Dramatic Tale of Roving Continents Pieced Together : Geology: Today’s Southwest evolved from volcanic islands in slow but violent process, scientists believe.

There was a time when all there was of the Pacific Southwest were a few volcanic peaks sticking up out of the ocean just a few miles from this modern desert city.

The Earth was already half its current age of about 4.6 billion years when those island mountains first appeared, but they were the beginning of the Pacific Southwest.

The giant chunk of the Earth’s crust, called a tectonic plate, from which they sprang gradually migrated north to its current position, where the mountains eroded and filled in the basins between the peaks.

There was no Nevada or California. That would come much later, when nature would fabricate the West Coast from materials imported from far away, and then twist and bend those rocks into the spectacular Sierra Nevada and, later, the coastal hills of California.

Aided by the most valuable tool in geological history--the theory of plate tectonics--scientists are now piecing together the story of the complex forces and conflicting events that shaped this corner of the continent into what it is today. It took every weapon in nature’s arsenal to create this grand design.

As always in geology, the story is revealed by the rocks. Even the absence of rocks can help fill in the blanks.

“There are no rocks here at all that represent the first half of the history of the Earth,” said William Dickinson, geology professor emeritus at the University of Arizona. This in itself reveals that the entire region is relatively young. “There are parts of the world in which very, very old rocks, as old as 3.5 and 4 billion years old, are found----places like Greenland, Australia and the Northwest Territory of Canada. But as far as we know, we don’t have any rocks of any kind older than about 2 billion years.”

The oldest rocks locally suggest that the Southwest began as a series of volcanic islands, called island arcs, that began to grow into a small continent about 1.8 billion years ago.

“By around 1.5 billion years ago, this had become all solid continental crust,” he said as he motioned out the window of his home in Tucson. About 1.4 billion years ago, the Earth began belching up mantle rocks, called magma, in a process called granite intrusion.

“Magmas came up in tremendous volume and injected themselves into these old island arc remnants and made large areas of granite,” forming a huge continent, Dickinson said. No one is quite sure why. One leading theory is that the sea floor to the south began subducting under the new continent, creating widespread volcanism similar to what is now happening in the Pacific Northwest.

A Supercontinent Forms

Keith Howard of the U.S. Geological Survey believes the volcanic activity may have been caused by “extension,” the stretching and thinning of the Earth’s crust that allows hot magma to work its way to the surface.

“But we haven’t seen other evidence of that,” he said during an interview in his Menlo Park office. “It remains a mystery.”

Then, curiously, “the construction process stopped,” Dickinson said.

“Nothing much (in the way of construction) happened for maybe a hundred million years,” Dickinson said. He said he believes the most likely explanation for the sudden end to the volcanism was that another land mass--what is probably now Mexico--crashed into the new continent and was too buoyant to sink, thus shutting off the fuel for the volcanoes.

Meanwhile, other massive continental blocks were heading this way, and eventually butted up against what is now North America. The additional land created a supercontinent, called Pangea, and most scientists believe all of the continents were clustered together there for about half a billion years.

“The North American continent became the interior of a much larger continent,” said Howell of the Geological Survey. That buffeted what became the Pacific Southwest from the tectonic activity along the shoreline, in effect shutting down geological activity in this region.

During that long period, the peaks eroded, leaving a continent that was as “flat as Kansas,” Dickinson said.

“The next big event we see was at about 1.1 billion years ago,” said the Geological Survey’s Howard. “That was a time when the North American continent was being rifted,” or torn apart. “It didn’t quite split all the way apart, it just tried to,” he said, resulting in a long rift down through the center of what is now the United States. The rifting created a great basin in the north which eventually became the Great Lakes.

Finally, between 600 million and 800 million years ago, something quite astonishing happened. A huge piece of Pangea was ripped away and sent drifting on its own way.

What became of that great chunk of land?

“Some people think it was Antarctica and Australia, and I think that’s a reasonable hypothesis,” said geologist Peter Coney of the University of Arizona. The evidence of that consists primarily of a rock outcropping found in those two Southern Hemisphere continents that is nearly identical to a long rock formation, called the Grenville Front, which extends across much of North America. Other experts, however, believe at least part of that giant chunk of real estate drifted off and became Siberia.

