GEOLOGY/GEOPHYSICS 101 Program 8
The summit of Mount Everest: five and half miles above sea level, buried in snow and ice are marine fossils, which were deposited in a shallow sea a hundred million years ago and uplifted by unimaginable forces. The story of how they got there is one of the most fascinating stories in geology.
Until the Theory of Plate Tectonics, the forces responsible for building large mountain belts like the Himalayas were entirely a mystery. It was obvious that it occurred, and there are many examples worldwide of these types of mountains, and the types and directions of the forces which generated them were pretty well understood from looking at the rock structures, and certain similarities are noted among all continental mountain belts, but the origin of the forces was entirely unknown.
Mountains are awesome features of the Earth's surface. The towering heights, craggy peaks, and majestic scenery have inspired great works of art, poetry, literature, and drama, and have also provided dramatic background and subject material for films of all kinds. But mountains are not forever. It seems to us like they're permanent features, but like all Earth features, they're born, evolve, and are eroded away.
It's kind of difficult to imagine a mountain range as a transient feature on Earth, but this is part of the problem we have in comprehending the emensity of geologic time. Most of these major mountain belts began as sediments on passive plate margins, the material deposited there having been eroded from older mountain ranges on the continents mixed with the remains of marine organisms, which settled to the bottom. This is all part of the rock cycle in which rocks are uplifted, distorted, and folded, and eventually leveled again by erosion.
Mountains also give us much information, not only about the movement of continents, but also the growth of continents. The process of differentiation of the Earth continues as continents grow by accretion at their boundaries. It also gives us a chance to study the relationship between the internal oceanic and continental processes. This is especially important since the oldest sea floor is only 200 million years old, and the rocks preserved in these mountain ranges on the continents give us information about ancient conditions on the Earth.
The lesson assignment for today is Lesson 7, Chapter 5, "Mountain Building. "
There are several objectives for this lesson, and I'll go over these with you briefly. In this lesson we want to discuss the characteristics of major mountain belts in terms of the following features:
All of these processes create slightly different mountain ranges. Before I actually introduce you to the lesson today, I have a couple of notes.
Everyone knows what mountains are. They're simply large terrain features that rise abruptly from the surrounding levels, but like people that come in all shapes, and sizes, and ages, and no two are exactly alike. There are five main categories of mountains recognized. Usually, the same types occur in the same region, but in a large mountain system like the Appalachians or the Himalayas, we find all groups present to some degree or another. I'll review with you these basic types.
The first type are volcanic mountains like we're familiar with here in Hawaii. These form from volcanic eruptions, where material ejected from the volcanoes simply piles up around the site of the eruption.
We'll cover these volcanic eruptions, volcanic mountains as a special topic in a later lesson because they are relatively important.
Another type of mountains are fault block mountains. Fault blocks are basically large pieces of Earth's crust that have been twisted, and distorted, and upended, and vary in the continental crust. They're usually bounded on at least one side by a large fault. The best example of this is the basin and range province in the Western United States, which comprises parts of Nevada, Arizona, and Utah. Here the crust is broken into hundreds of pieces, and the pieces are tilted to form these fault block mountains, sort of like the edge of a box buried in the sand. It's also been described as an army of caterpillars marching across a carpet. Along with the tilting and faulting are usually igneous activities, including both volcanic action and intrusions. These fault block mountains are thought to form either by upwarping from magma pushing up from below followed by cooling as the magma cools and the crustal rocks subside, but it may also have to do with changes in plate movements. The Grand Tetons in Wyoming and The Sierra Nevada in Western California are also examples.
Upwarped mountains show the greatest diversity. There are several examples in the United States: the Black Hills in North Dakota, the Adirondacks in New York, and in Colorado around the region of the Colorado plateau, where the Grand Canyon is down cutting through these. In these upwarped mountains the sediments are generally not deformed or deformed only very little, and they usually overlie a granite core. These are probably pushed up by rising magma still deep underground. Mountains can also be formed by erosion.
