Rocking, Rolling Southwest Is Geologic Time Machine : Science: Dynamism of Earth’s forces in region makes it a natural laboratory for study of shifting geography.

From the majestic peaks of the Sierra Nevada to the blistering deserts of the Southwest to the gentle hills of the California coastline, the awesome forces of nature have twisted and warped this part of the Earth and left it cloaked in robes of splendor.

It took every process known to nature to make what it is today the southwestern corner of North America--this vast land where once there was no land at all, these mountains that rose from the floor of the sea to tower over all else.

It is a region that remains one of the most geologically active in the world, a natural laboratory that attracts geologists by the hundreds. Its every shudder and shake is a reminder that the volatile history of the Southwest continues today. Yet it is also one of Earth’s great mysteries, strewn with clues of past cataclysms that leave scientists intrigued and baffled at the same time.

Great volcanoes once spewed their innards across the landscape, creating ranges of mountains, but the winds and the rains came and washed the peaks away, leaving only some of the rocks to be used again to build other mountains in different ways. As the volcanoes died, new forces went to work, pushing the land together, molding it into new mountains, most of which have also washed away.

Some young mountains produced by that process are still around, such as the San Gabriels and the San Bernardinos that form the backdrop for the Los Angeles Basin, but many more have vanished.

While the land was being squeezed in some areas so tightly that mountains squirted up like toothpaste out of a tube, it was being pulled apart in others, forming a seemingly endless range of basins and mountains that would look to astronauts many years later like hundreds of caterpillars inching their way north from Mexico. Powerful forces also caused great chunks of land to tilt and roll over, forming mountains such as the Sierra Nevada.

Some of the changes were, in the beginning, relatively subtle. The huge region that was to eventually form the Basin and Range Province of mountains and basins covering much of what is now Nevada, Utah and Arizona dropped, tilting the land and allowing a sleepy river to carve deeply into the soil below. The river created a natural wonder that would become known as the Grand Canyon.

The wind and the rain chipped away at the rocks, slowly reducing great mountains to grains of sand, and for millions of years much of the region was covered with deserts greater than the Sahara, blowing in amid new peaks and becoming embedded as layers of rock. Later, the winds and rains would carve into those layers of sandstone and limestone again, creating plateaus that stand out like tabletops over the surrounding valleys.

From time to time, the great seas came, covering much of what is now the Pacific Southwest, leaving marine deposits that would someday be thrust thousands of feet above sea level as new mountains formed.

And finally, almost yesterday in geological time, the entire character of the region would be changed, possibly because of events on the other side of the globe, and a great fault system would be formed 25 million years ago. Known today as the San Andreas, it shaped and molded much of what became California and the Southwest.

All of that is ancient history in human terms, but the story is by no means over. Even today, new mountains are being built while old ones are eroding in a process that is nearly as old as the Earth itself.

Although scientists disagree over the details, much of the history of the Southwest is fairly well understood today because of two developments. The first, and more important, was the emergence of the field of plate tectonics within the past three decades, which gave scientists the framework for understanding the forces that have been at work here.

Virtually all earth scientists believe today that the surface of the planet is shaped primarily by the interaction between the giant tectonic plates that make up the outer crust. As they move around the Earth, these plates grind together in some areas and pull apart in others. And in some regions, oceanic plates, which are younger than the plates that make up the continents, are actually forced under the thicker continental plates.

Each of those interactions produces different results. A subducting, or diving, oceanic plate, for example, creates volcanoes like those that line the shores of the Pacific Northwest and Northern California.

The West Coast of North America is known as a “plate margin,” because the plate that lies under most of the Pacific Ocean is colliding with the North American plate.

Unlike the situation in the Pacific Northwest, along the coast of Southern California the Pacific Plate grinds past the North American Plate in a process that has created the San Andreas Fault.

That has turned the Pacific Southwest, and especially California, into a natural laboratory that is without parallel. And while the theory of plate tectonics was not born here, this is where it came of age.

“Plate tectonics was much better received in California than anyplace else, and I’ve always thought that was because that’s where the action is,” said Peter Coney of the University of Arizona. “You could see it happening.”

The visible evidence included earthquakes, an inevitable byproduct of mountain-building processes, although most people mistakenly think of it as the other way around. Earthquakes do not cause the mountains to rise. The growth of the mountains produces the earthquakes.

“Earthquakes don’t cause growth,” said Ben Page of Stanford University, one of the grand old men of geology. “Even geophysicists make that error. They say the Loma Prieta earthquake caused an uplift. Well, that’s backward. (The uplift) was facilitated by a fault slip which produced an earthquake. Whatever causes deformation of the Earth’s crust causes earthquakes as kind of a symptom.”

