Sam Spade's San Francisco

An earthquake and a parade - what a way to start a new lunar year!

It was a wild San Francisco Chinese New Year Parade for 2008!

The parade took place faithfully as it does every year despite strong gusts of wind and a steady rain on the parade. The bright colors of the lion dancers, the lions themselves and even the venerable and historic dragon of San Francisco, Gum Lung, were protected by transparent plastic raincoats. Gum Lung's raincoat was 201-feet long.

In the photo: The Bottle Rock Power Plant at the Geysers. Photo courtesy of The Geysers Geothermal Assn.

To help celebrate the Year of the Rat, Mother Nature, deep within the hidden bowels of the Earth, decided to rumble and grumble and shake things up a bit. At 9:32 PM on Saturday, after the parade was over, a 4.1 earthquake struck almost 2.5 miles below the surface of the planet. The epicenter of the earthquake was The Geysers, which is about 25-miles from Santa Rosa. The exact longitude and latitude is 38.818°N, 122.809°W.

The Geysers is an active geothermal area within the Clear Lake volcanic field in Northern California. The Geysers is a steam field that is comprised of 30 square miles along the Sonoma and Lake County border. It is the largest complex of geothermal power plants in the world.

Of the 21 power plants at The Geysers, Calpine owns and operates 19 of them. They generate about 750 megawatts of electricity - enough to power 750,000 homes, or a city the size of San Francisco. The towns of Pine Grove, Cobb, Forest Lake and Whispering Pines, all along Highway 175, are located right on on the edge of the geothermal field. The town of Anderson Springs pretty much sits right on top of the field.

The Geysers geothermal field is actually a place where maars come very close to the surface of the earth. Maars are flat-floored craters that are formed after extremely violent expansion of super-hot magmatic gas or steam. Often the intense pressure drills upward through solid rock and creates vent holes, or diatreme, that carry the super-heated steam under intense pressure to the surface where it spurts out as steam or hot water geysers.

The largest visible feature of the geothermal field is Mount Konocti, which is not a normal mountain at all. It is a composite dacitic lava dome along the South shore of Clear Lake. It is dacitic in that when Mount Konocti is active it spews out molten dacite, which is about 65% silica. The flows erupt at the surface between 1470 and 1830 degrees Fahrenheit (approximately 800-1000 degrees Centigrade).

There is no reason for immediate alarm, however. The last time Mount Konocti erupted was about 10,000 years ago. Tourist literature often refers to Mount Konocti as an extinct volcano. That is absolutely not true. It is not extinct. Konocti is active - just very, very patient between eruptions.

Mount Konocti, however, is a soundly sleeping giant. Beneath it's slopes bordering Clear Lake on one side and world-class vineyards on the other, rests a large dacitic magma chamber with gurgling and bubbling molten lava. As water seeps down into the earth it encounters intense heat coming from the magma chamber. That heats the water to steam and it makes its way to the surface. The result is the world's largest producing geothermal field.

Yesterday's steam-boiler earthquake was little more than a burp in the grand life of the living, breathing Planet Earth as we know it here in Northern California. We are, after all, part of the Great Pacific Ring of Fire that is the most active volcanic area on Earth.

Resources:
Map of The Geysers - after opening, click on the map to enlarge
The Geysers and Clear Lake Volcanic Field
Calpine - The Geysers Geothermal Production Field
History of Geothermal Production at The Geysers
The Geysers Geothermal Association
U.S. Geological Survey (USGS) - for more information on recent earthquakes
Mount Konocti - Wikipedia article
The Pacific Ring of Fire - map from the USGS

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