Nature Moved Mountains to Carve California : Geology: Twisting and tearing of Earth’s crust over millions of years created a land of extraordinary beauty.

California has become home to so many immigrants that it is perhaps only fitting that the land on which they dwell is also composed of immigrants.

The mountain ranges that make this state one of the most spectacular regions in the world are relatively young, but many of the rocks from which they are built came here long ago and formed far away.

More than a billion years ago the huge chunk of the Earth’s crust that lies under the Pacific Ocean, called the Pacific Plate, was dotted with volcanic islands similar to the Philippines or the Aleutians, called “island arcs.” As the Pacific Plate pushed under the western margin of the North American Plate, which lies under the North American continent, these islands were plastered against the edge of the growing continent and became part of the Pacific Southwest.

Magma, hot molten rock from the Earth’s mantle, pushed to the surface and flooded the basins between the islands with granite, some of which eroded and mixed with debris washing down from the slopes of the volcanoes to form giant sedimentary basins that would one day provide fertile lands to grow crops to feed a new nation.

Powerful tectonic forces chopped some of those basins and volcanoes into pieces and built new mountains, some of which still stand today. By studying those chunks of debris, and examining the relationship of rocks of differing ages, scientists today are able to reconstruct the geological history of California with some degree of confidence.

But the picture is extremely complex and much of what we know today will undoubtedly be altered by new information gathered by the thousands of geologists who have come to this active region to study one of nature’s most dynamic laboratories.

It is a developing story in which tectonic forces continue to torment the land, knocking great chunks of rock over and pushing deeper, older rocks on top of younger formations, moving mountains back and forth as though they were rafts drifting on a choppy sea. An enormous sheet of rock was tilted over on its side, forming the Sierra Nevada, and part of the land ripped apart, creating the Salton Sea, as diverse processes reshaped the region into one of nature’s most spectacular showcases.

In some areas, the land was literally turned upside down as new mountains pushed up over nearby land, creating ranges such as the San Gabriel and the San Bernardino mountains.

Nowhere is that more apparent than in the Los Angeles Basin, a land built from rocks and sediments that once rested on the bottom of the ocean. Over time, the sediments were captured by tectonic forces that carried them aloft, moving the fossilized remains of sea creatures to new settings thousands of feet above the ocean.

The picture is somewhat clouded by erosional forces that have carried sediments down the mountain slopes to bury part of the record beneath thousands of feet of debris. But as scientists examine oil wells and old rocks for clues to a violent evolution, the geological history is emerging.

Leon Silver, a geology professor at Caltech, came across one of the most startling discoveries years ago while pioneering the field of “geochronology.” Silver developed the technique of comparing the ratio of minerals, such as uranium and lead, that decay at different rates, to determine the age of rocks. While studying the rocks of the San Gabriel Mountains east of Los Angeles he found that some were much older than neighboring rocks.

The rocks were so old, in fact, that they must have formed long before the land accreted to the North American continent to form California, and further studies of the Earth’s magnetic field that was frozen into some of those rocks convinced Silver that they had probably formed somewhere in the Southern Hemisphere. So Silver reached the dramatic conclusion that the fundamental building blocks of the San Gabriels actually formed south of the Equator and then migrated here for their new assignment.

Not every expert concurs with that conclusion, but he believes the record is convincing.

“The oldest rocks in the San Gabriel Mountains formed in the great episode of continent building between 1.7 billion and 1.62 billion years ago,” he said. Hundreds of millions of years later, the rocks became part of a chain of mountains. “Since then, the San Gabriel Mountains have traveled all over the Southwest. We have evidence that they once were as far north as Lake Tahoe and as far south as Mexico before reaching their present position north of Los Angeles.”

The San Gabriels, and their sister mountains, the San Bernardinos, probably formed as a single chain, Silver believes. But about 5 million years ago the San Andreas Fault jumped inland and dissected the chain. Today, the San Bernardinos are on the continental side of the fault and the San Gabriels are on the seaward side, and the two chains are separating as the oceanic plate moves north.