Virtually all experts agree today that the driving force is the collision of tectonic plates, a process so violent that huge chunks of rock can literally be turned upside down, building mountains with the oldest rocks on top of younger rocks.

Earthquakes result from that process. California’s earthquakes are vivid evidence that the mountain-building process is very much alive.

David G. Howell, an expert on those forces, who is with the U.S. Geological Survey in Menlo Park, believes there are only four ways to build mountains.

“We see all four processes in the Southwest,” Howell said. “And they keep repeating themselves.”

The forces are compressional, tensional, transcurrent and thermal.

Southern California’s transverse range, which includes the San Bernardino and San Gabriel mountains, was created by compressional forces. That occurs when two large chunks of real estate are pushed together. The process forced one chunk to ride up over the other, twisting layers of rock as it went and producing earthquakes.

Transcurrent is a similar process, and it produced most of the coastal mountains of California. But in this process, instead of one slab riding up over the other, the land simply buckled. You can achieve the same result by pushing the opposite ends of a sheet of paper together until the center buckles upward.

Tensional is the opposite of both of those in that it occurs when the land is being pulled apart. The process is similar to pulling both ends of an art gum eraser apart until it splits into jagged chunks. The lengthening of the land causes huge blocks of Earth to tilt over on their sides, producing sharp peaks like those of the Sierra Nevada and the Rockies.

Oddly enough, since the crust is actually being stretched, the peaks resulting from tensional forces may be of lower elevation than the land from which they formed. But the overall relief will be mountainous, and the change in topography will cause erosional patterns that will wash away surrounding areas, leaving the peaks much higher than adjacent territory.

Perhaps the easiest mountain-building process to visualize is thermal, which causes volcanic eruptions that leave the familiar cinder cones that are so prevalent in the Pacific Northwest. Most volcanoes are formed when an oceanic plate subducts under a continental plate. As the subducting plate rubs under the overriding plate, it heats the underside, causing hot rock to flow toward the surface, sometimes with violent results.

Other volcanoes, such as those in Hawaii, are caused by hot spots deep within the Earth’s mantle that rise toward the surface, melting through the crust and flooding the landscape with molten rock. Much of the Pacific Southwest once had such volcanoes.

So if there are only four ways to build mountains, it would seem that it should be relatively easy to piece together the history of the Pacific Southwest. But nothing could be further from the truth because the region has changed dramatically over the past few tens of millions of years, superimposing one process on top of another and eroding away the evidence of what happened first.

The Southwest “is a very complicated place right now,” said Keith Howard of the U.S. Geological Survey. “And it was probably equally complicated in the past. That’s why we’re having so much trouble.”

The processes may be fairly well understood, geologists say, but why those processes may have occurred is unclear. Why, for example, did the region that now runs from Arizona and through Colorado, which had been under compression for millions of years, suddenly begin lengthening, or stretching out, more than 20 million years ago in a process that created the mountains of Arizona and Nevada?

“You can’t fill a phone booth with people who agree on why it happened,” said Arizona’s Coney. Yet clearly it did happen, despite the fact that the West Coast was being compressed at the same time.

The clues to all of that are found in the most mundane of objects: rocks. Rocks are to a geologist what X-rays are to a surgeon. They reveal the dynamic processes that created the land we see today and they offer the geologist his best window on the past.

Little that was here a hundred million years ago is here today. Only some of the rocks remain, some nearly half the age of the Earth itself, and they are the precious clues that tell us today of what happened so very long ago. Geologists today comb through the hills and the valleys of the Southwest in search of those rocks, nature’s own time capsules that tell so much about what happened in the distant past.

To most, they would appear to be undistinguished rocks, but to a trained eye they are priceless treasures with stories to tell, real and imagined.

When he was a young man, the Geological Survey’s Howell came to California to see what the rocks could tell him. Years ago, while having lunch as he sat at the foot of a high bank near the Salton Sea, he realized that the bank was actually a fault scarp created when the mighty San Andreas suddenly lurched in what must have been a great earthquake.

He thought for a moment about the chances that the great fault would move again, even as he sat there eating his lunch, and he realized he could be buried beneath tons of rubble.

Now one of the U.S. Geological Survey’s leading experts on mountain formation, Howell recalled that moment in his Menlo Park office. He said the scientist leading the expedition had asked if he realized he could become a victim of the San Andreas.

