On the Art of Mineral Identification

Hardness is an important feature used for mineral identification, but it is not the only one. 

Fig.1. Lecture in mineralogy, from Bartholomäus Anglicus "Über die Eigenschaften der Dinge" (1390-1400), on the Characteristics of Things.
 
May the chemist Torbern Bergmann (1735-1784) was one of the first naturalist to discuss mineral-hardness, however, as he believed that clay is also a mineral, he assumed that hardness was strongly influenced by the humidity of the environment and therefore not very useful. In 1784 the geologist A.G. Werner published his textbook "Von den äußerlichen Kennzeichen der Fossilien" (The external characteristics of fossils; fossils as anything excavated from the ground) introducing six hardness-degrees that could be distinguished with simple tools, like a knife, a file or steel-tools, all things available to miners or amateur rock-hounds. Mineralogist René-Just Haüy (1743-1822) introduced also test-minerals, like calcite (Mohs Hardness 3) and quartz (Mohs Hardness 7) for hardness identification. Finally mineralogist Carl Friedrich Christian Mohs published the modern 10-degrees scale in 1822.

 

 

Fig.2. Hornblende (Moos in Passeier, South Tyrol).

The streak, the color of the mineral-powder, is mentioned already by Georg Pawer (1494-1555), better known as Georgius Agricola, in his books on mining techniques. Also Werner considers the streak one of the most important features, where Mohs considers both streak as color of the crystal equally important. However it was Werner to introduce a classification scheme for crystal colors, using terms like steel-gray and apple-green to describe the colors of minerals.
Curiously to get the powder the crystal had to be crushed or damaged, only in 1865 streak plates were introduced.
 
Specific weight was used already by Arabic scholars to distinguish gemstones from fake stones.

 

Fig.3. Apatite (Lodner, South Tyrol). 

Also the reactions of minerals with chemical solutions can be very important. A sort of marble was known already as "Bitterspat", "Murakalzit" and "marble tardum" by Carl von Linné (1707-1778). However in 1791 the French naturalist Deodatus Sylvain Guy de Tancrède greatet de Dolomieu (1750-1801) noted that this rock doesn´t react with acid like common limestone and limestone-marble does. He published this observation and later the Irish chemist Richard Kirwan identified and named the new mineral dolomite - a Ca-Mg-carbonate.

Fig.4. Dolomite from the Dolomites, South Tyrol.

Today many other features, like magnetism, luminescence and radioactivity are used to identify minerals. However until the 18th century only some hundred minerals were known, mostly ores or gemstones, and the described identification methods were more than appropriated for everyday use.

Bibliography:

KORTINIG, S. (1988): Der Strich. Der Aufschluss, Jhg. 39: 221-225
KORTINIG, S. (1988): Die Härte der Minerale. Der Aufschluss, Jhg. 39: 371-378
KORTINIG, S. (1988): Die Farbe und der Glanz der Minerale. Der Aufschluss, Jhg. 39: 295-299
KORTINIG, S. (1988): Die Dichte der Minerale. Der Aufschluss, Jhg. 39: 376-378
MÜCKE, A. (1988): Die Seiten für den Anfänger. Der Aufschluss, Jhg. 39: 35-38

Mineral Classification Made Easy - Mohs Hardness Scale

Talc – Gypsum – Calcite – Fluorite – Apatite – Feldspar – Quartz – Topaz – Corundum – Diamond -  “Mohs Scale of Mineral Hardness ” should be familiar to rock-hounds and earth-science students alike, as it lists common minerals in the order of the relative hardness (talc as the softest and diamond as the hardest mineral). Almost all  basic classification charts include this scale, as mineral hardness can be a quite useful criteria to identify unknown minerals and can be easily tested in the field (a steel blade corresponds to fluorite and a piece of glass to quartz).

Mohs scale  is appropriately named after the German mineralogist Carl Friedrich Christian Mohs (lithograph by Joseph Kriehuber, 1832), born January 29, 1773 in Gernrode (at the time located in the principality of Anhalt-Bernburg), son of a middle-class family.

After school he worked in his father’s business as merchant, but in 1796 he went to the University of Halle to study  mathematics, physics and chemistry. He continued his studies at the famous Royal Saxon Mining Academy of Freiberg, where he studied under the even more famous geologist Abraham Gottlob Werner. Werner had published in 1787 a “Kurze Klassifikation und Beschreibung der verschiedenen Gesteinsarten” (Short classification and description of the various rock types), a classification guide that used – unusual at a time when most rocks were classified based on the complex rock-chemistry - easily recognizable features (like color)  to identify minerals and rocks.

