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Stephanie Mills

M.S. Student/Teaching Assistant

Stephanie Mills
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“It has long been an axiom of mine that the little things are infinitely the most important.” ~Sherlock Holmes

An image taken with the cathodoluminescence (CL) detector on University of Maine’s School of Earth and Climate Science’s scanning electron microscope (SEM). The large grain with all the dark splotches and lines is quartz; the dark splotches and lines are the focus of my Master’s work. The small, brightest grains on the edge of the quartz are k-feldspar and the rest of the grey grains are plagioclase.

An image taken with the cathodoluminescence (CL) detector on University of Maine’s School of Earth and Climate Science’s scanning electron microscope (SEM). The large grain with all the dark splotches and lines is quartz; the dark splotches and lines are the focus of my Master’s work. The small, brightest grains on the edge of the quartz are k-feldspar and the rest of the grey grains are plagioclase.

 

georgian bay sz

A small (less than a meter wide) shear zone that runs through the bottom right corner of the photograph. Notice how material seems to bend into the shear zone; that is because the material in and near the shear zone was deforming like silly putty so it bends and flows rather than breaking or fracturing. This is on an island off the Georgian Bay in southern Ontario.

I like to study the little things about rocks, which is why I like microstructures. What makes little microstructures important? Well, every grain got its composition, shape, orientation, etc. by some process, so microstructures give us clues to the kinds of processes that rocks have undergone. For example, if a grain is elongate, perhaps it got that way because it was squished.

 

Now the squishing of one little grain isn’t earth shattering, but the squishing of millions of grains can move rocks several kilometers! This is how we build mountains. Where the rocks are too hard to squish, they shatter, and if enough of them shatter all at once, they make an earthquake. So in a sense, microstructures really are earth-shatteringly important.

 

georgian bay

This is Georgian Bay in southern Ontario. Yes, there were once giant mountains here, but they have been eroded away so that the rocks once deep inside/below them are now easy for us to grab. The last stage of glacial erosion left some breathtaking sculpting.

The rocks that I study come from Ontario’s shores of the Georgian Bay, not too far from the land of my heritage, Michigan. These rocks are important because they come from what was deep below mountains that were once much like the Himalaya are today. While we can’t directly measure what is happening beneath the Himalaya, we can study these rocks that are preserved from a similar environment so that we can understand the processes that are building mountains today.

 

The microstructures that I study are in quartz and I look at them using the cathodoluminescence (CL) detector on a scanning electron microscope (SEM). I have identified several different kinds of microstructures; my goal is to determine what processes formed these microstructures and determine what role those processes may have played in the building of mountains.

The three different CL-dark microstructures I have identified in quartz. The bright grains are plagioclase.

The three different CL microstructures I have identified in quartz. The bright grains are plagioclase.

CL images of quartz. The arrows point to examples of the “thin lines”.

CL images of quartz. The arrows point to examples of the “thin lines”. These lines correspond to the borders of areas within the grain that are slightly misoriented with respect to each other, kind of like subgrains.

CL images of quartz. The arrows point to examples of the “thin lines”.

CL images of quartz. The arrows point to examples of the “thin lines”. Note that there are a few nearly straight “thick lines” that cut through the thin lines.

A good example of the mantle on the grain boundary (top of image) and some of the thick lines running roughly NW-SE. These thick lines correspond to fluid inclusion trails which are interpreted as healed fractures.

A good example of the mantle on the grain boundary (top of image) and some of the thick lines running roughly NW-SE. These thick lines correspond to fluid inclusion trails which are interpreted as healed fractures.

A group of quartz grains displaying all three microstructures.

A group of quartz grains displaying all three microstructures.

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

(And if you really want to know why I’m studying rock microstructures, it’s because they’re pretty!)


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