The Pacific Plate does not glide smoothly along the continental margin. Instead, it pushes into the continental plate north of Los Angeles, generating powerful compressional forces that are squeezing the land. The mountains are riddled with faults that drop down at an angle from the surface. As the land is squeezed together, great chunks of rock--called fault blocks--are pushed up some of those “thrust” faults, riding up over their neighbors in a continuing mountain-building process.

Today, topped with snow for much of the year, the San Gabriels tower over the basin from whence they came. They are the dominant part of a series of mountains, including the Simi Hills and the Santa Monicas, that are called the Transverse Range because they are oriented across the normal pattern of mountains in the Southwest.

The process that built the San Gabriels is different from the technique nature employed to build other ranges in California, including the Sierra Nevada and the gentle rolling slopes of the coastal mountains.

The result has been the creation of a natural laboratory that is so dynamic that geologists have flocked to this area to see what they can learn about a planet that is constantly rebuilding itself. One of them was Ben Page.

When he was a young geologist, Page moved from Nevada to Stanford University to be closer to the action. Intrigued by the new field of plate tectonics, which had emerged during the 1960s as the most plausible explanation for how the planet evolved to its present state, Page wanted to study the coastal range that runs from Ventura County in the south through Northern California. Today, most geologists defer to him as the leading expert on how that range of mountains formed.

But as he gazed recently toward the hills that rise just west of the Stanford campus, he said he isn’t all that sure how they got there. He believes, however, they came about because of a subtle change on the other side of the Pacific Ocean.

About 3 1/2 million years ago, the Pacific and North American plates converged slightly--by about 10 or 12 degrees--rather than sliding past each other as they had for the 24 million to 28 million years before that, Page said.

Page is not sure why that happened, but he believes it resulted from plate interactions in the southwestern Pacific.

When the direction of the movement of the Pacific Plate changed, he said, it squeezed the sedimentary land along the coast, which was riddled with faults because of millions of years of plate interactions. The compression, he said, caused the range to rise just like a long “welt” along the coastline.

“The land squeezed up as though it were putty,” usually along existing fault lines, he said. Unlike the San Gabriels, which rose as one block rode up over another, the coast range just sort of oozed up like toothpaste out of a tube.

“And that induced the rise of the coastal ranges,” Page said. As the mountains rose, other areas were “left behind,” like the San Francisco Bay and the Santa Clara Valley.

There is considerable variety in the coastal ranges, however, because not all of them are built out of the same materials. The Santa Cruz Mountains, for example, contain debris from an old sub-sea volcano.

“It crossed a large part of the Pacific Ocean on the Farallon Plate, just riding piggyback, and then collided and crashed” into the North American plate, said David G. Howell of the U.S. Geological Survey in Menlo Park.

Volcanic debris can be seen in several areas along the coast. Morro Rock, which towers over the entrance to Morro Bay, formed as the “plug” in an old volcano and was left standing when everything else eroded away.

Most rocks in the coastal ranges, however, are vulnerable to erosion, and that is the reason the mountains are relatively modest. The growth of the coast ranges has been fast enough to have produced a far more spectacular chain of mountains had it not been for that fact, Page said. Had they been built of tougher stuff, like the Sierras, the coastal mountains today might rival their kinfolk to the east.

But nature used a different tool to create the majestic Sierra Nevada range. It also used different materials.

The materials that eventually became the Sierras probably began as volcanic islands that plastered themselves to the edge of the continent and then were pushed upward by plate movements, according to William Dickinson of the University of Arizona.

About 25 million years ago, the San Andreas Fault began to form along the continental margin west of its present location. That “changed the geometry,” causing the land that became the Sierras to rise even higher, said N. King Huber of the U.S. Geological Survey. Exactly why the birth of the fault should have had that much impact remains a geological mystery, he added.

The changes in stress patterns brought about by the new fault and uplift eventually caused a huge block to lean to the west, creating mountain peaks as it tilted over onto its edge. And at least 6 million years ago, the Owens Valley began to drop, adding to the relief.

The tilting of the range caused rivers to aggressively cut into the land, creating deep canyons. Meanwhile, glaciers moved restlessly over the granite, cutting the U-shaped valleys and polishing the sides of towering sheets of granite that would frame one of nature’s crown jewels, the Yosemite Valley.