No matter what eventually became of that land, the rifting of such a huge area and the breakup of Pangea had a profound effect on what was to become the Southwest. It created the Pacific Ocean and a “continental margin” along the West Coast, exposing the region to the dynamic forces caused when an oceanic plate pushes against a continental plate. This set the stage for a period of profound change beginning about 250 million years ago. At times the ocean rose, and then it fell, flooding what is now Arizona for millions of years, and then drying it out. It repeated the process over and over as North America drifted into tropical regions, and then back to its present location.

That conclusion is based on many fossils found in this region that could only have survived in a tropical setting. Other fossils had to survive under water, thus showing that the land was flooded. That is why to this day seashells can be found atop mountains throughout the Southwest.

Each change brought different types of sedimentation, with different fossils, that would someday be carved into spectacular red-rock canyons.

That would come much later, but the forces of nature stood poised for a dramatic restructuring of the landscape. New mountains would rise where there had been none. A dynamic new landscape was being formed.

“The expanse here (in the Southwest) is about as dramatic as it gets,” said Howell.

It is equally as complex, because nature has continually switched signals and redirected the geological processes, leaving a very confusing story.

“What is complicated is the overlay of one process on top of another,” Howell said. “That makes it very difficult to read the history.”

About 200 million years ago, for some unknown reason, the Pacific Plate began subducting under the North American continent. No one is sure why.

“Since then, all kinds of stuff has been going on at a tremendous tempo,” Dickinson said.

“It starts to get dramatic,” said Howard. “There had to be both slipping and subduction” along the new plate boundary.

He said the evidence for that, predictably, is found in rocks that formed near the Grand Canyon beneath a shallow sea. “You find these rocks extending clear over to the eastern Mojave Desert, and then suddenly they are gone. The next thing you see are deep water sediments. So (a great piece of land) dropped, and probably there is a piece missing. There’s been a suggestion that some piece was carried away, probably by faulting, but we are not even sure whether it went north or south.”

What is clear from the rocks, however, is that a period of intense volcanic activity began, creating mountains such as today’s Cascades in the Pacific Northwest.

“They came all the way down into Mexico,” Howard said. “It was a dynamic era. These were live volcanoes, and lots of them. It was big stuff.”

The volcanism indicates that the Pacific Plate had begun subducting under the southwestern edge of the North American Plate.

When the oceanic plate was pushed under the continental plate, it melted the underside of the overriding plate, sending up plumes of molten rock. The mantle continued to melt its way toward the surface, creating the volcanism.

The rising hot rock broke through to the surface, flooding the surrounding countryside. Thus this corner of the continent was growing thicker and higher as great seas of granite that would one day form spectacular mountains continued to thicken.

And then, about 80 million years ago, nature changed the rules again.

For reasons that are not clearly understood, the North American continent began gliding over the Pacific plate at an accelerated rate, causing the land to the east to be stretched as if it were a rubber band.

The immensely thick continent was like a pile of putty, coaxed by gravity to flatten itself out and suddenly relieved of the pressure that had caused it to grow higher and higher. That gave birth to the Rocky Mountains.

“Suddenly, all hell had broken loose,” said the Geological Survey’s Howard. “Everything had just started splitting apart.”

The Sierra Nevada probably started to form about then, but that process would not be completed for millions of years.

Then it all came to an end, about 40 million years ago, when nature changed the rules again. The forces that were molding the landscape of the Pacific Southwest were suddenly altered, quite possibly because of events on the other side of the globe.

The Pacific Plate, which had been grinding underneath the North American Plate, changed its course and began moving in a more northerly direction. That brought an end to the subduction along the southwestern coastline, and scientists have debated fiercely among themselves over why that happened.

The best evidence for that sudden change lies buried beneath the Pacific, thousands of miles to the west. The present island of Hawaii sits over what geologists call a “hot spot,” a mysterious region deep within the Earth’s molten mantle that causes magma to rise toward the surface. The Hawaiian Islands were created when the magma melted through the crust and sent rivers of basaltic lava flowing across the surface.

The hot spot remains in the same location as the Pacific Plate moves over it. Thus, the youngest island--Hawaii--is directly over the spot. Islands to the west are increasingly older because they were formed many years ago as that part of the plate passed over the hot spot.

Only a few of the islands stick above the water, but scientists have charted a chain of subsea mountains extending northwest of the present islands. But these seamounts do not lie in a straight line, as one would expect if the Pacific Plate had always moved in the same direction. Hundreds of miles to the west of the Hawaiian Islands, the chain of seamounts suddenly turns north.

Scientists have dated that change in direction at about 43 million years ago.

