The Wallowa Mountains in NE Oregon have the most complicated and longest geological history of any range in Oregon. In this podcast, we talk with Dr. Kim Ely, a geologist, about this unique and beautiful area. Learn how to “untangle what you see” when riding in the Wallowas!
Transcript
“The high Wallowas are composed of metamorphic rock and granitic rocks of the Wallowa Batholith. The Batholith formed when huge blobs of magma were injected into the rocks of the exotic terranes about 130 million years ago. Because the magma was injected many miles below the surface, the molten rock cooled slowly, allowing the characteristic coarse salt and pepper crystal texture of granite to form.
However, the hills around the valley, which were carved out during the last glacial period, are made up of Columbia River Basalt intermixed with limestone.”
Kevin
I’m Kevin English. And today with me is Kim Eli, our resident rock doctor at Dirty Freehub. She has a Ph.D. in geology from the University of Melbourne in Australia. Welcome, Kim.
Kim
Hi, Kevin. Thanks.
Kevin
So I want to talk about a mountain range in northeast Oregon, the Wallowa Mountain Range and to me, this mountain range looks different than almost any other mountain range in the rest of Oregon. Am I imagining this, or does it actually look different?
Kim
No, you’re absolutely correct. The Wallowas is really quite unique in Oregon.
Kevin
How does that mountain range look different than what else I see in Oregon?
Kim
One of the reasons why the Wallow Mountains looks really different to other mountains in Oregon is because of the types of rocks that make up the mountain range. They are different from most of the mountains in Oregon, which the people will be familiar with our volcanic mountains. Here, these mountains are made up of metamorphic rocks and granite.
Metamorphic rocks started out as some other type of rock, but have been substantially changed from their original igneous, sedimentary, or earlier metamorphic form. Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot mineral-rich fluids or, more commonly, some combination of these factors. Conditions like these are found deep within the Earth or where tectonic plates meet.
Kevin
And so when I typically think of Oregon mountains, I’m used to like these blocky lava flows that kind of crumbled together, look black and charred. The Wallowas don’t look that way, they look almost silvery or white and granite colored. Is that do I have that correct?
Kim
Yeah, a large part of the Wallowas is made up of what is called the Wallowa batholith which is a really large granitic intrusion. The other parts of the Wallowas, are a series of layered rocks that have been metamorphosed and folded and actually in some parts along the high mountain peaks where there’s big cliffs and no vegetation, you can actually see the layers of rock that have been folded around into huge folds going right across the side of the mountain.
A batholith is a large mass of intrusive igneous rock that forms from cooled magma deep in Earth’s crust. Batholiths are almost always made mostly of intermediate rock types, such as granite and quartz.
Kevin
So when you say folds, am I looking at the rock and actually seeing structural layers that look folded?
Kim
Absolutely. So these rocks would have originally been either layers of very old basalt or sediments, layers of limestone. So originally these would have been laid down in flat pancake like layers and then forces with movements caused by plate tectonics have led these to be heated up and then squashed together, which makes them deform and change their shape into folds.
Basalt is volcanic rock (or lava) that characteristically is dark in color (gray to black), contains 45 to 53 percent silica, and is rich in iron and magnesium.
Kevin
Are these light layers really tightly squished compared to the volcanic black basalt we see? Is that what happened? It got squished really, really hard and became a really hard rock?.
Kim
Yeah, absolutely. This would have happened at quite, quite deep below the surface of the earth at the time. And so there was high pressure as well as high temperature that enabled this folding to happen.
Kevin
If it happened deep within the earth. How did it get up to the surface of the earth?
Kim
Well, much later, after the folding and after the intrusion of the granite, there was tension in the crust of the area that’s now Oregon that started to pull things apart. And so the crust started to fold and deform, allowing layers to, like, rise up above, above each other.
Kevin
And when you say it was deep, are we saying it was miles deep underneath the surface of the earth? And then you had the plate tectonics that pushed things up and moved it by miles? How many years does that take? Are we talking hundreds of thousands of years to make this happen?
Kim
Yeah, we’re talking millions of years. So the original formation of these rocks and the intrusion of the granites occurred maybe about 150 million years ago. And so there’s been a long time for erosion from the surface to remove surface rocks and allow these rocks that were once deep within the crust to rise up.
Kevin
And in geological times, it or measurement is 150 million years a long time ago? Or is that like not very long ago in terms of geological measurement of time?
Kim
It’s actually not very long ago. So the earth is about four and a half billion years old and many of the oldest areas of continents that we know about are often older than two and a half billion years. So rocks that are 150 million years, they’re just teenagers.