Mohs was impressed by the approach of Werner and in 1804 published himself a “student-friendly” classification chart for minerals, based on his experience in the mining district of the Harz and as consultant for wealthy mineral-collectors.
In the work”"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) Mohs combines various physical properties of minerals (like color, hardness an density) with 6 classes of crystal shapes  (in part in use even today) to identify 183 different minerals.

Fig.2. and 3. Specimens of Quartz (Mohs hardness 7) and Calcite (Mohs hardness 3), both minerals are common and can be very similar in shape and color, however they are easily recognizable by the different hardness, calcite can be scratched with a knife blade, quartz not.

After 1812, now as a professor in the Austrian city of Graz, he continued to improve his mineral classification scheme and to publish guidelines for mineral identification. In 1818 he succeeded Werner and became professor in Freiberg and between 1822-1824 Mohs finally published his famous hardness scale in the book  “Grund-Riß der Mineralogie” (Essentials of Mineralogy).

 
Bibliography:
 
HÖLDER, H. (1989): Kurze Geschichte der Geologie und Paläontologie – Ein Lesebuch. Springer Verlag, Heidlberg: 243
WAGENBRETH, O.(1999): Geschichte der Geologie Deutschland. Georg Thieme Verlag: 264

Geologists in the land of the Kangaroo

Terra Australis - the southern continent had been “discovered” by Europeans already in 1606, but only in 1642 the size of the new “island” becomes clear and the first geological observations  were made only in the early 19th century.

October 1800 two ships – the “Geographe” and the “Naturaliste” – set sail from the harbor of Le Havre, France. Under the command of Captain Nicolas Baudin (1754-1803) geographers, astronomers, artists, naturalists, zoologists, botanists, and 2 mineralogists – Louis Depuch (1774-1803) and Charles Bailly (1777-1844) – were instructed to explore, map and eventually claim for France new territories of this new world. In the last moment also the young zoologist, and trained paleontologist, Francois Auguste Peron (1775-1810) joined the expedition.

The geological observations made by Depuch (died during the expedition) are known from various reports send to Baudin. Bailly will publish some notes after his return to France and Peron included his research in the official report of the expedition.

In May 27, 1801 the bare land of Cape Leeuwin was in sight and the naturalists went on land along the Wonnerup Inlet, where they collected the first specimens of Australian animals, plants and rocks.

 

Fig.1. The “Baudin” – expedition, route drawn on Louis de Freycinet´s (1779-1842) “Carte générale de la Nouvelle Hollande”, published in 1811 as part of the results of the 1800-1804 expedition.

A storm forced the men to remain on land for several days and one man died during a failed attempt to reach the ships (during the entire expedition 32 men died, 13% of the crew, a surprising low percentage considering the period). The storm separated the two ships, the “Naturaliste” proceeded to the island of Timor, a Dutch colony at the time, where the crew fell ill with Malaria and other tropical diseases. The “Geographe” approached in November 1801 the island of Tasmania, where the expedition will stay for three months.
April 1802 the “Geographe” meet the British vessel “Investigator“. The expedition of the “Investigator” will map large parts of South-Australia and prove that Australia is one large continent, not two islands separated by a sea strait, as some geographers assumed. This was a disappointing discovery for captain Baudin, as there was no apparent geographic separation between the territories already claimed by British explorers, the entire continent had to be considered of British domain.

Captain Baudin, the crew and the naturalists could now only hope to gain some fame with the scientific results of the expedition...

The geologists Depuch and Bailly used a rock classification scheme, developed by the famous French geologist Déodat de Dolomieu, with four categories. They recognized primary rocks, such as granite; secondary rocks, such as stratified sandstone and limestone; alluvium (recent deposits) and volcanic rocks, such as basalt. The presence of these rocks in Australia was an important discovery, it proved that the classification scheme developed in Europe could be applied worldwide.

 

Fig.2. Charles-Alexandre Lesueur´s and Nicolas-Martin Petit´s depiction of Van-Diemen´s-Land for the “Voyage de decouvertes aux Terres Australes“. The two young men – unskilled workers at the beginning of the expedition -  were invited by Baudin to illustrate the logbook  -  both will become the most skilled artists for animal- and plantlife of the time. The granitic rocks found on the island of Tasmania convinced Peron and the other geologists that the most ancient – the primary – rock was Granite, forming the basement of all continents.

