Chapter 4 – Obduction and Tsunamis

Chapter 4 – Obduction and Tsunamis

Obduction and Tsunamis:

Learning Objectives

Welcome to Chapter 4. the end of this chapter you will be able:

Identify the three types of tectonic plate boundaries and the major landforms found within them
Define the physical processes responsible for plate tectonic movement
Compare and contrast the three basic tectonic styles (Push-Together, Pull-Apart, and Slide-Past)
Compare and contrast subduction and obduction
Identify the causes of tsunamis
Describe the geometry of tsunami waves in the deep ocean versus the shallow marine environment
Explain why the location of the Rocky Mountains is an anomaly, and how this likely occurred.

The Smokies—and State College?

sunset over the Appalachian Mountains with a tree in the foreground. — Scenic view of the folded Appalachian mountains in Great Smoky Mountain National Park, North Carolina and Tennessee. Credit: R.B. Alley

The Great Smoky Mountain National Park of North Carolina and Tennessee includes 16 mountains over 6,000 feet (about 2,000 m) high, making this generally the highest region in North America east of the Mississippi River. Gatlinburg is a mile (1.6 km) lower than Mt. Le Conte, a relief not much smaller than in many of the great mountain parks of the west, where the peaks are higher but so are the valleys. The Smokies were preserved in a park in 1926, with much of the funding for land purchases provided by J.D. Rockefeller. The Great Smokies, today, are the most-visited national park, because they combine spectacular scenery, rich biological and historical diversity, proximity to major population centers, the lure of a quick stop-off on the drive from the northeast to Florida, and a shortage of other nearby national parks to draw off the crowds. (Although we should not forget Shenandoah, connected to the Smokies by the Blue Ridge Parkway, a beautiful park in its own right.)

Map of U.S. with Great Smoky Mountains National Park highlighted mid-way along the North Carolina/Tennessee border. — Great Smoky Mountains Location Credit: R.B. Alley

In case you’re interested, the top-ten most visited national parks in 2012 included great Smoky Mountains with 9.7 million, Grand Canyon with 4.4 million, Yosemite with 3.9 million, Yellowstone with 3.4 million, Rocky Mountain with 3.2 million, Zion with 3.0 million, Olympic with 2.8 million, Grand Teton with 2.7 million, Acadia with 2.4 million, Cuyahoga Valley with 2.3 million. Cuyahoga Valley may seem the odd-one out; it is a recent addition, seems to have been justified as a national park in part so that Ohio would have a national park, and seems to have a lot of day-picnickers from nearby Cleveland who increase the attendance a whole lot, based on Dr. Alley’s observations during a recent visit. But, Dr. Alley also believes that the Park Service is doing great things with it, and it is well worth the visit! Over 282 million people visited US national parks in 2012 (bear in mind that is with ~2 million fewer visitors than expected as a result of park closure caused by Hurricane Sandy). Note that this is nearly 90% of the whole US population. Although some people will have visited a few parks whilst others visited none, overall people are enjoying their parks!

Much interest in the Smokies centers on the historical aspects. For example, how did the early European settlers survive and flourish in this region? At Cades Cove, wonderful relicts of a bygone lifestyle are maintained in a living museum. Many visitors are also seeking to learn about the earlier Native Americans. Biologically, the Smokies host an amazing array of tree species, flowering bushes (azaleas, rhododendron, and mountain laurel in particular), wildflowers including many orchids, and more. Approximately one-third of the park is covered with “virgin” timber that was not cut by European settlers, and the regions that were logged are growing back rapidly.

Abundant rainfall and snowfall “scraped” from the sky by the high peaks feed numerous cascades and waterfalls, with trout in the pools and kingfishers by their banks. Rainfall is roughly 50 inches (1.3 m) per year in the valleys, and more than 85 inches (2 m) per year on the peaks, so the Smokies share some characteristics with temperate rainforests of the west such as in Olympic. Especially during “off-peak” times, you can get lost in the Smokies, and imagine what the Appalachians must have looked like without humans; approximately 3/4 of the park is wilderness.

Obduction Zones – The Push-Together Boundaries

The Smokies are a small part of the great Appalachian mountain chain, which extends along the coast of North America from Newfoundland through the Smokies, and then bends westward into Oklahoma. In the Great Smokies, the mountains display a truly remarkable feature—older rocks sit on top of younger rocks! (See the diagram below.) The very high peaks are composed of hard, resistant, old metamorphic rocks (which we will explain soon), of the sort that one finds deep in a mountain range. Beneath them are younger, sedimentary rocks that were deposited in shallow seaways. Between these is a surface called a thrust fault or push-together fault. Thrust faults often show evidence of sliding—scratches and polish indicating motion in one direction, crushing or breaking of rocks, etc. In some cases elsewhere in the world where deformation is still active, thrust faulting has been observed during earthquakes. In the Smokies, the older rocks have been shoved as much as 70 miles (110 km) to reach their present position on top of the younger rocks. The picture below the diagram shows two very much smaller thrust faults, with the upper rocks shoved up to the right only a few inches, but the idea is the same.

Explained thoroughly in caption and text. — Diagram showing how push-together forces moved older rocks above younger ones along a thrust fault in Great Smoky Mountains National Park; Cade’s Cove, a famous location in the park, is a valley eroded through the older rocks and the thrust fault into the younger rocks beneath. Credit: R.B. Alley

You may recall that we started with pull-apart faults at Death Valley. As shown in the diagram above, thrust faults are of the push-together type. Squeeze from either side, and one set of rocks will be pushed over another set. Each set is right-side up, but where they meet, the older rocks are on top of younger. This is seen clearly in the Great Smokies.

