The vagaries of the Eastern Sierra

It’s no secret that Husband and I own property in the Eastern Sierra Nevada mountains, about five miles south of the intersection of highways 108 and 395.  On our property,  cliffs of granite come close to or against the hwy 395 road; this is called the Devil’s Gate, for the muddy grief it gave travelers during the pre-hwy-395 years.  There are springs there. There’s a Spring creek there. Very annoying water, if you’re trying to take a wagon through the Gate in June, and there’s no hwy 395 to help.

Relatively near the gate, there’s an alluvial fan with a creek that runs for a couple of months in the Spring.  We’ve chosen a building spot on the fan, well uphill from the creek, but not so close to the granite wall that it could drop large rocks on us.

Down by hwy 395, there’s a low spot.  It wasn’t there before the construction of hwy 395 in the ’20s — the road builders should have put in a culvert — and we got a county permit to fill it in with the excess excavation material from our home site.  Then someone complained that we were filling in a wetland.

The expert we hired from an independent firm has not yet released her results, but her preliminary finding was that it was probably not a wetland under Federal law, but might well be under California law.

At this point we’re just hoping we don’t have to mitigate the damage already done to the “wetland”.  We have REAL wetlands on our property on the other side of hwy 395, fed by those springs, and we’re as determined as anyone to preserve them.  But this is just a Spring mudhole.

Grunble grumble mutter mutter,

Karen

A Personal Post from Karen

I have a cousin by marriage — we’ll call her Mary for the sake of this post, not her real name.  She has lots of health issues, though the most dire one is bipolar disorder.  It keeps her from holding down any sort of job.  She’s married to a guy who has troubles of his own, and is often on disability.  Family members say he’s amazingly lazy; I only met him once, at their wedding, so I can’t say.   But they definitely have trouble making the rent AND eating.

I suggested she put up a web page (another relative, who runs a small ISP, will host one for free), explain the situation, and put up a Paypal donate button.  I, and I suspect many of her friends, would be glad to sign up for small Paypal subscriptions.  I got no response.  Perhaps she’s having a “down” episode.

I want to help Mary help herself, but I don’t know how.  She does fantastic things when she’s “up”,  and struggles mightily when she’s “down”.  She does take meds, but they don’t seem to help much.

So I ask you, readers of this fine blog, for suggestions.

Thanks, Karen

The Mystery Fault, Part 3

In Part 2, we’ve established that the Silver Creek fault is, in fact, the southwestern boundary of the Evergreen basin.  But what’s it doing there, anyhow?  Back sometime around 10 to 15 million years ago, an early Hayward fault formed (an estimated 100 km (62 miles) south of where it is now) with a right stepover in the southeastern most end of it.  Basically that’s where a fault  stops and then starts again some distance away, in this case to the right.  .Now, both the Hayward and the Silver Creek faults are right-lateral strike-slip faults.  This means if you stand facing the fault, and it slips, the ground on the other side of the fault will move right.  When you have a right step in a right-lateral fault system, it tends to pull the ground between the faults apart like so:

pullApartBasin

And thus creating a pull-apart basin.  So the basin started to be created in the Miocene.  Since then there’s been 175 km (109 miles) of slip on the Hayward Fault, of which 40 km (25 miles) of slip was on the Silver Creek fault itself.

Now, the Hayward fault is a very active fault.  Parts of it are creeping at the surface, but the whole fault system is locked at depth right now and is one of the prime candidates for the next Northern California Big One.  Scary thought, and keeps geologists and emergency managers awake at night.  But what about the Silver Creek fault?

Well, it turns out that about 2 million years ago, give or take a few thousand, the slip that was happening on the Silver Creek fault moved over to the Calaveras fault, which runs east of the Hayward fault.  (See the faults pic in Part 1.)  That doesn’t mean the fault isn’t still moving, but it’s moving at a much slower rate than the Calaveras and Hayward faults, and probably won’t generate any big earthquakes. Probably.

There’s some evidence for a couple of magnitude 6-ish quakes in 1903 down near the south end of the Silver Creek fault, but they’re not very well located, and it could very well be that they were produced by the Calaveras or another fault.  Because there is another fault, one that I haven’t talked at all about yet, and which is completely buried and only inferred.  But there’s some pretty decent inference for inferring it.  I’ll talk about it in Part 4.