Paleontologist Peron noted along the west coast of Australia horizontal sand- and limestone layers (the Tamala-Limestone) and concluded, based on similarities to recent sediments, that these layers were deposited along an ancient beach, implying substantial variations in the sea level during geologic time:

“One of the greatest achievements of modern geology research and also one of its most indisputable, is the certain knowledge that, in the past, the level of the sea was higher than at the present time. At almost all places in the old and the new world is the proof of this phenomenon as numerous as it is evident. Only in les Terres australes was this still to be ascertained as, by virtue of its immense areal extent, it could have proved to be an important exception to the universality of the former domination of the ocean over the land.” 

(PERON & FREYCINET 1816)

Unfortunately the return to France will be disappointing for Peron. Captain Baudin dies on the island of Timor and French authorities will show little interest in the 220.000 samples of animals, plants and rocks, the 73 living animals, 3 kangaroos, 2 emus and 3 wombats brought back to Europe.

Peron publish his report “Voyage de decouvertes aux Terres Australes” only in  1807, after a long struggle for money and dies just three years later, before the completion of the second volume. However the sea shells collected during the expedition will be studied by an important French naturalist – Jean-Baptiste de Lamarck. In 1804 Lamarck publishes his theory about the transmutation of species, based in part of the observation that the fossil shells found in the sediments of France are similar, but not identical, to shells of living molluscs collected in Australia.

 

Fig.3. Peron discovers on the shores of Tasmania a living clam with a peculiar triangular shape – Trigonia antarctica – a genus of bivalve known only from fossils found in the sediments of the basin of Paris. He notes the similarities of this living specimen with fossil specimens – an important step to consider a relationship between fossil and extant species. Image of Trigonia sp. from Cretaceous sediments of Bavaria.

Unfortunately for Lamarck – and the naturalists of the Baudin expedition – he mixed his careful observations with wild speculations. Lamarck noted variations of organisms in time, however he could not explain why such variations occur or why certain organisms went extinct or survived – apart invoking a final cause and implying a sort of supernatural scheme. Geologist Charles Darwin will later regard Lamarck’s work as “useless“...

Fig.4. Geological map by Jules Grange, published in 1850, surprisingly little was known of the geology of Australia until the 20th century.

Bibliography:

GLAUBRECHT, M. & MERMET, G. (2007): Josephines Emu oder Die Geschichte einer vergessenen Expedition. GEO Nr.6/2007: 98-122
MAYER, W. (2008): Early geological investigations of the Pleistocene Tamala Limestone, Western Australia. from GRAPES, R.H.; OLDROYD, D. & GRIGELIS, A. (eds) History of Geomorphology and Quaternary Geology. Geological Society, London, Special Publications 301: 279-293
MAYER, W. (2009): The Geological Work of the Baudin Expedition in Australia (1801-1803): The Mineralogists, the Discoveries and the Legacy. Earth Sciences History Vol.28 (2): 293-324
RUDWICK, M.J.S. (2005): Bursting the limits of time – The reconstruction of Geohistory in the Age of Revolution. The University of Chicago Press, Chicago, London: 708

From the Contracting Earth to early Supercontinents

“What are they? Creations of mind?- 
The mind can make Substance, and people planets of its own  
With beings brighter than have been, and give  
A breath to forms which can outlive all flesh.”
“The Dream“, Lord Byron (1788-1824)

Already when the first maps of America were published (1507 and after), geographers and naturalists alike noted the similar shape of the west-coast of Africa and the east-coast of South America.

In 1620 the English philosopher Francis Bacon claimed in his “Novum Organum” that “it’s more then a curiosity”. In 1658 the cleric Francois Placet published a small booklet entitled “The break up of large and small world’s, as being demonstrated that America was connected before the flood with the other parts of the world.” He argued that the two continents were once connected by the continent of “Atlantis”, submerged and lost forever during the biblical flood.

The idea of a flood to explain the shape of continents will remain very popular for the next 250 years.

Fig.1. Illustration from Thomas Burnet´s book “The Sacred Theory of the Earth“, published in 1684, where he tries to explain the shapes of the continents by the biblical flood. The homogenous primordial crust of earth is shattered (first drawing) releasing water from the underground. This water covers the entire planet (second drawing) and finally flows back in the fissures, leaving behind fragments of the crust that now forms the modern islands and continents (last drawing).

The great French palaeontologist Buffon in his “Les Epoques de la Nature” (1717) not only addresses the age of earth, but also speculates about a former land bridge connecting Ireland and America to explain the distribution of fossil shells found on both sides of the Atlantic Ocean.

