Monday, April 25, 2011

Geologic Study of Montezuma Well, Arizona

In an arid state like Arizona, water is king. And that would be a Meso-American king if you happen to live in the Verde Valley. As one of the state's most scenic watering holes, Montezuma Well is a gem of a landscape feature that in many ways defies easy description. Last week two geologists gave a series of lectures and field trips at the Well that shed some light on this curious feature.

(By the way, the famous Aztec king, Moctezuma, never had anything to do with this area or its landforms. However, when Anglo pioneers first entered the Verde Valley in the 1870's they assumed that the many ruins found scattered throughout the valley must surely have something to do with the fabled leader. Although his true name was Moctezuma, that was apparently too hard of a sound to replicate and the name was corrupted to Montezuma).

Cross-section showing the flow path of groundwater from the Coconino Plateau and Mogollon Rim (upper right) through the Redwall aquifer (dark blue) to Montezuma Well (labeled)

Ray Johnson, a hydrologist with the USGS, and Ed DeWitt, a geologist, completed their study and gave the series of public programs. Their official report will be released next month but you can read an excellent summary article in the Camp Verde Bugle by Steve Ayers here. Steve is a science writer who has the ability to capture the essence of a topic and make it understandable to regular folks. He has published many pertinent geology features in the Bugle through the years.

The study by Johnson and DeWitt states that the water in Montezuma Well originates as precipitation on the Coconino Plateau  and travels a lengthy journey within rocks beneath the Mogollon Rim. This subsurface journey, which lasts up to 13,000 years before emerging in the bottom of the Well, is routed through the Redwall Limestone. The Redwall serves as a regional aquifer in northern Arizona and was laid down in a tropical sea some 340 million years ago.

Paleogeography of the Four Corner states at 340 million years ago when the Redwall Limestone was deposited. Figure from "Ancient Landscapes of the Colorado Plateau" by Blakey and Ranney

Along the way, the groundwater dissolves some of the limestone, making the water calcium rich and creating carbon dioxide in the process. The water encounters a basaltic dike some 750 feet below the Well floor and this forces some of the water up to the surface. The evidence for the dike comes from tiny olivine crystals that are found in the Well sands. Olivine is a common mineral found in basalt.

This is an excellent study with many avenues available for public understanding. In my opinion, this is the best kind of science. I leave you with a quote about this from George Otis Smith, the director of the US Geological Survey in 1921:

"I am convinced that at its best, science is simple - that the simplest arrangement of facts that sets forth the truth, best deserves the the term science. So the geology I plead for is that which states facts in plain words - in language understood by the many rather than the few."

Saturday, April 23, 2011

Earthly Musings Chosen as Nature Site of The Week

A pleasant surprise came to me this week. Emma Springfield, who writes a blog called Nature Center Magazine found my site Earthly Musings and awarded it "Nature Site of the Week". You can read about the distinction here. It is nice to get noticed from so far away (Nebraska) and to be told that your offerings are meaningful to others. Thank you Emma and good luck with your work to help people connect with the natural world! We need people like you more and more as time goes by.

Saturday, April 02, 2011

The Mojave National Preserve - A Geologic Paradise

When some people hear the word Mojave, they may think of snakes, centipedes, or spiders. Maybe even cactus, sand, and desolation. But lovers of the Mojave well know its charms, perhaps more difficult to discern in July but here nevertheless. In March, I traveled to the Mojave National Preserve with 11 former students to learn about this varied landscape. What we found more than anything else was the unexpected. Sand yes, but snow too. Cactus and other desert vegetation but all growing next to some pretty fantastic rocks. The weather was challenging but all good things have a rightful price.

We entered the Preserve on the south at Essex Rd. The preserve was created in 1994 to help stop the wanton destruction of this lush desert locale.

Our first stop was at Hole-in-the-Wall, located to the east of the Providence Mountains.  Pyroclastic rocks are exposed here and have weathering into fantastic shapes.
We followed a trail into a slot canyon that has been carved during the numerous summer thunderstorms that rack this landscape. The rocks are mostly resistant but cannot withstand the hard boulders washed through here.

A set of ring bolts helps hikers negotiate the steep drop-offs. The trail was bout one mile in length and we got an appreciation for the power of explosive volcanism.

At Hole-in-the-Wall, tafoni textures permeate the rocks. These features begin to form long before the rock is exposed to erosion. Groundwater moves through the ash flow and can dissolve some of the silica. No hole is created at this time - only pockets of weakened rock. Some of the dissolutioned silica is redeposited adjacent to the weakened areas. When the rock is exposed to erosion, the weakened areas form holes while areas with extra silica cement surround them!

Taking a break along the trail. The banded range in the distance is the Woods Mountains and is the center of this explosive volcanic complex. It erupted 18.5 million years ago, making is the same age as the famous Peach Springs Tuff in Arizona. Neither the Mojave nor the Basin and Range were in existence at this time. Rather Woods Mountains Volcanic Complex is one of the last vestiges of the Mogollon Highlands which stood here.

Volcanic stratigraphy exposed near the Hole-in-the-Wall Visitor Center.

During the first night's camp at 4,500 feet, it rained a lot. Thankfully, it did not start until we were snug in our sleeping bags, then let up just before sunrise - Camelot! As we poked our heads from out under the rain fly, this is what we saw - snow in the Mojave! We had come here in hopes of finding Spring, but winter still lurked in the air.

Just 500 vertical feet above us, there was snow that had accumulated to about 3 inches. We drove through this wonderland of volcanic rock and snow.

Pinto Mountain on the southern fring of the New York Mountains. Unfortunately, this meant that we could not visit, hike, and camp at Carruthers Canyon.