Farther north, in the State College, Pennsylvania area, where Drs. Alley and Anandakrishnan teach and where Dr. Alley wrote most of this text, we see a different way that rocks can respond to push-together stresses. There, in addition to some push-together thrust faults, many folds occur. Take a piece of paper, lay it on your desk, and squeeze the opposite sides towards the center. The paper will buckle into a fold. You may achieve the same effect by trying to push a carpet along the floor. Clearly, there are push-together forces involved here.

Small thrust faults, with one bed of sandstone thrust a few inches over another in a cliff below the Glen Canyon Dam in Arizona. — This picture shows two very much smaller thrust faults, with the upper rocks shoved up to the right only a few inches, but the idea is the same. Credit: R.B. Alley

Just as pull-apart forces occur at spreading ridges, we should expect push-together forces at subduction zones, or at other collision zones. Today, the Appalachians and the east coast of South America look across the quiet sea floor of the Atlantic, across the spreading center of the mid-Atlantic ridge, to the coastlines of Africa and Europe. The coastlines on either side of the Atlantic are parallel to each other and to the mid-Atlantic ridge—slide the new and old worlds back together again, and they fit like a jigsaw puzzle. You can put all of the modern continents back together jigsaw-puzzle style. This fact, and especially the wonderful fit across the Atlantic, has figured prominently in suggesting the idea of drifting continents to scientists and other observers almost since the first decent maps were available of the Atlantic coasts. More importantly, putting the continent shapes back together jigsaw-puzzle style puts the “picture”—the geology—back together as well for events that happened while the continents were joined. For example, the tracks of a glacier run out to sea from Africa, and glacier tracks run from the sea into South America; put the continents back together, and the tracks fit together in showing the path of a single ice flow.

The oldest rocks on the Atlantic sea floor are about 150 million years old, approximately the same age as sediments that were deposited in a Death-Valley-type setting in the Newark Basin of New Jersey and elsewhere along the U.S. east coast. Evidently, the modern situation of a spreading Atlantic began about then, splitting apart a supercontinent to form the Atlantic Ocean in the same way that Baja California is being split off to open the Gulf of California.

See caption. — Photo of a road cut along State Route 322 just east of Penn State’s University Park campus. As the proto-Atlantic ocean closed, subduction-zone volcanoes formed. Ash from these volcanoes can be found in the rocks exposed in the road cut. Credit: R.B. Alley

But, the Appalachian Mountains are much older than that. A story begins to emerge of a cycle—older push-together forces led to closing of a proto-Atlantic ocean that produced the Appalachians. When the proto-Atlantic was closing, subduction-zone volcanoes formed and spread ash layers across the land, much as Crater Lake/Mt. Mazama and Mt. St. Helens did more recently. (You can find some of those ash layers in many places, including the road cut along the Route 322 expressway just south of East College Avenue in the State College, PA area; see the picture at left.) Sometimes, the proto-Atlantic subduction zones formed offshore and then their volcanoes collided with the North American continent.

When the pushing stopped, the giant pile of the Appalachians, with deep, hot rocks beneath, began to fall apart in Death-Valley style, and red sandstones and mudstones accumulated in Death-Valley-style valleys along parts of the U.S. east coast. The drop in pressure as the Appalachians fell apart probably caused a convection cell in the deep mantle to rise right there, eventually forming the Atlantic Ocean.

Such a cycle, with subduction or continental collisions building mountain ranges that then spread Death-Valley style and eventually split to make ocean basins, has been played out many times over the history of the Earth. And, the fun isn’t over yet; North and South America are cruising westward toward Australia and Asia, as the Atlantic widens and the Pacific narrows. Africa is still bumping into Europe and pushing up the Alps, and India has not yet grown weary of ramming Asia to raise the Himalaya. At fingernail-growth speed, the next 100 million years or so should lead to a lot of geological high drama, but the next 100 years won’t see a whole lot of change. A good animation of this cycle(link is external) can be found on YouTube.

The key to most of this is that you can sink old, cold sea floor, but you can’t sink a continent. Island arcs and continents float on the mantle too well. So, rather than going down the subduction zone with the oceanic lithosphere, the island arc or continent rides across the subduction zone for a major collision. In such a collision, called obduction, layers of rock are bent into folds such as those of the State College area, or broken into thrust faults such as those under the Blue Ridge and the Great Smokies. In the case of the Appalachians, the thrust faulting was very efficient, with older rocks sliding tens or hundreds of miles (or kilometers) over younger ones.

The Three Structural Styles

You have now seen, at least briefly, the three structural styles that are possible: pull-apart (Death Valley, spreading ridges); push-together (Crater Lake and Mt. St. Helens subduction, State College and the Great Smokies obduction); and slide-past (the San Andreas Fault in California). Pull-apart behavior involves stretching of rocks until they break, forming pull-apart or gravity faults (after being pulled apart, gravity pulls one block down past the other). Pull-apart action occurs at the spreading centers, probably where the convection cells deeper in the mantle spread apart. Push-together behavior occurs at subduction and obduction zones, and produces squeeze-together folds and faults, with the faults also known as thrust faults. Slide-past boundaries, also called transform faults, occur where two large blocks of rock move past each other but not toward or away from each other. Slide-past motion produces earthquakes without mountain ranges

See caption. Diagram described in text. — A bend in a “slide-past” (transform) fault can create locally push-together or pull-apart conditions, and such behavior is observed in California along the San Andreas fault. Credit: R.B. Alley

Now, you might imagine that we have oversimplified just a little. There is no law that rocks must move directly toward each other (push-together) or exactly parallel to each other (slide-past); sometimes you see an oblique motion with rocks approaching on a diagonal. Or, the rocks may pull apart on a diagonal. And a bend in a slide-past boundary may produce pull-apart or push-together features, depending on which way the bend goes relative to the motion, as shown in the diagram above. A large bend in the San Andreas Fault just north of Los Angeles gives push-together motion and some impressive mountain ranges.

