"Where do you live?"
"Alpine County, Calif."
"Where is that?"
"Near Lake Tahoe, in the Sierra Nevada."
That is a typical dialogue between a resident of Alpine County and most anyone else. In other words, Alpine County remains a relatively unknown geographic area with few full-time residents. It is not Lake Tahoe, although the Lake is a well-known landmark, nor is it the Carson Valley of Nevada, where most Alpine County residents have commerce.
And if it is not the Lake or the Valley, what is the county of Alpine? What geologic forms are we seeing as we drive the scenic routes of highways 88 and 89?
A young woman who grew up in Alpine County and became a certified geologist for California offered a brief interview during her holiday visit to her family home in Markleeville. Jenny Thornburg brought maps and mining publications from the California Geological Survey to get me started on understanding the basic geology of Alpine County.
"What am I looking at, Jenny?" I began.
"The whole mountain range to the west is part of the huge batholiths of granite that extends some 500 miles in California. That process occurred 100 or 200 million years ago," Thornburg explained.
Another process is known as "normal geologic block faulting," which occurred 2 to 3 million years ago, formed Lake Tahoe by drop faulting the Sierra and Carson Range on the east by normal faulting. We can visualize slabs quaking some 20 kilometers deep.
Since faulting is a term I associate with earthquake activity, I asked Thornburg about the fault line through Alpine County. "The Genoa fault line is pretty straight down the Carson Valley. Then it steps to the east through Alpine County. The fault lines look like parallel stair steps, except the connecting lines become complicated and fractured," she said.
What is a fault? Andrew Alden, writing for About.com, defines a fault as a large crack in the Earth's crust where one part of the crust has moved against another part.
If normal faulting created the mountains and Lake Tahoe, are faults still active at the Lake and here? First of all, Thornburg explained, an "active fault" is defined as one that has moved in the last 10,000 years and so is likely to move again.
And yes, there are at least three active faults submerged at the Lake, the West Tahoe fault, the Stateline-North fault, and the Incline Village fault.
The Geological Society of America published an article in June 2009 after studying the faults at the Lake and faults through Carson Valley and Alpine County. Using seismic imaging (CHIRP) and multi-beam and light detection bathymetry the group of scientists from UC Davis, UC San Diego, San Diego State, and the University of Nevada, Reno, found that the faults at the Lake were moving consistent with the Genoa fault-slip rate. Thornburg was aware of that finding, and noted that her boss in Sacramento has a wise saying about findings in geology: "Interesting, if true." Geology and its sister sciences deal with a rather "messy" Mother Nature, who doesn't always seem to follow patterns and rules.
Carson Valley and the Lake Tahoe Basin show similar earthquake events as recorded by valley colluvial (a loose collection of rock and debris at the foot of a slope) wedge deposits or submerged offset-fan delta stratigraphy. The delta fans are evident along the east slopes of the Sierra - in Alpine County that would be the Emigrant Trail area. Older glacial deposits in much of Diamond Valley show that the fault in that area was less active than the western one.
To summarize, millions of years ago an enormous slab of magma resurfaced, cooled, and crystallized into a 500-mile chunk of granite: the batholiths that dominate the Sierra. Then pressure created faults and volcanic activity a few million years ago.
Along with granite, there is much volcanic rock in Alpine County. It is evident in cliffs along the Carson River and in eastern parts of the county. The volcanoes left rugged domes, plugs, and cones in the southern part of the county.
The volcanic rock covering the eastern part of the county and many of the higher peaks have yielded great mineral wealth, including precious metals and sulphur.
Common knowledge is that areas with volcanic rock provide great agricultural land. I asked Thornburg why Alpine County does not have that richness when it does have volcanic rock. She replied, "Well, rain is important to decompose the rock and grow plants." Yes, we live in high desert.
While most of the county has giant granite rocks or volcanic rock, there are some glacial moraines. One is evident from my back deck, looking from Highway 88 toward Woodfords Canyon. From a distance, it looks like an engineered road, sloping from behind Monroe Ranch to the canyon. Actually, Thornburg explained, it is a lateral moraine, a pile of rock left on the side of the glacier as it moves forward. A glacier pushes rocks out in front and off to the side as it moves.
Many geologists today presume that the whole deep Earth has its own cycle, including the down-welling of old and upwelling of hot plumes. Plate tectonics remains a powerful concept. The Genoa fault has been mapped and shown to be moving. Alpine County's faults may be connected to those under Lake Tahoe.
Geologists think in terms of thousands, millions, or billions of years: the faults may or may not become more active in the next 10,000 years. The only certainty is that the earth is changing. The evidence of change is available in the geological formations in our own Alpine County.
Thanks to Jenny Thornburg, the California Geological Survey, and the GSA Bulletin for their information and knowledge.
