The Voyage of the Beagle – Focus on Geology

I long associated Charles Darwin’s round-the-world voyage solely with biology and evolution of living creatures, but that focus came after his return to England.  Certainly, Charles spent a large portion of the five years observing, collecting, classifying, experimenting with, and speculating about the animals and plants he found.  This activity tapped his life-long passion for observing and collecting from the natural world.  As he matured into a would-be naturalist under the guidance of Professor Henslow and others, Charles turned passion into profession.  As a naturalist, however, Charles was also interested in the inanimate components of nature, especially the fascinating variety of rocks and landscapes and climates.  It was typical of naturalists of his time to be interested in everything, looking at the whole even as they collected and described its parts.  The more sophisticated naturalists were natural historians and philosophers, speculating continually on the who, what, when, where, how and why of the natural scenes their educated curiosity drove them to investigate.

  

Putting Lyell to the Test in South America

 

Just in the few months before the voyage of the Beagle, Charles was introduced to the emerging science of geology by Professor Adam Sedgwick of Cambridge University.   In the three centuries before the 1830s, natural philosophy had advanced toward what we think of as scientific method and evidence-based theory, first in astronomy and physics, then chemistry.  Charles did not feel capable of mastering the mathematics needed to fully participate in these scientific specialties, but he appreciated their power to explain the material world in material terms – focusing on proximate causes that a person can directly or vicariously appreciate with the body’s senses, rather than on ultimate causes beyond human comprehension.  Charles joined his older brother, Erasmus, in doing chemical experiments in a shed near his house, often generating obnoxious fumes that earned Charles the nickname “Gas” among his schoolmates.  He learned what it means to experiment and to predict the outcome from prior experience and to be open to the possibility of surprises that force revision of future predictions.  Charles also saw how much sophisticated scientists and their useful knowledge were admired by well-educated men and women of social standing.  Esteem could be achieved by a career in such science, and geology was just then transforming into such science, and Sedgwick made the practice of this science accessible to Charles. 

 

Armed with the first volume of Charles Lyell’s Principles of Geology, Charles realized during the first landing of the voyage, in the Cape Verde Islands, his opportunity to become an esteemed contributor to the emergence of scientific geology.  The distinctly different layers of rock, lying one upon another, presumably in the same sequence in which each was deposited, told a geological history of the island of São Tiago. 

 

Mainstream thinking of the time, conforming to the Biblical story, maintained that the land was stable in the forms originally created and only the level of the sea had risen and fallen over time; the time periods allowed for inundation were relatively short and few.  Lyell (building on the work and words of others, of course) proposed that the land itself had been transformed over time by the same geological processes experienced in the present age – erosion and deposition, volcanic eruptions, and earthquakes.  Moreover, whole blocks of land the size of islands and countries and even continents could rise and fall relative to each other and the level of the oceans.  Lyell proposed cycles of erosion of rocks of varying chemical composition, deposition by water and wind of the eroded rock and organic material in lakes and shallow seas, transformation of these deposits into new types of rock under the weight of overlaying deposits and water, perhaps followed by volcanic activity that spread layers of lava from deep in the earth over the earlier deposited layers, then all uplifted in a series of small events accompanied by earthquakes, until all that had been submerged in the sea now stands inches or feet or even miles above the current sea level.  Acknowledging that all this complex process must require substantial intervals of time, Lyell proposed that the earth was millions, not thousands of years old. 

 

Lyell’s proposed principles were like a dictionary enabling Charles to read the story of the rocks and the layers of São Tiago.  The story fit the facts in front of him together so well that he was “convinced of the infinite superiority of Lyell’s views over those advocated in any other work known to me” (from his Recollections).  This particularly excited Charles, because neither Lyell nor other British geologists had visited South America, giving ambitious Charles the opportunity to be the first to apply the principles to the landscapes he would soon see first hand.  He could test the explanatory power of Lyell’s principles– perhaps even refine and add to this grand theory of the earth. 

 

The Coasts of the Southern Cone — a Continent on the Move Upward

 

After four months on the coast of Brazil, based at Bahia and Rio de Janeiro, the Beagle expedition spent nearly two years (July 1832 to May 1834) working up and down the coast of Argentina, from Uruguay to Tierra del Fuego, with a couple of visits to the Falkland Islands.  Charles left the Beagle several times to make long-distance excursions inland.  All along the shore of the essentially unbroken Argentine and Patagonian plains, he found thick deposits of alluvium (shingle, gravel and mud) that looked very much like they had been carried by rivers and laid to rest in smooth layers underwater in the estuaries where these rivers met the South Atlantic.  Yet these smooth layers of alluvium now stood a few feet to several hundred feet above sea level.  Were they gradually uplifted from the sea, as Lyell would suggest, or had the sea suddenly risen at some point to flood these plains and then withdrawn?  The smoothness of the plains and the sea mollusk shells embedded fairly evenly and deeply in the alluvium gave evidence of gradual uplift from under the sea.  Charles also found that the marine species represented in these layers were quite similar to the species found living in the present estuaries, which indicated the uplift had been fairly recent.  Was it possible that the whole of this long coast was rising up from the sea?  This question made Charles eager to see the mountainous Chilean side of the “southern cone” of South America, hoping for further evidence that the whole of southern South America was rising.

