Welcome to Module 4!
Mountain Building, Obduction, & Tsunamis
Introducing Obduction
"No matter how sophisticated you may be, a large granite mountain cannot be denied—it speaks in silence to the very core of your being."
—Ansel Adams, The Spirit of the Mountains
Video: Introduction to Module 4 (1:29)
The Great Smokies are geologically attached to the whole Appalachian mountain range, including the ridges near Penn State’s University Park campus. There, if you’re so inclined, you can visit the beach in the mountains—all thanks to Africa.
Dr. Richard B. Alley: We're up on Tussy Mountain not far from Penn State's University Park campus, in an old obduction zone. And this is an old beach. This is a Sandstone that was deposited on the edge of a sea when there was a mountain range being formed behind us. And what eventually turned into the giant collision when North America ran into Europe and Africa during the closing of the Proto-Atlantic.
Now you know about Beaches. They do not form like this, they form like this. But this one is now turned up on edge, like this. And what happened in the great collision is, there was a squeezing and sometimes the squeezing even broke, and then streams could get down in those cracks, and they could erode things, and they eroded way down to make a valley in the middle, which is where Penn State was founded. And now, the layers are sitting here like this. This particular layer goes way up. It's cut off. It comes back down and makes the top of Mount Nittany and then it comes back down, way on the other side, and makes Bald Eagle. And then it goes down and under rocks that are running off to the west of us. And people come up here and they go skiing and they go mountain biking and have a great time in this old obduction zone.
We saw in Module 3 that an old, cold sea floor is subducted beneath the warmer sea floor or a continent, but what happens when a high-floating continent or island arc tries to go down beneath another continent or island arc?
In this module, we’ll see that the answer is obduction, a BIG collision. The Great Smoky Mountains, Mt. Nittany near Penn State's University Park campus, and all the rest of the Appalachians were formed by just such a collision, between North America on one side, and Africa and Europe on the other side. Folding and thrust-faulting in the collision zone thickened and shortened the crust and upper mantle. This produced high mountains—probably about as tall as the Andes today. And, as we will discuss, high mountain peaks float on deep roots. When erosion lowers a mountain range, the root floats up, bringing metamorphic rocks to the surface that have been "cooked" by heat and pressure deep within the Earth.
Before we go any further, look at the following short video introduction by Dr. Anandakrishnan.
Video: Duck Mountain (2:23)
[RUNNING WATER]
Dr. Sridar Anandakrishnan: Oh, hi. Welcome to the GEOSC 10 Tectonics III Section, Mountain Building. We're going to be talking about how the Appalachians were built, and why they're still as high as they are now. And we're gonna talk a little bit about icebergs. Now, what does all that have to do with rubber duckies, and why am I in the bathtub surrounded by rubber duckies? Well, let's find out.
So why are we in a tub with rubber duckies? Imagine that these ducks represent the mountains that surround us right here in Happy Valley. This is Mount Nittany. Here's the Great Smoky Mountains. And these are all the ridges and valleys that spread up and down the east coast of North America known as the Appalachian Mountains.
About 300 million years ago, North America and what's now Europe and Africa all collided to form a supercontinent known as Pangaea. And when these continents collided, because they're both about the same density, neither one subducted beneath the other. In fact, as they crashed together, they formed larger and larger mountains that wrinkled up. And that's what we had 300 million years ago with the Appalachians. This big duck represents the mountains at the end of that mountain-building phase.
So after that collision, we have the proto-Appalachians. They were big. They were tall. They were high.
But for the last 300 million years, they've been eroded and eroded and eroded. But still, we have mountains around here that are a couple of thousands of feet high. If you go further down towards Tennessee and Kentucky, some mountains are 4,000 or 5,000 feet high. Why are they still high? Well, it has to do with buoyancy.
[RUNNING WATER]
And....A Word About Tsunamis
Plate tectonics causes earthquakes, volcanic explosions, and steep slopes that can experience landslides. If any of these happen under the ocean or in a deep lake, or if a landslide falls into an ocean or lake, a lot of water can be moved in a hurry. The resulting great waves are called tsunamis and can have catastrophic consequences. Fortunately, warning systems can be devised to reduce the loss of life, and we can use our knowledge to build in ways that increase safety for people and property. We'll look at some of these issues as we wrap up our multi-week exploration of Plate Tectonics and Mountain Building.
Learning Objectives
- Identify the push-together faults and folds formed in obduction zones when continents or island arcs collide.
- Explain the three basic tectonic styles: pull-apart, push-together, and slide-past.
- Explain how obduction zones can produce thickening of the crust that leads to the formation of metamorphic rocks.
- Explain how erosion can reveal metamorphic rocks formed in obduction zones.
- Explain how tsunamis are caused, and why they are dangerous.
What to do for Module 4?
You will have one week to complete Module 4. See the course calendar in Canvas for specific due dates.
- Take the RockOn #4 Quiz.
- Take the StudentsSpeak #5 Survey.
- Continue working on Exercise #2: Geology is All Around You.
Questions?
If you have any questions, send an email via Canvas, to ALL the Teachers and TAs. To do this, add each teacher individually in the “To” line of your email. By adding all the teachers, the TAs will be included. Failure to email ALL the teachers may result in a delayed or missed response. For detailed directions on how to do this, see How to send an email in GEOSC 10 in the Important Information module.
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