Erosional remnants like mesas and buttes often occur in desert areas, where the resistant rocks simply remains after the surrounding land has been leveled by erosion.
The largest and most complex type of mountains are folded mountains. These comprise all the world's major mountain ranges like the Alps, and the Urals, and the Himalayas, and the Rockies, and the Appalachians. It's these mountain belts that we're concerned with, so I'd like to clarify terms with you for a second.
A mountain is an individual peak. A mountain range, on the other hand, is a group of closely spaced mountains or parallel ridges. The mountains belts are large chains of folded mountains thousands of kilometers long, and I can show you some examples of where these are located on the globe. Beginning in Southern Asia with the Himalayas, a mountain belt, including the Caucuses extends all the way into Europe where it becomes the Alps. On the other side of the world in North America, we find the Rockies, which extend Alaska, to Canada, the Cascades, and the continuation of the Rockies, and in the Eastern part of the United States, the Appalachians. There are also the Andes in South America.
We recall that the continental crust is made out of granite and is thicker than the oceanic crust. The crust is thicker under the continents, and the continents themselves are covered by a thin veneer of sediments in most areas, but it turns out that the continents themselves consist of two fairly different kinds of structural units.
One of these is called "cratons." The other are the orogenic belts or the mountain ranges. The cratons are vast regions on the interior of the continent, which have attained tectonic stability. That means that they haven't been changed much over hundreds of millions of years.
Some continents have more than one craton: Australia and Africa, for example. It's as if two separate continents have been welded together. These cratons are usually stable for long periods of time, hundreds of millions of years. The rocks that comprise the cratons have been deformed from ancient tectonic processes, and it's thought that they represent the deep cores of ancient mountain ranges. It's these cores around which the continents have grown.
Orogenic belts, and the word "oro" means mountain, so orogenic belts are basically mountains belts, are elongated regions of continental crust that have been intensely folded, and faulted, and metamorphosed. They're also intruded by massive amounts of magma, which cool under ground during repeated cycles of mountain building. These orogenic belts differ in age, size, history, and exact origin, but they were all once mountainous regions that are now eroded to various degrees.
The Appalachians, for example, are nearly reduced flat; whereas, the Himalayas still extend 29, 000, 30, 000 feet above sea level. The cratons themselves may be either exposed or covered. The exposed portion of the craton is called the "continental shield."
The continental shield is usually near the center of the continent and consists of the most ancient igneous and metamorphic rocks; in fact, it's within these continental shields that we find the oldest rocks anywhere on Earth. They may be as old as 3. 6 billion years old. That means that they're about three quarters as old as the Earth itself. The continental shields have little or no sedimentary covering. In North America the Canadian shield is exposed all the way from Hudson Bay in Eastern Canada to Lake Superior around Minnesota. Here the continental shield is exposed by continental glaciation, where glaciers have scraped off the overlying sediments and deposited them someplace else.
The rocks in the Canadian shield are generally older than 2 billion years; in fact, there are no rocks older than about 1. 8 billion years located in the Canadian shield. By contrast, the stable platform is covered by a thin layer of sedimentary rocks, and by thin here I mean only a few thousand feet. These sediments are largely undeformed by any tectonic processes although there may be some large scale upwarping or downbowing. Here the basement rocks, or the rocks underlying the sediments, represent the cores of younger mountain belts which surround the cratons.
In other words, the cratons are very old, and surrounding them are the continental shields, which represent the cores of yet younger mountain ranges that have grown outward from the cratons. In the United States the Great Plains between the Appalachians and the Rockies comprise the continental shield.
The orogenic belts have many features in common, which suggest a common origin for all of them. They usually consist of parallel ridges of folded and faulted sedimentary rocks. Portions of them, especially deep down, are strongly metamorphosed and also intruded by younger igneous rocks. The sedimentary rocks, which form the folded portion of the rocks, formed from sequences thousands of feet thick, sometimes exceeding 30, 000 feet, and these rocks were deposited before the orogeny; in other words, before the mountain building phase.