Kevin
Wow. Okay. And what was in this area? Was an ocean at one time, or was it a jungle forest or do we know what was actually there a 150 million years ago or what was there to help make this rock all get squished together and form these layers?
Kim
Yeah. Well, it’s not that long ago geologically, what we know now is Oregon did not exist. The edge of the North American continent was somewhere around the Idaho border. So way out in the ocean there was lines of volcanoes that we call volcanic arcs. And in today’s planet, you can imagine somewhere like Tonga or the Aleutian Islands coming out from the west side of Alaska. And what is going on there, it’s what we call a subduction zone where you have lines of volcanoes caused by one tectonic plate subducting under another, and then melting, forming magma. And these volcanoes are very explosive types. And often in a tropical ocean setting, they’ll be surrounded by coral reefs. And it’s a very dynamic geological setting, so there’s sediments coming off these volcanoes and forming sedimentary layers in the ocean. So this type of process would have gone on for tens of millions of years, but at the same time, plate tectonics were moving these sections of the Earth’s crust around. And at some point one of these volcanic arcs collided with another volcanic rock. And this was starting the process of burying these rocks deep in the crust and then folding them and heating them up. This continued and this package of arcs that had already collided with each other continued to move towards the edge of the North American continent. And that’s where basically they came to rest, packed up against the continent. But that action of them moving and colliding with the continent is what caused a lot of what we see now, evidenced by these giant folds, this defamation that happened.
Kevin
Okay. Okay. So if I got this right, the edge of the continent was Idaho, and Oregon was underwater.
Kim
That’s absolutely right. Yeah.
Kevin
So we’ve got really a special type of mountain range in Oregon. So can you just run me through that a second time of how what do we end up with these. What’s special about these metamorphic rocks and how do we end up with these folds? I’m starting to piece these elements together. But I need you to bring me back through to second time.
Kim
So what makes the Wallowas so interesting and so different, is it’s an area in Oregon that’s probably got the longest geological history and also the most complicated. So we had a line of volcanic islands out in an ocean. So along with those volcanic islands, there were coral reefs. There were other sedimentary rocks. Then plate tectonics, which is always moving plates around the earth, transports this volcanic rock towards the North American continent, which is like a backstop if you imagine in the geological world. So the island arc is getting transported kind of like on a conveyor belt, if you like, by forces underneath with within the mantle that’s bringing the arc towards the continent. And then it rams into it and it can’t go underneath the continent and it’s not going to push up on top of the continent. So they just crash into each other. And, you know, if you can imagine anything crashing into another solid object, it’s like a giant crumple zone. But for rocks. So things that were flat layers get squished up into folds. And a lot of this happened quite deep under the surface. So that was high pressure and it got quite hot, allowing these rocks to fold. Then there was also granite intruding into that system, But then we had about 120, 130 million years in which over that time there would been erosion from the surface and also faulting which brought these once deeply buried rocks up to near the surface.
Tectonic plates divide the Earth’s crust into distinct “plates” that are always slowly moving. Earthquakes are concentrated along these plate boundaries.
Kevin
Okay, so if I got this right, we basically had two cars that crashed into each other and crumpled and folded. But this happened way deep inside the earth was there was a bunch of pressure and then it finally got exposed due to erosion over time and came to the surface.
Kim
Yeah, that’s right. And then most recently, the area was glaciated. And so in the Wallowas there was about nine really big glaciers, up to about ten miles long. And those carved out those really long, linear U-shaped valleys that we see in the Wallowas leaving those huge vertical cliffs that are on the high peaks.
Kevin
Okay. So we had this big car crash where things kind of bumped into each other and then it kind of got pushed to the surface due to erosion. And then we had the glaciers come through and carved these deep valleys. Am I doing okay so far?
Kim
Yes, you’re doing really well. And one of the one of the kind of signature glacier landforms that’s left in the Wallowas now is Wallowa lake, which is you’ve probably seen it. It’s a really long lake and it’s bound by these two really high ridges that are quite odd looking landforms compared to the other parts of the the lake. And what these are is they’re called glacial moraines. And so they were before they had vegetation grow on them, they were just two enormous piles of boulders and crushed up rock and everything left by the side of the glacier.
Kevin
The other thing you mentioned to me, when we are talking offline is that the foothills are this kind of combination of basalt and limestone. How did that happen?
