Thursday, April 5, 2012

"Why would you want to study rocks?" Part 2: The Little Things

In part one of this series I put the big picture on display.  This time I'll be focusing on smaller things that make geology interesting.

Let's start by looking at a few minerals, the building blocks of rocks:

Sheets of biotite.
Image credit:

Image credit: Wikipedia

Smoky quartz.
Image credit: Mineral-Tivadar

 Image credit: Amethyst galleries

The most obvious feature of these minerals is that they're very pretty.  If you take a closer look you may also notice that they're structurally distinct, with the exception of photo #2 and #3, which I will come back to later.  Why is this?

A detailed answer is outside the scope of this post as it requires some in-depth chemistry, but I will provide a simplified answer.  When atoms come together to form a mineral they form electrical bonds, and these bonds result in patterns.  These patterns repeat and grow into the shapes you see.  Different combinations of atoms result in different patterns, which in turn result in the formation of the different shapes.

What about cases where there is a change in color, but not in structure, you may wonder?  This is often a result of chemical impurities within a mineral.  If an ion is similar enough in size and charge to another type of ion, it can occasionally break the bond formed between that type of ion and others and 'replace' it, which result in a variant on the more common color(s) normally seen in that mineral. (At this point I would like to note that I am, again, over-simplifying some pretty complex chemistry.)

I should note that these mineral photos are idealized in the sense that it is uncommon for the right conditions to occur to produce such spectacular results.  In many cases minerals can only be seen in their 'true' shape in thin section under a microscope, and you will often find them deformed, even under the lens.

Speaking of that, what you can see under the lens is another great reason to study geology.

Two sets of sand grains under a microscope. 
Image credit: Michael Welland at Through the Sandglass.

Take a look at these sets of sand grains that have been magnified.  What's the first thing you notice?  Likely the odd shapes and colors of the set on the left, which consists of sand from a beach.  The photo on the right consists of sand from a sand dune.  Putting aside the surprising beauty of something as seemingly mundane as sand you should notice that the shapes are very different.  This speaks to their history.  One set of sand has traveled more than the other.  Can you figure out which one? (Geobloggers, give the outsiders a chance!)

Let's take a look at some other things put under a microscope:

Gabbro in thin section with cross polarizing filters on.
Image credit: OESIS

Fossiliferous limestone in thin section with polarizing filters on.
Image credit: OESIS

The two images above consist of thin sections of gabbro and fossiliferous limestone, respectively, and display some of the fascinating characteristics that rocks reveal when placed under the microscope.  The gabbro's kaleidoscopic array of colors and shapes consists of a set of interlocking minerals that form the rock the thin section was taken from.  Ain't it pretty?  By way of contrast is the psychedelic landscape below, which is a combination of worm tunnels, shells, and rock that turns out to be...limestone.  What is revealed is much more complex than what one would expect from a cursory glance at limestone, is it not?

I stressed the importance of the big picture in my first post addressing why one would want to study geology, but the little things are just as fascinating.  They're also necessary if you want to fully understand the big picture.

For those of you interested in seeing more rocks under the lens, there is a free petrographic microscope online.  Go play!

Part 1, Part 3, Part 4

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