And farther south, the land was being literally torn apart.

About 5 or 6 million years ago, plate movement changed, causing the San Andreas Fault to migrate from somewhere offshore to its present position inland. This had a profound impact on this corner of the continent. In addition to generating forces that would create mountain ranges north of Los Angeles, the fault transferred what is now the Southern California coast from the North American Plate to the Pacific Plate.

Scientists believe that development also caused Baja California to pull away from Mexico, forming the Gulf of California in a rift zone. One dramatic result of that was the creation of the Salton Sea as that region pulled apart, a process that is still going on today. It probably also had some effect on the formation of the Mojave Desert to the north, and it reshaped the landscape from Los Angeles to the tip of Baja California.

The University of Arizona’s Dickinson believes development of the spreading center that created the gulf also might have had another impact. The stretching of the land that created the mountains of Arizona has mysteriously ended. Perhaps, he said, the spreading along the Salton Trough has relieved the strain that had been pulling Arizona apart.

While Arizona may be geologically quiescent, that is not the case for much of the Southwest’s interior. The Colorado Plateau is an elevated region that covers much of Utah, Arizona, Colorado and New Mexico, and it is very active because it is so high.

Peter Coney of the University of Arizona thinks the crust there grew thicker during the years when North America was collecting the land that became California. The process accelerated in the last few million years, pushing a modest chain of mountains up more than a mile, creating the grand relief that is today’s Rocky Mountains.

Coney contends that the land to the west stretched and broke into blocks that began forming the Basin and Range Province about 17 million years ago. That entire region sank, altering the river drainage of the plateau. Since it had been relatively flat prior to the formation of the Basin and Range Province, rivers had probably meandered slowly across the region, causing little erosion.

“The Colorado River was presumably just running around on the surface, and as the drainage systems dropped in the Great Basin and in the Gulf of California, the river just cut right down” into the land below it, he said.

The river sliced through the sandstone and limestone that had been laid down during the millions of years that the region was under the sea, carving deep canyons as it went. The walls of the canyons caved into the river, leaving extraordinary views of layered rock of many colors, and thus the Grand Canyon was born.

“The Grand Canyon is probably no more than a few million years old,” he said.

Today, there may be no other single natural feature in the world that is as widely known as the Grand Canyon, but it is merely the most spectacular manifestation of a process that has created many vistas. Rivers and wind have carved through the plateau in southern Utah, slicing off chunks of rock and leaving high, flat mesas that rise over the surrounding countryside in stark relief.

It seems ironic that an arid land should have been shaped so much by water.

“Some of the rivers were here before the mountains,” said Bart Kowallis of Brigham Young University. “The rivers were there as the mountains came up, and they just cut through.”

Glaciers also did their part to reshape the land. Except for an occasional earthquake, it is a peaceful land today, but Kowallis is not sure it will remain that way.

Between 15 and 37 million years ago, the region had enormous volcanic eruptions. “And I mean enormous,” Kowallis said.

These explosive eruptions were three or four thousand times greater than Mt. St. Helens, he added, and they released so much ash into the atmosphere that day turned to night around the Earth.

There is ample evidence that such an event could occur again, he added.

“We still have a very active area like that today up in Yellowstone,” he said. The last major eruption there was about 1,000 times greater than Mt. St. Helens. So we are still not out of the woods.”

All the mysteries are not solved. Nor have all the mountains been built, or all the canyons cut, or even all the volcanoes shut down. The Pacific Southwest will continue to change as nature stretches, compresses and twists this tormented land.

The Puzzling Plateau

Scientists have long been mystified by the Colorado Plateau, an elevated region that covers parts of Utah, Colorado, Arizona and New Mexico. During the past 15 million to 20 million years, a series of mountains and valleys known as the Basin and Range Province was formed, but for reasons that remain unclear, the plateau to the east was left intact.

Outcome: As the Basin and Range Province, which covers much of Nevada, Arizona and Utah, formed, it had several effects:

* The plateau tipped slightly.

* The drainage of major rivers changed.

* One of the rivers, the Colorado, began cutting more deeply into the ground, ultimately carving out the Grand Canyon in a million years.