- [Teacher] What does it mean to be curious?

For my students, curiosity creates opportunities for a bigger and brighter future.

(bright music) (gentle music) (bright music) - Back country hiking today up to Lake Serene at the base of Mount Index.

Look at that, 3,500 vertical feet of index granite, famous in geology, famous from rock climbers around the world.

The whole valley's full of index granite, but the uppermost part of mount index is not granite.

It's far older rock that formed at the bottom of the ocean.

(bright music) (water splashing) (bright music) What a river.

The Skykomish and these boulders, big and small, the Index granite.

Well, that's Mount Index over that shoulder, 33 million years old.

It's a batholith, the Index batholith, which simply means there's a hell of a lot of granite here in this area.

It was originally magma chamber rock underground in the dark.

We remove the heat that magma turns to solid rock, and then it's brought to the surface so that we can see it.

There's more than a hundred square miles of the Index batholith exposed, not just at that mountain, but Philadelphia Mountain, many of the other ridges and hills and many of these boulders along the river, they're all Index batholith.

To get a very careful and close look at the detail of this rock, let's head over to a very famous rock quarry, (bright music) (leaves rustling) The towering walls of the Index granite.

This is a historic place, a rock quarry, where we can get a good closeup look at this beautiful granite.

Back more than a hundred years ago, they quarried the granite out for the construction of Smith Tower in downtown Seattle and even the State Capitol building in Olympia.

Rock climbers love this place because of the texture of the granite.

You can see the white feldspars, the gray quartzes, the black biotypes, the black hornblendes.

It's geology 101 here.

This is textbook granite.

But it's the walls themselves that are striking and they're visible here on this side of the valley and back across the way to Mount Index itself.

(bright music) (birds chirping) (rocks rustling) (water splashing) These are the majestic shear walls of the Norwegian buttresses, as they're called.

Mount Index looks pretty familiar.

It's just like the rock quarry across the valley, right, Index granite, 33 million years old, but above Lake Serene, we have the same sheer walls and there's a glacial story.

Lake Serene and this beautiful cirque was carved by glacial ice.

You can see these amazing striations and this polished bedrock.

So the ice age erosion did a good number on eroding a lot of this rock for us to take a peek inside of the mountain.

But this magma chamber, 33 million years ago, is shallow level and the magma is eating its way into older rock that's at the top of Mount Index.

What is that rock at the top of Mount Index?

(bright music) (birds chirping) (bright music continues) Well, we saved the biggest surprise for the end of this episode.

The upper one third of Mount Index is not granite.

It's this stuff, gabbro which is a very rare rock.

It's basically a black granite, mostly dark colored plagioclase feldspar crystals.

But the shocking thing is that the upper third of Mount Index was formed under the ocean floor in the Pacific Ocean more than 140 million years ago.

Yes, magma chamber rock, but the magmas were below the ocean floor in the Pacific.

You cool off that magma, you form this gabbro more than 140 million years ago, you add it to the edge of North America before the Cascades ever thought of getting started.

Then 33 million years ago, you have the Index granite eating its way up through when it was a magma, but doesn't get to the surface.

And what are you left with?

A mountain that violates one of the most basic laws in geology.

I thought the deeper you go, the older you get and the shallower you go, the younger you get.

Not true in this case.

The older stuff's at the top, the younger stuff's at the bottom, Mount index, two completely different kinds of rock making up the same mountain.

Wow.

(bright music) - [Voiceover] This series was made possible in part with the generous support of Pacific Science Center.