At least a handful of times in the last 500 million years, 50 to more than 90 percent of all species on Earth have disappeared in a geological blink of the eye.
According to the U.S. Geological Survey, more than 600 volcanoes occur in the San Francisco Volcanic Field, and they have erupted at various times over the past 6 million years.
据美国地质调查局,旧金山火山带上大约有600座火山,在过去的6百万年里,它们爆发在不同的时期。
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According to the U. s. Geological Survey, more than 600 volcanoes occur in the San Francisco Volcanic Field, and they have erupted at various times over the past 6 million years.