

“Of course the valley must be older & it is this one that has finally left the neighbourhood of Praya in the state we now find it.-How long a time intervened between this period and the deposition of former beach it is impossible to say.-during it three great phenomena occurred, the flowing of the lava.-the upheaving of the coast. & the great beds of diluvium collected in the older valley.-To what a remote age does this in all probability call us back & yet we find the shells [in the 'former beach'] themselves & their habits the same as exist in the present sea.“

In the final note Darwin considers the possibility that the similarity between the fossil shells and the recent ones could also be explained by a short interval of time between the formation of the white rock and the deposition of modern beach deposits (so there was no time for a faunal turnover).
However accepting a young age for the fossil beach deposits and the even younger eroded remains of the volcanic island of St. Jago (the coastal lava shields are covering Darwin´s white rocks and therefore according to stratigraphic principles are younger) would invoke some unknown – and presumably catastrophic – geological event in the not-too distant past to explain its actual deep incised valleys.

“I conceive it to be clear, from the pieces left standing and from the corresponding appearance on each side of the valley, that the country was originally covered with a uniform bed of this rock.-and that after being shattered by some great force: these valleys were formed by the agency of large bodies of water: To this latter force the valleys nearer the coast give abundant evidence.“

Darwin will admit in his diary “what a confusion for geologists.“

To be continued…

Bibliography:

CHIESURA, G. (2010): A Santiago sulle orme di Darwin. Darwin – Bimestrale di Scienze No.40: 32-36
CHIESURA, G. (2013): Isole di Darwin – Un curioso in mezzo al mare. CD-Rom
HERBERT, S. (2005): Charles Darwin, Geologist. Cornell University Press: 485
JOHNSON, M.E.; BAARLI, B.G.; CACHAO, M.; da SILVA, C.M.; LEDESMA-VAZQUEZ, J.; MAYORAL, E.J.; RAMALHO, R.S. & SANTOS, A. (2012): Rhodoliths, uniformitarianism, and Darwin: Pleistocene and Recent carbonate deposits in the Cape Verde and Canary archipelagos. Palaeogeography, Palaeoclimatology, Palaeoecology Vol.329-330: 83-100
PEARSON, P.N. & NICHOLAS, C.J. (2007) : ‘Marks of extreme violence’: Charles Darwin’s geological observations at St Jago (Sao Tiago), Cape Verde islands. in WYSE JACKSON, P. N. (ed.) Four Centuries of Geological Travel: The Search for Knowledge on Foot, Bicycle, Sledge and Camel. Geological Society, London, Special Publications, 287: 239-253

"Mad about Geology" - Geologizing with Darwin

"A journey of a thousand miles begins with a single step."
Chinese proverb

January 16, 1832 the H.M.S.Beagle, with Charles Darwin on board, arrived to the barren "Quail Island" (today Island of Santa Maria, Cape Verde Islands). It was the first time that Darwin geologized alone in a foreign country, however he was well prepared...

In 1831 Charles R. Darwin went on a life changing field trip – not to mention the voyage on board of the Beagle later in that year. The botanist John Stevens Henslow introduced the 22-year old Darwin to 46-year old Adam Sedgwick, self-educated naturalist and professor for geology and botany at Cambridge University. Even if Darwin was a student at Cambridge, he seems not to have attended Sedgwick´s lectures on geology, as he regrets in an autobiographic note that

“Had I done so I should probably have become a geologist earlier than I did.“

At the time Sedgwick was studying the geology of Wales and invited Darwin to join him in a field trip from Shrewsbury, Darwin’s hometown. Sedgwick was especially interested in the stratigraphic succession exposed in North Wales (Sedgwick will later use his observations to define the geologic epoch of the “Cambrian“) and Darwin was interested to acquire the basics of geological field work. Darwin wrote in July to a friend

“I am now mad about Geology & daresay I shall put a plan which I am now hatching, into execution sometime in August, …[]“

Darwin was well equipped for his geological field investigation. He purchased a new clinometer with an incorporated compass for structural analysis and a geological hammer for the collection of rocks.
 

He visited Llanymynech (located west of Shrewsbury) alone and "on my return to Shropshire I examined sections and coloured a map of parts round Shrewsbury", mapping outcrops of sandstone and coal measures.