The period of orogeny itself, the mountain building process, commonly exceeds a hundred million years in time. The rock structures that make up these sequences suggest that the deformation of the rocks proceeded in a landward direction; that is, it started from the sea and worked its way inward toward the continent. The deep water sediments were the first to be deformed. The rocks that we find preserved as these deep water sediments take the form of greywackies and volcanic debris of various kinds and shales.
We'll learn more about these rock types as we progress in later lessons. These rocks were intensely folded, and faulted, and metamorphosed. It's almost like a vice with a moving jaw moved in from the sea compressing the sediments ahead of it. There are also numerous intrusions of magma, which generate this igneous and metamorphic core that we see preserved in the cratons and the continental shields.
During the process of mountain building general thickening of the crust as its compressed causes the rocks to ride above sea level, and we find that the thinner shallow water deposits that represented the continental shelves are shoved inward toward the continental interior, sometimes producing giant thrust faults hundreds of miles long. It's as if the one block of continental crust was moved over top of another one.
These rocks consist mostly of sandstones, limestones, and, well, some shales. They also later, as part of the later stages of the orogeny folded and broken along smaller thrust faults. Today's video shows us many pictures of mountain ranges to help us visualize these processes and keep in mind the idea of the model. Keep in mind plate tectonics as a model to see if you can't get a sense of how plate tectonics relates to the formation of the mountain ranges. The video also tries to illustrate for us how the continents grow by accretion of mountain ranges at the edges, so with that in mind, let's watch the video.
Music. Major funding for "Earth Revealed" was provided by the Annenberg CPB Project. Music.
The majestic sight of a mountain range is an endless source of wonder and beauty. To most people, mountains are synonymous with great size and permanence, but are mountains really permanent?
Rivers flow out of nearly every mountain range on earth carrying sand and rock that were eroded from the mountains themselves. This process would eventually remove the mountains from the landscape unless somehow they were being maintained by uplift. Mountains are built by tectonic processes that cause portions of the Earth's crust to rise. These processes are fueled by the escape of heat from the interior of the Earth, causing crustal uplift by volcanic activity and by movement along faults that, in turn, is responsible for the formation of mountains.
Mountain building processes like these are concentrated at the boundaries between tectonic plates and are especially active where the plates are moving apart or converging. By studying the origin of individual mountain belts, geologists are helping to unravel the tectonic history of our planet. With the development of the theory of plate tectonics, geologists finally had an explanation for what causes mountains to grow.
In addition, the geographical distribution of mountains also began to make sense. Most of the worlds great ranges lie not at the center of continents, but instead close to their margins. In general, the centers of continents consist of stable regions of very old crust. These regions called "cratons" or shields are deeply eroded, mostly low lying, and level.
One of the world's largest cratons lies in the middle of the North American Continent, a great rolling flatland. To the north, this flatland has been stripped bare of much soil and sedimentary cover by past ice sheets. This is the Canadian shield, and within it is a vast region, the superior province, over 1, 500 kilometers across.
Here the rock ranges from 2. 6 billion to 4 billion years old, the ancient heart of North America. Cratons this ancient are significant for they hold clues to the birth of continents. The typical rocks of the superior province are granulite and greenstone. "Granulite" is a high grade metamorphic rock and makes up most of the landscape.
It is too severely metamorphosed to provide much information about the past, though its exposure at the surface indicates that the crust has been deeply eroded. Scattered throughout the granulite are intricate belts of volcanic and sedimentary rocks called "greenstones."
Geologists interpret these as the remnants of numerous small island arcs, which have been closely packed together. Initially, in the early Earth, the crust of the Earth must have been principally oceanic crust surmounted by a thin veneer of water and a very dense atmosphere. Gradually, due to convergent plate motion, they were probably pretty small thin platelets back in early times. We're talking three, four billion years ago.
