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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. ==Geology== 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.〔 p. 167.〕 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.〔Hill, Mary (1999), p. 168.〕 Beginning about 200 million years ago, tectonic pressure forced the sea floor beneath the American continental mass.〔Hill, Mary (1999), pp. 168-69.〕 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〔, pp. 195-196.〕 to become the granite rock found throughout the Sierra Nevada and other mountains in California today.〔Hill, Mary (1999), pp. 149-58〕 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.〔〔Hill, Mary (1999), pp. 174-78.〕 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.〔Hill, Mary (1999), pp. 169-173.〕 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.〔Hill, Mary (1999), pp. 94-100.〕 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.〔Hill, Mary (1999), pp. 105-110.〕 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Gold in California」の詳細全文を読む スポンサード リンク
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