Meanwhile, those of you who live in California, there are fault systems in both the northern and southern parts of the state primed to go off and produce the Next Big One.  Plan for a few days of food, water, no electricity, etc.  Expect cell phone systems to be down.  Expect land lines to be overloaded.  Expect hospitals to be backed up.  But, as Douglas Adams says, don’t panic.

 

References:

Wentworth, C.M., Williams, R.A., Jachens, R.C., Graymer, R.W., Stephenson, W.J., 2010, The Quaternary Silver Creek Fault Beneath the Santa Clara Valley, California, U.S. Geological Survey Open File Report 2010-1010, http://pubs.usgs.gov/of/2010/1010/, accessed 4/9/2013

 

 

The Mystery Fault, Part 2

In Part 1 I gave you the setting of the Silver Creek fault.  Here I’ll talk about how scientists at the U.S. Geological Survey figured out where the buried part of the fault runs.

The earliest (I think) tool these scientists used to analyze the geology of the Santa Clara Valley was gravimetry.  Gravimeters, developed in the ’70s-’80s time frame, measure gravity to such a fine degree that they can be used to determine what’s underground.  Valleys are usually covered, and to some extent filled in, with alluvium that local streams have eroded out of the surrounding hills/mountains.  They can get very filled-in, because over time what’s loose-ish near the surface compacts under the weight of the overlying alluvium and subsides.  This makes more room for more alluvium!  But still, even the compacted alluvium is not as dense as the surrounding mountain rocks.  A gravimetry survey  can distinguish rock from alluvium — and the deeper the alluvium, the lower the microgravity.   First, here’s the Google Earth map of the modern Santa Clara Valley:

SCValleyToday

And here’s the microgravity map of the Santa Clara Valley:

gravity

 

By golly, there are TWO valleys under some of that alluvium!  The one on the northeast, with all the deep, deep blue areas, is called the Evergreen Basin.  And while we now know that it is bounded to the west by the Silver Creek fault, that wasn’t known when the gravimetry survey was done, it was just suspected.  (Sorry about all the  “after” photos, but these guys like to publish when they’re certain they know what they’re talking about.)

The next clue, which I used to have a pic of, but can’t find, is about hydrology.  The Santa Clara valley, before it was Silicon Valley,  was an ideal place to grow fruit trees.  The whole valley was filled with agricultural activity.  And since it doesn’t rain in coastal California in the summer, they pumped groundwater to water those trees.  A lot of groundwater.  When towns and small cities started to spring up, and grew, and grew, they pumped more and more groundwater.  There wasn’t that much to pump.  Land started subsiding.  In downtown San Jose, it subsided as much as 16 feet in some places.  Obviously, this couldn’t last, and the valley now gets its water from the rivers that drain the Sierra Nevada mountains and cross California’s Central Valley.  There’s still pumping going on, though, and percolation ponds to counteract it; that’s just how the water is managed. So every summer there’s a couple of centimeters, give or take, of recoverable subsidence.  Except it STOPS, to the east, at an invisible barrier.  There’s this annual subsidence in San José… but it abruptly stops at the boundary of the Evergreen Basin.  Any geologist worth her hammer would be yelling, “fault!”   Faults often form hydrologic barriers.  So that’s the next piece of evidence for the Silver Creek Fault.

To really determine whether the barrier is a fault, the U.S.G.S. decided to run a seismic reflection profile.  To do this, they run a line of sensors designed to detect seismic reflections.  Then, using a truck with a BIG weight in the back, they smack the ground really hard. This makes a seismic disturbance that is reflected back from the different layers of alluvium and rock, to give an idea of where layers are beneath the surface.  What makes these layers?  They’re simply layers of slightly different composition of alluvium — sand vs. clay, for instance — or in rock, changes in rock type or rock density.  In valleys like the Santa Clara where all the alluvium has been deposited by streams, layers naturally vary in density and composition, as streams move around, have floods, create graded banks, and carry on like this for thousands of years.

