Thanks to Subaru of America for supporting this series of SciShow.

Their all new 2020 Subaru Outback helped us stay safe and warm during filming. [♪INTRO].

Stefan: Welcome back to our road trip! We're in the middle of a three-part series about the geology of Olympic National Park.

Alexis: Last week, we talked about the Mt. Olympus, the tallest mountain on Washington's Olympic Peninsula. But now, let's switch gears and talk about one of my favorite stories here: the story of an ancient island called Siletzia.

Stefan: Oh right, let's go!

Alexis: So, here's a question for you. There are no volcanoes on the Olympic Peninsula, but there is a ton of basalt, which is a type of volcanic rock. So, how did it get here? Well, it wasn't the Cascades off to the east.

Instead, scientists believe it got there because about 50 million years ago, a volcanic island called Siletzia collided with North. America.

Stefan: The story goes like this. More than 50 million years ago, in the northeast Pacific Ocean, an island made primarily of volcanic rock appeared. Today, geologists call it Siletzia. Siletzia was attached to an oceanic tectonic plate.

And over time, that plate moved — as they tend to do. Specifically, this one went east and was slowly traveling underneath the North American tectonic plate.

Alexis: Or subducting, if you watched our Mt. Olympus episode. As the tectonic plate moved, Siletzia moved along with it. And eventually, about 50 million years ago, Siletzia ran up against North America.

In other scenarios, this might have actually been the end of the story. Siletzia might have just disappeared under the North American plate, melted, and never been heard from again. Which happens.

Stefan: But! Recent studies suggest that Siletzia was too hot and buoyant for it to subduct. So instead, it accreted onto North America — meaning that it got stuck to the side of the tectonic plate and eventually spread throughout the landscape.

Alexis: That's why the Olympic Peninsula and the. Pacific Northwest have a bunch of volcanic basalt! Not because of a bunch of volcanoes, but because of an ancient, traveling, volcanic island!

Stefan: Geologists have figured most of this out by studying the rocks around here and by using models, like one published in 2014 that used a bunch of data from rocks in Washington and Oregon.

Alexis: And you can see evidence of this all over the place.

Alexis: The Siletzia basalt stretches from Oregon to British Columbia, and it varies from 10 to 32 kilometers thick. If you know what to look for and have a good map, you can see it for yourself on certain hiking trails or other rock outcroppings around here.

Stefan: So this is all so really cool, especially if you know the story behind what you're looking at. But there's one question we haven't answered yet: Where did Siletzia come from? How did that island get there in the first place?

Alexis: Stefan, I am very glad you asked! Because that is one of my favorite parts of this story! Okay, so tectonic plates are moving all the time. Which means that 50 million years ago the North American plate and the oceanic plate that contains Siletzia were on the same place on the globe as they are today.

They were actually much farther east.

Stefan: Okay, okay.

Alexis: So, about 50 million years ago… off the west coast of what's now Washington… was the Yellowstone hotspot.

Stefan: Wait, Yellowstone as in “Yellowstone National. Park?”, like no where near Washington!

Alexis: Yes! 50 million years ago, the same hotspot that powers all of the cool thermal features at Yellowstone National Park was most likely located under the eastern Pacific Ocean. And over time, it released a bunch of volcanic material that became the Siletzia basalt.

Stefan: That is wild!

Alexis: Admittedly, this is somewhat up for debate because it's pretty hard to prove, and there are other hypotheses. But there's a lot of good evidence for this. And it's just mind-blowing to think about.

Stefan: So, the story of Siletzia is super cool, because if you didn't know the story behind it… well, you could just look at a bunch of basalt and not think twice about it. But if you do know what you're looking at, it's seriously amazing.

Alexis: And there are more stories like that out here too, too! So, before we head home, let's make one more stop.

Stefan: Before we wrap up, though, we'd like to say thanks to Subaru for making this road trip adventure possible!

Alexis: When we filmed this series, it was the beginning of January, and we got a chance to take advantage of the Outback's all-wheel drive, ground clearance, and — of course — the heated seats and steering wheel.

Stefan: We are so glad we had heated seats.

Alexis: Seriously. After a cold day of filming, it was very nice to get into a warm car.

Stefan: If you want to learn more about the 2020 Subaru. Outback, you can check out the link in the description. [♪OUTRO].