Hi, I’m Niba, Notes by Niba and I’m so excited to join you guys here on SciShow Here’s a fun fact for you!

Alaska has more active volcanoes than any other state in the US. They’ve got over 50 that are considered recently active, but turns out, geologists think that’s just not enough.

That’s because there’s a 400 kilometer gap between Mount Spurr at the end of the Aleutian Islands and the Wrangell Mountains that’s missing its volcanoes. Deep beneath the ground, the Pacific plate is subducting beneath North America, which would normally mean some pretty spectacular volcanic activity up above. Except there’s hardly any.

This is the Denali Volcanic Gap, named after the tallest mountain in North America, which sits right in the middle of it. And Mount Denali isn’t a volcano… even though everything about the area makes scientists think it should be. [SciShow Intro] All over the world, you can find volcanoes right above where oceanic tectonic plates subduct beneath continents. That’s because when these plates plunge down into the Earth, they bring water with them.

And this is held between sediments, or even within minerals themselves. As the plate subducts and the temperature and pressure builds around it, the plate releases its water to the surrounding mantle. Water lowers the melting point of rocks in the mantle, so magma can form.

And this rises up to the crust, eventually breaking through the ground and causing a big, showy volcanic eruption. So the ingredients to make this kind of volcano are pretty simple. You need a tectonic plate with lots of water that subducts to the right depth.

And that sweet spot depth seems to be around 100 kilometers. The Denali Gap seems to have all of these ingredients. Scientists can see the subducting plate down at least 150 kilometers underground, so we know it’s far enough down to make volcanoes happen.

And that same plate has no problem making volcanoes at this depth on either side of the gap. So for a while, researchers have been stumped about what’s kept these volcanoes away from this spot here, right in the middle. In fact, scientists still aren’t certain what is stopping the volcanoes.

But they have a couple hypotheses. So a good place to start is to identify what makes this spot different from other subduction zones. And right under the gap, they’ve found that the Pacific plate has what’s essentially a gigantic speed bump.

It’s called the Yakutat terrane, and it’s a seafloor plateau that had sediments piled up on it by an ancient tectonic collision. Around 10 million years ago, it also started subducting beneath North America and it completely changed the geometry of the subduction zone. Yakutat acts as this big buoyant blob on top of the Pacific plate, so it stops the plate from sinking as quickly.

This is called flat slab subduction, and it means that the angle of subduction is much shallower here, so the Pacific plate doesn’t get down to those hot depths quite as fast. So one idea is that the speed bump makes it take so long for the plate to subduct that it completely dries out at shallow depths. Which would mean that when it eventually does get deep enough, there’s just no water left in it to form magma.

The issue with the no water idea though is that it would mean there never would have been volcanic activity in this area, at least not since the Yakutat Terrane formed. And there actually is a little bit of volcanic activity in the area. Buzzard Creek is home to lakes that started as volcanic craters.

And these formed recently, only about 3000 years ago. So this means the subduction in this area is capable of forming magma… at least sometimes. And there’s another hint that this area does form magma – earthquakes.

The earthquakes in the area look very similar to those you’d see when subducting plates release fluids, or when magma oozes around underground. So this leads to the second big idea, which is that magma actually is actually forming, it just can’t get to the surface. See, the continental crust is particularly thick in this area.

So that thick crust could be blocking the magma. But there are other spots around the world like Bolivia and Tibet where the crust is really thick, but those volcanoes can still form just fine. So there must be something unique that’s blocking the magma in Alaska.

Which brings us back to the Yakutat terrane. The flat subduction caused by Yakutat means the Pacific plate cools down the mantle as it makes contact. So a cooler mantle means less magma gets made in the first place.

And flatter subduction also means magma accumulates over a wider area beneath the crust. So if the angle of subduction was steeper, this magma would be more likely to concentrate in a single place and form a volcano above it. But Instead, the magma is forming in a kind of puddle and it just gets trapped there And subducting this gigantic bump also compressed the North American crust.

This compression likely played a big role in building the mountains of the Alaska Range in the first place, including Denali which grew rapidly over the last 5 million years. But compressing the crust also likely made it harder for magma to get through it because it would push closed any gaps magma could travel through. Like taking a nice fluffy cake and squishing it flat.

And by chance, a 2023 study found some pretty compelling evidenc that the magma is there, it just can’t get to the surface. Scientists set up more than 300 recording devices called geophones, one every kilometer. And for a month they listened to the vibrations of the Earth beneath the Denali Volcanic Gap.

They weren’t looking for magma specifically, they just wanted to get a clearer picture of what was going on down there. They found an area 12 kilometers beneath the gap where seismic waves travel about 15% slower. This could be a couple of things, but the researchers think it’s most likely a magma reservoir that the compression of the mountains above is keeping buried.

So this combination of a speed bump and compressed crust may have put a lid on this volcanic mystery. And even if there is magma down there, Denali isn’t about to erupt anytime soon. A lot of factors would need to change.

For one thing, that flat subduction would need to get steeper. It would take millions of years for that to happen, so at least for now, the Alaskan volcano gap is here to stay. We hope you liked this episode of SciShow.

If we got you curious about all things volcanic, you might like our recent episode we did on why it’s harder to see volcanic eruptions in the dark. We even got to chat with a geologist to break down the science of volcano-watching. The link is down below.

Thanks for watching! [ OUTRO ]