Sedgwick arrived to Shrewsbury on the 2nd August, visiting in the next days some outcrops located south-west of the city, where he recognized limestone and volcanic rocks. It’s not clear if he met Darwin already, for sure both geologist left Shrewsbury on August 5th venturing north. They spend a week trying to find Old Red Sandstone. Sedgwick was interested in the geological formations underlying the Old Red Sandstone (Silurian to Carboniferous in age), as the age of these rocks was still unknown and according to the large-scale geological map published by George Greenough in 1819 such rocks should be found in the area. However despite their combined efforts and a meeting in Llangollen with another great geologist, Robert Dawson, no Old Red Sandstone was found.

In his autobiography Darwin affirms that he left Sedgwick at Capel Curig, however it may be possible that he visited with Sedgwick the island of Anglesey and even made a short trip to Dublin (as Sedgwick did, on Anglesey he found also the Red Sandstone he was after). During his voyage on the Beagle, Darwin will recognize on the Cape Verde Islands Serpentine, this kind of rock he could have only previously seen on Anglesey - or maybe he used Sedgwick notes.

Fig.1. Geology of North Wales, after Reynolds 1860, 1889, Woodward 1904 (click to enlarge), with the route of Darwin and Sedgwick after ROBERTS 2001. The first part of the route, starting from Shrewsbury, follows the contact of the Silurian limestone (pink-coloured) and younger sediments (blue colour; Carboniferous to Permian), as both geologist hoped to find the Old Red Sandstone formation. Sedgwick found it (dark-orange) only on the island of Anglesey.

Twenty pages of notes made by Darwin during this tour are still today conserved – in his autobiography he will later remember: “This tour was of decided use in teaching me a little how to make out the geology of a country…”
 

When Darwin returned home to Shrewsbury August 29th a letter by botanist John Stevens Henslow, in name of Captain Robert FitzRoy, was offering him a position as gentlemen companion and naturalist on board of the Beagle. 

Read on...

Bibliography:

HERBERT, S. (2005): Charles Darwin, Geologist. Cornell University Press: 485
ROBERTS, M. (2001): Just before the Beagle: Charles Darwin’s geological fieldwork in Wales, summer 1831. Endeavour Vol. 25(1): 33-37

From Nukes to Ship Disasters - how Forensic Seismology helps to understand Catastrophes

On July 25, 1946 the United States detonated the first underwater nuclear weapon in history – code name “Baker” – at the Bikini Atoll. The explosion generated a gas bubble that pushed against the water, generating a supersonic shock wave which crushed the hulls of nearby target ships as it spread out. Seismic waves of this test were observed at seismograph stations around the globe and it was realized that these waves could be used to detect and potentially characterize a nuclear explosion.

Fig.1. Photography of the underwater “Baker” nuclear explosion of July 25, 1946 showing the white sphere of water and vapour formed by the shock wave of the explosion (image in public domain).

The U.S. performed also the first fully underground explosion – code name “Ranier” – that was detected by about 50 seismic stations; however, it was confused in part with a “normal” earthquake.
 

With the ban of nuclear weapon (well, sort of…) testing in the year 1958 it became necessary to install an effective worldwide monitoring system. Three years later the set-up of the WorldWide Standardized Seismographic Network (WWSSN) began and in 1966 almost 112 stations were working in the monitoring project “Vela“. Vela provided a large quantity of supplementary seismic data used to answer three questions: Where is the seismic event located? What is the source type (artificial or natural) of the event? How large is the event?

“It appears increasingly doubtful that an atomic-weapons test of significant dimension can be concealed either underground or in outer space. A five-kiloton nuclear explosion in an underground salt cavern near Carlsbad, N.M., in December was clearly recorded by seismographs as far away as Tokyo, New York, Uppsala in Sweden and Sodankyla in Finland. The seismograph records included tracings of the ‘first motion,’ considered critical in distinguishing between earthquakes and underground explosions.” from “Scientific American“, February 1962

The signature of a natural earthquake shows a distinct pattern: a seismometer will first detect the Primary and Secondary Waves, followed by the more destructive Surface or Rayleigh Waves.
Seismic P Waves are compressional waves, similar to sound waves in the air. Secondary or Shear (S) Waves are transverse waves, like those that propagate along a rope. A sudden explosion generates a “sphere” of compressional waves travelling in all directions. In contrast an earthquake is caused by the sliding of rocks along a fracture and it will generate shear waves concentrated in a certain direction. Therefore an explosion will show a strong and sudden signal of P-waves, with a similar signal recorded by all the seismometers collocated around the explosion. An earthquake will show a more complex pattern, depending of the position of the seismometer, characterized by strong S-Waves and R-Waves.
Also an underground explosion does not generate very strong surface waves as a natural earthquake does.