Limestone is a sedimentary rock that is formed in one of two ways. It can be formed with the help of living organisms and by evaporation. Ocean-dwelling organisms such as oysters, clams, mussels and coral use calcium carbonate (CaCO3) found in seawater to create their shells and bones. As these organisms die, their shells and bones are broken down by waves and settle on the ocean floor where they are compacted over millions of years, creating limestone from the sediments and the pressure of the ocean water.
Kim
So there’s actually two different components there. So the limestone would have been there as part of the original volcanic arc formation. So those rocks would be quite old and came along with this arc that collided with the North American continent. The basalt is actually a whole lot younger, and that basalt was related to the Columbia River Basalts that cover much of northeastern Oregon.
The Columbia River Basalt Group is the youngest, smallest and one of the best-preserved continental flood basalt province on Earth, covering over 81,000 square miles in mainly eastern Oregon and Washington, western Idaho, and part of northern Nevada.
Kevin
And so having that mix of limestone and basalt that is that a unique occurrence to the will? Or do we see that elsewhere in Oregon?
Kim
We don’t see it too many places in Oregon. Having the limestone is not very common. And because around the Wallowas, other similar rocks occur in the Klamath Falls area as well. That has an area that is most similar in geologically to the Wallowas and completely different to most of the rest of Oregon.
Kevin
Okay. So can you help help me out just a little bit since I’m really new to rocks. Described to me in a sentence or three, what is basalt? And then describe to me in the sentence 1 to 3 sentences What is limestone?
Kim
Okay
Kevin
So you’re really trying to help the neophyte here.
Kim
Sure. So basalt is a volcanic rock and it’s extruded from a volcano. So it comes out as a lava which can flow across the surface of the earth’s. It then cools and hardens into these flat layers of hard black rock. And sedimentary rocks are more commonly made of sand and silt. And those sort of materials. Limestone is quite special in that it’s made often by coral reefs. So you’ve got creatures that build coral reefs and as they do that, they just keep on layering it up. And the remnants all the material that’s below the actively growing reef becomes limestone.
Kevin
So when we have that basalt and limestone integration in the Wallowas, we’ve got two radically different aged rocks with the limestone being probably much, much older than the basalt. Is that true?
Kim
That is true. And that’s a result of the older Wallowa geology existing, having uplifted and eroded and then as those enormous vessel flows occurred, they just flooded the landscape. They filled in all the valleys, they just built up these big pancake layers. And so on the original base of that newer basalt, you’ve got the contact between the new rocks overlying the old rocks.
Kevin
This is really interesting to me. And it gets it gets complex to me at least really quickly because we’ve got these different ages and we’ve got billions of years versus millions of years and we got rocks that were compressed into limestone by layers and then we’ve got the volcanoes exploding. So help me out of this a little bit more. So when we talk about granite, what type of rock is granite?
Kim
Granite is what we call an intrusive plutonic rock. So this is a rock that has its origins deep under the surface of the earth in what we call a magma chamber. So that’s an area of molten rock, which is formed under really high temperatures, and then it intrudes up into the crust, but it doesn’t come to the surface. So when lavas come to the surface, they become volcanoes. When they rise up into the crust, but then stop that is what we call an intrusive rock, and that then cools slowly and makes big crystals. And that’s the signature of granite.
Kevin
So in the Wallows then we’ve got granite, limestone and basalt.
Kim
Yes. And also some other metamorphic rocks. It’s pretty complicated.
Kevin
So the Cascades are like simple arithmetic in terms of complexity and the Wallowas feel like calculus to me in terms of are highly complex. Is is that a good analogy or not?
Kim
I think that’s an excellent analogy. The Wallowas have a really long, complicated history. There’s numerous processes that have over printed previous processes, so it’s actually really quite difficult to untangle the exact history of that area.
Kevin
I really like that word untangle because you’re untangling, like you said, untangle the history, which is really cool. So when we go to north northeast Oregon, we’re looking at a really special place in terms of geology that we’re trying to untangle with our eyes when we look at it.
Kim
Absolutely, Ya.
Kevin
Would we say the Wallowas are an older mountain range than volcanoes of what we find in Central Oregon? How would we classify a date on the Wallowas?
Kim
Yeah, they’re definitely older. They are some of the oldest rocks in the state.
Kevin
I don’t know much about rocks, but I’ve learned a lot about rocks today. And this is actually the most I now know about rocks. And I’m getting fascinated by it. The more I learn about rocks. I really appreciate you taking the time and being super patient with me on this podcast. Thank you very much.
Kim
You’re welcome. I look forward to talking again about more rocks.
Kevin
Thank you
