Earthquakes and The Great Western Idaho Coastline

The colors don’t actually mean anything, it’s just the only way to get people to look at geologic maps

By Bryce Johnston,

Idaho doesn’t have a coastline,”

you may be mumbling to yourself as you read this,

the only seafood I get is the shrimp platter from Albertsons,” which, most locals cannot personally recommend.

Just trust us, by the end of this blog post we’ll have turned your McCall duplex into beachfront property.

Welcome to Jurassic Idaho!

Imagine you’re in central Idaho about 100 million years ago, the geologic era is the Jurassic, and unfortunately Jeff Goldblum is nowhere to be found. Fortunately, familiar dinosaurs such as the allosaurus, brachiosaurus, and stegosaurus could be found. Climatic conditions would rival something similar to the Pacific Northwest tropical environments, and it wouldn’t be the last time either (humans really picked the wrong era to move to Idaho.) Higher levels of carbon dioxide meant that there were probably larger deciduous plants creating canopies instead of the lodgepole pine forests that now tower over us.

All of this began to change with the movement of a few plates. To refresh your memory, the Earth’s crust is composed of different tectonic plates that slide around on the mantle beneath us. This movement and ensuing collisions with plates is what causes volcanoes, mountains, basins, trenches, basically every large scale geological feature. Take two pancakes, slide them across the syrup lake you’ve made on your plate towards each other, and watch. Carbohydrate ridges and valleys formed, butter glaciers slide through canyons, syrup is visible through tears in your flapjacks. Welcome to tasty, tasty tectonic action, and remember playing with your food is only acceptable for geologic purposes.

The Great Western Idaho Coastline

From Pangaea to modern day North America, there have been a lot of face lifts for our little island. In the Jurassic period, Idaho was in a very different situation. There was a coastline. On the Pacific Ocean. This might not sound very absurd to people living near the coast, but Mountain West residents see the sea as often as they have mild winters. Now, how do we know there was a coastline? And Idaho of all places? It’s actually relatively simple.

The red line is the believed coast (sorry Lewiston)

The line corresponds with a stretch of rock that’s mylonitic. Mylonitic rock occurs when existing rock is extremely sheared or pulled by the friction of plates sliding against each other. This line formed when the continental plate of North America collided with the lithographic (oceanic) plate of the Pacific over millions of years, at the movement of about 2 inches a year. This dragged and ripped the rock where it slid over dozens of millions of years.

The horizontal streaking is the shearing layer

Going back to your pancakes (if you ate the last ones, you can always eat more), imagine sliding one under the other. On the higher one a long bump forms perpendicular to the collision. In non-pancake terms, a giant welt formed on the former coast and this was the base of all of the mountains east of it – the White Clouds, the Pioneers, the Bitterroot, the Boulder, the Sawtooths, and many others.

Going on down to Shakedown Street

If you are currently living in southern-central Idaho, you are probably familiar now with a ritual text from your friends:

“Did you feel that?”

Idaho has been feeling quite a bit of seismic activity recently. On March 31st, the pancakes stopped playing nice and caused a 6.5 earthquake that people experienced all over the state. This happened about 20 miles northwest of Stanley, ID, near the Cape Horn area. Since then we have felt multiple aftershocks between magnitude 2 and 5, near the same area. These earthquakes are called strike-slip, meaning that the plates are moving side-to-side against each other. Geologists are still unsure the fault causing it, as the Sawtooth fault is not the kind of fault to cause strike-slip movement. When the Pacific plate collision occurred, central Idaho folded up like an accordion, creating a lot more plate movements and fault action inside that area, leading down the long path of deep time to our modern day earthquakes.

The geology of the Sawtooths is long and storied, and this blog post only covered until about the mid-Jurassic. Head here for a blog post explaining how the mountains got their signature granite and shape. One last note about our coastline: you may be curious how Oregon and Washington stole the coast from Idaho. They seem to be comprised of different oceanic islands that were slowly pulled to their current day location due to a trench located off the former coast.

Now, with all this knowledge, you can safely jack up property rates in McCall with the promise of ‘historic beachfront views’. When you come up to Stanley, visit our Redfish Center and Gallery to learn more about the many natural wonders of the Sawtooths. Make sure to eat your pancakes!


Alt, David D., and Donald W. Hyndman. Roadside geology of Idaho. Mountain Press, 1989.

Fleck, Robert J., and Robert E. Criss. Location, age and tectonic significance of the western Idaho suture zone (WISZ). US Department of the Interior, US Geological Survey, 2004.

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