Terra Sierra: GRANITE: THE STONE HEART OF THE SIERRA

by Frank DeCourten

On a recent trip home from the valley, I was driving east over Donner Summit at sunset when the bright flash and glittering sparkle pierced the corner of my eye.  From the low road cut on my left, the reflected sunlight glistened with such brilliance that it seemed somehow electrified. How different, I thought, was this glittering outcrop from the monotonous dull brown exposures of volcanic rocks farther east, or the drab slate gray of the roadside rocks miles behind me.  The beauty of the rock reminded me of the words used by John Muir in 1894 in his loving description of the Sierra Nevada as a mountain range:

“ … so luminous, that it seems to be not clothed with light, but wholly composed of it, like the wall of some celestial city.”

Muir’s famous allusion to the Sierra as the “Range of Light” and the burnished outcrops of rock along Donner Summit both signify one of the most fundamental aspects of Sierra Nevada geology: granite.  So common is granite in the Sierra Nevada that the range is often described (inaccurately) as a mountain system comprised solely of it.  In fact, though granite comprises the foundation of the Sierra Nevada, other types are rocks are equally abundant at the surface, especially in the portion of the range north of Yosemite National Park.  Nonetheless, granite, or rock very similar to it, is widely exposed on the surface, and does comprise most of deeply buried and unseen foundation the Sierra Nevada.  Geologists estimate that the granite exposed at the surface of the Sierra Nevada extends downward some 20 miles or more before it gives way to different kinds of rocks in the deeper earth.  Granite may not be the only rock in the Sierra, but it by far the most characteristic, especially in the central core.  Granite is, without question, the stone heart of the Sierra. 

Because of its widespread use as tile and monument stone, most people are familiar the speckled crystalline appearance of granite, though the overall color may range from white, to tan, to reddish-brown.  The hardness, density, and durability of granite is legendary, and the word alone represents strength and permanence in popular culture and language.  And yet, despite the familiarity of granite, most of us do not fully appreciate the fascinating story behind the origin of this stone.  Granite is as  under-appreciated as it is common in the Sierra Nevada.   What is it, exactly, that makes this everyday rock so engaging?

As a starting point, let’s briefly consider the origin of granite.  Granite is an igneous rock, meaning that it formed from the cooling and solidification of magma, a hot subterranean fluid representing molten rock.  Bodies of magma develop underground, from about 30-150 miles below the Earth’s surface.  In this deep

subsurface realm temperatures approach 1,500º C, hot enough to form pockets or  “bubbles” of liquid rock.  The bodies of molten rock are surrounded by hot, but still solid material.  The cooler rock above the zone of melting is hard and rigid, forming what may seem to be an impenetrable barrier between the magma bodies below and the cold surface above.  No granite, or any other type of igneous rock, would ever form unless the magma somehow moves into a cooler realm where it can lose heat to the surrounding environment and eventually solidify.  As long as the bubble of magma remains in the hot zone where it originated, it will remain in the molten state indefinitely. Magma must move up, toward the cold surface of the earth, to encounter the lower temperatures necessary for cooling and solidification to occur.   As unlikely as it seems, there are several mechanisms that work simultaneously to allow magma to penetrate the overlying barrier.  The first mechanism arises from the fact that magma bodies are fluids under high pressures.  The pressure deep underground results from the weight of the overlying materials pushing down under the influence of gravity, similar to the pressure you may have felt in your sinuses at the bottom of a swimming pool.  The water of the pool squeezes your head the same way that rocks in the earth squeeze materials buried deep underground. 

Underground pressure, commonly called lithostatic pressure, intensifies with depth due to the increasing weight of the overlying rock.  On the surface of the earth, there is no lithostatic pressure.  But 60 miles underground, in the realm where magma is produced, the lithostatic pressure is 35,000 times greater than the atmospheric pressure at the surface.  Because all liquids move toward lower pressures, magma bodies are actually “squeezed” toward the earth’s surface.  The pressures within a magma chamber may propel the molten fluid into cracks in the overlying rock with enough force to widen them, resulting in an upward surge of additional magma.  Imagine the body of magma as a water balloon that changes shape when it is squeezed.  This forceful distention is similar to way magma works its way upward toward the earth’s surface.

Magma also contains dissolved gases such as carbon dioxide and water vapor.  Much like the gas from a carbonated beverage, these gases separate from the liquid as the fluid migrates upward in zones of lower pressure.  Thus, as the magma moves upward, gas pressure within the fluid increases, even as lithostatic pressure decreases. This internal pressure can help drive the magma even closer to the surface.   

Finally, because magma is molten rock, it is also slightly less dense than the surrounding solid rock, at least where it initially formed.  The density contrast between the magma and its surroundings causes to the magma to “float” upward against the ceiling of overlying rock, at the same time that the external and internal pressures are also driving it toward the surface.  As the magma body migrates into the overlying rock, it passes into cooler and cooler material as it ascends.  Throughout most of its upward

journey, the fluid is hot enough to melt some of the surrounding rock, assimilating the resulting liquid in the process.    

