Posted by: Rob Viens | April 23, 2012

Brazil’s Gneiss Rocks

After more than 2 weeks on the road, on April 23rd Darwin returned “home” to Rio only to have more bureaucratic trouble.  He writes:

“The number of pretty & gay houses showed our approach to the city. — During the day we passed through a wood of Acacias, the finely pinnate foliage makes for the sky a most delicate veil. — And casts on the ground a pleasing kind of shade; from the softness of the leaves, no rustling is heard when a breeze moves them. We arrived in the evening at Praia Grande, where owing to having lost our pass ports, we were plagued to prove that our horses were not stolen.” (Apr 23)

Macaé excursion – April 23

Map of Darwin's Expedition – April 23, 1832

It would be another day before he reached the Beagle and his “rental” cabin on Botafogo Bay – but he was effectively “out of the woods” for the time being. I can remember doing field work for a couple of weeks at a time in Alaska.  It always felt good to come back into town – where you could get a pizza or refrigerated food like ice cream for the first time in weeks. Or just hear the news or mail letters.  Even though it was not “home”, it still felt pretty good. However, it never took too long for the urge to return to the wilderness to come back!

I do have to wonder – once he was back in Praia Grande, how did Darwin contact the Beagle and/or get back to his cabin located on the far side of the bay? It would be easy today – use your cell phone (or a pay phone) to call someone to come pick you up. But I have to constantly remind myself, not only where there no cell phones in 19th century Brazil, there were no land lines either.  For that matter – no radios, walkie-talkies, or telegraph cables. I imagine it must have involved finding someone to run a message (maybe by boat) across the bay to the ship or chartering a boat across the bay.  In either case, I bet it involved a lot of waiting around. It makes me think of people who get upset when their cross-country airplane trip is delayed by two hours. Anyway…

In his geologic notebook Darwin touches on a couple aspects of the geology of the Macaé expedition:

“Whilst riding to the Rio Macaè. I passed through an extensive tract of country lying NE of Janeiro & collected the following scanty geological notes.” (Geologic Notebooks)

Along with a discussion of the lagoons (covered a couple days ago) and a description of how rocks weather in the tropics (which I will come back to in the future), Darwin describes the rocks that are found in this part of Brazil.  Here is an except – keep in mind these notebooks are Darwin’s “raw” thoughts:

“The formation is entirely either granite or gneiss: it generally contains much feldspar becoming porphyritic from large lamellar crystals: Near the Rio St. Joao x 1/2 way between this & the R. Macae, the rock gneiss likewise contains crystals of Hornblende. in one place I found a bed of that rock. — Near to Rio the granite gneiss was crossed by veins of a finer kind, & in many places by large ones of quartz. — The hills assumed very various forms being either rounded & massive or peaked. — generally there was an apparent entire absence of stratification or cleavage.” (Geologic Notebooks)

I realized in my earlier discussion of the South American Craton that I didn’t really say much about gneiss (which is found everywhere in Brazil. So today, I thought I’d turn the spotlight on this “high grade metamorphic rock”. Let me break it down….

First, most people probably remember that there are three basic types of rock – igneous, sedimentary and metamorphic. Metamorphic rocks start out as other types of rock and undergo a change in their chemical structure – a metamorphosis (meta = after, morph = form, -osis = change). The minerals in metamorphic rocks start out as one thing and “after” an exchange or rearrangement of their atoms during metamorphism, “change” into a new “form”.

To understand what happens during metamorphism you need to know that all minerals are “stable” under certain temperatures and pressures.  When you change the temperature and pressure they are exposed to, a mineral can “change” its “form” and become new mineral (i.e., they undergo “metamorphism”).  For example, when exposed to high pressure and temperature, the carbon atoms in the mineral graphite rearrange themselves into a more compact structure resulting in the formation of a new mineral – diamond.  Likewise, under the right temperatures and pressures minerals such as garnet or feldspar or hornblende can form, too.  (If you then take these minerals to the surface, where they are exposed to lower temperatures and pressures, they will undergo the opposite of metamorphism – chemical weathering. This results in the formation of clay minerals and rust minerals. But that part is another story.)

Verda Maritaka – a commercial variety of gneiss from Brazil (from Wolfgranite.com):

Verda Maritaka variety of gneiss from Brazil

Metamorphic “grade” is a reference to how much temperature and pressure that a rock is exposed to,  “High grade” means that it was exposed to high temperatures and pressures (say 600-800°C and 7-10 kilobars of pressure).

By the way, a bar of pressure is basically the weight of the atmosphere on your head right now (unless you are on a tall mountain in which case it is less). You don’t really think about it as pressure, since it has always been there and seems normal.  A kilobar of pressure is 1000 times the weight of the atmosphere.  To experience a kilobar of pressure you would need to go down about 10 km in the ocean (about how deep James Cameron went a couple of weeks ago – not an easy feat).  Alternatively, you could bury yourself under about 3 km of rock.  At that rate, 10 kiobars is the pressure you would experience if you went down about 30 km in the Earth (on average).  That is pretty deep.

Gneiss from the North Cascade Mountains in Washington State (from nature.nps.gov). Notice the banding running up and down – that is a characteristic of high-grade metamorphism.  The white band is an igneous intrusion cutting across the gneiss. These rocks formed about 24 km below the surface.

"Skagit

What does that mean in the context of Brazil?  Well it means that the rocks found at the surface in Brazil today once formed 20+ km below the surface.  That means that they had to have 20+km of other rock erode off the top of them before seeing the light of day.  Fortunately, they are also very old, so there has been a lot of time for that erosion to occur.

Darwin had some concept of the scale and timing of this erosion, and even alludes to it when discussing granites in Brazil – (see Neptunists, Plutonists and the Significance of Granite). Granite also forms deep in the Earth’s crust, so it is pretty common to find them in the same place a gneiss.

Thus endith the lesson for today…this is pretty deep stuff :). (RJV)

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Responses

  1. For those that might have read this post as an email – I fixed an error (in the text above) to the linguistic breakdown of the word metamorphosis (meta = after, morph = form, -osis = change). The minerals in metamorphic rocks start out as one thing and “after” an exchange or rearrangement of their atoms during metamorphism, “change” into a new “form”. (Thanks Leslie!)

  2. […] surprisingly, Darwin probably saw a bunch of granite and gneiss on his trip.  In Brazil’s Gneiss Rocks I discussed a little bit about the general concepts regarding the metamorphic rock gneiss.  Today, […]

  3. […] undergone great changes due to heat and pressure. I discussed metamorphism earlier this year (see Brazil’s Gneiss Rocks), but that was mainly in the context of gneiss.  Since Darwin mentions two other types of rock […]


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