Rock Blogging
greengabbro | October 26, 2008 So far, Thomas M. is the only one to take advantage of my Donors Choose fundraising gimmicks. So, this rose quartz cobble, which I picked up while hiking in the hills near Santa Fe, shall henceforth be known as Thomas.
When you find something so well-rounded in a sedimentary deposit it's always hard to say precisely where it came from - especially when you subsequently forget where, exactly, you were hiking that day. Northern New Mexico is home to a number of pegmatites, coarse-grained igneous veins that host spectacular crystals and economically useful deposits of lithium and beryllium.…
greengabbro | April 11, 2008 Coal doesn't burn completely. Here's what's left over.
(Do I apologize for the weak blogging? No! No apologies! I am still trying to work "elusive", "obsequious", and "vapid" into a discussion of bedrock hydrogeology. "Propinquitous", though, that's taken care of. Also, I am contemplating cheesy steganography.)
greengabbro | April 4, 2008 I was going to write up a proper post on marine evaporite sequences, and how they relate to the deliciousness of expensive salt vs. cheap salt, but, um, I didn't. Hey! Look! Pretty picture!
Enough salt for nine lives... Originally uploaded by aleske
greengabbro | March 27, 2008 This one's in honor of the new activity at the peak of Kilauea. For pictures and updates, see the Hawaii Volcano Observatory homepage. For more geoblogospheric coverage, check out the posts at Magma Cum Laude and the roundup at Geology News.
To get Pele's hair, you need to throw around some lava. More precisely, you need your volcano to work like a cotton candy machine: take a bit of liquid rock, and spin it out until it looks like, well, hair. Or cotton candy (seriously, Internet, why do you not contain any magnified images of cotton candy for me to link to here?). Though these images…
greengabbro | March 21, 2008 So sand is just little weensy rocks, anyway. And this is a weensy volcano made of sand, in Peru. It’s about a meter (0.33% 0.9% of a football field thanks LL!) across.
Normally, layers of sand and silt underground bear the weight of whatever’s on top of them through a network of contacts between individual soil grains. During an earthquake, this network is disrupted. But the stuff on top is still there, bein' all heavy. Wacky hijinks ensue!
If the jiggling sand happens to be wet, all that overburden pressure is transferred to the water. Water, being incompressible, is not happy about being…
greengabbro | March 14, 2008This is an outcrop of Bishop tuff, an ash deposit created 760,000 years ago when the Long Valley Caldera exploded - though “exploded” is, if anything, an understatement. The photo was taken 15 miles (25 of your Earth kilometers) away from the eruption; it contains no persons for scale, but the outcrop is about 10m high. Ash deposits from the same eruption are found all over Southern California and as far east as Nebraska.
Tuff is what happens when a pile of hot ash (”ash”) and fragments of exploded crap (”breccia”) consolidates and hardens under its own heat and weight. If the proportion of…
greengabbro | March 7, 2008
Picture courtesy reader Martin. Or maybe Martin doesn't actually read this blog, and it's just Wren. Anyway, thanks, Wren and Martin!
Today's rock is a geopuzzle: What's up with these ridges? How did they get there, and what determines their size?
I don't actually know the answer, so this is the best hint you're going to get out of me.
greengabbro | February 29, 2008 If you kick a dark pebble in the middle of the desert, you will sometimes find that it is light underneath. What this means is that you have disturbed a pebble that has been sitting there untouched for thousands of years. During that time, it accumulated a thin coating of windblown gunk - mainly clay dust, and manganese and iron oxides - known as desert varnish.
Desert varnish is not difficult to scratch through, and petroglyphic sgraffito is a popular artistic medium for native desert-dwellers and idiot tourists alike.
Desert varnish has a complex internal structure; there are thin…
greengabbro | February 22, 2008 This is a thin section from some Colorado shale. It's part of the Green River Formation, which is a series of rocks laid down about fifty million years ago when the West was wet. The shales come from a set of lakes that occupied part of what is now Colorado, Wyoming, and Utah.
If you look carefully - behind the white blotches, where the contrast is too blown out to say much but they might be grains of sand or bits of shell that fell into the lake where this was forming - you'll see that the shale was deposited in alternating layers of dark stuff and light stuff. The dark stuff is organic…
greengabbro | February 15, 2008
Waterfall and Columnar Basalt © Joe Decker. Used with permission.
I finally found a piece from my nature photographer friend Joe Decker that would make a suitable subject for rock blogging. Y'see, the problem with fine art photographers is that they often forget to do things like add a rock hammer for scale. Also, they can have entire portfolio sections devoted to the Carrizo Plain without once showing an offset stream channel! It must be a very strange way to see the world.
Anyway, this image is of part of a lava flow in Iceland. As a new blanket of rock cools, it contracts. If it cools…
greengabbro | February 8, 2008
gabweb
Originally uploaded by kevinzim
I have a confession to make: I have absolutely no idea what this picture means. And most of you probably don't either, which is okay, because you're not running around the Internets pretending to be a geologist.
This is what's colloquially known as a thin section - a piece of rock sliced so thin that you can shine light through it, and then stare at it under a microscope until you get a headache. Minerals that look similar in a hand sample will refract light very differently in thin section, which makes it useful for obsessive mineral-…