Mountain ranges correspond directly to the main boundary types. Fault-block mountains—the Sierra Nevada, the Wasatch Range, the flanks of the great rift valleys of Africa, and the mid-oceanic spreading ridges—form at pull-apart margins. The mountains are high because the rocks beneath them, in the mantle, have expanded because they are the hot upwelling limbs of convection cells. Volcanic-arc mountain ranges form over subducting slabs, where some of the downgoing material melts and is erupted to form stratovolcanoes; smaller ranges (such as the Coast Ranges of the Pacific Northwest, including Olympic National Park) may form from the sediments scraped off the downgoing slab just above the trench. Continent-continent or continent/island-arc obduction collisions occur at convergent or push-together boundaries as well, producing folded and thrust-faulted mountain ranges.

Why the Great Smokies Are Still So High

Remember that crustal rocks are the low-density “scum” that floats on the denser mantle. When obduction occurs, this crustal scum is crunched—it goes from long and thin to short and thick, in the same way that the front end of a car is changed when it runs into a brick wall. Then, just like an iceberg floating in water, a mountain range is a thick block of crust floating in the mantle, with most of the thickness below and only a little bit sticking above.

With an iceberg, about 9/10 of the thickness is below the water and 1/10 above. If you could instantly cut off the 1/10 that is above water, the iceberg would pop up to almost as high as before. A 100-foot-high berg would have 10 feet above the water and 90 feet below. Cut off the top 10 feet, and it is a 90-foot berg with 9 feet, or 1/10, above the water and 81 feet below. So removing 10 feet from the top shortens the ice above water by 1 foot, and the ice below the water by 9 feet. With mountain ranges, the density contrast between crust and mantle is larger than that between ice and water—only about 6/7 of a mountain range projects down to form the root, and 1/7 projects up to form the range.

It remains, however, that if you erode a mountain range, some of the root is freed to float upward. Only by eroding the equivalent of 7 mountain ranges can you eliminate the mountain range entirely. So the Appalachians, despite having been deeply eroded, are still high because they still have a root.

The idea that things on the surface of the Earth float in soft, denser material below is called isostasy, which means “equal standing”—that each column of rocks on Earth has the same weight or standing. Lower-density columns then must be thicker to weigh as much as thinner, higher-density columns. The continents stand above the oceans because the silica-rich continental crust is lower in density than the silica-poor sea-floor crust. The mountain ranges stand above the plains because the thick, low-density roots of the mountains have displaced some of the high-density mantle that is found beneath the plains, or because the rocks beneath the mountains are especially hot and so low in density.

Put a big weight on a piece of crust (say, an ice sheet, or the Mississippi Delta, or a mountain range) and that piece of crust sinks, pushing up material around it in the same way that the surface of a water bed sinks beneath your posterior when you sit down, while the surface is pushed up around you by the water that is shoved sideways. The rising and sinking of the land are slower than for a water bed—thousands of years rather than seconds—because the hot, soft, deep mantle flows a lot slower than water does. But for a mountain range over 100 million years old, a few thousand years doesn’t mean much.

Notice something else fascinating; when a mountain range is being eroded, the top is taken off, and rocks below bob up. Those are taken off, with their place taken by more rocks from below. Pretty soon, the rocks at the surface have come from far down in the Earth, where temperatures and pressures are high. And as you might imagine, high temperatures and pressures change rocks. The rocks around State College, PA have not been “pressure-cooked” much, but the rocks around Philadelphia have been; they tell the story of a great mountain range that fell apart, leaving the remnant that we know as the Appalachians. The rocks in Rocky Mountain National Park are similar to those in Philadelphia, having been deep and now occurring at the surface. Let’s go take a quick look.

The Rocky Mountains

Snow-covered mountains behind a lake. — Bierstadt Lake in Rocky Mountain National Park, Colorado. Credit: R.B. Alley

Mountain Building and Metamorphism

Rising high above Estes Park, Colorado, and almost within shouting distance of the population centers of Boulder and Denver, Rocky Mountain National Park is a natural destination for the crowds that throng to this mountain playground. Long’s Peak, at 14,256 feet (about 4300 m), dominates the south-central part of the park; the peak was first climbed in 1868, by a party that included John Wesley Powell, the man who later commanded the first boat passage of the Grand Canyon and then led the United States Geological Survey. Numerous peaks over 13,000 feet (4000 meters) in Rocky Mountain lure climbers.

Small and rapidly shrinking active glaciers still carve the mountains, and much greater glaciers of the past left the numerous tarn lakes, moraines, and other features that decorate the park. Trail Ridge Road surmounts the high tundra of the park, giving the visitor a first-hand look at periglacial processes and ecosystems (those of cold regions; more on this later). The Colorado River rises on the west slopes of the park, and lovely little trout streams such as the St. Vrain flow down the east slope. Bighorn sheep and elk attract traffic jams in Horseshoe Park.

Map of U.S. with Rocky Mountain National Park highlighted in north-central Colorado. — Rocky Mountain National Park location Credit: R.B. Alley

It is a tad embarrassing to say that we don’t fully understand the Rockies yet, including those of Rocky Mountain National Park. Oh, the long history of mountain building, erosion, glaciation, etc., is well-known—we can tell the story. But most mountain ranges hug coasts, or are trapped inland only by the destruction of the ocean to which they once were coastal, whereas the Front Range of the Rockies is as far as almost 1,000 miles (1600 km) from the coast, yet the Rockies are not the direct result of obduction.

The U.S. West is a complicated region (see the Enrichment section for a little more on this). The continent has been approaching and overriding the East Pacific Rise spreading ridge. The San Andreas Fault is the product of the rise running into the trench. Before these met, subduction had been occurring from push-together motion, but with a little slide-past motion thrown in. After the meeting, the subduction stopped, so the push-together stopped, but the slide-past remained to make the San Andreas. To the north of the San Andreas Fault, subduction is still active, forming the Cascades including Mt. Rainier and Mt. St. Helens.