Small island arcs began to form along and above convergent plate junctions due to the rise of melt from the downgoing slab up into these primitive arcs. As those island arcs formed they were swept together by this continual sea floor spreading process, probably involved a lot of small rapidly overturning convective cells in the upper mantle, so very rapid growth accompanied the early Earth, and as these accreted together they formed enlarging eventually supercontinental assemblies.
Those assemblies were kneeled over time, and gradually they formed relatively larger plates capped by continental crust. The continental crust itself is an amalgam of these smaller island arcs, which had been all swept together. Continental crust, however, consists mostly of granitic rocks and not greenstone lavas, so a mechanism must have existed for transforming the composition of the crust.
Some geologists speculate that the greenstone lavas weathered to form sediments rich in potassium, aluminum and silica. These sediments could have been incorporated by magmas erupting at a later date altering the molten rock to a more granitic composition. Or perhaps repeated partial melting of the lithosphere underneath the greenstone islands produced a more granitic crust. Whatever the reason, the world of greenstone belt volcanism did not last.
At about 2 billion years ago the general tectonic pattern appears to have changed, where there was a higher organization of larger plates and continental margins, which began to move into more of a rigid plate tectonic domain, and that change may have been related to the development of a thick lithosphere.
Early in Earth history, there may have been a much thinner lithosphere, which promoted a much more random and faster shorter cycle pattern of convection to give us these ancient protocontinents, which we call the greenstone belts. But later on as perhaps the Earth cooled, and the lithosphere thickened, then we began to drive around thicker plates, and the continents began to embed themselves into plates that had mantle roots, and they began to develop their own unique lithospheres as well.
For almost a billion years, the superior province was almost all that existed of North America. Then, other regions were added: the Churchill Hudsonian Province, 1. 8 billion years ago, the Central Province, 1. 6 billion years ago, and the Grenville Province, about 1 billion years ago. In these regions geologists find rocks similar to those making up modern mountain ranges, and in the very youngest parts of North America, mountains are still growing.
These observations suggest that mountain building, a process geologists call "orogeny" is an essential part of continental growth.
Generally speaking, the further one travels from the craton, the younger a continent becomes, and the youngest, most rugged parts of a continent lie at the edge of the ocean. In fact, the essential key to mountain building lies in understanding this link between ocean basins and continents.
In the Eighteenth Century Scottish Geologist, James Hutton, recognized that much of the rock that composed his native land originated beneath the sea as sediment accumulating quietly on the ocean floor. Rocks today exposed in the Earth's mountains tell us how those mountains came to be. James Hutton, I think, was the first to make a very giant step with regards to the growth of mountains. He found that wherever he went into the Earth's mountains, he found sedimentary rocks. Sedimentary rocks that were formed always formed on the bottom of the ocean floor. He realized that there had to be a connection between marine deep sea sedimentation and mountain building.
What was once on the bottom of the ocean floor was later destined to become at the tops of mountains. Hutton had no explanation for how marine sediment could be uplifted and added to dry land. He simply observed that it had occurred. He also recognized that as the rock of mountains is eroded down it supplies new sediment to the sea floor which, in turn, can be converted to rock and uplifted forming new mountains.
The fact that a great deal of material in nature is recycled over and over was one of Hutton's most brilliant insights. It showed that the land grows from the sea. The Theory of Plate Tectonics developed nearly two centuries after Hutton's time explains how the ocean floor rises to become mountains.
Because of differences in density and makeup of continental and oceanic lithosphere, the edge of a continent is a likely place for a convergent plate boundary to form. If enough pressure is applied, the lithosphere splits. Heavier than continental lithosphere, the oceanic lithosphere begins sinking or subducting beneath the adjacent continent as the new plates are pushed together.
An orogeny begins as volcanoes form an island arc or belt along the continental margin. Sea floor sediment between the trench and shore is caught in the squeeze, crumpled up and uplifted. Ultimately, so much pressure and heat may accumulate that the soft sediment recrystallizes into durable metamorphic rock, the very rock that makes up vast areas of older continental crust.