Now, this wasn’t the first, nor the last, seismic reflection profile that has been done in the Santa Clara Valley, but what makes it unique is that it was done through downtown San José, against the backdrop of lots of other seismic disturbances: big trucks, construction, trains, etc.  But the smart geophysicists were able to filter out most of that, and produce this profile:

reflectionProfile

 

All the squiggly, mostly horizontal lines are reflections from various layers.  Just to make sure you can’t miss it, they’ve marked the Silver Creek fault in red;but if you look closely, you can see it in the profile.  To the left of the line is the “noise” that comes back from solid, uniform-composition rock; to the right is the multitude of little lines that represent alluvial layers.  “Franciscan Basement” refers to rocks of a group named “Franciscan”.  You can see there’s no nice, gradual, left edge to the Evergreen Basement; the transition is very abrupt, and clearly indicates a fault.  Bingo!  The Silver Creek Fault forms the western boundary of the Evergreen Basin.

Great.  So there’s proof that the Santa Clara Valley — Silicon Valley — has its very own fault, a less than reassuring thought to the people who live and work here.  So what kind of fault is it?  Has it moved a lot in the past, and will it do something nasty any day now?  Stay tuned for part 3.

References:

Wentworth, C.M., Williams, R.A., Jachens, R.C., Graymer, R.W., Stephenson, W.J., 2010, The Quaternary Silver Creek Fault Beneath the Santa Clara Valley, California, U.S. Geological Survey Open File Report 2010-1010, http://pubs.usgs.gov/of/2010/1010/, accessed 4/9/2013

 

 

 

The Mystery Fault, Part 1

Sorry for the long hiatus, everyone, but life sort of caught up with me for a few months.  To attempt to make amends, I’ll tell you a geologic mystery story that was just solved — provisionally — a few years ago.  It regards a fault that crosses my home base, the Santa Clara Valley.  It’s called the Silver Creek fault, after a creek that roughly runs along some of its length, though the rocks it cuts aren’t the type to provide silver ore.

Let’s start with some regional perspective.  The Santa Clara Valley is at the south end of San Francisco Bay in California.  Anyone even slightly familiar with the local geology knows that the San Francisco Bay Area has all sorts of faults.  Most of them, or at least most of the ones geologists worry about, are the result of two tectonic plates sliding along past each other:  the Pacific plate, to the southwest, and the Sierra Nevada-Great Valley microplate, to the northeast, are moving right-laterally relative to one another.  That is, if you stand on either plate and look at the other one, it appears to be moving to the right.

OverviewFaultMap

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The map above, from Simpson and others (2004), shows the major faults in the south and central parts of the San Francisco Bay Area.  Ignore the color codes; they’re only important for the original document.  Also, these are fault zones; they often have little strands and cross-faults and are generally messy things.  Ground breaks reluctantly and makes messes when it does.

Finally, these are just the major strike-slip fault zones. The rocks to either side of these faults primarily slide past each fault.  But there’s always some motion of the rocks either pulling apart from each other or pushing up one side over the other, and these motions are called dip-slip.  Dip-slip, it turns out, will be important to the story.

The map above also shows the Silver Creek Fault.  But, drawn in 2004, it postdates at least part of our mystery.  Here’s a 2005 map from another USGS paper (Wentworth and Tinsley, 2005)  that shows the visible part of the Silver Creek fault in a continuous line and the inferred part in a dashed line:

of2009-1165

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The southeastern part of the Silver Creek fault is visible in the rocks along the southeastern end of the valley.  The northwestern part of the Silver Creek fault is inferred because it is covered in valley alluvium.  From the surface, there’s no trace of the fault.  Which brings us to the mystery questions: so how did the scientists figure out where the trace really is?  What kind of an effect does the fault have on the valley?  And is it going to kick off a nasty earthquake that will devastate the Santa Clara Valley?

Stay tuned for Part 2.

 

References:

Simpson, R.W., Graymer, R.C., Jachens, D.A., Ponce, C.M., Wentworth, C.W., 2004, Cross-Sections and Maps Showing Double-Difference Relocated Earthquakes from 1984-2000 along the Hayward and Calaveras Faults, California, U.S. Geological Survey Open-File Report 2004-1083, http://pubs.usgs.gov/of/2004/1083, accessed 4/7/2013.

Wentworth, C.W., and Tinsley, J.C., 2005, Geologic Setting, Stratigraphy, and Detailed Velocity Structure of the Coyote Creek Borehole, Santa Clara Valley, California, in Asten, M.W., and Boore, D.M., eds., Blind comparisons of shear-wave velocities at closely spaced sites in San Jose, California: U.S. Geological Survey Open-File Report 2005-1169, http://pubs.usgs.gov/of/2005/1169/of2005-1169.pdf, accessed 4/7/2013.

