Monday, July 13, 2020

Arizona's Granite Mountains

     Our previous story detailed how common granite was in the mountains of the American West.  Such granite mountains certainly show up across Arizona too. The hump of famous Camelback Mountain in Phoenix is composed mainly of granite.  Strangely, the head of the camel is composed of sedimentary rock, which forms from particles deposited on the floor of a body of water.   In between the formation of the hump and the head of the camel lies a vast gap in geologic time that’s even greater than Great Unconformity visible at the Grand Canyon.   Payson and Prescott are both areas with outcroppings of granite boulders.  The following is a May 14, 2016 article by Greg McKelvey from the Payson Roundup detailing the granite boulders of both communities. 

“Payson and Prescott have at least one thing in common, besides the longstanding rodeo wars.

We’ve both got granite fit to send a photographer into a frenzy.

Both offer the most distinctive of granite landscapes — a “dell” outfitted with mysteriously rounded, giant, sculptural granite boulders.

In Prescott, this landscape sets off Watson Lake and Willow Creek Reservoir, nearly as popular as Sedona with photographers. Easy to reach, convenient to lodging and good eats, these man-made lakes are known for their geomorphic feature, The Granite Dells.

Payson’s Granite Dells don’t get as much attention, but offer one of the region’s scenic treasures — which hikers, mountain bikers and horseback riders would love to turn into one of the region’s scenic attractions.

We encounter granite every day in many forms — including kitchen countertops. Igneous rocks are classified by their mineral composition and granite is but one of many igneous rocks. Stone slabbers and Home Depot give these same rocks more personal names like Norway blue. Gabbro, monzonite and the other geo stone words do not sell. That said, the Payson Dells are correctly classified a granite with just the right amount of quartz, two types of feldspar, mica and some iron-bearing minerals including magnetite.

Normally, to create a rounded boulder you have to tumble it down a river or stream. But granite often naturally erodes into giant spheres. Spheroidal weathering of rock occurs all over the world. OK, Watson, spheroidal weathering is a mouthful for the process that makes rocks round by ever so patiently removing all the corners.

The1.4-billion-year-old Dells’ granite in Prescott is a magma that formed a molten blob of rock some two miles beneath the surface. Still buried under tremendous pressure, the magma cooled slowly enough to crystallize as a uniform rock. As uplift pressed the layers toward the surface, the overlying rock layers eroded away. The stress of that rise created stress joints that cracked the rocks into cubes. Once the buried layers of once molten granite reached the surface, weathering along joints produced the rounded boulders typical of granitic terrains — including Payson’s own Granite Dells.

As an aside the Watson Lake Dells granite also boasts an unusually high uranium content.

Spheroidal weathering that creates secondary minerals of a larger volume. Water will penetrate the bedrock along joints, increase in mineral volume, swell up and widen the fractures. Eventually, these softer infiltrated minerals erode away. The weathering is greatest along the corners of each block, followed by the edges, and finally the faces of the cube. This process is not mechanical weathering, it’s chemical. Like Bunsen Honeydo said in “The Muppet Show,” better erosion through chemistry. 

The high water marks, called bathtub rings, are mineral deposits composed largely of calcium deposited as waters evaporated upon receding.

Our Payson Granite Dells waited like forever to be exposed to the surface and now they too are being reduced to sand. The Dells are dominated by large rounded boulders or outcrops rounded on the edges with piles of coarse sand piled up around the base, with the sand made of the same and only a sand at the base. A course sand, not like the fine sand on your favorite beach. Quartz, Feldspar, mica and black minerals as the granite. The minerals formed as the hot magma cooled in place to become granite. That happened deep under the surface.

The buried granite eventually made its way to the surface, where shifts in the earth created wide cracks.

Time continues to slip away and the cracks, called fractures, allow the slow, but oh so relentless, attack of water to channel down the fractures, removing the minerals grain by grain. There are no minions out there at night with hammers and chisels rounding the corners, it is the water! Life-giving water reduces even the hardest rocks to sand.

What makes the Dells so unique — both in Prescott and Payson — is that the landscape is made of just the big round boulders or sand, with little in between. The weathering of other types of rock like sandstone and limestone produces lose rocks of all sizes, from pebbles to boulders.

The result of spheroidal weathering at the Payson Dells is an open forest, since the weathered granite produces poor soil for the plants. This is work in progress and long after the last Roundup edition is published, there will be more sand and fewer boulders. The sand washes away as the mountain flattens. It is the water! So Dells need a dense granite, wide fractures, water and time to produce the landscape we see today. Eventually, the bits of granite will return to the sea where tectonic plates may well push them back under a continent to be melted, pushed up as hot magma, and start the cycle again.

The challenge to the photographer is the mainly the lighting. The boulder edges and fractures offer shadow and lines. Water reflections are just downright stunning. If there ever is a place for early morning and late afternoon tripod photography, it is Watson Lake.

I would like to try some night work, accepting that the lights of the greater Prescott metroplex will dim out some stars, there should be some good angles and perhaps even moonlight shots to take.

Payson’s own Granite Dells also offers a great place to photograph these natural, abstract sculptures.

If you want to wander further afield try the Stronghold granite in the Chiricahua Mountains, I-10 east of Benson, and along Arizona Highway 87 just before you reach the Valley.”

Link  - https://www.paysonroundup.com/outdoors/features/the-power-of-water-and-time/article_c3542962-fc51-5473-ade9-0aa29127627d.html


Camelback Mountain
The Dells of Prescott
Prescott Valley granite out croppings
Credit:  Payson Roundup
Credit:  Payson Roundup
Chihuahua National Monument - Credit: NPS


   

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