At high enough temperatures the sediment will even melt, eventually forming igneous rocks such as granite. In the course of mountain ranges, we could see in the metamorphic rocks that sediments are folded, and metamorphosed, and actually involved and melted into typically granitic types of materials.
We also look within the igneous and metamorphic core and see evidence of magma that move directly out of the mantle, which will be more basaltic composition, and, in many instances, promoted the melting of the crustal rocks. We also see evidence of older continental crust having been heated up, and melted, and rejuvenated into younger new continental crust. These hardened rock types slowly wear away contributing new sediment to the sea. Much of the rock does not wear away, however; instead, it remains attached, adding new mass to the continent. New material is also added as magma from the underlying mantle and subducting ocean floor rise up into the crust to cool and harden or to erupt as lava and ash.
The continuous transformation of material from sedimentary, to metamorphic, to igneous and back again, is called the rock cycle.
Throughout a growing mountain range, the complete rock cycle may be active creating new continental crust.
Oceanic crust represents material, which has come to the Earth's surface directly by magmatism out of the Earth's mantle. As the most common type of magma to come out of the Earth's mantle, it comes out as a composition of basalt, which is a silica magnesium rich rock. The continents represent material that has been recycled numerous times by igneous activity, metamorphism, sedimentation, deformation, melted mountain building, deposition into basins, and recycled recycled. It basically came from proto- oceanic crust, but it has undergone many different changes through these processes through time to make it distinct and compositionally much lighter, which is a composition more similar to granite.
The mountain building process causes continents to increase in size over time. We know from radiometric dating of rocks that the central portion of the North American Continent is composed of very old rocks, all of which formed over a billion years ago. The Appalachian Mountains were then built on to the eastern margin of the craton in a series of collisions ending about 250 million years ago. The Sierra Nevada Mountains were then added to the western margin of the continent in a process ending about 80 million years ago, followed by formation of the Cascade Range, which continues even today.
In this way, the North American Continent has incrementally grown by accretion in a concentric pattern with the oldest rocks in the center surrounded by younger and younger mountain belts.
Subduction of sea floor is the most common reason mountains form, but it is not the only way. Another method is "accretion" the joining together or separate land masses. When two masses of continental lithosphere, such as India and Asia are brought together by the complete subduction of an intervening ocean basin, the collision raises huge mountains.
Sea floor and sedimentary deposits caught in the squeeze are metamorphosed and converted in part to igneous rock. This process glues the once separate land masses together. The accretion of two such large bodies of lithosphere as India and Asia is not a common geological event, but there is evidence that much smaller bodies of land are frequently added to the margins of continents by subduction.
Small continental fragments, such as Madagascar and the Fiji Islands are scattered throughout the world's oceans. Because of their buoyancy, such fragments cannot be subducted. They merely become glued by metamorphism and igneous activity to the continents they collide with. As this happens, the position of the subduction zone jumps to the seaward side of the added land mass.
Oceanic sea mounts and islands also may not be subducted, being too thick to pass into the trench at the convergent boundary. Much of the landscape of Western North America is made up of continental fragments, sea mounts and island arcs. These land masses have attached to the continent during the subduction of Pacific Ocean lithosphere over the past 150 million years.
Such fragments are commonly described by geologists as "exotic, " "suspect," or "accreted" terranes.
The term "terrane" refers to an area of rocks having continuous strata or structure and a distinctive composition. The boundary between an accreted terrane and the main body of the continent may be marked by a fault zone or in places by a belt of oceanic rock, which was not subducted but caught in the squeeze between the colliding land masses. To determine the ultimate origin of an accreted terrane geologists look for specific clues in the field. The kinds of clues geologists can use are fossils, which fossils in a terrane, which would be very different from the fossils of a neighboring terrane or the neighboring continent.
