Metamorphic Core Complex...while on vacation

I am a snow snob...I admit it. This winter has seen less-than-great ski conditions and so when the temperatures in Southern Arizona were in the mid 70 degree range we thought a visit was in order.

Our major objective was to enjoy the warmth, ogle the Saguaro cactus and hike as much as possible while wandering the hillsides above Tucson we started to notice the abundance and great examples of crystalline rock, both igneous and metamorphic rock. The outcrops we examined were not anything we were familiar with beyond a very general way. The granites appeared to have been drastically changed and the gneisses looked like they had been put through the wringer.

The mountains we were hiking in were formed in mid-Laramide time when a large detachment fault allowed a decidedly large rock mass to slide while the Catalina Mountain were arching upwards. The material that moved along the fault was metamorphosed considerably, giving us some awesome outcrops to examine. Since then, erosion and weathering has moved materials from the mountain tops into the broad valleys. Many of the hiking trails use these alluvial fans to gain access to the higher elevations. Garry at geotripper has also just recently written about these mountains

Hiking in the Catalina Mountains with examples of granite that had not been altered during the formation of the Catalina Mountains.

Further out from the mountains we came across this outcrop of mylonitic gneiss, a former granite that was abused by its passage along a detachment fault as the Catalina mountains were being uplifted 25ish million years ago.

A fun outcrop of gneiss, another remnant of movement of granite along the fault.
You can never be too careful riding your bike around interesting geology.
A trail crew with an eye for aesthetics and geology. An assortment of metamorphic rocks making a wall.
Hiking across an alluvial fan; Saguaro and geology at the same time.

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.

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.

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

Trilobites before lunch

The last time I tried this blogger decided to stop working.

Earlier this month I had the opportunity to hike across the Grand Canyon from rim to rim. Wow! what a trip. I went with a group from the Grand Canyon Field Institute taking a geology class (of course) The trip met all of my expectations both as a backpack trip and as getting to know the geology of a part of the Colorado Plateau I am not familiar with. Most of my time is spent around Canyon lands where I am quite familiar with the Mesozoic sedimentary sequence. This Paleozoic stuff was all new to me. Hence the title of this post. We started with the Kaibab limestone and ended in the Cambrian Tapeats sandstone where we spent some quality time during lunch looking for trilobites. This is just the trip down. It gets better!

Looking over the edge the night before our descent into the canyon.

The Coconino sandstone. A layer that scares the hikers traveling back up to the rim. This Permian aged sandstone creates a 300 foot cliff with some great cross bedding showing the "petrified" sand dunes .
Lots of chert nodules throughout the Redwall limestone, another formidable cliff former!
Ribbon falls, notice the travertine dome at the base of the falls. Calcium carbonate rich water has created a fantastic dome similar to a stalagmite in a cave.
Next will be about time spent at the bottom of the canyon.

A new campsite for geology field class

Last week I spent an enjoyable 4 days floating down a nearby stretch of the Colorado River looking for new places to take my geology-by-canoe class. There is a campsite almost 1/2 way down this particular stretch that has a name made for a geology field class. "Fault Line".

These images show the Kayeta-Wingate combination being bent downward by the actions of the fault. The canyon just to the left of the fold continues upward right along the fault. It is an easy hike with the exception of minor 15 foot pour over that requires a touch of scrambling.

How easy is it to find this fault line? The BLM post identifying the campsite. Unfortunately the site is found on an outside bend and with the fast water of last week the landing was not pretty. We stayed dry but just barely.
The view across the river from the campsite. Although not as dramatic as the big fold in the first image, I think the fault line is easier to see. The right side (upriver) shows the mid-Jurassic Entrada sandstone. Just downstream from the fault is the early-Jurassic Kayenta formation.
As an added attraction, the cobbles on the gravel bar encompass examples from all directions in the Colorado Rockies. And, if that wasn't enough the channel in the fore ground makes for an awesome float. I love having my students become particles and float down stream.

It truly doesn't get any better

Serendipitous geology

I can't believe it has been over a month since I last posted anything...well I can believe it I guess. It is a combination of it being summer and constantly on the move and being a bit lazy when it comes to posting.

Last week I was poking around an un-excavated Pueblo III site in the Montezuma Valley near Mesa Verde NP. I am quite used to looking at cliff dwellings all around the 4-corners area, this was the first Pueblo aged, non-cliff dwelling that I had spent any time in. What struck me first was that the Mancos shale does not have much of the favorite building materials- sand stone blocks. The Mancos shale is an incredibly deep layer of marine sediment left over from the Cretaceous Seaway. It is not known for it's sandstone building blocks. However, as the inland sea retreated somewhat, there were deposited three units that can be used for building blocks: the Point Lookout sandstone was deposited followed by the Menefee formation, a coastal plain region and a local source of coal and lastly the appropriately named Cliff house sandstone. The Cliff house is where most of the famous cliff dwellings are found.

However, the Montezuma Valley is dominated by the Mancos shale.

Lucky for the former inhabitants of this little pueblo the Menefee formation, is not only rich in sandstone but has a habit of slope failure creating sandstone deposits on the valley floor. The search for local building materials just got easier. A little bit of driving around the nearby county roads found a potential sand stone quarry.

To be a more complete geology day and completely overshadow the archeology was the discovery of fossil shells in the sandstone building blocks. My knowledge of paleontology is quite limited and I won't even hazard a guess at the names of the fossils we found.

A section of wall peeks out from beneath some vegetation with Mesa Verde in the background.

Some of the pottery shards found near the site.
A chunk of ancient building materials complete with even more ancient fossil evidence.

...and more
A close up of the surviving Pueblo wall.