 

From June 1834 to September 1835, the Beagle expedition worked up and down the coast of Chile and Peru.  The southern coast of Chile, from Chiloe Island to Tierra del Fuego, is an archipelago of mountainous islands.  The line of mountains continues northward and inland from the coast north of Chiloe Island, becoming the highest Andes.  When Charles hiked into the coastal hills of central Chile near Valparaiso, he looked out across low mountains and valleys intervening between the coast and the Andes.  The valleys were filled by fog, and the mountains protruded from the sea of fog like the mountain islands of the southern archipelago.  This image gave visual substance to Charles’s hypothesis that the flat valleys had been the bottoms of ancient inlets and deep bays like those farther to the south, later drained as they were uplifted above sea level.  What about the high cordillera of the Andes in the hazy distance? 

 

Evidence in the Andes

 

Charles twice ventured into the Andes from Valparaiso, the second time doing a complete traverse by one alpine pass to the Argentine side, then back to Chile by another pass north of the first.  At 13-14,000 feet elevation, he found fossil shells of marine mollusks! Around 7,000 feet, Charles made an even more remarkable discovery – a standing grove of tree trunks petrified in white silica and calcareous spar, emerging from volcanic sandstone that had once entombed the trunks and then eroded away to reveal them transformed to stone.  Though he had little trouble interpreting this scene, Charles was still astonished by his conclusion.  From his Journal of Researches, “I saw the spot where a cluster of fine trees had once waved their branches on the shores of the Atlantic, when that ocean (now driven back 700 miles) approached the base of the Andes.”  The evidence showed that the land with its upright trees had been “let down to the depths of the ocean,” covered by sediment, and this again by enormous streams of submarine lava, then more sediment followed by more lava, repeated five times.  All this was uplifted so that he “now beheld the bed of that sea forming a chain of mountains more than 7,000 feet in altitude.” “Vast, and scarcely comprehensible as such changes must ever appear, yet they have all occurred within a period recent when compared with the history of the Cordillera; and that Cordillera itself is modern as compared with some other of the fossiliferous strata of South America.”

 

Eruptions and the Earthquake

 

The Andes testify to both the violence and the speed of geological processes, though belied by long periods (in human terms) of apparent calm.  But Charles and the Beagle crew actually experienced the episodic violence that drives mountain building.  Between his two journeys into the Andes, Charles sailed south with the Beagle to survey Chiloe Island and the Chronos Archipelago during the Southern Hemisphere summer (still the weather was rough and tempestuous for weeks).  On the night of January 18, 1835, the volcano Osorno, a hundred miles inland, lit up the sky with a spectacular eruption.  They learned later that the volcanoes Aconcagua, 480 miles to the north, and Coseguina, another 2,700 miles farther north, had erupted on the same night.  Just four weeks later, while Charles and his servant, Sims Covington, were ashore near Valdivia, the area was struck by a massive earthquake.  “It came on suddenly, and lasted two minutes; but the time appeared much longer.  The rocking of the ground was most sensible…There was no difficulty in standing upright, but the motion made me giddy…Captain FitzRoy and the officers were at the town during the shock, and there the scene was more awful; for although the houses, being made from wood, did not fall, yet they were so violently shaken that the boards creaked and rattled.  The people rushed out of doors in the greatest alarm.”  Charles concluded that “the world, the very emblem of all that is solid, has moved beneath our feet like a crust over a fluid; one second of time has conveyed to the mind a strange idea of insecurity, which hours of reflection would never have created.”

 

Two days later the Beagle sailed into the harbor of Concepcion and found the city utterly destroyed by the earthquake.  The epicenter had been just offshore, and a massive tidal wave had swept the shoreline of structures, livestock and people.  Charles devoted several pages of his Journal of Researches to description of the devastation.  While lamenting the loss of life and property, Charles was also deeply interested in what the patterns of destruction told him about the movements of the earth and the ocean during the quake and its after-shocks.  “The most remarkable effect (or perhaps speaking more correctly, cause) of this earthquake was the permanent elevation of the land.”  FitzRoy’s detailed survey work before and after the quake provided “a mass of evidence in proof of such elevation, far more conclusive than that on which geologists on most other occasions placed implicit faith.”  They calculated that, since the previous massive earthquake of 1751, the land had risen four fathoms – 24 feet!  With enough time allowed, Charles had no problem understanding how the Andes had been lifted up.  The earthquake had been accompanied by simultaneous eruptions of a train of volcanoes in the Andes of central Chile.  Charles concluded that the earthquake and volcanic activity formed parts of one great phenomenon underlying an area onshore and offshore that measures 700 by 400 miles.  In line with Lyell’s theory, all the evidence pointed Charles toward the conclusion “that a vast lake of melted matter, of an area nearly doubling in extent that of the Black Sea, is spread out beneath a mere crust of solid land.”