The American president (of the Academy and College of Philadelphia) and naturalist Benjamin Franklin explained marine fossils found on mountains in a letter to French geologist Abbé J. L. Giraud-Soulavie in 1782 as follows:

“Such changes in the superficial parts of the globe seemed to me unlikely to happen if the earth were solid to the center. I therefore imagined that the internal parts might be a fluid more dense, and of greater specific gravity than any of the solids we are acquainted with, which therefore might swim in or upon that fluid. Thus the surface of the earth would be a shell, capable of being broken and disordered by the violent movements of the fluid on which it rested.“

The great German naturalist and geographer Alexander von Humboldt explored South America in 1799-1804 and observed that the similitudes between the two coastlines were not only restricted to a morphological pattern, but also to the geological features: mountain ranges that seemed to end on one continent continued on the other, the Brazilian highland is similar to the landscape of the Congo, the Amazonian basin has it’s counterpart in the lowlands of Guinea, the mountain ranges of North America are – geologically – very similar to the old European mountains and rocks in Mexico resemble those found in Ireland.

Fig.2. Columnar Jointing in the basalts of Regla, Mexico, as depicted in Alexander von Humboldts (1810) “Pittoreske Ansichten der Cordilleren und Monumente amerikanischer Völker.” (image in public domain), the accompanying text explains: “The basalts of Regla, which are presented on this copper plate, are an incontrovertible proof of this identity of forms, which is noted on the rocks of different climates. Travelled mineralogist need only to look at this drawing to recognize the basalt forms in Vivarais, in the Euganean Mountains or in the foothills of Antrim, in Ireland. The smallest coincidences observed in the European rock-pillars are also found in this group of Mexican basalts. Such a great analogy let us assume a similar principle of formation acting under all climates in various temporal epochs, the basalts covered by compact limestone and clay-slate must be of different age than those who are resting on layers of coal and boulders.”

But even Humboldt still argued that the Atlantic Ocean represents a large and ancient river bed, flooded subsequently by the biblical catastrophe.

The French zoologist Jean-Baptiste Lamarck developed a surprisingly new hypothesis. To explain the discovery of fossil marine animals on dry land he proposed that the continents “move” slowly around the globe in a very peculiar manner. The eastern coastlines of the single continents are eroded by the sea, but in the same time new sediments were deposited on the western coasts, so the continents apparently move around the globe and the sea becomes land.
Unfortunately, also for the lack of evidence for his theory, Lamarck was not able to find a publisher for his “Hydrogéologie” and printed in 1802 on his own behalf 1.025 copies, but only a small number of books were sold.

In the early 19th century another hypothesis was proposed to explain the shape of Earth: the Contracting Earth theory formulated by the American geologist James Dwigth Dana explained mountains and continents as products of a cooling and subsequently shrinking earth. Like an old and dry apple the shrinking surface of earth would develop fissures (basins) and wrinkles (mountains).

Austrian Geologist Eduard Suess published in his multi-volume work “Das Antlitz der Erde” (1883-1909) this hand coloured map, showing the supposed remains of the primordial continents – preserved “cores of crust” surrounded by younger basins today filled with oceans. Curiously he suggested also that the deep-sea trenches, found along the borders of the Pacific, are zones where the ocean floor was pushed under the continents (!).

Fig.3. Hand coloured map showing the primordial continent -”cores” according to the Austrian geologist Eduard Suess, published in “Das Antlitz der Erde” (The Face of the Earth) 1883 to 1909 (image in public domain).

But the Contracting Earth theory couldn’t explain the irregular distribution of mountains on earth and why there are regions with strong tectonic movements and earthquakes and also “quiet” areas. According to this theory, such features and events should to be distributed randomly on the surface of a homogenous cooling and shrinking planet.

Already in 1858 the French naturalist Antonio Snider-Pellegrini (1802–1885) published a reconstruction of America and Africa forming a single continent on a planet with a fixed volume. But Snider-Pellegrini couldn’t propose a convincing mechanism, apart the great flood described in the Bible, to explain the forces needed to move entire continents.

Fig.4. This 1858 reconstruction by Antonio Snider-Pellegrini is the first map showing a former supercontinent.

Bibliography:

FRISCH, W.; MESCHEDE, M. & BLAKEY, R. (2011): Plate Tectonics – Continental Drift and Mountain Building. Springer-Publisher: 212
MILLER, R. & ATWATER, T. (1983): Continents in Collision. Time-life books, Amsterdam: 176

"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