Land of Giants

During a recent Desert Wonder Tracker Meetup, we had the pleasure of walking in the midst of giants... giant boulders that is.  Geological processes have graced our area with an abundance of enormous boulders and precariously balanced rocks (PBRs), creating the illusion of being on another planet or an episode of the Flintstones. When trekking through such an area, the puzzled expression associated with wonder always begins to ripple across faces and one burning question invariably comes up.. how did these boulders get here? 

To begin to understand the how, first study the composition of the boulders.  They are all granite, which is igneous rock crystallized from molten magma below the surface of the earth.  Granite has both large and small crystals made of light colored elements and minerals, mostly quartz and feldspar, with some mica and other minerals tucked here and there. The design of earth's rock cycle is quite elaborate as we scratch it's dusty crust to see the wonders below.

Granite boulders such as we see in north Scottsdale are the result of a two stage process. These giants once rested underground in a subsurface that trickled with chemically active water that seeped into joints and fractures, causing weathering and breakdown of rock along those weaker surfaces.  Over time, the remaining stronger rocks became sphere-shaped core stones beneath the land's surface.  Spheres are the only shape that has no single weakest point and no single strongest point, and in the case of these boulders, the more vulnerable edges and points have eroded away.

The second stage began when water, wind and gravity stripped the loose rock material away, leaving the spherical core stones in place.  Some of those core stones happened to be resting on stones beneath them, and because our area experiences very little seismic activity, remain standing today. Scientists are still working to understand how factors such as slope, bedrock properties, spacing of joints, as well as weather, impact these amazing natural sculptures. 

Across Arizona, there are several sites with terrain similar to north Scottsdale. Granite Dells in Prescott, Chiricahua National Monument in southeast Arizona are two excellent examples. Rock fields seen along state route 87 between Phoenix and Payson or along Interstate 10 between Benson and Wilcox also offer good views of precariously balanced rocks. 

When you go, be sure to take a magnifying glass to study the composition of granite. Take time to imagine huge boulders buried underground before they gradually made their appearance as you see them today.  Look for fractures and weak places on the boulders where erosion will continue to shape each one. See how many different sizes of granite boulders, rocks and stones you can find. Most importantly, let wonder take you away.

Geology Terms

Felsic - Descriptive term for igneous rock having light colored elements and minerals, such as Quartz and Feldspar

Igneous - One of three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or lava.

Intrusive - Body of igneous rock that has crystallized from molten magma below the surface of the Earth.

Porphyritic - Contains both large and small crystals.

References and Resources
Science Kids - Igneous Rocks
Arizona Geological Survey - Nature's Balanced Seismometers
Quora - Why are Spheres so Strong?

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