“I told him I couldn’t think of a better way to die,” Howell said. What he could not have known then was that the embankment towering over his head contained the secrets that would someday help scientists understand how mountain ranges hundreds of miles away were formed.

The San Andreas Fault, most experts believe today, had a profound role in that. But in those days, when the field of plate tectonics was still under hot debate, no one really understood that. All they had to work with was their rocks.

Like scores of other scientists, Howell has searched the world for rocks that tell of events long past.

“Your first concern is, ‘What are the rocks?”’ said Howard. “As you learn about them, it becomes more clear how the mountains formed. You start with the rocks and build up a story.”

By studying the Earth today, geologists know how different types of rocks form. And assuming that the same rules of physics have always applied, when they find a rock of a certain type they can deduce the history of the region in which it formed.

Sometimes, the rock tells far more than simply how it was formed. Many rocks contain fossils, sometimes so small they show up only microscopically, and each fossil has a story to tell.

“One fossil is worth 10,000 geophysical measurements because you know the age,” Howell said. The fossil record is complete enough that scientists are able to use the fossil to determine the environment in which the creature lived. A marine fossil, for example, could only form in a water-rich environment, and many had to come from the sea.

“So you can infer all kinds of things,” Howell said. “That fossil was not a freak of nature. You know within a million years or so how old the rock is, if it was in a tropical setting, or in deep water. It’s a very powerful tool.”

Yet the best tools of all lie in the mountains themselves, those twisted, tormented pillars of rock formed in the Earth’s most powerful crucible.

Making Mountains

There are essentially four ways to build mountains, and all of them have played a part in creating the topography of California.

COMPRESSION: The huge chunk of Earth’s crust that lies under the Pacific Ocean, called the Pacific Plate, grinds against the North American continent, and that creates enormous strains in the rock formations that make up California. The compressional forces push large slabs of crust up sloping faults and on top of adjacent areas, causing the mountains to grow taller.

The local result: The San Gabriel Mountains north of Los Angeles, part of the Transverse Range.

TRANSCURRENT: Compressional force causes rocks to move up along nearly vertical faults, like toothpaste squeezed out of a tube. Scientists believe this process occurs along the coast because of a slight change in the direction of the movement of the Pacific Plate 3.5 million years ago, which caused the Pacific Plate to grind harder against the North American Plate.

The local result: The Coastal Range from Ventura to Northern California.

TENSIONAL: Mountains can be formed when the land is stretched until it breaks into pieces. This process, also known as EXTENSION, occurs when the land is pulled apart and huge blocks of rock tilt.

The local result: The Sierra Nevada in Northern California, and the Peninsular Range, which runs from Santa Ana to the tip of Baja California.

THERMAL: Many mountains are of volcanic origin, formed when the Pacific Plate was pushed under the North American continent in a process called subduction. Friction between the two plates caused rocks on the bottom of the overriding plate to melt and flow to the surface in a volcanic eruption.

The local result: California’s Mt. Lassen and Mt. Shasta.

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Creating California

California has it all, from barren deserts to spectacular mountains and inland lakes. They were created by myriad geological processes, but the major player in this continuing drama has been the San Andreas Fault.

* WHEN: The great fault was created about 25 million years ago.

* HOW: The tectonic plate that lies under the Pacific Ocean began moving northwest along the edge of the North American continent. Then, about 5 million or 6 million years ago, the San Andreas migrated from offshore to its current location, extending from the Salton Sea to Point Reyes.

* WHAT: The birth of the San Andreas dramatically changed the landscape of the Southwest, causing mountains to form around the Los Angeles Basin and influencing geological events hundreds of miles inland.

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ROCK IN MOTION

The western part of Southern California is moving slowly to the northwest. The San Andreas Fault system bends around the base of the Sierra Nevada before heading northwest again, raising mountain ranges along its path. The Salton Sea was created in a trough on the fault.

Source: Scientific American

The View From Above

A mosaic of four images transmitted by the Landsat satellite provides a view of the geographical vista that is Southern California. The triangular shape in the lower left of the photo highlights the junction of the San Andreas and Garlock faults. Over millions of years, activity along the San Andreas shaped and molded much of what would become California and the Southwest. The area outlined by the two faults is the Mojave Desert. Due to computer color enhancement, dry lakes, areas with snow and some other land areas appear blue.

Source: Robert E. Crippen, Jet Propulsion Laboratory

Compiled by Times researcher Nona Yates

Strike-Slip Fault

The San Andreas is a well-known “strike-slip” fault. When a quake occurs, one side of this kind of fault slips horizontally in one direction, while the other side moves in the opposite direction. Arrows demonstrate what happens.