Long ago, the west-coast subduction zone started in the usual way, with old, cold ocean floor going down into the deep mantle. But as the continent approached the spreading center, the down-going ocean floor became progressively warmer and more buoyant. The ocean floor didn’t “want” to go down, but it was still attached to the older, colder floor ahead of it that was going down. So, the ocean floor went under the continent, but stayed high rather than sinking, and rubbed along the bottom of the crustal rocks rather than plunging steeply into the mantle. The friction between this buoyant subducted ocean floor and the crust above, in turn, caused thrust-faulting and crustal thickening far inland (see the figure just below). Because the western part of the country has been built up of many old rock bodies and sediment piles bulldozed from the Pacific, there are scars of many old faults and other geological features that have been reactivated by the recent events, so mountains and valleys have formed along the old weaknesses in response to the new pushes.

Formation of the Rocky Mountains. Diagram explained in text. — The Rocky Mountains are surprisingly far from the coast for mountains linked to a subduction zone. The diagram shows the most-likely explanation, which is that the subducted slab did not sink as rapidly as normal for a while, and friction along its upper surface rumpled the overlying rocks of North America to raise the Rockies. Credit: R.B. Alley

As to exactly how this came to produce 14,000-foot (4300-m) peaks in the Rockies, geologists can tell the story, but it isn’t clear that any geologist could have predicted this story without seeing the rocks first. Science moves from explanatory (easier) to predictive (harder), so we still have some work to do. (And, we’ve oversimplified a bit here; see the Enrichment for more.)

If you drive west from Boulder up the slope to Rocky Mountain National Park, you will go through sedimentary rocks, made from sediments brought down from the current Rockies and from earlier versions of the Rockies. As folding and faulting pushed up the mountains farther west, and as erosion of those peaks allowed “bobbing up” of their thickened deep root, the sedimentary rock layers were tilted (see the figure below), so you actually will be driving into older and older rocks as you go. Eventually, you will reach the heart of an old (Precambrian) mountain range that also forms the heart of the modern one. Even to a casual observer, the rocks here have been “beaten up.” Follow a single layer in a rock, and you will see that the layer twists and bends, doubles back on itself, or even pinches out in places. Analyze the rocks chemically, and you will find a composition similar to the common sedimentary “mud” rock called shale, but the rock clearly is not shale.

Folding and faulting of the Rockies. Diagram explained thoroughly in text. — At Rocky Mountain National Park, and along the nearby Rockies, the highest peaks were folded upward as a sort of giant rumple. Erosion has removed the younger rocks on top, exposing old igneous and metamorphic rocks in the center of the mountain range. Younger sedimentary rocks were tipped up along the sides of the mountains, and can be seen by tourists driving west toward the national park. Credit: R.B. Alley

Cooking the Earth

Think about cooking. If you mix up a bunch of ingredients to make cake batter, throw the mixture into a pan and put it into a warm oven, the cake you obtain will not be very similar to the mixture you started with. Grill a steak, and the original cow part will come out quite different. Marinade the steak before grilling, and more differences appear. It is common knowledge that a material that is stable in one environment will change if it is placed in a different environment. This is true of everything (and everyone!) on Earth.

The Earth clearly has a great range of conditions. The inside of a mountain range is hotter, higher-pressure, and less affected by acidic groundwaters than is the surface. Materials that are stable at the Earth’s surface (such as the clays in a piece of shale) are not stable deep in a mountain range. The minerals change, grow, and produce new types even without melting. This process is called metamorphism. Metamorphism makes rocks that many people consider to be especially pretty, produces some wonderful gems, and contributes rock names that make good puns. (The Geoclub at Wisconsin used a metamorphic rock, a volcanic rock, and a sedimentary rock in claiming that geologists are “gneiss, tuff, and a little wacke.”) You can read a little more about rocks and minerals in the Enrichment section.

Where Tectonics Meet People: Tsunamis

We’ve been looking at the ways the planet moves rocks around and makes mountains, and some of the ways that mountain-building can be dangerous to humans. Volcanoes and earthquakes are sometimes truly dangerous and damaging. But it is worth remembering that, in the developed world, only about 1% of us die in “accidents,” and car crashes greatly dominate those deaths (so 99% of us die of other things, such as heart disease, cancer, etc.). With good scientific warnings, good zoning codes, trained medical personnel, hospitals and ambulances to take care of us, nature just doesn’t kill that many of us. (In the less-developed world, this is, sadly, less true.) For the developed world, things we do to ourselves (smoking, eating and drinking too much, not exercising enough) are far, far more destructive of health and life than anything the planet does to us.

But, it is still wise to know about the dangers from the Earth. And now that we’ve completed the tour of mountain-building, we will look at another hazard. Tsunamis are not directly related either to the Great Smokies or the Rockies, but anything that makes earthquakes, volcanoes, or really steep slopes in or near the sea might be involved in a tsunami. And tsunamis can be truly horrific. We’ll start with a surprising hot-spot tsunami, and then look at some others.

On the flanks of many of the Hawaiian Islands, including Lanai, Molokai, and Maui, to at least 1,600 feet (500 m) in elevation, there are deposits of broken-up, mixed-up, battered corals, other shells, and beach rocks. Corals are undersea creatures, and surely don’t grow 1,600 feet above sea level. Some corals grow just below sea level and later are raised above the water by mountain-building processes; however, these Hawaiian deposits occur on islands that are sinking as they slide off the “hill” made by the rising motion of the hot rock of the Hawaiian hot spot, and the deposits are geologically too young to have been raised so far by mountain-building processes. Clearly, something strange happened.