So instead we drove south to a lower elevation to hike the Kelso sand dunes. You can see them far off in the distance glowing in the sunshine.

A trail (of sorts) leads to the top of the dunes. I remember hearing in grade school that there were more stars in the universe than grains of sand. When you hike on a dune field like this, it seems almost unimaginable. And then we heard on this trip about a new book, "The Hidden Reality", by Brian Greene, that talks about parallel universes and the possibility that there are many other universes - some that may be just like this one. Sand dunes have a way of making everything seem big.

The sand comes from the Mojave River as it flows into Soda Lake north of here. As the river dries in it's bed, the wind picks up the loose sediment and blows it south towards the Granite and Providence mountains. These ranges create a barrier that slows the wind and the sand grains drop here on the desert floor. Look for pictures and a story about the Mojave River below.

Dark colored minerals such as mica, hornblende, and pyroxene are layered within the more common quartz grains. Settling and deformation of the laminae creates this unusual and striking pattern on some dune faces. In reality, all of the grains reflect the mineralogy of the San Bernardino Mountains, where the Mojave River originates. As it flows north from there, it picks up sediment from other ranges - the Rodman Mountains, Cady Mountains and so on. The sand that arrives at Kelso dunes is the eroded remains of these ranges, piled exquisitely into fashionable dunes.

The Providence Mountains lie east of the Kelso dunes and were laden with wet clouds on the first day of spring, March 21, 2011
The Granite Mountains to the south of Kelso dunes

From the top of the dunes, we could see our next destination on the north horizon, the famous Cima Dome.

We hiked 1.5 miles on the Teutonia Peak Trail to get this view of the Cima Dome from the east. Cima Dome originated as a giant mass of Mesozoic granite that was emplaced as hot magma deep in the crust. The granite was uplifted and exposed to chemical weathering, where the feldspar were degraded, causing the other mineral grains (quartz and mica) to become liberated. This chemical weathering was perhaps aided by fracturing and jointing. As the weathered debris was plucked from the mountains, it could not travel far in the dry desert and the reaming granite mass became buried in its own debris! The Cima Dome.

This is said to be the densest stand of Joshua trees in the world. And my friend and botanist Orlando Mistretta says it is a subspecies too, The Jaeger Joshua (Yucca brevifolia jaegeriana)

Joshua trees and the Ivanpah Mountain in snow.

Next on our geologic tour was a visit to the Cinder Cones and Lava Flow area in the Preserve. Basaltic volcanism occurred here between 7.5 million years ago and 8,000 years ago and left behind 33 craters and cones. Here Louise poses in front of a large volcanic bomb on the slopes of one of the cinder cones.

This is a fantastic area to explore and the scenery was fabulous. Another eruption is likely but no one knows when.

Our final day in the field saw us join up with Dr. Norman Meek of Cal State San Bernardino who showed us some wonderful Ice Age lake evidence in the Mojave. Here is a shingled beach deposit from the shore of Lake Manix.

And a view from this strand line to the west. All of the low ground where the interstate exists today was under lake water about 19,000 years ago.

Note the bed of the Mojave River flowing away from the photographer to the upper right. To the left of the river bed and slightly above it are green clays laid down in Lake Manix. The earth tone deposits on top of these (and in the left foreground) are delta sequences that prograded from the west onto the lake sediments. A glorious sequence indeed!

While standing on top of the delta sequence, we could look across old Lake Manix and see a wave cut terrace of the former lake etched into the north side of the Cady Mountains. Most folks probably don't think of "lakes" in the Mojave Desert, but the Ice Age climate was pluvial at this time.

Dr. Meek pointed out tufa deposits that line rocks along various shorelines of old Lake Manix. Such deposits are still found along the shore of Mono Lake north of here.

Students walking on a ridge towards Lake Manix deposits.

Three sequences can be seen from here in the walls of a side canyon to the Mojave River. The light colored deposits below are granite rich fan material that originated to from the north and were washed south (right) before the river was here. These are overlain by volcanic rich deposits (gray-purple beds just below the ridge top) that came from the Cady Mountains to the south of here. These in turn are overlain by the green lake beds of Lake Manix seen on the ridge top to the right. The source terrain for each fan sequence is still visible in the area and shows what was present in this valley before the lake developed.

Here we see a lake bed remnant on top of the fan sequences. The green color comes from reduction of the iron minerals in the deposit.

Dr. Meek shows evidence for the fluctuation of the lake shore. In the lighter colored bed behind his right hand is a course cross-bedded sand. The greener beds behind his left hand are finer grained clays. The interpretation is that when the lake shore dropped, sand was washed into the area, and as the lake expanded, the lake beds covered the area.

The Cady Mountains fan sequence (brown, irregular eroded beds below) topped with Lake Manix clay sediment (upper green beds). Afton Canyon can be seen in the backgound.

A marvelous sequence that would be entertaining even to non-geologists. Knowing the origin of the scenery just seems to add to the enjoyment of this special place.

Our next stop was up into Afton Canyon, along the track of history.

The Manix Fault rips through the canyon and has upended many of the once flat-lying rocks here. Rainwater then runs down the vertical bedding creating this interesting texture.

It almost reminds one of Bryce Canyon on a small scale.

Afton Canyon

Runoff has carved some spectacular slot canyons into the old alluvial fan sequences.

This canyon became so narrow, you literally needed a flashlight to see in it. Thank you to Dr. Meek for meeting with our group in the desert to share his expertise on Lake Manix and its history.

This is a long blog and I'll end it here with a picture of the Providence Mountains, looking southeast from the Cinder Cone area. It was great trip with challenging weather but the Mojave National Preserve is worth a visit. Bring a geologist with you!