Sunday, May 24, 2009
Alan V. Morgan
Thursday, February 12th, 2009.
Surely today has some significance? My computer countdown clock chimed to inform me that it had.
I was up early, on a not so sunny day at Sani Lodge in the tribal lands of the Yasuni on the Napo River, one of the major tributaries of the Amazon. The day was important because exactly 200 years earlier, Charles Darwin had been born in Shrewsbury, England, setting events in motion some 22 years later which would resonate through various fields of science to the present. I refer to Darwin’s voyage on H.M.S. Beagle, a ten gun brig that sailed from Devonport, Plymouth, in southwestern England on the 27th of December, 1831.
As I mentioned in an earlier issue of Wat On Earth (see: Winter 2006) Darwin’s travels and interests have crossed with my own in many places in various parts of the world. However, this was my first visit to South America where Darwin spent the bulk of his time as a “part-time gentleman” companion with Captain Fitzroy and for the next two months was able to cross many different parts of his route. I thought I would bring his observations and my own together in this article.
Of course Darwin never got into the headwaters of the Amazon, and probably the closest he came to the region was when the Beagle made landfall in the Galapagos Islands some 1,500 km to the west. Nevertheless, his travels in Brazil, Argentina and Chile must have first introduced him to the vast diversity of tropical life and also set the stage for his observations and thoughts that radically changed biological concepts over 20 years later in 1859.
In order to follow “in the footsteps” I have commented on some selected localities visited by Darwin in the period from 1832 to 1834. Many of these are quite distant from each other and not necessarily in the order that Darwin visited them.
Darwin’s first lengthy stay in South America was in Brazil and he spent some time at Botofogo (currently Botafogo) Bay in what is now part of Rio de Janeiro. His residence was quite close to the near vertical mountain, Corcovado, about 4 km from both the renowned Sugarloaf and from Ipanema Beach.
Our visit to Rio was short, but like most visitors I was truly impressed by the startling topography around the city, much of which has also been enhanced by the recent drowning of the coastline. In the Journal of Researches Darwin had commented:
Everyone has heard of the beauty of the scenery near Botofogo. The house in which I lived was seated close beneath the well-known mountain of the Corcovado. It has been remarked, with much truth, that abruptly conical hills are characteristic of the formation which Humboldt designates as gneiss granite. Nothing can be more striking than these huge rounded masses of naked rock rising out of the most luxuriant vegetation.
Sugarloaf at 396 m, Corcovado at 710m and the many and varied monolithic hills of Rio are wonderful examples of inselbergs. These are residual hills derived from the tropical weathering of feldspathic granite plutons of late Precambrian age(about 570 million years old).Chemical weathering and “spalling”— the exfoliation of surficial layers of rock, have helped to produce the steep to near-vertical sides seen on the hills. Similar features can be seen in Yosemite Park; for example “Half Dome”, and just north of Vancouver near Squamish, the “Stawamus Chief” although both North American examples have been modified by glacial activity and this is not true of the Brazilian peaks.
After reaching the top of Sugarloaf (some fairly obvious feldspathic dykes easily seen in the approach to the summit station) we returned to the middle station on Urca Mountain. Here workers had been blasting and a number of fresh exposures illustrated the large feldspar phenocrysts present in the granite. We then walked down the southern side where we saw some really excellent palled slabs that had moved away from the exposed rock surfaces and had settled onto the coastal pathway.
Our journey then took us south to the estuary of the River Plate and the cities of Buenos Aires in Argentina and Montevideo in Uruguay. It was in this area that Darwin had first found fossils of Quaternary age, some of which were new to science. His fossil mammal finds his visit to the Falkland Islands and some additional observations in South America will be described in separate articles.
Eventually all sailors travelling westward around South America either have to pass around the infamous Cape Horn, or through either the Straits of Magellan or the Beagle Channel. Both of these short cuts eliminate the potentially stormy passage around the Cape.
We were fortunate in rounding the Horn in relatively calm seas eastbound, but were prevented by very stormy weather from doing so westbound. Cape Horn, perhaps unknown by many, is not part of the South American mainland. It is actually the southernmost promontory on Isla Hornos, part of the Hermite Islands south of Tierra del Fuego (which in itself is also another large island south of South America).