 

My state’s official rock is a mineral!

I live in California.  Our official State Rock is serpentine.  The only problem with that is that serpentine is a mineral, not a rock.   Rock consisting mostly of serpentine is called serpentinite.  AAAAGGGGHHHH!  (Jumps up and down in frustration.)  Admittedly, where serpentinite occurs, it’s really, really, full of serpentine.  But still…mutter grumble grumble mutter.

So I’ll talk a bit about serpentine.  It starts out life as olivine, a mantle rock.  Another name for gemmy olivine is peridot, a lovely green stone you may have seen in jewelry.   Olivine is the base rock in the undersea spreading zones where mantle rock is erupted to generate new earth-surface material.  But seawater eventually penetrates the base rock, and then Stuff Happens.  To understand, I have to introduce a little chemistry.  Don’t let your eyes glaze over; it’s simple, really.  Olivine is actually a chemical series: (Mg,Fe)2SiO4.  This means that Mg (magnesium) and Fe (iron) are interchangeable in the chemical composition of the stuff. Where there’s a lot of magnesium, the mineral is called forsterite; where there’s a lot of iron, the mineral is called fayalite.  Forsterite is green, while fayalite is dark brown, the color being dictated by how much magnesium versus iron is in the rock.  But forsterite, fayalite, or somewhere inbetween,  olivine is a dry silicate.  In the presence of seawater, it begins to turn into serpentine, (Mg, Fe)3Si2O5(OH)4  Again, magnesium and iron readily swap out for one another.

Those (OH) molecules make serpentine into a wet silicate.  That means it can survive  a very long time before being scooped up somehow onto the  dry land, most often in a subduction zone where some of the rock gets scraped off the subducting seafloor, and ends up on the land where we humans can study it.  The conversion takes time, though, which is why we find and mine peridot (gemmy forsterite) on the land.  (Beader Karen adds: that’s why peridot beads are so $%#@& expensive!)

I have a lovely sample of serpentinite.  I can’t find the damned thing; this house is so full of rocks it’s bulging at the seams.  However, Blogger Garry Hayes (http://geotripper.blogspot.com/) has an excellent specimen pic he’s allowed me to use:

Serpentinite

Serpentinite

This is better than my sample, which is mostly plain green; this is a mix of mineral variations with lots of different magnesium/iron induced color. Thank you, Garry!

A good field geologist can often spot serpentinite even without an exposed outcrop.  Most plants don’t like serpentiniferous soils.  There’s a place in the Santa Teresa Hills in the eastern part of my Santa Clara Valley where you can be wandering along a park trail, and all of a sudden the plants around you change.  I’m not a plant expert; just about everything I know about plants comes from a gardening manual.  But even I had  no trouble noticing the distinction.  The shrubs living in the serpentiniferous soil were definitely different from the shrubs living in other soil.

Now, a few readers are going to ask, “but isn’t serpentinite asbestos?  Well, serpentine is, sometimes.  Like some other minerals, it can form in long thin, fragile threads; in that state it’s described as asbestiform, and it that form it has a different mineral name, chrysotile.  Chrysotile crumbles easily and you really, really don’t want to breathe it.  Supposedly it isn’t as toxic as other asbestiform minerals, but I don’t recommend taking chances.  However, most of the serpentinite you see commercially is not asbestiform, and it doesn’t pose any sort of health hazard.

Serpentinite is a popular material in the bead trade, where it goes by various names.  Some are fairly honest, like “serpentine”; others are outright dishonest, like “new jade”.  Real jade is one of two minerals, nephrite or jadeite, both of which are much rarer, stronger, and far more resistant to wear than serpentinite.  Buyer beware!  Also, for some unfathomable reason, serpentinite is sometimes sold dyed red, orange, purple…  why take something so pretty and dye it?

Here’s a random sampling of some serpentinite beads I bought awhile back.

Serpentinite beads

Serpentinite beads

The little (6 mm) spotted round beads are from Russia; the others are from China. You can see the variation in colors, mostly produced by variations in the magnesium vs iron content of particular places in the beads. Some of the color differences come from other minerals, too; whatever the seller says, this is serpentinite.