 

Volcanic Islands and Coral Atolls — Rising from the Ocean and Under the Waves Again 

 

Later in 1835, the Beagle expedition left the coast of South America at last and set sail for home – westward across the vast Pacific Ocean.  The Galapagos Islands were the first stop en route.  The expedition spent a bit over one month surveying the islands; Charles and Covington and some others of the crew were put ashore for a few days on several of the many islands.  Charles knew beforehand that the islands were of recent volcanic origin, evidenced by the activity of volcanoes and the vast expanses of recent lava flows.  He could see for himself that the volcanically very active western islands are younger than the eastern islands, which are lower, more eroded, more vegetated, and showing no present-day volcanic activity.  He correctly concluded that these islands had all originated as subterranean volcanoes that built upward until they broke the ocean surface.  In addition to uplift from beneath the ocean, Charles also had opportunity to see evidence of subsidence – volcanic craters partially submerged to form crescent bays, like what is now named Darwin Bay on Tower Island (Isla Genovesa).  These observations no doubt encouraged Charles in his budding hypothesis about the origin of coral atolls, though he had only read about them up to this point.  The Galapagos waters are too cold for corals (despite being right on the Equator), but Charles saw many of these thin circles of low-lying land as the Beagle moved on through the Tuamotu (or Dangerous) Archipelago to Tahiti.

 

After stops in New Zealand and Australia, the Beagle made a detour to survey the Keeling or Cocos Islands, in the Indian Ocean southwest of Java in Indonesia.   The main island is a lagoon atoll of coral formation similar to the Tuamotu atolls.  The deep soundings of the survey work on the ocean side of the lagoon island verified that live corals could not be found at depths in excess of 120 feet (coral polyps depend on sunlight penetrating the water), and at greater depths, they found only calcareous rock or sediment formed from dead coral, the same material of which the atoll’s islets were composed.

 

Charles’s theory was that all coral reefs start as fringes of the tropical shore of rocky land.  If the land rises, the exposed coral dies, but new living reef is formed along the new shore to the depth of 120 feet.  If the land subsides, it sinks into the water slowly enough to allow the tiny coral polyps to build their reef upward to remain just below the ocean surface.  As the gently sloping shore sinks into the water, however, the coral polyps, being most active on the side toward the open ocean waves, build vertically toward the sun on the calcareous base created by their sunken predecessors; thus, the rocky shore gradually withdraws from the reef in contact with the open ocean, leaving a shallow lagoon in between.  In the case of large islands, like New Caledonia, or even continents, like Australia, these became great barrier reefs.  In the case of isolated oceanic islands, which rise as submarine volcanoes from the seafloor and then subside back under the ocean surface, these become coral atolls.  The reef first forms a fringe around the rising volcano; the subsiding volcano then leaves a lagoon encircling the sinking volcanic peaks still above water (like spectacular Bora Bora).  Then even those remaining peaks sink below the lagoon surface, leaving only the outer reef and a thin circle of calcareous materials piled up by the ocean’s pulverizing waves into islets just behind the outer reef, barely high enough to be visible from a distance – an atoll in the middle of the open ocean.

 

His theory was a masterpiece of deductive reasoning.  From Charles’s Recollections, “No other work of mine was begun in so deductive a spirit as this, for the whole theory was thought out on the west coast of South America, before I had seen a true coral reef.”  This theory, which captured the admiring attention of Charles Lyell himself upon Charles Darwin’s return to England, overturned all others and still stands today.  Charles was rightly pleased with his theory – how he derived the ideas from his geological observations in South America and how they were verified by the evidence of coral reefs around the world.  In his Journal of Researches, Charles built on the theory of reef formation to show that the distribution of the three types of coral reef – fringing the shore, encircling islands with lagoons, and forming barrier reefs and atolls – indicates that large blocks or plates of the earth’s surface (including seafloor) are tilting, so that one end is rising while the other is subsiding.  Charles thereby extended his theory of reef formation, built on his personal verification of Lyell’s principles, to even more dramatically demonstrate “the never-ceasing mutability of the crust of this our World.”

 

The next and last post on the voyage of the Beagle reviews the major biological and anthropological observations of Charles Darwin during 1831 to 1836.

 

Copyright 2008 by Chris Dunford.  May be quoted in part or in full only with attribution to Chris Dunford (www.darwinwatch.wordpress.com)

 

 

 

 

 

 

 

 

 

 

 

 

Advertisements

0 Responses to “The Voyage of the Beagle – Focus on Geology”



  1. Leave a Comment

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s




Welcome to DarwinWatch

This blog by Chris Dunford explores the meaning of Charles Darwin's life, work and words in relation to the Science-Religion Debate. It is committed to intellectual honesty and historical perspective. Please click on the "Why this Blog" tab under the banner photo to learn more. Started in July 2008, this has been a very slow work-in-progress. Be patient with me and check in occasionally, if only to enjoy the banner photo!

%d bloggers like this: