Geologists Try to Unlock Secret of the Mountains

After Garrett Hazelton and Jonathan Nourse show you their favorite rocks on the mountainsides near Mt. Baldy and in the foothills below, it forever changes how you see the landscape of the eastern San Gabriel Valley.

Geologists, like the very rich, are different from you and me. As Hazelton and Nourse cruise along mountain roads, you understand this immediately. Their minds, their words delightfully glide across eons--millions and billions of years--to explain their latest hypothesis, their answer to a tiny, specific geological riddle.

Detectives of the Earth, these two geologists have methodically combed the hills, valleys and ridge tops in a quest to solve a mystery many people would find meaningless:

What is the connection between volcanic rock found both high in the San Gabriel Mountains and miles below in the foothills around Glendora, La Verne, Pomona and San Dimas? And what might it tell us about why the Earth has shifted, shook, rumbled and belched over these many years in Southern California?

It could be years before Hazelton, a graduate geology student at San Diego State, and Nourse, a Cal Poly Pomona lecturer, know for sure.

Still, they search for minute pieces to fit into an even grander mosaic: how the Los Angeles Basin and the San Gabriel Mountains, considered among North America’s most complex ranges, melded together and took shape over the past 1.7 billion years.

“The great thing about the San Gabriel Mountains is there is a lot of geologic history locked up in here. People have spent lifetimes studying them and the pieces of the history,” Hazelton said as he and Nourse, both dressed in hiking boots, scrambled like mountain goats over the scree of eroding rock near Mt. Baldy, north of Claremont.

Maybe, said Hazelton, 27, “when I’m an old man somebody will come along and say what all the puzzle pieces mean. That’s the nature of geology.”

Geological things, he said, take a long time--whether it is research or the evolution of a planet.

Right now, Nourse, 32, and Hazelton, who studied under Nourse at Cal Poly Pomona, feel sure they are on the right track. Indeed, they got support for their work two weeks ago in Reno, where they presented a paper on the volcano premise at a meeting of the Geological Society of America.

The two geologists told their colleagues that volcanic rocks, known as the Glendora Volcanics and easily seen from the roadside in Glendora, La Verne, Pomona and San Dimas, match those far to the north, near Mt. Baldy and in San Gabriel Canyon.

The scenario Nourse and Garrett have developed goes like this:

About 30 million years ago, an ocean ridge far out in what today is the Pacific collided with the continent’s edge, generating incredible heat.

The heat was so powerful its effects were seen over the next 15 million years, partly in the form of volcanoes that formed during the collision of the two plates. The Earth’s outer crust is fragmented into about one dozen major segments, or plates. For most of the Earth’s history its plates have been shifting.

The coastal volcanoes, some submerged and some above the ocean’s surface, erupted in the area of what is now the Salton Sea near the Mexican border. Beneath those volcanoes was the very ancient rock that would one day become the San Gabriel Mountains.

Over millions of years, movement of the Earth’s plates caused the underlying rock to rise and to tilt. By 8 million to 9 million years ago, repeated earthquakes and fault activity had caused the volcanoes to gradually slide downslope six to 12 miles.

“It probably was not a catastrophic slide where the volcanoes instantly slid off the mountains,” Nourse said. “They basically moved over time. It was many, many earthquakes . . over a period of 100,000 years to several million years.”

As the volcanoes sunk into the ocean, they were covered by a thick marine sediment.

About 5 million to 6 million years ago as the San Andreas Fault emerged, these volcanoes and the material that would eventually form the San Gabriel Mountains began to head northwestward from the Salton Sea, pushed along by the movement of the earth’s plates.

The rock eventually formed the northern edge of the Los Angeles Basin.

To show off the raw building blocks of their assumptions, Nourse recently charted a four-wheel-drive van along Mission Boulevard where it cuts through Elephant Hill in Pomona.

The vehicle passed the Humane Society, the police station firing range, the vacant chunk of land where developers want to put 38 luxury condominiums with a view, and the land where developers already have put hundreds of apartments and condos with red tile roofs.

The winding road, amid foothills sprouting buff-colored grass and dotted by outcroppings, seemed unremarkable to the geologically uninitiated, just another standard-issue piece of Southern California where a hillside had been cut open to make way for progress, including the nearby mini-mall.

But the landscape took on new meaning as Garrett said: “Basically, we’re driving through the center of an old volcano.”

The top of the volcano is long gone, he said, just as with the other volcanic remnants that can be seen in the foothills around Brackett Field in La Verne or Puddingstone Reservoir in San Dimas or in northeastern Glendora.

When Hazelton explains all this to his girlfriend or his parents as they sit in their homes in La Verne, he tells them: “There were volcanoes erupting right where we are now sitting.”