Here’s a necklace I made for a friend,  with various serpentinite beads and peach moonstone (a gemmy version of a feldspar), along with sterling silver:

Serpentinite and Moonstone

Serpentinite and Moonstone

Hope you enjoyed my musings on my favorite rock.

Karen

Obsidian

UPDATE:  One commenter wanted to know what I do with obsidian beads.  So I’ve added a photo of one of my beaded necklaces at the end of the post.  It’ll be up for sale at http://www.etsy.com/shop/gemmyjoy in a week or two.  /UPDATE

Rocks are my friends.  I especially like the ones I can pick up, look over in my hand ( maybe with a handlens) and say “this is cool!” If it’s something that somebody can make beads out of, so much the better — I’m a beader.  Obsidian qualifies.

Obsidian is an extremely felsic (feldspar/quartz-rich) lava with a chemical composition similar to rhyolite. Think silica-rich.  Really, really, really silica-rich.  But while rhyolite congeals into microscopic crystals, obsidian doesn’t, and has a glassy composition.  For a long time, the “accepted wisdom” was that obsidian cooled too quickly to crystallize.  Anyone who’s ever seen an obsidian dome next to a rhyolite flow (there’s one at Medicine Lake, California) can tell that explanation is a bunch of hooey.  I haven’t kept up with the literature in recent years about new theories for obsidian formation, so if anyone can point me to a paper in the comments I’d be grateful.

There are obsidian domes all over the eastern and northeastern parts of California, my home state.  Most of these are associated with what are still considered active (quiescent) volcanoes.  The biggest obsidian dome I’ve seen, though, is the Big Obsidian Dome at Newberry Caldera in Oregon.

So I’ll start off with some Newberry pics:

Big Obsidian Flow

Big Obsidian Flow

Here’s a shot of the iconic Big Obsidian Flow from Paulina Peak, with East Lake in the background.  There are better pics on the web, but I like to use my own photos; that way I don’t have to worry about infringing on anyone else’s copyright.  Big Obsidian Flow erupted about 1300 years ago.  Note that it doesn’t look black, like obsidian is “supposed” to look; nor is it covered in snow, because this pic was taken in July 2003.  Obsidian flows are pumicy.

There’s a trail leading up and into the Big Obsidian Flow, and the day we visited there was even a docent at the top, answering questions.  Very cool.  So we wandered up the trail, looking at the mix of black obsidian and gray pumice, with ever-clumsy me trying hard not to cut myself on any exposed sharp surfaces.  Here’s an example of what the rocks along the trail look like, with reluctant spouse for scale:

Mike along the trail

Mike along the trail of the Big Obsidian Flow

Lumpy stuff, not very picturesque, with sharp edges that will gladly send you rifling through your purse/backpack for Band-Aids or worse.  But then there are sights like this one:

Obsidian with tree

Obsidian with tree

Gorgeous, black obsidian with white streaks, topped by a little tree trying to grow in the flow.  I was absolutely charmed by this picture.

But so far we haven’t seen any hold-it-in-your-hand samples.  So I’ll offer a couple from our collection.  Note that these were NOT collected at Newberry; Newberry is a national monument and I believe that collecting is prohibited.  Our samples came from domes in California on National Forest Service land where collecting for personal use is permitted.

Obsidian sample

Obsidian sample

This piece is about two feet long.  The blue color is an artifact of the photographic lighting; the piece is quite uniformly black, with a few lighter streaks.  In the upper-right side of the picture you can clearly see the conchoidal (glassy) way the material breaks.

pumicy obsidian

pumicy obsidian

Here’s a more classic sample from an obsidian flow.  A bit of black shiny goodness, with a lot of inclusions and pumicy “stuff”.  This piece fits comfortably in your hand, and doesn’t even have many sharp edges, either!

I did mention beads in the beginning, didn’t I.  So I must show you some from my embarrassingly extensive collection.

Snowflake Obsidian Beads

Snowflake Obsidian Beads

The White inclusions in snowflake obsidian are cristobalite, a high-temperature version of quartz. These beads are about a centimeter and a half in the long direction.

Mahogany Obsidian Beads

Mahogany Obsidian Beads

Mahogany obsidian has red inclusions. I’m guessing iron is involved, but I don’t really know. They do make lovely beads, though, and I find myself using them often when I’m beading.  These are 10 mm across.   Both the snowflake and mahogany obsidian beads are from China.

