Holiday Gifts For You

When I decided to go back to school to study geology, I really had to start at the beginning with the upper-division undergraduate courses, since my previous education had been in computer and software engineering.  The first class I took was Earth Materials, where I learned to recognize various rock types and incidentally fell in love with petrology.  We studied a lot of hand samples, and during finals week I took some photos of my favorites.  I really wanted to use them as computer wallpapers, but I hate tiled wallpapers that repeat awkwardly.  So I fired up a photo-editing tool called The Gimp, and made smoothly-repeating tiles that wrap both horizontally and vertically.

The original samples I photographed are all the property of San Jóse State University in San Jóse, California, USA.  The images themselves are copyrighted by me (Karen Locke) and are available for all non-commercial uses without attribution.

The first tile is a granodiorite from the Sierra Nevada mountains of California.  This I know the most about because I’m very familiar with the rock type; I spend a fair amount of time in the Sierras.

"Sierra White" granodiorite

“Sierra White” granodiorite

This granodiorite (and granodiorites in general) are made up of quartz, various feldspars, and “accessory minerals” like hornblende and biotite mica.  It formed underground in a magma chamber that slowly crystallized and became solid rock over a very long time.  The magma chamber that ultimately produced this rock was hot, active, and feeding a volcano back in the Mesozoic.

The next tile is a schist.

Mica schist

Mica schist

The shiny bits the photograph as bright white are light-colored micas.  Without looking at the rock again, I can’t really identify the other minerals, though there’s undoubtedly some chlorite in there somewhere.  Schists most often start out life as mud that, under high pressure and temperature, progressively gets metamorphosed into mudstone/shale, slate, phyllite, and finally end up as schist.  Along the way minerals get converted to other minerals, which then get converted to other minerals…

Here’s a gneiss.



Gneiss is another metamorphic rock, produced at moderately high temperature and pressure.  Under those conditions the minerals in the rock actually migrate, producing the light and dark colored bands you see in this sample.  The parent rock was probably something like a granodiorite.  The orange stuff is probably iron staining from weathering; most of the dark minerals in rocks like this are iron-rich.

And finally, here’s one of my favorite rocks, eclogite:



Sometimes called “Christmas Tree Rock”, eclogite consists of garnets embedded in pyroxene.  It’s created when basalt is subducted all the way into the mantle, because it requires mantle pressures to form.  It’s fairly rare to find it on the surface; it must be exhumed fairly quickly (in geologic time) to prevent the minerals from changing into other minerals at lower temperatures and/or pressures.

So, assuming the editor hasn’t clipped my artwork, any of these should give you a seamless wallpaper when you select “tiled”.  Enjoy!

Happy Holidays,