After a day of island exploring, March 30th found the Beagle on its way to Rio:
“All to day we have been cruizing in sight of the Islands & have been employed in sounding & taking angles. — I have been most pleasantly employed in working at yesterdays produce. — We are now (at night) sailing with a fine breeze abaft the beam for Rio.” (Mar 30)
I love the use of the word “produce”. The island really was grocery! And for those non-nautical folks (like me) – “abaft ” means behind and the “beam” are the lumber that runs across the deck from side to side. In other words, the wind was blowing from directly behind the ship, propelling them forward.
So let’s head back to the story of the Abrolhos Islands. Today a little about the “harder parts” of the islands – the rocks and corals. First the geology…
Darwin’s (and FitzRoy’s to some degree) descriptions of the geology of the islands are less helpful than those from previous locations. They are descriptive (as Darwin always is) but don’t seem to give enough of the picture to tell what is happening.
First, FitzRoy attempts to describe the geology but only gets the story partially right:
“Their geological formation, Mr Darwin told me, is of gneiss and sandstone, in horizontal strata.” (Narrative, FitzRoy)
Okay, as we’ll see from Darwin’s description there are sandstones here, but there is no gneiss. Although it is quite common on the continent (see Geologizing on the South American Craton) it is highly unlikely that there would be any on these islands. (FitzRoy was probably remembering something from the mainland when he wrote this.)
A complete aside but this “mistake” sort of reminds me of a story I heard about the Apollo 11 astronauts (I think it was from Andrew Chaikin’s excellent book called A Man on the Moon). Apparently Buzz Aldrin was looking at some rocks and commented that the mineral in them looked like biotite. Back at mission control (in the back room, since they were probably not allowed on the floor) a group of geologists all snickered. There couldn’t possibly be biotite in the moon rocks because biotite is hydrous (has water in it) and everybody knows there is no water on the moon. Come on, Buzz, get a clue. (though to be fair he said “like biotite”.) The astronauts on the next mission (Apollo 12) responded by only calling geologic material “stuff”. I bet these geologists would have made fun of FitzRoy’s geologic comments, too.
Anyway – back to 19th century Earth…
Darwin refers to the islands being composed of “sandstone and trap”. He describes the sandstone as ” fine grained, white, & cemented by ochreous particles”. This sounds like a yellowish quartz sandstone – fairly typical of the type you might find on a continental shelf. I really don’t know enough about the geology of the region to say why it would be found at this elevation – possibly deposits from times of higher sea level or uplifted at a later date?
“Sandstone” cliffs in the Abrolhos Islands (from waittfoundation.org/ abrolhos-mpa)
Darwin also describes the curious way in which the sandstones weathers:
“The sandstone, partly from this cause, weathers in a very extraordinary manner: the whole surface being honeycombed either by hexagonal or square or circular cavities.” (Geological Notebooks)
These honeycombed structures are are not uncommon along coastlines (especially were birds help to expand weathered cavities for nesting).
Abrolhos “Trap Rock” (from About.com)
The “trap” rock is a little easier to explain – this is an old term that effectively refers to columnar basalt (Darwin later supports this by referring to their “regular prismatic columns”). Much like Fernando Noronha (where we also see columnar basalt), these rocks are most likely the result of hot spot volcanism (see Fernando de Noronha II). From what I can tell, the Abrolhos Islands were above the Trindade Hotspot about 40 million years ago. As the continent continued to move westward over the hotspot it produced the chain of undersea volcanoes that you can see extending out to the east of the Abrolhos Islands (see the map posted yesterday). The hotspot is currently under the small, volcanically active island of Trindade located at the east end of the chain. (By the way, the term “trap” comes from the Scandinavian word for “step” – describing the step-like nature of the basalt columns.)
South American hotspot tracks – TRN refers to the Trindade Hotspot and FER is the Fernando de Noronha Hotspot (from Origin of igneous rocks: the isotopic evidence by Gunter Faure):
Before I get too long in the tooth, I better mention the corals. The Abrolhos Islands are said to have some of the most extensive reefs in the South Atlantic and therefore, have become a popular diving site. According to the Scuba Divers blog:
“there is no other place on earth where you can see the type of coral reef pinnacle prevalent in Abrolhos waters, locally known as “chapeirão” [sha-payr-an-o]. The species looks like a giant mutant mushroom and is overgrown with fans of fire coral and round knobs of brain coral also unique to this archipelago.”
Other sources support the uniqueness of the the Abrolhos reefs. This includes unique species such as the main reef builder Mussismilia braziliensis, which is endemic to to these reefs. And it also includes the unique reef structure – the chaperiroes – noted above.
Darwin described these corals, too:
“The bottom of the adjoining sea is thickly covered by enormous brain stones; many of them could not be less than a yard in diameter: Without being in the immediate presence of limestones how extraordinary it is that these Polypi should be able to obtain such an enormous stock of Carb of Lime. “
In a later book, Darwin also notes:
“Round many intertropical islands, for instance the Abrolhos on the coast of Brazil surveyed by Capt. FitzRoy, and, as I am informed by Mr. Cuming, round the Philippines, the bottom of the sea is entirely coated by irregular masses of coral, which although often of large size, do not reach the surface and form proper reefs. This must be owing either to insufficient growth, or to the absence of those kinds of corals which can withstand the breaking of the waves.” (The Structure and Distribution of Coral Reefs)
Corals of Abrolhos – the big brain coral in the bottom center is a Mussismilia braziliensi (from waittfoundation.org/ abrolhos-mpa)
Interestingly, Darwin is speculating that corals need a nearby source of limestone in order to grow. They are, in fact, made of the same stuff as limestone, but what he seems to be unaware of is that calcium carbonate is dissolved everywhere in the oceans. So there is no need for a local source.
The “brain stones” are brain corals. Making all of his observations from land (or the boat), Darwin would not have been aware of the diversity of other types of corals found there, too. But you can explore the reef second-hand via the following video clip made of the reef by divers (from YouTube posted by danielkuschnir)
Alistair Dove of the Georgia Aquarium has a great series of blog entries on coral reef research he conducted in the islands – try starting with this post first – Abrolhos, here we come!
Before moving on, one last example of Darwin’s sense of humor (that I just don’t want to leave out). Last night as he returned from the islands with the mids he writes:
“Whilst pulling back to the ship we saw a turtle; it immediately went down, nothing certainly could be imagined worse for surprising an animal than a boat full of midshipmen.” (Mar 29)
Onward to Rio, mail from home, and shore leave! (RJV)