Posted by: Rob Viens | November 26, 2012

Old Schist and Other Gneiss Rocks

On Sunday, Darwin went riding with his “old” friend Mr. Parry.  Parry (mentioned for the first time back in July as “a leading merchant here”) is referred to on at least 2 or 3 other occasions – usually as someone Darwin dines with in Montevideo.

“Rode with Mr Parry to Las Pietras; a pretty village so called from some rocks of a singular shape.— One calls a village pretty in this country, if it possesses a dozen fig trees & is situated a hundred feet above the general level.” (Nov 25)

Las Pietras is located about 10 miles inland, due north of Montevideo.  It was the site of a pivotal battle in 1811 during the war of independence from Spain. As Darwin mentions the rocks, I thought I’d give a brief the geology of the region.

Montevideo (and much of Uruguay) is built on the foundation of the Rio de la Plata Craton. (If you recall, cratons are the very old, stable parts of the continents – for a refresher see Geologizing on the South American Craton.) Much like the craton Darwin traversed in Rio de Janeiro, the basement rocks here consists of metamorphic rocks.  In Uruguay, these rocks primarily formed during two main metamorphic “events” – one about 2 billion years ago, and a second one about 600 million years ago. Darwin describes them below:

“The rocks here consist of several varieties of gneiss, with the feldspar often yellowish, granular and imperfectly crystallized, alternating with, and passing insensibly into, beds, from a few yards to nearly a mile in thickness, of fine or coarse grained, dark-green hornblendic slate; this again often passing into chloritic schist.” (Geological Observations of South America)

He even comments on a couple of specific locations such as “The Mount”:

“The Mount, a hill believed to be 450 feet in height, from which the place takes its name, is much the highest land in this neighbourhood: it consists of hornblendic slate …  Near the summit the hornblende-slate gradually becomes more and more coarsely crystallized, and less plainly laminated, until it passes into a heavy, sonorous greenstone, with a slaty conchoidal fracture.” (Geological Observations of South America)

These rocks are all metamorphic rocks – rocks that have 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 – slate and schist – I’ll say a word or two about them today.

Metamorphic rocks can (in a simplistic sense) be broken into two major groups – those with a foliation (i.e., layering) and those without. Which type you get depends primarily on two things – (1) the type of rock you start with (called the parent rock) and (2) the type of pressure you apply to the rock during metamorphism.  All of the rocks here could have come from the same basic parent rock, so let me concentrate on the type of pressure they experienced.

Foliated rocks require “directional pressure” – pressure that is greater in one direction. (In contrast, lithostatic pressure refers to pressure that is applies equally from all directions, such as what you might experience underwater.) The easiest way to get directional pressure is in the collision of two continents (much like what is happening in the Himalaya today). The slate, schist and gneiss Darwin mentions are all foliated metamorphic rocks. What this means is that at about 2 billion and 600 million years ago the rocks in this part of the world where caught up in a continental collision. Pretty cool thought, eh?

Slate (from U. of Pittsburg Geology Departent)

slate

Now slate, schist and gneiss all exhibit different types of foliation. Slate tends to “cleave” or break into sheets (a property known as slaty cleavage).  The foliation in schist is defined by the alignment of flat or platy minerals such as “mica” and chlorite. Gneiss’ foliation is defined by alternating layers of light and dark minerals that give it a zebra-stripe appearance.  Since they all could have formed from the same basic parent rock, and all experienced directional pressure, you might ask “why do they exhibit different types of foliation?”  Good question – thanks for asking!

Garnet Muscovite Schist from Manhattan (from About.com)

garnet mica schist

The different between these types of foliation is the grade of metamorphism (degree of temperature and pressure) to which they have been exposed.  Slates form under low grade conditions – let’s say the temperatures and pressures you might find about 8 km deep. (There are ways to determine a more exact depth but we won’t go there today.) Schist might form in the 15 km range, and gneiss in the 25 km range. So these samples represent rocks pushed up from a variety of depths. (Now that being said, I am pretty sure what Darwin referred to as a “hornblende slate” would today be considered a type of schist.)

The modifiers to these names – e.g., “hornblende slate” and “chlorite schist” –  simply refer to predominant minerals found in the rock.  So a chlorite schist is a schist hat contains the mineral chlorite.  These minerals can help determine the “parent” of these rocks. The chlorite, for example, suggests a more “mafic” parent – like a basalt. That would also be consistent with the final rock Darwin mentions – greenstone.  Greenstone forms when a basalt is exposed to low grade metamorphic conditions.

One final thought for today…

Although it was still more than half a lifetime away for Darwin, I realized I missed an important anniversary this weekend – Origin of Species was published 153 years ago on November 24, 1859. A couple of years ago, master visualizer Ben Fry created an interesting visualization of the Origin of Species that shows how it “evolved” though Darwin’s lifetime. The chart shows the chapters of the book from left to right and the colors on the image show changes that where made in later editions.  Click on the image to go to an interactive graphic that allows you top see the entire text color coded by which edition it added in. (Note – the graphics takes a little while to fully load.)

Ben Fry Origin of Species Visual

(RJV)

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Responses

  1. […] In the world of decorative stone, almost everything is called either granite or marble. However, as I  often point out to my students, names used for marketing stone are not always the same as rock names used in geology. To geologists, marble has a much more specific meaning – it is a metamorphic rock that forms from the exposure of limestone (a sedimentary rock) to high temperature and pressure. Limestone is made of basically one mineral – calcite (calcium carbonate). So when that rock is metamorphosed, there are not a lot of different elements present to rearrange into new minerals.  So during metamorphism the atoms simply recrystallize, forming new calcite crystals which are, in many cases, coarser then the calcite grains they started out as in limestone. (For a lesson on metamorphic rocks see Old Schist and Other Gneiss Rocks.)  […]


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