So — more, and yet less, than you probably every wanted to know about obsidian.

Karen

UPDATE:  Since one of the commenters asked to see my beadwork, here’s a necklace, about 28 inches long, draped over that long piece of obsidian you saw a few pics above.

Mixed obsidian necklace

Mixed obsidian necklace

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The snowflake obsidian is cut into to star shapes, which actually works for such a dark rock. The beads between the snowflake and mahogany obsidian are glass. I always string on plastic-coated, 49-strand steel wire; that allows a natural, relaxed feeling to the finished piece, while still being resistant to wear from burrs that occur in the holes of rock-based beads. I always finish with gold-filled or sterling silver crimps, too, on these long pieces.

Lava Butte, Oregon

I last visited Newberry Crater, Oregon, and it’s flank cinder cone Lava Butte, in the summer of 2003.  Husband and I met up with his parents in a campground near Bend, and introduced them to volcanoes.  Newberry Crater is interesting to potter about — especially its Big Obsidian Flow — but it has such fascinating underlying geology (hint: it isn’t a classic subduction zone volcano) that it deserves a blog post of it’s own.  Soon.  For now, I want to talk about Lava Butte, a classic cinder cone.About 7000 years ago (the day before yesterday in geologic time) Lava Butte erupted.  It probably started as a fissure that threw up volcanic ash and cinders, and kept erupting them until it had built a cone some 500 feet (152 meters) above the surrounding ground.  Not content with ash and cinders, it also “leaked” lava from its base, toward the south.  Thick, viscous lava that forms lumps and blocks — what the Hawaiians call “aa”, pronounced “ah-ah”.  Lots of it.

Now, cinder cones are one-shot deals.  They do their pyromaniac routine, leak their lava, and they’re done.  Sort of a geologic burp.  Newberry Volcano may only be quiescent, but if it decides to make another cinder cone, it’ll be somewhere else.

There’s a road of sorts — though FSM help you if you encounter an RV going the other way as you drive it — to the top, and a visitor’s center.  There’s a trail that goes around the rim, and offers some great views of Cascade volcanoes.  There’s also a trail that was painstakingly dug/blasted/ground through some of the lava, letting visitors see this aa stuff close up.  It’s impressive. (I’ve heard at least one geology professor address it in less flattering terms, encountering it in southeast California: “Oh, hell.”)

Lava Butte from the lava trail

Lava Butte from the lava trail

I didn’t think to get a clear shot of the cinder cone from the parking lot, and it’s somewhat obscured here by the lava along the trail.

Lava Butte Crater

Lava Butte Crater

There are trees growing inside the crater!  Shrubs, too.  I can’t imagine how a plant could get a start inside a cinder cone, where water just goes right through, but these plants are doing nicely.  Amazing.

A blocky aa section

A blocky aa section

Here you can see how blocky the lava is.  That’s my mom-in-law posing for scale.

A lava "valley"

A lava “valley”

The lava flow isn’t regular, either; it has peaks and valleys.

Lava field

Lava field

And lest you think this is just a few lumps of rock next to the cone, here’s a shot of the lava field.  You can see the line of trees where it stops.  Though I have to admit, I’m almost more impressed by the plants than the lava.  How could any growing thing colonize that?

Some geologic burp, indeed.

Karen

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

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:

Eclogite

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,

Karen

Space and Time to Heal

If I can figure out how to do it, I want to post a delightfully science-ific parody of the “Gangnam Style” video here:

NASA Johnson Style

Okay, that didn’t seem to work.  But do click on the link, and enjoy a few minutes of sciency silliness.

Why?  Because I need time to think.  The first salvos against the USian violence culture are clear: keep assault-type weapons at gun ranges, and make mental health care easily accessible and a positive thing to access.  Just those will be difficult enough.  But Dana has called for nothing less than the eradication (or at least a great reduction) in the violence culture of our country, and thinking of ways to go about that makes my head hurt.

Meanwhile the video reminds me that there are lot of people doing good work for the sake of our country and our world.  They don’t negate the shooters of the country; they don’t reduce the violence.  But they do good, and in this climate, it’s proper to remember that some people go to work every day and do good (other than teachers; they go to work every day and do fantastic.)

Karen