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Gold became highly concentrated in California, United States as the result of global forces operating over hundreds of millions of years. Volcanoes, tectonic plates and erosion all combined to concentrate billions of dollars' worth of gold in the mountains of California. During the California Gold Rush, gold-seekers known as "Forty-Niners" retrieved this gold, at first using simple techniques, and then developing more sophisticated techniques, which spread around the world.
Geologic evidence indicates that over a span of at least 400 million years, gold that had been widely dispersed in the Earth’s crust became more concentrated by geologic actions into the gold-bearing regions of California. Only gold that is concentrated can be economically recovered. Some 400 million years ago, rocks that would be accreted onto western North America to build California lay at the bottom of a large sea. Subsea volcanoes deposited lava and minerals (including gold) onto the sea floor; sometimes enough that islands were created. Between 400 million and 200 million years ago, geologic movement forced the sea floor and these volcanic islands and deposits eastwards, colliding with the North American plate, which was moving westwards.
Beginning about 200 million years ago, tectonic pressure forced the sea floor beneath the American continental mass. As it sank, or subducted, beneath the western margin of the North American plate portions of the sea floor and overlying continental crust heated and melted, producing large molten masses (magma). Being lighter and hotter than the ancient continental crust above it, this magma forced its way upward, cooling as it rose to become the granite rock found throughout the Sierra Nevada and other mountains in California today. As the hot magma cooled, solidified, and came in contact with water, minerals with similar melting temperatures tended to concentrate together. As the magma solidified, gold became concentrated within hydrous silica solutions and was deposited within veins of quartz.
As the Sierra Nevada and other mountains in California were forced upwards by the actions of tectonic plates, the solidified minerals and rocks were raised to the surface and subjected to erosion. The surrounding rock then weathered and crumbled, and the exposed gold and other materials were carried downstream by water. Because gold is denser than almost all other minerals, this process further concentrated the gold as it sank, and pockets of gold gathered in quiet gravel beds along the sides of old rivers and streams.
The California mountains rose and shifted several times within the last fifty million years, and each time, old streambeds moved and were dried out, leaving the deposits of gold resting within the ancient gravel beds where the gold had been collecting. Newer rivers and streams then developed, and some of these cut through the old channels, carrying the gold into still larger concentrations.
The Forty-Niners of the California Gold Rush first focused their efforts on these deposits of gold, which had been gathered in the gravel beds by hundreds of millions of years of geologic action.
The early Forty-Niners panned for gold in California’s rivers and streams, or used "cradles" and "rockers" or "long-toms," forms of placer mining. Modern estimates by the U.S. Geological Survey are that some 12 million ounces (373 t) of gold were removed in the first five years of the Gold Rush (worth approximately US$7.2 billion at November 2006 prices).
By 1853, the first hydraulic mining was used. In hydraulic mining, (which was invented in California) a powerful stream of water is directed at gold-bearing gravel beds; the gravel and gold then pass over sluices, with the gold settling to the bottom. By the mid-1880s, it is estimated that 11 million ounces (342 t) of gold (worth approximately US$6.6 billion at November 2006 prices) had been recovered via "hydraulicking."
The final stage to recover loose gold was to prospect for gold in the flat rivers of California’s Central Valley and other gold-bearing areas of California (such as Scott Valley in Siskiyou County). By the late 1890s, dredging technology (which was also invented in California) had become economical, and it is estimated that more than 20 million ounces (622 t) were recovered by dredging (worth approximately US$12 billion at November 2006 prices).
Gold-seekers also engaged in "hard-rock" mining, that is, extracting the gold directly from the rock that contained it (typically quartz). Once the gold-bearing rocks were brought to the surface, the rocks were crushed, and the gold was separated out (using moving water), or leached out, typically by using arsenic or mercury. Eventually, hard-rock mining wound up being the single largest source of gold produced in the Gold Country.
There were decades of minor earthquakes, more than at any other time in the historical record for Northern California, before the 1906 San Francisco earthquake. Previously interpreted as precursory activity to the 1906 earthquake, they have been found to have a strong seasonal pattern and due to large seasonal sediment loads in coastal bays that overlie faults as a result of mining of gold inland.