SOU student find Rogue Valley landmark to be relatively young.

Ever since settlers began arriving in the Rogue Valley in the mid-1800s, Pilot Rock near the Siskiyou Summit has been a guiding landmark.

Just as many passers-by on Interstate 5 do today, those early travelers no doubt wondered about the origin and age of the stone behemoth.

But it wasn't until graduating Southern Oregon University geology students Manuel Silva and Lia Knauss began studying the volcanic plug that its origins were finally unearthed.

Turns out the rock is a mere 25.6 million years old, just a kid in geological time. Seems it's also a geological intruder, a magma flow shooting up through a weak spot in the Earth's crust to form a plug that wasn't revealed until erosion wore away softer rock.

"Pilot Rock has always been interesting and unique to me," said the Medford-born Knauss, 36, of Central Point.

"Everything I had heard from living here, and reading about the rock, was that it was an ancient volcanic neck.

"We've found out that wasn't the case," she added, noting it didn't erupt like a volcano as others had theorized. "The lava didn't reach the surface. We called it a plug."

Although Silva, 32, of Ashland, was born in Mexico and reared in Arizona, he was equally intrigued about how the local rock was formed.

"We wanted to see what we could find out about this local landmark," he said. "Everybody had an idea of what it was. We wanted to find out more.

"The more you learn, the more you want to learn," he added of geology in general. "We've barely scratched the surface."

Silva will be the first in his family to graduate from college when he receives bachelor degrees in both geology and environmental studies upon completing an SOU summer field course in July.

Knauss graduates Saturday with a master's degree in teaching with a science endorsement. She already has a bachelor's degree in geology from the university.

The students' work adds another valuable piece of information on how the local mountains were formed, observed Jad D'Allura, geology professor in SOU's environmental studies department who served as a mentor for Silva and Knauss on the project.

"Erosion is the whole reason we see it today," he said. "Pilot Rock is very young."

In comparison, Mount Ashland is about 160 million years old, D'Allura said, adding it is not volcanic. Another local well-known volcanic plug is Roxy Ann, a butte poking up east of Medford that is about 30.8 million years old, he added.

Jutting up to 5,910 feet above sea level, Pilot Rock was originally called Emmons Peak after Lt. George F. Emmons, a U.S. Navy officer who led an exploratory scientific mission through the region in 1841. The rock is now part of the Soda Mountain Wilderness Area in the Cascade-Siskiyou National Monument.

The students sent samples taken from Pilot Rock to Oregon State University to have its age determined through a process known as argon-argon dating. They also studied the rock's mineral composition.

Their study concluded that Pilot Rock is slightly younger than the rocks it intruded. The rock also has vertical columns knows as "columnar joints" like the much larger Devil's Tower in Wyoming.

However, Pilot Rock is an unusual andesite (lava) rock in that it contains black hornblende crystals, indicating there was enough water in the mix to create crystals, she said.

"It has the biggest hornblende crystals I've ever seen," she said, noting some are more than an inch in diameter.

Upon graduating, Knauss hopes to teach science locally at the middle or high school level.

When he graduates, Silva will be among the last 10 SOU students receiving degrees in geology. The geology department is being absorbed into the environmental studies department.

A McNair Scholar, he plans to take a job with the U.S. Forest Service later this summer, then attend Arizona State University where he has been accepted in a graduate study program. His goal is to earn a doctorate in geology.

For him, geology began as a hobby.

"One of my uncles got me started," he recalled of his childhood in Arizona. "Every time we would go out hiking, he'd pick up a rock and tell me a little story about it, that it may have gold or something in it.

"I now realize he had no idea what he was talking about," he added with a chuckle.

For D'Allura, their project reflects the geologic riches that await scientific study in the region.

"There is no better place in the whole state to study geology," he said. "Take a rock and throw it in any direction — you'll find something interesting to discover."

Reach reporter Paul Fattig at 776-4496 or e-mail him at