OAKLEY • On Oct. 21, College of Southern Idaho professor Shawn Willsey addressed a crowd of students at City of Rocks. This is one of the most geologically interesting areas in the United States, Willsey said.

“This may be the premiere attraction in all of southern Idaho,” he said, pointing to the soaring granite formations in the distance. “But I might be biased.”

Snake River Canyon, Craters of the Moon, Balanced Rock — they’re all landmarks that help set south-central Idaho apart. But what’s the geology behind them? The Times-News talked with Willsey to get a brief geological overview of five of the area’s most famous features.

Having a beginner’s understanding of geology has helped Willsey’s students appreciate Magic Valley’s features. Scott Bolen of Filer can’t help but note the rocks as he travels through the area.

“It drives my wife crazy,” Bolen said as he took a break from examining crystals in the rocks.

Classmate Jake Semons  agreed. “I definitely notice it more,” he said. “My wife tells me to shut up sometimes.”

You don’t need to take a course on geology to appreciate the basics. Here is a quick take on what makes Magic Valley’s rocks so special:


City of Rocks

What you’re looking at: Some of the oldest granite in North America. To see other rocks this old in the west, you’ll have to go to the bottom of the Grand Canyon, Willsey said.

How old is it? The oldest rocks are 2.5 billion years old, Willsey said. Other spires come from a separate magma chamber that formed about 27 million years ago.

How did it form? There are multiple formations at City of Rocks. One of the most famous, Twin Sisters, is a perfect example of the unique geology of the area — and the massive timespan over which it formed.

At first glance, the two rounded rocks that protrude from the ground look similar. But take a closer look: The sister rock to the south is more ragged and fractured, and is darker in color than the north rock. 

That’s because the south sister is 2.5 billion years old, while the younger one is 28 million years old — nearly 100 times younger than her “twin.” The theory behind the formation: The second rock protruded from that 27 million-year-old magma chamber, pushing the older sister up as it rose from the ground.


Balanced Rock

What you’re looking at: When most people think of Balanced Rock, they think of the area’s namesake: The Africa-shaped formation is certainly unique, but the columns in the surrounding area are neat in their own right.

The formations are made from rhyolite, a silica-rich volcanic rock formed from slow-moving, viscous lava. 

How old is it? About eight million years old. 

How did it form? The columns formed as a result of cooling. “Basically, when hot volcanic rock cools and contracts, it develops these vertical cracks in it,” Willsey said. Rhyolite cracks in a 120 angle, giving them the angular appearance.

As for Balanced Rock, it’s not a product of wind erosion like most people assume, Willsey said. 

“Wind is pretty wimpy,” he said. In order to effectively erode rock, it has to carry sediment. Instead, the biggest factor was frost getting into cracks and fracturing off pieces of rock.


Malad Gorge

What you’re looking at: A deep, narrow passage carved into basalt rock, with the Malad River at the bottom.

How old is it?: That’s a point of contention, but one theory is that floods cut into the gorge 15,000 years ago, Willsey said. The source of the flood’s water is in question, but it’s clear a swollen river cut the canyon at one point.

How did it form? The floods that most likely formed the gorge weren’t the same as the Bonneville Flood from Bonneville Lake, Willsey said. Instead, the rush of water came from glaciers that let off water.

The Malad River was much stronger back then — strong enough to rip large boulders out of the canyon walls and carry them downstream.

“There’s no way the river in modern times could move something that big,” Willsey said.

Still, the river’s sediment continues to erode the canyon. The waterfall near the bridge will eventually retreat and be under the freeway, Willsey said.


Snake River Canyon

What you’re looking at: Two types of volcanic rock make up the Snake River Canyon near Twin Falls: Rhyolite from explosive volcanic eruptions, dating back to when Magic Valley was located over the Yellowstone volcano, and basalt from slower-moving lava.

Mind you, this only applies to the canyon near Centennial Park and Shoshone Falls.

“It’s very different if you go upstream or downstream,” Willsey said.

How old is it? The rhyolite is eight to 10 million years old, while the basalt is less than a million years old.

How did it form? The canyon existed before the Bonneville Flood 14,500 years ago, but was much more shallow, Willsey said. But when Lake Bonneville, a large freshwater lake, started spilling over, the extra water headed down the Portneuf and Snake rivers. The huge rush of water filled the canyon to the brim, then excavated it even deeper.

Other factors have affected the Snake River’s route. Every new volcanic eruption and lava flow has changed the river’s course.

“The poor Snake River is kind of like the whooping boy for all these volcanoes,” Willsey said.


Craters of the Moon: 

What you’re looking at: The youngest lava field in the lower 48. Craters of the Moon formed from slow-moving lava, much like you see in Hawaii. 

“It’s a tourist-friendly kind of eruption,” Willsey said.

And the monument is also tourist-friendly. Throughout the area, you’ll see signs that explain the geology and science behind the black rocks. 

How old is it? The volcanic activity happened over the course of 15,000 and 2,000 years ago.

How did it form? It depends on the feature you see. The lava tubes formed when large rivers of lava cooled at the surface, insulating the interior and allowing it to continue flowing.  

The cinder cones — cone-shaped hills — formed in areas where magma had more gas in it. The volcano threw out tiny pieces of lava and formed the hill.

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