Washington, April 24 : Geologists studying deposits of volcanic glass in the western United States have found that the central Sierra Nevada largely attained its present elevation 12 million years ago, roughly 8 or 9 million years earlier than commonly thought.
The finding has implications not only for understanding the geologic history of the mountain range but for modeling ancient global climates.
According to Andreas Mulch, who was a postdoctoral scholar at Stanford when he conducted the research, a variety of studies over the last five years have shown that the presence of the Sierra Nevada and Rocky Mountains in the western United States has direct implications for climate patterns extending into Europe.
"If we did not have these mountains, we would completely change the climate on the North American continent, and even change mean annual temperatures in central Europe," he said.
Mulch and his colleagues reached their conclusion about the timing of the uplift of the Sierra Nevada by analyzing hydrogen isotopes in water incorporated into volcanic glass.
Volcanic glass is an excellent material for preserving ancient rainfall. The glass forms during explosive eruptions, when tiny particles of molten rock are ejected into the air.
The researchers analyzed volcanic glass at sites from the Coast Ranges bordering the Pacific Ocean, across the Central Valley and the Sierra Nevada and into the Basin and Range region of Nevada and Utah.
The ratio of hydrogen isotopes in the glass reflects changes that occurred to the water vapor content of air over the Pacific Ocean as it blew onto the continent and crossed the Sierra Nevada.
By determining the ratio of heavier to lighter hydrogen isotopes preserved in volcanic glass and comparing it with today's topography and rainwater, researchers can estimate the elevation of the mountains at the time the ancient water crossed them.
Until now, researchers have been guided largely by geophysical evidence indicating that the range reached its present elevation approximately 3 or 4 million years ago, owing to major changes in the subsurface structure of the mountains. "There was a very dense root of the Sierra Nevada, rock material that became so dense that it actually detached and sank down into the Earth's mantle, just because of density differences," said Mulch. "If you remove a very heavy weight at the base of something, the surface will rebound," he added.
The rebound of the range after losing such a massive amount of material should have been substantial.
But, according to Mulch, "We do not observe any change in the surface elevation of the Sierra Nevada at that time, and that's what we were trying to test in this model."