Hazelton said: “That’s the kind of thing people can understand.” The good news, he said, is the volcanoes are extinct; the bad news is the earthquakes aren’t.

As they drove near the county fairgrounds in Pomona, Nourse and Hazelton stopped to look at the rocks revealed by a road cut on Fairplex Drive.

There, seen in the morning sun, the embankment displayed exquisite shades and textures of rock, baked in the Earth’s furnace and then left out to weather over millions of years. Gradations of tan, silver, purple, black and gold shone from the Earth, just as they do in the Elephant Hill rocks.

“The ocean covered up this whole area 15 million years ago,” Hazelton said. “We’re standing about at the shoreline. This is beachfront property.”

Nourse, fleshing out the flashback, said: “There were volcanoes erupting. The ocean went all the way to where Japan is now.”

The rocks, which are suspected to be kissing-volcanic-cousins to the ones on Fairplex Drive and Elephant Hill, can be found near the small cabin where Nourse lives at Mt. Baldy Village.

Had he not lived there, Nourse doubts he would have become so fascinated in the volcanic connection, nor would he have gotten the idea to puzzle over the rocks on high and those below.

Nourse’s thinking also was aided by his six years of graduate studies at Caltech in Pasadena, including work with noted geologist Leon Silver. Nourse’s thesis focused on fault lines, dating from 20 million years ago, in northern Mexico. “That’s how I developed my feel for how things fit together.”

He rubbed his tanned hand on an ebony ribbon of rock, known as andesite, that forms a distinct band cutting across the lighter granite gneiss on the steep mountainside. “I’ve become pretty fond of these rocks.”

The band, Nourse explained as he pointed to example after example slicing across the canyon face, was once a dike--a conduit for magma that fed a volcano. The magma broke up through much older rock.

The dike, which now is more than 4,000 feet above sea level, fed a volcano whose upper remnants now may be found to the south in foothills whose elevations are 800 to 1,000 feet, he theorized.

Nourse and Hazelton have been mapping the dikes both in San Antonio Canyon around Mt. Baldy and San Gabriel Canyon above Azusa and Monrovia.

“Nobody has really looked at the dikes and tried to connect them to the volcanics in the valley,” Hazelton said.

Nourse and Hazelton, however, point out that they are by no means the first to study Southern California’s volcanics.

In 1955, long before today’s theory of plate tectonics, a geologist from Pomona College mapped and named the Glendora Volcanics. Three decades later, a Cal State Northridge geologist studied volcanic rock throughout the region, including the 23-million-year-old Vasquez Rocks and the 15-million-year-old Zuma Rocks.

But Nourse and Hazelton say they are the first to make the connection between the rocks in the San Gabriels with those in the foothills and to propose how it happened.

Still, the geologists must undertake extensive dating of the rocks both in the mountains and valley. They are applying for grants to finance the work to determine the age of the rocks and to study the faults.

Their work is not just an academic exercise. “It’s useful to understand what this area looked like before the San Andreas Fault was here,” Nourse said. “It’s like a window back in time.”

Said Hazelton: “If you took a snapshot 15 million years ago, we hopefully have now produced a picture of what it would have looked like.”

Beyond that, Nourse said Southern California has been tectonically active, not just today, but for millions and millions of years.

“We have history of violent activity,” he said, ticking off cataclysmic eras, starting with volcanoes 200 million years ago and ending with today’s earthquakes aplenty.

“That’s exciting when you compare the geological history of California to other places in the world where nothing much has happened for billions of years. This just gives a little bit more of the piece of the puzzle.”

The Volcanoes of Glendora Two geologists believe they have found the link between volcanic rocks in communities of the eastern San Gabriel Valley with apparently identical rocks high above in the mountains. Here’s how they believe the separation came about:

MID MIOCENE: 1. Fifteen million years ago, volcanoes along the coast, some submerged and some above the ocean’s surface, erupted in the area of what is now the Salton Sea. Beneath those volcanoes was the rock that would one day become the San Gabriel Mountains. LATE MIOCENE 2. Over millions of years, movement of the Earth’s tectonic plates caused the underlying rock to rise. By 8 million to 9 million years ago the volcanoes somehow were sheared off and slid downslope 6 to 12 miles. 3. About 5 to 6 million years ago, these volcanoes and the material that would form the San Gabriel Mountains began to move northwestward from the Salton Sea, pushed along by the movement of tectonic plates. The rock eventually formed the northern edge of the Los Angeles Basin. Today, volcanic rocks seen from the roadside in San Gabriel Valley communities of Azusa, Glendora, La Verne, Pomona and San Dimas apparently match volcanic rocks far to the north near Mt. Baldy Village and in San Gabriel Canyon.