High Sierra Topix

This is a call to the geologists of HST. In a nutshell, the accompanying photo of a decomposing chunk of granite prompted a lot of idle speculation by my buddy and me: our question was how long it had taken for this rock to crumble and create the artistic pattern shown below. Yes, I understand that several variables influence the pace of this decomposition, but since geologists operate on unfathomable time scales (with large uncertainties), how's about a ballpark estimate for this example. Are we talking a century? A millennium? More? Less? OK, your turn! Cameron

I studied geology in college but have worked in IT all my life, so this is only an amateur opinion...

I see that this boulder has decomposed on top of a glacially polished granite slab. It looks to me like a glacial erratic left there as the glacier retreated. That would make it about 10 to 15 thousand years that it has been sitting there, gradually rotting away.

But, I can't see what is just outside the photo. Is there a steep slope that could generate falling, bouncing boulders? In that case, this could be a rock that fell much more recently.

-Phil

My first question, is it really granite or granitic in nature or is it metomorphic/sedimentary/basalt.

Phil/Jim,
Wow, you guys are quick! As to Phil's query: this example was a LONG way from any hillside with exposed rock (and, for any rock to roll to that location would require skipping over a small lake), so your inference that it may have been deposited there by retreating ice sounds reasonable to me. Ergo, your time frame may seal the deal. As to Jim's query, I'm hardly an expert, but both my buddy and I thought that it looked pretty "granitic" to us, but I agree that the appearance in the photo makes it look more metamorphic. Maybe that's part of the aging process for granite? I'm open to other insights! Cameron

I missed this post. There are in fact ways of measuring the erosion rate of a rock surface through geologic time (especially using radionuclides produced by cosmic ray bombardment), so studies have been done on these sorts of things. Erosion rates of rock surfaces such as that have been measured in the order of hundredths to thousandths of mm/yr. There have been specific studies on granite surfaces in the Sierra Nevada that have recorded erosion rates (rates averaged over tens to hundreds of thousand years) and they have returned the erosion rate range given above.

So let's take a rate of 0.01 mm/yr--it would take a 100 years to erode 1 mm which you wouldn't see. 10000 years for 100 mm (say about 4") which is getting a bit more visible. 100,000 years gives you 1 m which would be quite noticeable. If we get off of relatively flat surfaces to hillslopes the erosion rates can be higher.

Not all granite is equal. Granite is an intrusive igneous rock, meaning that molten magma cools underground. Granite has a range of percentages of particular minerals, and varies in color as well as hardness on the Mohs scale. The longer the period of cooling, the more time minerals of the same compound have to migrate, find each other, and form larger crystals. Basalt is made of the same mix of minerals as granite, but as an extrusive igneous rock, forming from rapidly cooling lava above ground, the minerals remain mixed and the crystals microscopic. Exposed plutons, e.g. Half-Dome, tend to have formed under greater pressure and over longer time, resulting in a higher Mohs number. Often, the large granule sand you see along the trail in the Sierra is decomposing softer granite.

The amount of time granite takes to decompose varies greatly according to various mineral percentages as well as the time and pressure under which the magma cooled.

(Speaking as the other card-carrying geologist on this thread beside GiantBrookie) The first thing to note is that a sample like this on a polished surface surrounded by debris is somewhat uncommon because the debris would be carried away (unless it is resting at the absolute bottom of a bowl) by surface wash over that polished surface in what are called "100 year storms," where there is torrential rainfall, a common occurrence in the Sierra. And I have seen LOTS of boulders in a similar position that were there a very long time that did not disintegrate in this way. I'm going out on a limb and suggesting that the normal weathering may have been enhanced by a lighting strike. Fun to think about anyway.

mokelumnekid wrote:Speaking as the other card-carrying geologist on this thread beside GiantBrookie....I'm going out on a limb and suggesting that the normal weathering may have been enhanced by a lighting strike.

Now that is something I would love to see! I have to wonder how all those electrons affected the chemical bonds within the minerals, but also how it affected the interface between crystals of differing minerals.

mokelumnekid wrote:(Speaking as the other card-carrying geologist on this thread beside GiantBrookie) The first thing to note is that a sample like this on a polished surface surrounded by debris is somewhat uncommon because the debris would be carried away (unless it is resting at the absolute bottom of a bowl) by surface wash over that polished surface in what are called "100 year storms," where there is torrential rainfall, a common occurrence in the Sierra. And I have seen LOTS of boulders in a similar position that were there a very long time that did not disintegrate in this way. I'm going out on a limb and suggesting that the normal weathering may have been enhanced by a lighting strike. Fun to think about anyway.

Regarding lightning strikes, I'm not sure how a strike would physically affect a disintegrating boulder but we see ample record of lightning strikes on rock types of all sorts throughout the Sierra and other mountain ranges. The most spectacular record of a lightning strike is melting from the strike--a tiny bit of the target partially melts then quickly cools to form a bit of glass. This is called a 'fulgerite'. These are most notable on summits of various peaks that get hit a lot. The best I've seen is atop Rodgers Peak (S border of Yosemite) where the target is metamorphic rock. The summit has all these green glassy coatings on various rock surfaces. As these weather a bit they become sugary-looking and it looks like someone spilled something on the rock. Once you've recognized this (Rodgers was the first summit I actually noticed it on) then you look for it and see it in many places, although I've never seen as much of it on a summit as I've seen on Rodgers Peak. Freel Peak near Tahoe, which is a granitic target, has a lot of it, too. You can also see fulgerite in an urban context, too. Lightning strikes on telephone poles commonly conduct down to the concrete so you'll see little glassy splotches in the concrete where grounding cables or similar stuff touches down. I remember seeing this on some sidewalks in the SF Peninsula area (Mt View and Palo Alto) on the day after a particularly spectacular thunderstorm. How long these urban ones last depend on how long its been since they've replaced or refinished the concrete, of course.

In some localized cases forest fires can affect rock.
The rock must be covered by a fair amount of dead and down material, so it burns and heats the rock for an extended time. Even then, it only affects the surface of the rock.
I am certain everyone has seen the effects of fires on soil. The brick red color of the soil underneath campfire rings, for example.

Does UV affect the rock? An exposed rock at high elevation versus a shaded rock under canopy at a lower elevation. I could understand some chemical reaction occurring over time from UV exposure.