What could have caused such a major reorientation of plate motion? Only recently have scientists come up with an explanation.

Evidence suggests that the subcontinent of India was thousands of miles south of its present location about 80 million years ago. The tectonic plate on which India sits moved north, and scientists believe that India crashed into Asia about 43 million years ago.

Since a continent cannot subduct under another continent, the two huge land masses ground together so violently that they produced the most awesome chain of mountains in the world, the Himalayas. The collision slowed the northward migration of India, causing the Pacific Plate to change its direction.

Rise of the San Andreas Fault

Meanwhile, dramatic changes were also taking place off the southwestern coast of North America. Tanya Atwater, a geologist at UC Santa Barbara, believes the change in direction of the Pacific Plate’s migration had a profound impact here.

Her research indicates that several smaller tectonic plates along the West Coast were being destroyed as they ground against the larger continental plate. About 30 million years ago, a relatively short time after the collision of India and Asia, a small plate off the coast of what is now Mendocino “just shattered,” she said. “It just got too small and broke into pieces.”

A similar process was taking place to the south, where the small Rivera Plate was being ripped apart.

The destruction of those two smaller plates, Atwater believes, allowed the Pacific Plate to begin slipping north alongside the continental plate, ending subduction here and setting up a new set of rules.

That change in forces, she believes, created the mighty San Andreas Fault, which would rule this corner of the globe, and even influence events hundreds of miles to the east, for decades. It probably was west of its present location, but it was there, establishing its own priorities for the Pacific Southwest.

The two tectonic plates slipped past each other along the great fault, thus easing the pressure on inland areas and allowing the land to extend toward the coast.

The land pulled apart very slowly, at about the pace of the growth of human fingernails, and as it stretched, something had to give. Great fractures began to form in the crust, dipping at low angles relative to the surface, cutting deeply through the granite and sandstone like cracks in an eraser that is being pulled from both ends.

Since the continental slab was being stretched toward the west, these long faults formed in a north-south direction as the land was literally pulled part. They were sort of like extremely wide on-ramps leading up to an elevated freeway, but in this case the faults provided the mechanism for the land rather than commuters to move.

Gradually, great chunks of the continent began to slip down those on-ramps, and they tilted on their sides as they slid down along the fractures. Massive granite peaks were thrust toward the heavens, standing out in stark relief as adjacent chunks slid down other faults, leaving ranges of mountains that extended for hundreds of miles in a north-south direction.

That allowed the Sierra Nevada to tilt to the west, creating one of the most spectacular mountain ranges in the world. It also created the Basin and Range Province that covers much of Arizona and Nevada.

But it was not a tidy arrangement along the continental margin. The crust moves in fits and starts, generating earthquakes that can level homes, and the plates do not glide past each other in a consistent way.

Next: California

The World Then and Now

Many millions of years ago, all the Earth’s continents are believed to have been grouped together in one supercontinent called Pangea. Here’s what happened:

* About 200 million years ago, parts of Pangea drifted off as the Earth’s crust broke into giant chunks called tectonic plates.

* Some of those areas formed new continents, creating the world we see today.

* The breakup of Pangea also exposed the western coast of North America to powerful forces as the tectonic plate that lies under the Pacific Ocean, called the Pacific Plate, was forced under the continental plate.

*

The Southwest is Born

The Pacific Southwest was created through most of the geological processes known to science.

A. Volcanic Islands: The oldest rocks were part of a series of volcanic islands, called island arcs, which formed on the huge slab of crust that lies under the Pacific Ocean.

B. Plate action: The crust under the ocean, called the Pacific Plate, was pushed under the North American continent by subduction, which pulled the Pacific Plate toward the North American Plate. As the oceanic plate moved under the continent, it brought the islands to the shore and caused North America to grow to the west.

C. Thrust faults: The conflicting forces caused by the interaction between the two giant plates also caused the land to fold like a piece of clay being squeezed from both sides. In some areas, massive chunks of rock were pushed up over faults that are inclined slightly from the surface, called thrust faults, thus creating mountains.

D. Volcanoes: Friction between the subducting oceanic plate and the overriding continental plate also caused rocks to melt and flow to the surface, creating volcanoes. The new volcanoes flooded the basins between the ancient island arcs with granite, forming the rocks that would later become mountains.

E. Extension: About 25 million years ago, the western end of North America began to pull apart in a process called extension. That caused huge blocks of crust to tilt onto their sides, creating mountains.