Magma always tends to move up from its deep source, then, for three reasons: it is “squeezed” toward the surface at the same time that it tends to “float” in that direction, all the while “welding holes” in the overlying rock ceiling.  In reality, the advancing front of the magma mass is probably highly irregular and uneven.  Fingers of pressurized magma may protrude from the main body as internal pressure forces the fiery fluid along cracks in the surrounding rock.  Chunks and slabs of the solid wall rock may fall into the molten magma and become partially liquefied in the hot solution.  After the magma solidifies, inclusions of exotic rock in the resulting granite are called xenoliths (“foreign rocks”).  Xenoliths are usually darker in color than the enclosing granite, as is the case in the many xenoliths embedded in the granite bedrock in the Donner Pass area.

Magma solidifies through the process of crystallization.  Once the magma has cooled to about 1,000º C, small crystals of minerals such as quartz, mica, and feldspar begin to form in the magma.  In the deep, subsurface environment the crystal growing process is very slow, requiring perhaps thousands of years of slow, gradual cooling for the crystals to attain visible size.  The crystals grow as atoms of silicon, oxygen, iron, magnesium, and other elements migrate from the magma to the crystals, much like tiny bricks entering the elaborate atomic structure.  As the crystals grow, they eventually come into contact with each other and begin to build an interlocking framework.  After a lengthy cooling period, the molten magma solidifies into a mass of hard stone composed of completely inter-grown crystals

This network of interlocking crystals is what gives granite its characteristic sparkle and glitter.  The flat, reflective surfaces of the randomly oriented crystals flash light to the observer in a twinkling pattern as the angle of light changes.  The actual size of the crystals in granite varies from barely visible to several inches, which explains why some granites appear more crystalline than others.   Granite, as defined by geologists, is comprised of a specific set of minerals dominated by quartz, mica, and feldspar.  Other igneous rocks may appear similar to granite, but contain a different assemblage of mineral crystals. Diorite, for example, has no quartz, a different kind of feldspar, and abundant dark-colored crystals of hornblende.   Gabbro contains large crystals of feldspar, olivine, and pyroxene.  As in most classification systems used in natural science, the names mentioned above represent our attempt to subdivide a continuum of variation in the mineral composition and texture of the igneous rocks.  The distinction between granite and diorite, for example, is defined more or less arbitrarily for human convenience, not because any natural process prevents other mineral assemblages from existing. Many rocks have mineral assemblages that place them astride the subjective boundaries we have superimposed on the natural continuum of variation. 

Geologists use terms such as granodiorite and monzogranite for these transitional rocks, expanding the lexicon of igneous rock names considerably in the process.   Much of the “granite” in the Sierra Nevada is actually granodiorite or diorite, but the distinction between these varieties and true granite is so subtle that only a geologist might notice (or care!).

Because granite forms underground from the slow gradual cooling of magma, it is known as a plutonic (from Pluto, the Greek god of the underworld) igneous rock.   Volcanic rocks result from the eruption of magma onto the earth’s surface (it is then known as lava), where it cools and hardens rapidly.  Such rapid cooling does not allow the development of large crystals, and most volcanic rocks exhibit a corresponding dull appearance.  Plutonic rocks occur in large underground masses known as batholiths, great irregular bodies of granite that represent, in essence, an entire magma chamber that cooled and solidified into solid stone.  Because batholiths form underground, their exposure on the surface is always an indication of uplift and erosion into the deeper levels of the crust.  Over the past several million years, as the Sierra Nevada rose to its present height, the rock overlying the batholith was worn away by erosion to expose more than 10,000 square miles of sparkling granite.  Because the southern Sierra Nevada is significantly higher than the northern portion of the range, more of the granite was unroofed by erosion south of Yosemite than farther to the north.   But, even as far north as the Tahoe region, granite exposures are extensive.  

The Sierra Nevada Batholith is one of the largest and most complex granite bodies in the world.  Granitic rocks of this batholith can be traced from the Feather River region to Tehachapi, a distance of more than 375 miles.  Geologists have determined that there are actually scores, perhaps hundreds of individual granite bodies within this enormous batholith.  So complex is the Sierra Nevada Batholith that its structure has been compared to a bag of marshmallows: many individual granite masses are huddled closely together to constitute a larger mass.  Each granite body represents a distinct bubble (a “marshmallow”) of magma that migrated upward into the Earth’s crust, primarily between 80 and 110 million years ago.  For some thirty million years, bodies of magma flooded upward into crust, cooling and solidifying adjacent to older batholiths that were already in place.  This explains the wonderful variety in color and texture of Sierra Nevada granite.  Our Sierra granite may be gray, white, tan, or rusty.  It may have many xenoliths, or it may have none.  It may be delicately crystalline or it may have monster crystals several inches long.  It may be fractured and crumbly, or it may be solid and massive. 

Our mountains are truly a granite wonderland.  Yes, other kinds of rock exist in the Sierra Nevada, and they are all fascinating in their own right.  But, the sparkling peaks and gleaming domes of the high country shine through the uniquely effervescent

glint of granite.   Without granite, a rock commonly overlooked and unappreciated,

Muir’s “Range of Light” would be much less luminous.

All photos by Frank DeCourten

Photo DonnerLake.jpg :  The light-colored granite (foreground)exposed near Donner Lake is typical of the Sierra Nevada bedrock.

Photo granitetext.jpg: The interlocking crystals of light and dark colored minerals give granite its characteristic flash and sparkle.

Photo xeno2: Dark-colored xenoliths, such as those in this photo, are especially abundant  in the granite of the Donner Pass area