One of the deposits in particular is the same age as a nearby, giant underwater landslide, as nearly as the age can be measured, which may suggest something. The Hawaiian volcanoes have rather gradual slopes above the water, where the hot, low-silica lavas spread out to make shield volcanoes. But when lava hits the water, the hot flows cool and freeze very quickly, and can make steep piles. Too steep, and eventually the side of the island fails in a great landslide, perhaps when melted rock is moving up in the center of the island and shoving the sides out to make them steeper. Surveys with side-scanning sonar have shown where several such slides have slipped. (Try saying the previous sentence five times fast!) Such slides can be miles thick, tens of miles wide, and over 100 miles long.

Now, if a chunk of rock miles thick and tens of miles wide suddenly starts moving, maybe at hundreds of miles per hour, it will shove a LOT of water out of the way. Where will the water go? The answer is that it will make a huge wave, or tsunami, that will race up any land it encounters after crossing the ocean. Imagine a wave that would run far inland and reach heights of 1,600 feet above sea level. Fortunately, the highest deposits in Hawaii are from a tsunami about 110,000 years ago, long before people were in the way there. Although many such tsunami-generating landslides have occurred, they typically are spaced thousands of years apart or more. But, we can’t absolutely guarantee that there won’t be another one.

The word tsunami comes from two Japanese words, for harbor and wave, a sort of shorthand for a wave that devastates a harbor. Most tsunamis are generated by undersea earthquakes, but undersea landslides or volcanic eruptions, and even meteorite impacts in the water, can generate tsunamis.

Tsunamis move rapidly across the deep ocean, with speeds of 300 to 500 miles per hour (600 to 1000 kilometers per hour). In the deep ocean, the “bump” of water that is the wave of a big tsunami may be only a very few feet high, but may extend well over 100 miles in the direction it is moving. Waves slow down as they enter shallower water, and the leading edge of a wave hits shallow water before the trailing edge. So, the leading edge slows as it nears the coast, the trailing edge catches up, and the wave goes from being long and low to being squashed and high. Even so, the tsunami wave is usually not a towering wall of water, but a strong surge, something like the tide coming in but higher (hence the mistaken name “tidal wave”).

See caption and text. — A tsunami wave is long and low in deep water, but “friction” with the ocean bed as the wave enters shallower water slows the front of the wave down while the back catches up, causing the wave to become high and short, as shown in this diagram. Credit: R.B. Alley

An especially nasty feature of a tsunami is that the water often goes out before it comes in (waves consist of troughs and crests, and some places get the crest first while other places get the trough first). The sudden retreat of water and exposure of the sea floor tempts people to walk out and look around. Then, the ocean returns faster than a person can run. The outcome is predictable and unpleasant.

Terrible tsunamis have occurred. Still horribly fresh in our memories is the Indian Ocean tsunami of 2004, which was triggered by the second-largest earthquake ever recorded, and which killed over 300,000 people. Japan was much better prepared for the 2011 Tohoku earthquake, but almost 16,000 people still died, mostly from the tsunami. The massive 1883 explosion of the volcano Krakatau in Indonesia essentially destroyed the island, with tsunami waves observed as far away as England. Floods raced miles inland on Java and Sumatra, with probably tens of thousands of deaths. Another volcanic eruption, likely in the 1600s BC, of the Greek island volcano Santorini, pushed a tsunami perhaps 300 feet (100 m) or more high across the coast of Crete, and may have contributed to eventual demise of the Minion civilization there. Many commentators have suggested that this is the source of the myth of Atlantis. The great 1964 Alaska earthquake generated a deadly tsunami that killed 118 people, with deaths as far away as California. In 1958, an earthquake-caused landslide in Lituya Bay, Alaska, caused a tsunami that included a wave 50-100 feet high out in the bay, which a father and son safely rode out in a boat. They watched in awe as the wave then ran 1,800 feet up an adjacent coast; five people were killed in the event.

There isn’t a whole lot that can be done to stop tsunamis, but loss of life and property damage can be limited. Tsunami warning systems are functioning in many places, and are being extended rapidly. When instruments (called seismometers) sense the shaking of the Earth from a large undersea earthquake, volcano, or other disturbance, if characteristics suggest that a tsunami is likely, communications are sent out to various agencies concerned with safety, and sirens or other warnings on beaches are activated to get people away from the coast before the tsunami arrives.

The Indian Ocean tsunami seems to have been especially deadly in places where human activities had caused damage to the coral reefs and coastal vegetation that would have blunted the strength of the wave, so maintenance of these natural buffers can help the people living there. And, scientists can figure out where tsunamis are likely, how big and how frequent they are likely to be, and then zoning codes can be enforced so that people build in appropriately safe ways on appropriately safe land if they want to live in an area.

GeoMations

The Rockies

Credit: Dr. Richard Alley

Transcript: We’ve been looking at how spreading ridges under the ocean make seafloor, which moves away from them and eventually gets old and cold and sinks down. And it will be sinking down beneath a continent, often, and it will scrape things off to make the Olympic. And then it will have a big volcanic range such as the Cascades, Mount Saint Helen’s, and so on.

And they will be sitting there next to the ocean, which we can draw in. And coming up from below, there will be melt to make seafloor out here. And coming up from below there will be materials that erupt at the volcanoes like that.

Now, that works all fine, but what happens when there is a little bit of a swinging down of the slab? We know that the slab is moving away from the seafloor spreading ridge, as I show here. But the slab really does have a little of this swinging down, as well, in some cases. And the continent moves to catch up with it.

And so pretty soon, the continent is going to end up getting close to the spreading ridge. And the stuff going down is then not going to be cold. It’s going to be getting warmer.

