Balancing Act EarthCache
Hidden : 4/4/2015
Difficulty:
3 out of 5
Terrain:
3 out of 5

Size: Size: other (other)

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Geocache Description:

Tons of granite resting precariously on a slender stone base...

About the Rock:

The Balancing Rock, has long been a natural attraction on Lake Tahoe's western shore. Visitors to the Lake Tahoe area in the late 1800's and early 1900's enjoyed being photographed next to this geological marvel. Today, the Balancing Rock is the feature attraction of a short, half mile self-guided nature trail in the northwest section of D.L. Bliss State Park. Did some forgotten civilization place this rock upon its pedestal? How on Earth did it get this way? Better put, how under Earth did it get this way?

Despite its odd appearance, the rock wasn't put here by ancient aliens, trolls, or other-wordly magic, but developed through Spheroidal Weathering, a form of chemical weathering that affects jointed bedrock and results in the formation of concentric or spherical layers of highly decayed rock within weathered bedrock. When exposed by physical erosion, these concentric layers peel (spall) off as concentric shells much like the layers of a peeled onion. Spheroidal weathering often creates rounded boulders, known as corestones or woolsack, of relatively unweathered rock within layers of broken down granular material called saprolite. When that saprolite breaks down and wears away, the boulders are left behind, often in rather dramatic formations. Such is the case with Lake Tahoe's Balancing Rock. This process is also called onion-skin weathering, concentric weathering, spherical weathering, or woolsack weathering.

How it Happens:

First off, the granite in the Lake Tahoe area is part of the Sierra Nevada Batholith. The Batholith is composed of many individual masses of rock known as plutons, formed millions of years ago as magma cooled deep underground. Indeed, granite is considered an intrusive igneous rock because of its underground formation. Over time, plate tectonics forced the Sierre Nevada mountains upwards, the soil and sedimentary material above them eroded away, ultimately exposing the dramatic and distinct granite peaks and domes of the Sierra Nevada we see today. When plutons like those in the batholith are buried beneath the earth's surface, they remain dense bedrock under significant pressure. However, as they near the surface and are exposed through erosion, that pressure lessens, allowing the rocks to expand slightly. Due to their naturally brittle nature, rocks don't stretch very easily. Instead they crack and break, forming fractures known as joints. These joints form a network of cracks running through the bedrock.

In rocks that are at or near the surface, water seeps along these joints, breaking down the component minerals. This exposure to water (hydrolysis) and air (oxidation) breaks down the minerals in the bedrock, causing the dense rocks to decompose and disintegrate at their edges, widening the joints, and allowing even more water to seep in along the surfaces. At corners where two or more joints meet, the chemical weathering occurs from multiple directions, resulting in more rapid decomposition of the rock structure. This accelerated disintegration at joint intersections turns sharp corners into rounded edges. The granular disintegrated rock along the joints is called saprolite, (literally rotten rock). Eventually, natural forces like running water, wind, and gravity, flush the soft saprolite from the joints, leaving the dense, unweathered portions of the rock behind in the form of rounded boulders (corestones) resting in their original positions.

Spheroidal weathering is most common among coarse-grained igneous rocks, especially granite and similar rock types. It is also found more often in warm climates, where mechanical weathering by freezing ice is less likely.

The landscapes that result from this process vary in nature. Some are fields of boulders scattered amongst a meadow. Other times you might find a large outcrop of rock with worn down sides or pile of boulders resting atop one another. But when the conditions are just right, the underlying joints don't completely disintegrate, and the surrounding rock and soil wear away in just the right fashion, you are left with these natural works of art: giant boulders held aloft on pedestals, impossibly connected by just the smallest bit of stone. Over time, both physical and chemical weathering will eventually break the connection and send them rolling on their way.

Take a look around and you will see all numerous of examples and varying degrees of onion-skin, woolsack, and spheroidal weathering in this area.
Getting There:

The easiest approach to GZ does not actually require you drive into D.L. Bliss and can be reached year-round. Simply park at the turnout off Highway 89 (see designated waypoint) and walk down the hill until you reach the Balancing Rock trail system and see the Rock itself. This is an easy way to reach this EarthCache, but does make for a long walk if you intend to explore more of the State Park. You can also enter the Park (when it is open) by the Visitors' Center and follow the signs to the marked trailhead.

Logging requirements:
(You can log your find as soon as the email is sent, but logs with insufficient answer may be deleted)

Send me a note through my caching profile with :
1. The text "GC51518 Balancing Act" on the first line
2. The number of people in your group (put in the log as well).
3. How tall is the Balancing Rock itself (not including the pedestal)?
4. How much do you think the Balancing Rock weighs? How did you determine that?
5. Look at the soil around you. Would you consider this saprolite? Why or why not?
7. Optional (yet encouraged) Post a photo of you and your party at the Balancing Rock.

Bonus Points for photos with balancing Geocachers!

You can find a smaller balancing rock in the Tahoe Mountain area: GC57Q7E: GT1440 - Near the Sphere.

I have earned GSA's highest level:

References:
Eastern Illinois University: Mechanical and Chemical Weathering
EarthSci: Weathering and Soils
University of Connecticut: Engineering Geology
Diagram borrowed from GC49VTX
UWSP Dept. of Geography and Geology

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