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| North and Basket Domes (L) and Half Dome (R) |
Towering 4,100 feet above the floor of Yosemite Valley, Sentinel Dome is one of literally thousands of granite domes projecting from the great Sierra batholith commonly known as central California’s Sierra Nevada range. Of the dozen or more domes lying within just a few miles of Sentinel Dome, the most famous are Half Dome, North Dome, Basket Dome, and Mt. Starr King.
The Sierra batholith is a huge expanse of granite shaped like a blimp. It has an exposed surface area of about 25,000 square miles, and is thought to have an internal volume of some 150,000 cubic miles. Many years before I was born, plumes of molten magma rose deep beneath the earth’s crust, much like weather balloons might fill an airplane hanger. Their ascent stopped by the impenetrable roof, the magma plumes retained their balloon shapes as they slowly cooled into what we now know as granite.
Meanwhile, the Pacific tectonic plate was bulldozing into the North American tectonic plate, lifting the Sierra Nevada range as if it were a trap door hinged on the west – its western side gently sloping to California’s central valley, and its eastern side dropping precipitously to the Owen’s Valley. Within the trap door, the granite balloons went along for the ride, producing the highest mountain range in the continental United States. Mt. Whitney, near the batholith’s southern end (210 trail miles south of Yosemite), rises to 14,493 feet above sea level. Note: For anybody interested in climbing Mt. Whitney, you should do so soon, before global warming raises the sea level and correspondingly reduces Mt. Whitney’s calculated elevation.
A few million years of rainstorms and a glacier or two eroded the top 6 or 8 miles of the mountain crust, exposing the Sierra’s great granite balloons. No longer restricted by the hanger’s roof, and having had the great weight of the upper crust removed, the granite, itself, continued to rise, complementing the uplift caused by the colliding plates. Geologists call this process “isostatic orogeny” in a futile attempt to make their science sound sexy. Just as a weather balloon expands as air pressure against it diminishes with altitude, the granite balloons expanded as the weight of the overlying crust eroded away. But granite doesn’t like to expand very much, so it cracked in concentric layers, much like the layers in an onion. A few more rainstorms and glaciers peeled away some of the outer layers, leaving the rounded tops we now refer to as domes.
About the time William Shakespeare was putting the final touches on Hamlet, a tiny seed from a Jeffrey pine was blown into a small crack on the top of Sentinel Dome. Finding some soil that had also been blown into the crack, the seed sprouted. Irrigated by years of winter snowmelt, the seedling grew into a stately Jeffrey Pine whose image was forever preserved by Ansel Adams in one of his famous photographs.
But nature is never finished. In the late 1970s, California was hit by several years of severe drought. Despite the efforts of numerous hikers who hand carried literally thousands of bottles of Perrier to the base of the pine, the famous Sentinel Jeffrey Pine died. It is unknown whether the pine died from the stress of the drought or merely from old age. Or maybe it was the Perrier. I hear that part of President Obama’s stimulus package includes a grant to scientifically prove that the tree’s death was actually a result of George W. Bush’s failed policies. What remains certain, however, is that the tree is no longer emitting vital signs.
The dead tree continued to stand for another 25 years, until 2003, when it morphed into a log. The log still lies atop Sentinel Dome, insulated from fire and far from the forest soil whose critters cause fallen trees to decompose. The primary natural risk the log faces is a well placed lightning strike, an unlikely event since it is no longer upright. In the 30+ years since its death, the once noble Sentinel pine has been subjected to more damage from tourists than from nature. In California’s arid climate, the log will probably remain where it is for 150 years or more.
In the meantime, other seeds have blown into the Dome’s cracks. Several have germinated – perhaps some are even descendants of the famed Sentinel pine. What are now thirty or forty year old trees may someday achieve the grandeur of their famous ancestor.
© 2010 Ken Klug
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