Fig.2. Schematic seismogram with Primary (P; compressional waves), Secondary (S; shear waves), and Rayleigh (R; surface waves) phases for an artificial blast and a natural earthquake.

As every atomic explosion will generate a unique pattern, distinct from natural earthquakes, seismology is a reliable tool to control the ban of nuclear test and to supervise countries that still test atomic weapons.

The information recovered from seismograms of nuclear blasts can be applied in forensic seismology also to study detonations of common explosives. Most spectacular cases in the last years comprise the reconstruction of the Oklahoma City bombing in 1995 (see this abstract by HOLZER at the AGU meeting in 2002) and the investigation in the explosion on the Russian submarine “Kursk” in 2000 (see KOPER et al. 2001; the blog “About.com Geology” hosts many other examples).

Seismic waves can be generated not only by shear movements along faults or by the expansion of plasma (nuclear device) or gas (conventional device) during an explosion, but also by the impact of objects with the ground.
Seismic signals were already used to identify the location of rock-falls and recent research suggests that the signals can help to characterize the dynamics and volume of a landslide, Dave Petley discusses the significance and use of seismograms in various posts published on his “Landslide blog“.

The analysis of seismic waves provided also insights on what happened September 11, 2001 in New York. Seismograph stations around the city recorded the signals generated by the aircraft impacts and the subsequent collapse of the two towers of the World Trade Center (the Lamont-Doherty Cooperative Seismographic Network provides a rich collection of datasets of the seismic activity around N.Y.). The collapse of the south tower generated a signal with a magnitude of 2.1 and the collapse of the north tower, whit a signal of magnitude 2.3, was recorded by 13 stations ranging in distance from 34 to 428 km.
Also these seismograms show a distinct pattern if compared to the pattern caused by a natural earthquake. There are no P or S Waves, but the impacts of the buildings on the ground generated a sudden peak of short-period Rayleigh Waves.

Fig.3. Seismic recordings at the seismograph station Palisades (N.Y.) for events at World Trade Center on September 11, distance of station from Ground Zero ~ 34 km. Note that impact 1 and collapse 2 relate to the north tower, and impact 2 and collapse 1 apply to the south tower. Expanded views of the first impact and first collapse shown in red. Figure from KIM et al. 2001, published here according to the Usage Permissions granted by AGU & authors.

The seismograms show also that the impact and explosion of the two airplanes generated a relative small amount of seismic energy. This confirms the observation that the collapses of the two towers were not a direct result of the impacts, but caused by the weakening of the supporting structures of the buildings due the subsequent fires.

Most energy of the collapses was dispersed into the deformation of the buildings and the formation of rubble and dust, only a small portion of potential energy was converted into seismic waves. The generated 2.1 and 2.3 M earthquakes were too weak to destabilize nearby buildings, most damage was done by the kinetic energy of the debris and the displaced air.

Also the collision of the cruise ship “Costa Concordia” on January 13, 2012 was recorded by the seismograph station “Monte Argentario“, situated on the Italian mainland. From the eyewitness testimony and the Automatic System of the ship the time of collision with a submerged rock was estimated at 20:45 (UTC). This time is confirmed by a sudden peak in the seismogram at 20:45:10 (the seismograph station is distant 18 km from the site of the collision, the seismic waves needed almost 3-4 seconds to travel this distance). The seismogram shows also after the impact the “noise” generated by the hull of the ship grinding along the rocky substrate.

Fig.4. Seismogram recorded at the station “Monte Argentario” (Italy) showing the seismic waves generated by the impact of the “Costa Concordia” on January 13, 2012 20:45 (UTC). An accurate analysis of “The seismic wake of “Costa Concordia” (23.01.2012) can even specify the speed of the ship at the moment of the collision. Figure used with permission and taken from the post “The earthquake of the Costa Concordia” by Italian seismologist Marco Mucciarelli, published January 21, 2012 on his blog “terremoti, sismologia ed altre sciocchezze“.