The magnetism of rocks in a terrane can also be distinctive. Geologists compare the magnetic direction recorded by rocks in an accreted terrane to the magnetic direction in rocks that formed on the continent itself. If these directions of magnetism are very different, that's a clue that the terrane has traveled a long way to get to where it is now. Geologists have discovered that not all accreted terrane come from plate drift across ocean basins. Some terrane are merely huge pieces of a continent, which are sliced off and shifted great distances by movement along large faults. Geologists can't always distinguish between terrane of this origin and those which truly come from across an ancient sea; in fact, many mountain belts seem to contain terrane of both types.
The notion that some terrane are exotic or suspect is controversial because it's unclear as to how far some of these terrains have traveled. Some may be only locally derived. I mean let's say portions of Baja, California were offset and then rafted to the edge of California. That would not be very far traveled. But let's say portions of China traveled all the way across the Pacific Ocean, and they were rafted into the sides of Oregon; that would be far traveled.
It's clear from paleomagnetic evidence, fossil evidence, that some bits of material, some from the ocean crust and some from other parts of continents have been brought great distance, some thousands of kilometers and accreted to the edge of continents.
When we look at modern day mountain belts, we recognize there are really two portions, an interior portion of metamorphic rocks and igenious rocks that are intrinsic to that continent and then an outboard portion on amalgamation of exotic or suspect terrane that have come from various distances and representing various ages of ancient rock.
Regardless of how they formed, mountain belts along convergent boundaries stop growing when subduction ends. They gradually deteriorate to become part of the low-lying craton itself. Ultimately, of course, mountain building ends, and that signals the end of convergent plate motion, a settling back or perhaps low angle distributive faulting occurs, which extend the mountain belt rather than compress it, and the forces of erosion once this constructional stage is over take over.
Gone, too, also is the volcanism that characterizes early and middle stages of many mountain belts, but the actual geologic mountains are then terminated by erosive processes. There may be later uplift, which provide strong relief and gives you topographic mountains, but this later process is not strictly speaking a mountain- building process; it is simply an uplift and erosional process.
In Eastern North America, the Appalachian Mountains continue to exist more than 200 million years after the plate collisions that formed them. Given rates of erosion, these mountains should have worn flat tens of millions of years ago; yet they still stand, indicating that some uplift must be continuing.
The cause of this puzzling late stage uplift was discovered in 1859 by British surveyor, G. B. Airy. While working in India, Airy discovered that plumb bobs, iron weights used to level sighting instruments were less attracted by the gravity from the nearby Himalayan Mountains than they should be if the Himalaya were directly underlain by the same dense rock presumed to form most of the Earth's interior.
This suggested there was less mass present beneath the Himalaya than previously thought. To explain this discrepancy Airy concluded that a low density root must lie beneath the range. Geophysical studies have since confirmed that the crust beneath the Himalaya extends to a depth of 75 kilometers, twice as thick as ordinary continental crust.
It's now known that most mountain ranges are underlain by crustal roots floating atop the hot plastically deforming mantle. The roots grow as a result of compression during plate convergence. As mountain ranges are worn down, their roots are buoyed upward by the mantle.
Because the mantle is far stiffer than the most fluid lava, the crust flows upward quite slowly sustaining a hilly topography in the landscape for hundreds of millions of years. As the crust rises, rocks from ever deeper levels inside the Earth are brought to the surface and worn away.
The floating of Earth's crust atop the mantle is termed "isostasy". This is similar to what happens at sea, where large icebergs float with more ice extending beneath the surface than small ones do. In the same way, tall mountains usually have roots extending deeper into the Earth than low mountains made up of the same rock type. In both cases, far more mass lies hidden from view than can be seen at the surface. Isostasy is the process by which different thickness and different density irregularities in the outer Earth float in gravitational equilibrium with one another.
When you build up a large mountain range, you're liable to have a root underneath and a lot of material piled up high on the Earth's surface, and, ultimately, if you don't have forces to keep it piled up, that is going to tend to want to equilibrate and float in gravitational equilibrium with the other areas around it.