And when that happens, we have to erase this, because now the materials don’t want to go down anymore. You’ll get something that goes more like this, with it running right underneath the edge of the continent and staying high. And where it runs under the continent and stays high, you’ll get a lot of rumpling, a lot of pushing happening in this way. And so then you might expect to see something that gets pushed up like this. And we’re reasonably confident that that sort of push up is what the Rockies are.

In a few cases, as in the west, where the San Andreas Fault is, in fact, the subduction zone has been pushed all the way out and has run over the spreading ridge. And the subduction zone and the spreading ridge annihilated each other. And then you’ve got the San Andreas Fault.

Icebergs

Icebergs

Credit: Dr. Richard Alley

Transcript: Out in the ocean, there’s a giant iceberg sitting out here. Big thing threatening the oil platforms floating around. And in the bottom of this very special iceberg, there is a really strange looking, googly-eyed space alien. And you have been given the task to go out there in your little boat sitting here in the water to get the space alien out.

Now, you’re a good geosciences 10 student, and you know that all icebergs have about 1/10 above the surface, and they have about 9/10 below the surface. So you know immediately what to do. You take your gimongous chainsaw and you chainsaw off the top of the iceberg and throw it away, because you know what this will cause is that the iceberg will come bobbing up, carrying the space alien with it.

And so after it gets done bouncing up and down for a little bit, you find that the iceberg is almost as tall as it was before. It still has, sitting way down in the bottom of it, the space alien that you’re trying to get to. So there’s a space alien down here. And it still has about 1/10 of its height above the surface, and about 9/10 of its height below the surface.

But what you find is it’s just a little bit shorter than it was, and it doesn’t stick down quite as far as it did. Now you wump the top off again, and you keep wumping the top off, and you keep wumping the top off. And after a long time, you get down to a little iceberg that doesn’t stick very far down.

Now it still is the same picture, that it has 1/10 above, and it has 9/10 below. And if you’re not careful, you’re sitting there admiring this lovely fact of science, and the space alien sticks out a giant tentacle, and it grabs your ship and throws it to the bottom of the sea. And so you’d better not do that.

However, there is a scientific piece to this. Suppose instead of space aliens, that we wanted to talk about mountain ranges. Now, we know that mountain ranges stick up above the plains. But you might not have known that they also stick down.

They have a root in the same way that an iceberg has a root that it’s sitting on. There’s a slight difference in that about 1/7 of a mountain range is up and about 6/7 of a mountain range is down, way down. The rocks have been heated. They’ve been squeezed. There’s all sorts of interesting things going on and new minerals being grown.

And at the surface, the streams are sitting here, busily trying to grind away the mountain range. As the streams grind away the mountain range, why, the deep stuff will come bobbing up towards the surface. And if you come much later and look at it, you’ll find that the rocks have barely any mountains left.

There’s still a little bit of root with 1/7 up and 6/7 down. But now what you’ll find is that the rocks that had been cooked way down, and bent way down, are very near the surface. And you can go see them.

Optional Enrichment Article

Raising the Rockies

As noted in the text, the history of the West is quite complex, with some big questions not yet answered, and more work to be done. The most widely accepted history, with the most scientific support, is sketched in the text and given in a bit more detail just below and in the Optional Rock Video Review, The Hanging Wall.

Much evidence indicates that beginning about 100 million years ago, the subduction zone in the west grew shallower. The Pacific sea floor that was sinking under what is now Seattle is called the Farallon Plate, and rather than going down somewhat smoothly into the deeper mantle, the Farallon began to move along just beneath the lithosphere of the North American plate. The text notes that the plate was slowly getting warmer over time and thus more buoyant because the ridge and trench were getting closer together, so the plate was having less and less time to cool off before it went down the subduction zone. Another reason, and perhaps a more important reason, may be that a huge volcanic outpouring on the sea floor more than 100 million years ago made a thick layer of not-very-dense rock, which began going down the subduction zone about 100 million years ago. Where smaller bumps on the sea floor are going down subduction zones today, as off Costa Rica, features similar to those seen in the US West are forming, so the bigger features of the West are easy to explain this way.

Anyway, friction between the top of this buoyant Farallon Plate and the rocks above it—the rocks that we see in the Rocky Mountains and elsewhere in the US West—began to squeeze, bend and break those rocks above. The breaks are thrust faults, with older rocks thrust upward and over younger rocks to make the mountains. You will recall that the rocks above the fault are the “hanging wall”, which gives rise to the pun in the Rock Video.

Where a fault didn’t break all the way to the surface, it often raised the rocks above (think of lying on your back in bed under covers, and then raising your knees—the cover draping over your knees are the unbroken rocks). If the rocks were raised in a more-or-less circular pattern when viewed from above, we call the feature a dome; if shaped more like a US football when viewed from above, it may be called an arch, and a few other names are sometimes used. Such uplifts gave us the Black Hills of South Dakota, the Waterpocket Fold of Capitol Reef National Park, the Kaibab Uplift that the Grand Canyon cuts through, the beautiful San Rafael Swell of central Utah, and many other features of the West. These events mostly happened soon after 100 million years ago, during what we now call the Sevier Orogeny, and somewhat more recently in the Laramide Orogeny—we see more faults from the Sevier, and more uplifts from the Laramide.

The squeezing and thrusting of mountains caused some downwarping as well as upwarping. One of the downwarps held the lake in which the pink limestones were deposited that we now see at Bryce and Cedar Breaks, and similar lakes gave us the Green River limestones that produce such fantastic fish fossils from farther north in Utah. Oil is found in the west in rocks that accumulated in such downwarps, too.

The Farallon Plate probably eventually broke, and a new, “normal” subduction zone started up, feeding Mt. St. Helens and Mt. Rainier in the west. The Farallon Slab is now sinking beneath the eastern part of the US. As it sinks, it creates space into which hot rock flows slowly, and this may be helping “rejuvenate” the Appalachians, so the Great Smokies are a bit higher than they would be otherwise because of processes traceable back to the Rockies and the Farallon. Why the Farallon “decided” to sink eventually may be because, as it ran into thick rocks beneath the west, either some of the low-density parts were peeled off so the higher-density ones could sink, or the low-density ones were shoved deep enough that the pressure changed the mineral structures, making denser things that can sink.

Anyway, there is still much to discover about the western US. But, to catch a light-hearted version of what we know, check out the tale of a geology student trying not to be hung on the Hanging Wall.

Still more about plate tectonics and related topics

What happens when you “cook” a rock metamorphically depends on how hot, and what chemically active fluids are present, and the pressure and deviatoric stress. Both pressure and stress have units of force per area, and represent a “push” on a material. Pressure is the part of the stress that is the same in all directions–it squeezes rocks or fluids to make them smaller, but doesn’t tend to change their shape. Deviatoric stress, sometimes just referred to as stress, is an extra push or pull in one or more directions, and does change the shape. Deviatoric stress has been involved in aligning the mineral grains of most metamorphic rocks into layers, or folia. Chemically active fluids—water, carbon dioxide, methane, etc.—can add or subtract chemicals, lower melting points, and dissolve and reprecipitate chemicals.

Materials that are stressed deviatorically can have one of three responses. The materials may bend and, when you release the stress, snap back (elastic deformation), they may bend permanently (plastic deformation or creep), or they may break. We have already seen examples of all of these. Earthquakes are caused by the snap-back of rocks near faults following bending. Faults such as the San Andreas are the result of breakage, and folds such as those around State College, or those in the rocks exposed in the heart of Rocky Mountain, are the result of plastic deformation.

Whether a rock bends elastically or plastically, or breaks, depends on the rock itself and on several other factors: heat (or more properly, how close a material is to melting), pressure, deviatoric stress, and even the chemically active fluids, acting over time. Elastic deformation is favored by low stresses and high pressures (to prevent breakage) and low temperatures (to prevent creep). Plastic deformation is favored by low stresses and high pressures (to prevent breakage) and high temperatures (to allow creep). Fracture is favored by high stresses and low pressures (to allow breakage), together with low temperatures (to prevent creep). Pressure matters in breakage because, to break a material, the two sides must be pulled apart, which increases the size of the material. When pressure is high, this is difficult to achieve. Fluids generally soften rocks and promote creep, although the details depend on the fluid and the rock involved. You will often see that the rocks in a region will have undergone both permanent folding and breakage, and may be bent and ready to snap back, with different modes of deformation more important in different places. Figuring this all out is fascinating, as well as useful (miners, well-drillers, and many others want to know what they’ll hit in the rocks, whether the rocks are about to break, what kind of cracks the oil or gas or ores may be hiding in, and much more).

Melting and Freezing with Chemistry

If you take water and freeze it, you obtain ice that is made of the same stuff as the water. Melt the ice, and you have the water back. Melting and freezing without separating chemicals is easy to understand, but is more the exception than the rule. If you take beer and start to freeze it slowly, the crystals that form will be almost pure water ice, although each crystal typically will start to grow on a small impurity particle in the beer. Filter the crystals out, and you will have cleaned the remaining beer marginally, and you will have increased its alcohol content. Hire some advertising agents, and you have a “new” product to pitch: ice beer. (If you’re under age, please substitute iced root beer. You’ll end up with a lot of ice and a little sugar water.)

In the world of rocks, things are even more complex than with (root) beer. Suppose you take a piece of granite (containing quartz, mica, potassium feldspar, and a sodium-rich sodium-calcium feldspar) and melt it by heating it hot enough to melt basalt. Suppose you then start to cool it. The first minerals to crystallize may be a little bit of olivine, and a calcium-rich sodium-calcium feldspar. Cool the melt a little more, and the olivine and remaining melt react to make pyroxene, while the feldspar and melt react to make more more-sodium-rich feldspar. Keep cooling, and eventually you will get the sodium-rich feldspar and the mica, followed by the potassium feldspar and the quartz.

This ideal sequence may not be observed in many situations, but portions of it are well-known in laboratory experiments and in nature. In general, the first things to crystallize are poorer in silica, sodium, potassium and aluminum, and richer in iron, magnesium and calcium than the melt from which they grew. (Note that there are MgSiO₃ pyroxenes and Mg₂SiO₄ olivines, or mixtures in which the Mg and Fe substitute for each other because they are almost the same size and have the same electric charge.) As the temperature drops, the early minerals react with the melt to make new minerals that are more like the original melt in composition. However, if the early minerals are removed from the melt, perhaps by settling to the bottom, they may be preserved, and exceptionally silica-rich rocks will be formed from the minerals that grow from the remaining melt. For more on the minerals, take a look at our “Sidebar” next.

Sidebar: Minerals and Rocks

If you throw a bunch of typical Earth chemicals into a pot, melt them, and cool them slowly, you will find that only certain things grow. You might, for example, find the mineral quartz (SiO₂), or the mineral pyroxene (FeSiO₃) or the mineral olivine (Fe₂SiO₄). You will not find something midway between olivine and pyroxene; it doesn’t exist. Nature puts the chemicals together in certain ways, and only certain ways. It is a little bit like building with Tinkertoys—there are only certain holes you can put the sticks into, which fix the angles at which you can build things.

Minerals are orderly—the same basic structure is repeated over and over and over (say, a silicon surrounded by four oxygens, each oxygen in contact with an iron that then contacts another oxygen that is one of four around another silicon, which is the structure of olivine). When minerals are allowed to grow freely, they assume certain shapes that look as if a gemstone-cutter had shaped them. The faces on such crystals are controlled by the underlying order of the chemicals. The classification of minerals is based on the chemical composition, and on the structure in those cases when a single composition can assume one of two or a few different structures.

Rocks are collections of minerals. One can have an all-olivine rock, or an all-pyroxene rock, or a mostly-olivine/some-pyroxene rock, or any other possible combination. We humans have chosen to classify rocks based first on their origin, and then on other characteristics such as their grain size, or their composition, or more details of their origin. The main subdivisions are igneous (rocks that formed from cooled magma or lava), sedimentary (those formed from pieces of pre-existing rocks, or from such pieces that dissolved in water and then crystallized from it), and metamorphic (those formed from igneous, sedimentary, or older metamorphic rocks by the action of heat, pressure, stress and chemically active fluids).

The classification of igneous rocks is next (we’ll do igneous and then metamorphic, and save the classification of sedimentary rocks for later in the semester). We distinguish coarse-grained rocks that cooled slowly from magma deep in the Earth, and fine-grained rocks that cooled rapidly from lava at the surface; the extreme case is obsidian, a glass that cooled too rapidly to allow the high-silica types. Low-silica rocks contain the minerals olivine, pyroxene, and calcium-aluminum-rich feldspars. High-silica types include quartz, potassium- and sodium- rich/aluminum-poor feldspars, and mica. Putting these together (grain size and composition) allows us to draw the following grid:

Grain Size and Composition:

—

Low Silica

Medium Silica

High Silica

Small Grains

Basalt

Andesite

Rhyolite

Large Grains

Gabbro

Diorite

Granite

Of these, basalt dominates the sea floors, andesite dominates island arcs, and granite to diorite are common in the hearts of mountain ranges. Many other types occur, but these are the most important ones.

Side Bar: Metamorphic Rock Classification

The commonest sedimentary rock is shale or mud rock, and the commonest metamorphic rocks are formed from sedimentary rocks. For our purposes, then, we will just list the metamorphics that are formed from shale. With increasing heat (or time), the crystals get bigger, and some new minerals are formed. The general trend is shale (essentially mud rock), slate (harder, clinks rather than thuds when you rap it, but with grains too small to see), schist (lots of micas, grains visible to the naked eye), and gneiss (minerals have separated into dark and light layers). All of these are foliated—they appear layered. The foliations in shale come from sedimentation of small clay flakes. Those in slate, schist and gneiss come from alignment of mica grains that grow in the rock, in a direction controlled by the squeeze in the mountain range. Contact metamorphic rocks—those around igneous intrusions—don’t have the squeeze of mountain ranges and so aren’t foliated. But contact metamorphic rocks are heated, and often made wet by water that comes from the magma or by surface water that is driven to convect through spaces in the rocks by the heat of the magma. The commonest mineral in many is amphibole, a water-bearing silicate with four silicons to eleven oxygens. The commonest rock of this sort contains a lot of amphibole, and is called amphibolite.

So Why Did North America Run Over the Spreading Ridge in the Pacific?

Good question. We’re not sure. But we do know that mid-ocean ridges are high because they are hot. Plates will slide off high spots, heading toward low places. So, North and South America are being pushed westward because they are sliding off the mid-Atlantic spreading ridge. If the spreading center in the Pacific simply sat still for a long while (which it probably did, more-or-less), then eventually the Americas would get to it, which they seem to be doing now.

Key Takeaways from Obduction and Tsunamis

Plate Tectonics III: Obduction

Continents and island arcs are too low-density to go subduct
When they run into each other, OBDUCTION results, with folding, push-together (thrust) faulting, and thickening.
This makes the biggest mountain ranges—Appalachians (still high after 200 million years), Himalaya, etc.
Can even push older rocks on top of younger ones.

A Little History

Appalachians formed as proto-Atlantic closed.
Had subduction-zone volcanoes with big eruptions, island arcs colliding with continent, etc.
This ended when Africa and Europe hit the Americas and pushed up the Appalachians (Great Smokies, State College).
When the push-together ended, the great, hot pile of the Appalachians spread under its own weight, with Death-Valley-type faulting.

The Three Basic Styles

PUSH-TOGETHER: subduction (Olympic, Crater Lake, Mt. St. Helens) or obduction (Great Smokies).
PULL-APART: rifting/spreading/sea-floor-production (Death Valley).
SLIDE-PAST: faulting (San Andreas).
Can have intermediates (push-together while sliding past, or pull-apart while sliding past).

Meanwhile, Out West:

Subduction under western US initially cold rock, but as continent moved toward Pacific spreading ridge, hotter rock was forced down, scraped along under US rather than sinking deep, and rumpled up the lithosphere to make Rockies, etc., far inland.
Where and when the push-together of the subduction ended, the pile of the western US spread under its own weight, giving Death Valley faulting.

Old mountains & metamorphism

When obduction collision thickens upper rocks, the mountains sticking up float on a root sticking down
Erode off top of mountains and bottom bobs up (isostacy).
Bobbing-up of eroding mountains brings rocks to surface that had been squeezed deep and hot.
Heating and squeezing turns rocks into metamorphic rocks, often pretty with ores or gems.

Tsunamis

Undersea earthquakes, volcanoes, or landslides, or meteorite impacts, can move lots of water.
Makes a wave (a tsunami) that is long and low in the ocean, but the wavefront slows down as it enters shallow water, and the back catches up and piles up.
Most tsunamis tiny, but can run up on land to elevations above 1000 feet; 2004 Indian Ocean tsunami killed over 300,000 people.
Can’t stop tsunamis, but can give real-time warnings
Can enforce zoning codes to build in safe places, and keep reefs and barrier islands healthy to break some of the tsunami energy.

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The Geology of North American National Parks Copyright © 2022 by Dr. Richard Alley, Evan Pugh, and Sridhar Anandakrishnan is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.