Wednesday, December 21, 2011

AW #41 a significant geologic event

From Ron: What we seek for Accretionary Wedge #41 is an account of a geologic event that you experienced firsthand. It could be an earthquake, a landslide, a flood, a volcanic eruption, etc. (but don’t feel compelled to stick to the biggies – weathering, anyone?) – some geologic process that you were able to directly observe and experience. The event itself need not have been dramatic or life threatening, or it may have been.

It has been said that as we grow older our tendencies to be high in the mountains change into spending quality time in the desert. I am in that stage where my summit time is being eclipsed by my canyon time. So it was just last summer on a blue sky day that I saw the very beginnings of a flash flood coming down the canyon.

Lucky for us, the catchment basin was small and so was the run off amount, but the idea to see the very tongue of red frothy water coming down the once dry stream bed was enough to make us sit up and take notice.

We were actually between two choke points of narrow slot canyons. So it was easy to step up and out of the way of the water. If we had been up or down stream just a little it would have been harder to get out of the way.
I have taught classes about the bed load and suspended load of streams. I have had my students put pfd's on and act as particles as we float down the Colorado River, but to see the red mud flowing around the corner where there was no water just moments ago was pretty cool. It was so cool, that we just watched and didn't take as many pictures (or video) as you might imagine.

The last part of the canyon, where we would rather not be during any water event flashing or not.

Sunday, December 11, 2011

A mid-continent shallow snowpack

Went snow shoeing this weekend near the summit of Red Mountain Pass where we were a bit disappointed with the snow depth and quality of the snow. However, I admit that I am a snow snob and expect quite a bit when I am recreating on snow.

The snow is always dry and light here, but today it was dry, light and bottomless. Any step off of the packed trail would send our snow shoes to the ground (albeit that was not far away). The snow crystals looked very much like the image below, large facets not anything like their original snow flake self. This is what happens with a shallow snowpack.

Snow crystals on a pine tree after a little bit of metamorphism.
This winter saw a very nice snowfall of at least 50 cm, a few weeks ago, but since then hardly anything. The weather, clear and cold each day and it can get cold at 10,000 feet. In fact, when we started that morning the temperature at the car was 9 F.

The metamorphosis of snow crystals depends on a few factors, not the least of them is the temperature gradient through the snowpack. What we were seeing was a large gradient. The bottom of the snow pack at the ground is usually thought to be at 0 C while the top of the snowpack is equal to the air temperature. Lately we have seen temperatures approaching -25 F or -31 C. Making a gradient of 32 C over the depth of the snow which originally was 50 cm. It seems that if the gradient is smaller than 10 C/m then the metamorphosis of snow will lead to a more rounded grain of snow that can connect with its fellow snow grains easily making for a more stable snow pack while gradients greater than 10 C/m will result in a more faceted grain which will not connect all that well with its fellow snow grains. Our gradient of 32C/0.5 m is certainly within the this-snow-grain-does-not-play-well-with-others category.

A fantastic picture of a snow flake "growing" a large facet. Image courtesy of the Utah Avalanche Center
It seems that the newly fallen snowflake will sublimate small amounts of water vapor from the tips of the crystal arms (where the vapor pressure is greater) . If the temperature gradient is small enough (<1 C/cm) then the vapor will re-condense in the areas between the arms (where the vapor pressure is less) effectively making the snow flake into a round snow grain. If the temperature gradient is too large (as in the example) then the water vapor will sublimate from the tips of the crystal arms and re attach onto a cold region of a nearby snowflake creating the large faceted crystal. These crystals will not connect with its fellow snow grains well creating an unstable snow pack.

This is the problem with the early season snow followed by a week or so of good weather. The snow pack is now made of a bunch of grains that will make a poor base for the coming winter.

Stay tuned for any avalanche reports as the snow pack starts to pile up. But, perhaps the La Nina will keep the snow depths to a minimum and we can worry about drought instead of snowslides.

Saturday, December 10, 2011

A tortured inner canyon

I accept the blame that my Grand Canyon trip was in early October and we are currently observing early December.

I think what impressed me the most about the Grand Canyon was the tortured rocks in the inner canyon. I am used to being around 1.7 billion year old gneiss and schist while floating the upper stretches of the Colorado River. The Vishnu schist and Zoroaster granite exceeded my prior experiences greatly.

Putting my hand on the Great Unconformity. An amazing amount of missing earth history under my hand.
An old river runner told me that most river guide books will identify any crystalline rock as schist so I wanted to visit the bottom of the Grand Canyon to see what was there. Take a look at the folding!
An intrusive Zoroaster vein. There are so many times I wish I had normal color vision to see these outcrops as others can. It is pretty vivid with my red/green problems.
The dark inner gorge