How Sierra Nevada rose from the jungle

Fifty million years ago, powerful forces deep underground launched a wave of mountain building that swept southward from British Columbia through Nevada and California, and on into Mexico.

It was the beginning of what would become today's High Sierra.

For the next 20 million years, as the mountains heaved upward, California rivers like the Feather, the Yuba and the American flowed to the Pacific from as far east as central Nevada, while tropical jungles flourished throughout the region.

This scenario of a distant past for California and the West comes from new findings by a team of Stanford scientists who have collected nearly 3,000 samples of fossil rainwater absorbed by ancient rocks and glasses formed from melting volcanic eruptions to re-create the geologic history of a region that once extended east to what is now Nebraska.

Hari Mix and Page Chamberlain, together with their Stanford colleagues, ranged across the territory to analyze clays in the rocks and in glasses formed by volcanic eruptions millions of years ago. They also studied rock samples and even fossil leaves collected by other Stanford researchers and geologists worldwide.

The mountains rose

The result was the most extensive record yet of how and where the mountains rose during geologic periods known as the Eocene and Oligocene.

"There have been many competing hypotheses about the rise of the Sierra Nevada in recent geologic time," said Chamberlain. "One view suggests there was once a huge plateau in the West known as the Nevada Plano; but all our isotope data, taken with other records, shows clearly that the wave of uplift beginning about 50 million years ago and ending some 20 million years later saw the Sierra rising on the west side of Nevada well before it reached its present height."

For example, the researchers concluded, about 49 million years ago, mountains in British Columbia had already risen as high as 14,000 feet, while between 31 million and 28 million years ago, California's northern Sierra rose by nearly 10,500 feet. Mount Whitney was still to grow another 4,000 feet before it reached its summit elevation today - exactly 14,494 feet.

Tectonic forces deep beneath the ground have also played a crucial role in this saga, Mix and Chamberlain said.

As they described it, a huge slab of rock many miles thick and thousands of miles wide, which geologists call the Farallon Plate, once lay deep beneath the ocean off the Pacific Coast. During that Eocene-Paleocene period the slab was diving ponderously down and east beneath the newly forming mountains of California.

As the plate moved deeper and farther inland for hundreds of miles, Earth's viscous mantle - like softened fudge on a hot day, only thicker - welled up into the crust, triggering a wave of volcanic eruptions that also played a major role in thrusting the mountains upward.

The heights

Mix used one of Stanford's mass spectrometer laboratories to determine the heights of the mountains long ago. Oxygen in the fossil water in the rocks and glasses made it possible.

As Mix explained it, oxygen atoms in water come in three different long-lasting forms known as isotopes, and each isotope weighs a tiny bit differently than the other, depending on the number of subatomic particles called neutrons that each atom holds in its nucleus.

The ratio of heavy and lighter oxygen isotopes in rainfall varies with the altitude of the rocks that have absorbed the rain, while dating the rocks is standard in geology. So Mix and Chamberlain - using their own records and many other records from studies of thousands of miles of the Far West - were able to determine the shifting ratio of heavier and lighter oxygen isotopes, and that revealed how high the mountains rose during geologic time.

Geologic basins

The West is now marked by broad and mountainous geologic basins, the remains of those long-ago periods when the mountains were rising and rivers were carving new channels - and Mix and his colleagues have explored them all.

One among many is the mountainous Elko Basin in far northeastern Nevada, where Mix and Chamberlain collected ancient volcanic glass from rocks around the abandoned gold-mining town of Jarbidge. The town, with a population of 51, a 10-room motel and a saloon, is at the end of a 26-mile, rutted, one-lane dirt road from the nearest paved highway.

"It's great and gorgeous mountain country, but with impassible snowfalls in winter," Mix said as he recalled the field work that led to his team's report.

"But millions of years ago, you'd be hacking your way there through jungles and crocodiles," Chamberlain said.

The two scientists are publishing their results in this month's issue of the journal Geology, and among their colleagues are former Stanford graduate students Andreas Mulch of Germany's Biodiversity & Climate Research Center and Goethe University in Frankfurt; and Malinda Kent-Corson, Bridgewater State College in Massachusetts.

E-mail David Perlman at dperlman@sfchronicle.com.

This article appeared on page C - 1 of the San Francisco Chronicle