Without being able to go ashore it was difficult to make out much of the geology of Cape Horn even through a 400mm lens, but it did appear that north of the Horn predominantly light coloured rocks (perhaps metamorphic or igneous) are penetrated by dykes of what appear to be, basalt or andesites. (See inside cover).
Travelling westward our next encounter with Darwin and his Beagle voyage was in the Beagle Channel. His comments are worth recording because of the implications of global warming.
January 29th 1833:
Early in the morning we arrived at the point where the Beagle Channel divides into two arms; and we entered the northern one. The scenery here becomes even grander than before. The lofty mountains on the north side compose the granitic axis, or backbone of the country, and boldly rise to a height of between three and four thousand feet, with one peak above 6,000 feet. They are covered by a wide mantle of perpetual snow, and numerous cascades pour their waters, through the woods, into the narrow channel below. In many parts, magnificent glaciers extend from the mountain side to the water’s edge. It is scarcely possible to imagine anything more beautiful than the beryl-like blue of these glaciers, and especially as contrasted with the dead white of the upper expanse of snow. The fragments which had fallen from the glacier into the water were floating away, and the channel with its icebergs presented, for the space of a mile, a miniature likeness of the Polar Sea.
Many people travelling through these southern Chilean fjords have expressed admiration for the intensely blue-coloured ice of the many glaciers that lie perched on the slopes above the Beagle Channel. Even Darwin commented on the “Berylblue” of the glaciers. The blue- ranging to turquoise-coloured ice often present in glaciers is due to reflectance of light. Red and yellow wavelengths are more easily absorbed by ice crystals than blue. When glacial ice has lots of bubbles in it, it tends to scatter all incoming visible wavelengths and so the ice appears grey or white.
Any Earth scientist traversing this region cannot help noting the large expanses of bare bedrock surrounding various glacier and ice field margins. These have to be ascribed to “modern” glacial retreat. It appears that the maximum recent extent of glacial ice in several parts of southern Chile was about 1860, some 30 years after Darwin’s voyage, and since then glacier margins have been rapidly retreating.
Our journey west along the Beagle Channel, a deep glacial fjord, followed Darwin’s route to the southwestern coast of Chile. It was here that he experienced (but did not recognize) the forces that manifest themselves in Plate Tectonics. Like Darwin and many other sailors in this region we encountered several major storms, and one could really empathise with Darwin who survived a similar gale.
In January 1833 he wrote:
“… On the 13th the storm raged with its full fury; our horizon was narrowly limited by the sheets of spray borne by the wind. The sea looked ominous, like a dreary waving plain with patches of drifting snow; whilst the ship laboured heavily, the albatross glided with its expanded wings right up the wind. At noon a great sea broke over us, and filled one of the whale boats, which was obliged to be instantly cut away. The poor Beagle trembled at the shock, and for a few minutes would not obey her helm; but soon, like a good ship that she was, she righted and came up to the wind again. Had another sea followed the first our fate would have been decided soon, and forever.” (… and although he did not know it, the course of science likely would have been changed, at least until Wallace’s ideas on evolution were expressed)!
Of course there was a vast difference in the size of our two vessels. The poor Beagle was 27.5 m long and 7.5 m wide and weighed 242 tons. The Star Princess is 290 m long, 36 m wide and weighs 109,000 tons! (The Beagle would have fitted into the central pool area of the Star Princess)! Nevertheless viewing the storm from the top deck, about 150 feet above the sea was quite impressive!
Our journey continued northward along the coastline of Chile, where we had the opportunity to sail between Chiloe, the fifth largest island in South America and the mainland coast of Chile. Darwin sailed the same route in the Beagle arriving in Chiloe on November 10th , 1834. It was here that Darwin had the opportunity of seeing three regional volcanoes in simultaneous activity. One of these (Osorno) is shown on p.14, but the 2300m Corcovado volcano (not to be confused with the peak in Rio de Janeiro) and the 2404m Minchinmávida were also active. All three volcanoes produce basaltic to andesitic lavas.
In the same area the 1122 m Chaiten caldera was in eruption while we were passing, but unfortunately we were too far away to observe the eruption cloud. Chaiten is an unusual acidic volcano that is extruding rhyolitic lavas and translucent grey obsidian that in former times was used by the prehistoric people of the region.
Darwin finally reached Valparaiso on July 23, 1834, and his travels to the north will be concluded in other summaries in Wat On Earth.