Bibliography:

ANDERSON, D.N.; RANDALL, G.E.; WHITAKER, R.W.; ARROWSMITH, S.J.; ARROWSMITH, M.D.; FAGAN, D.K.; TAYLOR, S.R.; SELBY, N.D.; SCHULT, F.R.; KRAFT, G.D. & WALTER, W.R. (2010): Seismic event identification. WIREs Computational Statistics Vol.2, July/August: 414-432
KIM, W.-Y.; SYKES, L.R.; ARMITAGE, J.H.; XIE, J.K.; JACOB, K.H.; RICHARDS, P.G.; WEST, M.; WALDHAUSER, F.; ARMBRUSTER, J.; SEEBER, L.; DU, W.X. & LERNER-LAM, A. (2001): Seismic Waves Generated by Aircraft Impacts and Building Collapses at World Trade Center, New York City. EOS Vol.82 (47)

KOPER, K.D.; WALLACE, T.C.; TAYOLR, S.R. & HARTSE, H.E. (2001): Forensic seismology and the sinking of the Kursk. EOS, Vol.82 (4): 37

Nicolas Steno and the Origin of Fossils

In October 1666 a large shark was captured by a French fishing boat in the sea of Livorno (today Italy, at the time County of Tuscany), pulled onto the shore, the animal was beaten to death and dismembered, as the body was quite too heavy to be transported and only the head was saved. In Florence the Danish anatomist and naturalist Niels Stensen or latinized Nicolas Steno (born January 11, 1638) was asked to dissect this head. 

Stensen observed various anatomical particularities: the skin was covered by glands, secreting a reddish slime. Steno assumed that this slime would keep the skin smooth, helping the animal to move in water (today the ampullae of Lorenzini are considered part of an organ to sense electromagnetic fields). He noted also the structure of the brain, arguing that it appeared quite too small to coordinate such a large animal, so also the spine must have some role in (unconscious) movements (like reflexes). Finally Steno studied the mouth, noting the ranks of sharp teeth.
 

Steno, after the anatomical description, adds a chapter comparing these teeth with common fossils - the Glossopetrae or tongue stones. It´s important to note that Steno was not the first to speculate over the organic origin of such fossils, already in 1616 the Italian Fabio Colonna (1567-1640) explained glossopetrae as shark teeth. However many naturalist argued that the organic origin of fossils could not explain how such remains of sea animals  could become entrapped in rocks, found on dry land and even high in the mountains. 

Fig.1. Recent, mummified shark head.
 

But Steno was for the first time able to explain why these fossilized teeth were found inside rocks, far distant to the modern sea.  Steno had already observed fossils hosted in the Royal Danish Kunstkammer (Copenhagen) and in a private note he writes "Snails, shells, oysters, fish, etc., found petrified on places far remote from the sea. Either they have remained there after an ancient flood or because the bed of the seas has slowly been changed. On the change of the surface of the earth I plan a book, etc."
 

Fig.2. Steno's figure of a dissected shark head, comparing the teeth of a modern shark to the fossil Glossopetrae, from "Elementorum myologiæ specimen, seu musculi descriptio geometrica : cui accedunt Canis Carchariæ dissectum caput, et dissectus piscis ex Canum genere" (1667).
 

He also studied outcrops of layered rocks in Tuscany, recognizing the sedimentary origin and a stratigraphic order. However only with the description of the shark head he combines all his observations in one "geo-theory":

• Fossils, resembling modern animals, are not found in recent soils of dry land. If fossils were of inorganic nature, however we should find them in every kind of soil and rocks.
• The layering was formed by sedimentary deposition, the soil where fossils are found once was therefore a sort of liquid mud, so that bodies of dying animals could become imbedded into it
• Those soils were deposited and therefore covered once by water, this explains why fossils resemble animals of the sea
• The sea can become repeatedly dry land by movements and disturbances of earth´s crust , the fossils in the mud are uplifted, the mud dries and becomes hard soil, therefore fossils can be found high in the mountains

However Steno's work, like the work of many others before him, was ignored for decades. Then a certain John Woodward, considered an amateur physician and naturalist by some, by others a quack, used/stole the principles formulated by Steno in his 1695 book "An Essay toward a Natural History of the Earth". The best part of work, thought to support the idea of the biblical sin flood as origin of the fossils, were the text passages copied from Steno.
However the book of Woodward and the principles of Steno used in it initiated a new interest in the study of sedimentary rocks.

Bibliography:

KARDEL, T. & MAQUET, P. (eds.) (2013): Nicolaus Steno - Biography and Original Papers of a 17th Century Scientist. Springer Publishing: 739

January 11, 1771: The Birthday of Lake Alleghe

The lake of Alleghe in the valley of Cordévole is today exactly 244 years old. The "birthday" of this lake is well known, at 7:02 in the morning of January 11, 1771 the river flowing through the valley became dammed by a landslide coming from the mountain Piz.

Fig.1. General view of the valley of Cordévole with the village and lake of Alleghe. On the right, on the mountain Piz, the scar of the landslide is barely visible in the forest, in the background the Civetta (3.220m).

The Alps-traveler Belsazar Hacquet (1739-1815) remembers a visit to the lake in 1780:

“The river Cordévole became my guide, by following him I would find the valley of Cadore. But only after some hundred steps the river was flowing in a large lake, existing here only for the last nine years. I walked around in eastern direction, to the villages of Sternade and Saviner until the mountain of Piz. First the lake was narrow, only by Saviner it became more than 100 venetian fathom [an old length unit used in the mining industry of these times, one fathom ca.1,8m] broad and more than thirty deep. The last mentioned village once was situated on a hill, before it in a broad valley there were four smaller villages…[]…which became flooded by the lake, but the fourth locality, named Marin, was buried with the village of Riete under the collapse of the mountain of Piz, last mentioned village situated previously on the top of the mountain.” Standing on the top of the mountain, I immediately noted that the mountain has a volcano on top of it, and it was possible to see how deep it went. After the mountain collapsed, it could be seen that its base was composed of limestone, build up by mighty layers, dipping from the west to the east with 45 degrees. The surface of the collapse is so smooth, that a man has difficulties to climb on it to the mountain.“

 

Fig.2. Historic depiction of the landslide-lake in the “Atlas Tyrolensis” of 1774 by Tyrolean cartographers Peter Anich and Blasius Hueber. Note the landslide-boulders on the southern shore of the lake, Anich and Hueber were one of the first cartographers to use signatures to display geomorphologic features in their maps (image in public domain).

The strange notion by Hacquet of an active volcano in the Dolomites in very recent times is based maybe on his discovery of volcanic rocks in the area, however – as we today know – these deposits are more than 235 million years old. At the time of Hacquet’s geologic investigation volcanic forces were also believed to cause strong and sudden movements of the terrestrial surface, effects that maybe could also explain a sudden disaster, like a landslide. Also notable how Hacquet describes the surface where the landslide “slipped away”.

Fig.3. Detail of the modern geological map (Carta Geologica delle Tre Venezie, Foglio 12 "Pieve di Cadore", 1940) showing the village and the landslide of Alleghe (with the lake in the lower part of the map) and the dammed lake.

The detachment area of the landslide is composed by Anisian dolostone (Contrin-fm) overlying limestone - layers and marls of the Ladinian (St. Kassian-fm).

 Lithology: pink = Anisian dolostone (Contrin-fm), blue= Anisian dolostone, lower succession (Moena-fm), brown= marls with bedded limestone-layers (St. Kassian-Formation).

Detail of the geological map (Carta Geologica delle Tre Venezie, Foglio 12 "Pieve di Cadore", 1940) showing the village and the landslide of Alleghe (with the lake in the lower part of the map) and the dammed lake, lithology: pink = Anisian dolostone (Contrin-fm), blue= Anisian dolostone, lower succession (Moena-fm), brown= marls with bedded limestone-layers (St. Kassian-Formation). - See more at: http://historyofgeology.fieldofscience.com/2011/01/11-january-1771-landslide-of-alleghe.html#sthash.3iyCIGcp.dpuf

The landslide of Alleghe killed 48 people and destroyed parts of the village of Riete and some farms. The landslide-lake inundated the village of Peron, only in February 1771 a new outflow formed and stabilized the lake level.

Bibliography:

HÖFLER, H. & WITT, G. (2010): Katastrophen am Berg – Tragödien der Alpingeschichte. Bruckmann Verlag: 144

January 6, 1912: Happy Birthday Continental Drift!

January 6, 1912 the German meteorologist Alfred Wegener presented in a lecture entitled “Die Heraushebung der Großformen der Erdrinde (Kontinente und Ozeane) auf geophysikalischer Grundlage” (The uprising of large features of earth’s crust (Continents and Oceans) on geophysical basis) for the first time his hypothesis of the ancient supercontinent Pangaea, from which all modern continents split apart.
Three years later he will publish his book “Entstehung der Kontinente und Ozeane“, translated in the third edition and published in 1922 as “The origin of continents and oceans.”
 

Wegener didn’t propose something completely new; as he based his idea on earlier observations and suggestions, but his work started a fierce discussion in the scientific community.

  

In 1889 and 1909 the Italian musician and naturalist Roberto Mantovani published a hypothesis based on his observations on the volcanic island of Réunion: cracks forming during volcanic eruptions could separate even large parts of an island, could it then be possible that entire continents split apart? Mantovani collected various evidence and published maps to show the shape of the hypothetical former continents (Wegener will use these maps to support his idea), however he explained the driving force behind the breakup of former large continents by the slow expansion of the earth.
 

In 1908 the self-educated geologist Frank B. Taylor proposed that the crust of earth was influenced by tidal forces of the moon and the continents were pulled apart in some regions and pushed together in other areas, forming folds like a carpet. However the involved forces were to weak and his explanation wasn’t deemed plausible. The Austrian geologist Otto Ampferer speculated in 1906 that the Alps were formed by folding of the upper crust, as driving force he proposed magma sinking into the mantle and pulling pieces of crust downwards (!). This “Unterströmungstheorie (also Subfluenztheorie)” lacked however a convincing source of energy and couldn’t explain all aspects of the genesis of the Alps, as it implied only pulling and not compressive forces needed to form folds and faults.

Wegener became interested in the idea of a single continent in 1910 by observing an atlas and noting the coasts of the Africa and South American. Some time later he read a paleontological paper discussing the similarities of terrestrial fossils between separated continents.
 
Wegener continued to collect various published evidence to support his theory of a single continent:
- Like a puzzle also the outlines of continents (especially the continental shelves) seem to fit together.
- There are various geomorphologic and geological similarities along the coasts of South America-Africa and Europe-North America.
- Fossil of land vertebrates and plants can be found on different continents, separated today by large oceans.
- Fossil evidence of ancient climates, today without a recognizable pattern, will form climate zones when the continents are put together.

Wegner considered the prevailing explanation for the patterns in the fossil record as impossible: ancient land bridges that connected continents and habitats (like the Isthmus of Panama today) were composed of light continental granitic crust, such pieces of less dense rocks couldn’t simply sink into the much denser oceanic basalts and disappear without trace.

He will explain in 1911 his idea in a letter to his father-in-law, Professor Wladimir Peter Köppen:

“You consider my primordial continent to be a figment of my imagination, but it is only a question of the interpretation of observations. I came to the idea on the grounds of the matching coastlines, but the proof must come from the geological observations.
These compel us to infer, for example, a land connection between South America and Africa. This can be explained in two ways: the sinking of a connecting continent or separation. Previously, because of the unproven concept of permanence, people have considered only the former and have ignored the latter possibility. But the modern teaching of isostasy and more generally our current geophysical ideas oppose the sinking of a continent because it is lighter than the material on which it rests. Thus we are forced to consider the alternative interpretation. And if we now find many surprising simplifications and can begin at last to make real sense of an entire mass of geological data, why should we delay in throwing the old concept overboard?“

 

  

Wegener hypothesis of continental drift (a catchy phrase adopted mainly by the critics, as Wegener talks more general of “displacement theory“) was received with mixed feelings. Most geologists regarded it as cherry-picking of anecdotes from the literature. However some geologist with field experience, especially in Africa and South-America, became soon convinced of this possibility.

Like Taylor also Wegener could not explain the forces necessary to move the continents in the crust. Wegener imagined that continents - like gigantic ice floes - swim on and are surrounded by the much denser oceanic crust.  He proposed gravitational pull, tidal and centrifugal forces, but the English geophysicist Harold Jeffreys demonstrated that these forces are much too weak or if strong enough, had to stop first earth’s rotation.
 
Wegner himself reacted to the critics and tried to respond to them in various editions of his books, however with moderate success. The greatest problem remained the lack of direct evidence for the movements of continents and the needed explanation for the mechanism and also the large amount of energy needed to move and deform rocks. Most importantly Wegener considered his work as starting point and stimulus for other or even future scientists, a message that wasn’t fully understand at his time.

 

Fig.1. – 3. “Eppur si muove!” Reconstruction of the former supercontinent of Pangaea and the subsequent breakup in various smaller continents from the Carboniferous to the Quaternary. From WEGENER, A. (1929): Die Entstehung der Kontinente und Ozeane. 4th ed.

Wegener will die in 1930. His continental drift hypothesis is in many aspects erroneous: not the single continents move but entire plates of the crust and the driving force comes from within the planet, not from outside. However his most important legacy is to have introduced the idea of moving continents to the scientific community and the public (even Lovecraft will became inspired by Wegener’s writings) – decades later this legacy will influence a new kind of theory: Plate Tectonics.

Bibliography:

MILLER, R. & ATWATER, T. (1983): Continents in Collision. Time-life books, Amsterdam: 176
SCALERA, G. (2003): Roberto Mantovani an Italian defender of the continental drift and planetary expansion. From Scalera, G. and Jacob, K.-H. (eds.): Why expanding Earth? – A book in honour of O.C. Hilgenberg. INGV, Rome: 71-74

A tribute to the Year of Crystallography - Haüy´s Models

"It would be good if the readers, who wish to follow the details of these demonstrations, make themselves or have made, in cardboard or any other materials, solids that represent the principal varieties of crystals"
Haüy, 1784 - Portrait of French naturalist Haüy with contact goniometer, an instrument to measure the crystal angles. Haüy refused to use any other type of gioniometer during his lifetime, even if after 1809 high-accuracy optical goniometers, using reflection of light to measure the angles, were introduced.

The Danish anatomist and naturalist Nicolas Steno (1638-1686) was the first to note in 1669 that the faces of a crystal (2014 was also dedicated to the science of crystals) are always arranged in specific angles and crystals display a characteristic symmetry. Mineralogist René-Just Haüy (1743-1822) used a mechanical or contact goniometer to accurate measure the angles between the faces, realizing that all the various shapes of crystals could be reduced to just a limited number of basic geometrical shapes. In 1784 he published his observations in the book" Eassai d´une théorie sur las structures des crystaux", introducing the idea of seven basic unit cells. From a single "forme primitive" (the first unit cell) by adding other unit cells a crystal could grow (this concept predates also the modern theory of crystal nucleation).

Fig.1. Haüy´s seven unit cells, note the numbering, from "Eassai d´une théorie sur las structures des crystaux" (1784).

 

Fig.2. & 3. Wooden crystal model based on Haüy´s work, made in 1805 in Paris. As certain symmetries are repeated in crystals of a mineral, Haüy concluded that a mineral is made up by smaller, basic chemical units - he called them "molecule intergrante" - symbolized here by the small cubes, forming both a larger cube as a rhombus (both characteristic shapes of the cubic crystal system). More than 500-1000 wooden models were made after 1801, some sets commissioned by Haüy himself. Most models show simple crystals with smooth faces, only 20 complex models, showing the structure with the unit cells, survive.

 

Haüy´s work was quite influential for later mineral classification. In his popular book "Über die oryktognostische Classification nebst Versuchen eines auf blossen äußeren Kennzeichen gegründeten Mineraliensystems" (The genetic-geological classification and an attempt to introduce a mineral-system based on superficial properties, published 1804) the German mineralogist Carl Friedrich Christian Mohs (1773-1839) combines various physical properties of minerals (like color, hardness and density) with crystal models to identify 183 different minerals. 
From there the use of simplified crystal geometry to identify minerals was quickly adopted by other naturalists and the classification of crystals based on the seven unit cells / crystal systems of Haüy is still in use today.

Fig.4. Carl Linnaeus "Systema Naturae", published in 1770, in his work Linnaeus didn´t not only classify animals and plants, but also minerals. One element used to identify minerals were the various crystal shapes, here still displaying a confusing variability.