As mountain belts uplift and late in their stages, they may begin to actually undergo extensional collapse or breaking apart at the high levels due to the force of gravity. At their deeper levels, there may be plastic flow underneath them or compensation by flow in the mantle in order to let whatever root that exists to equilibrate and to come to gravitational equilibrium with the mantle and a lower crust around it.
During this stage of ultimate isostatic equilibration, if there are no longer major forces uplifting the mountain range, then erosion will ultimately win out over the uplift process, and the mountain belt will be beveled to a much flatter lower relief surface. At this stage the mountain belt is well on its way to becoming part of the craton.
Through geologic time, the amount of continental material on Earth has slowly grown in size at the expense of the ocean basins. But tracing the history of growth on individual continents is a great challenge for each continent today has been joined to other continents in the past.
The general pattern in continents is to find the oldest material in the interiors of the cratons, and this is because the cores of the continents formed and then successive mountain belts and continent-edge accretions occurred around their margins. But geologists find that pattern to be imperfect because continental masses tend to break and rift apart during their growth. And so if they break apart, they may break apart across older interiors of continents across younger mountain belts, and then subsequently they may form a new mountain belt across a broken edge, so that leaves us with a competing series of processes of marginal growth, and breaking apart and drifting, and then colliding back together and growing again.
Mountain ranges, newly forming and ancient, mark the growth of continents in response to plate movements. Floating on Earth's plastic mantle, these gigantic topographic features disappear slowly as their low-density roots are buoyed up. So mountains owe their existence to two factors: the heat that drives plate tectonics and the effects of gravity.
In time, mountains wear flat, adding new crust to the cratons, the oldest, most stable lands on the planet Earth.
One of the benchmark discoveries in geology over the last half-century is the origin of mountain ranges. Continents and oceanic crust have collided or subducted at tectonic plate margins. Mountain ranges have been formed, and processes of erosion have torn them down. Eventually, the continents are split apart by renewed plate divergence and are on their way to new collisions, often forming a supercontinent. This tectonic cycle, sometimes referred to as the "dance of the continents" has been repeated many times in the geologic past with each complete cycle lasting several hundred million years. Some aspects of this tectonic dance have surprisingly complicated steps. Alaska, for example, is largely composed of plate fragments that have been packed together by successive collisions. Some of these terrane have been tectonically transported thousands of kilometers by sea floor spreading and strike slip faulting before colliding with North America to form Alaska.
The Mediterranean Sea is a shrinking ocean basin caught in a collision between the colliding continents of Africa and Europe, the famous volcanoes and earthquakes and intensely deformed mountains of this region are evidence of the profound mountain building that accompanies the death of an ocean.
Tectonic cycles and mountain building are nearly as old as the Earth itself, and the forecast for the geologic future is continued change, change in the ocean basins, and continents, and mountain ranges, that together are the face of the Earth.
A map of the world a billion years from now will be a scant resemblance of the world we know today.
Music Major funding for "Earth Revealed" was provided by the Annenberg CPB Project.
This is such a fascinating process, this mountain building, and when you think about it, it's amazing that we, as geologists, could figure this out at all considering how little of the Earth we can see at any one time and how many different ways we have to put all this information together, but can't you see how nicely this fits into the model of plate tectonics?
That much of the facts about mountain building: the forces, the folding of the rocks, the igneous intrusions, the metamorphisms. All of these things were known long before the plate tectonics theory came around. The model seems to provide the mechanism which ties everything together.
We also have new evidence as well that goes into examining the mountain building process, and we'll try to come back and look at some of that later on. From the video we see that mountain belts start out as thick piles of sediment, as much as 10, 000 meters thick; that's six miles thick.
This sediment is accumulated mostly on passive continental margins, which have since become active.
I'll remind you that passive continental margins are those found on the trailing edges of continents, where usually it's tectonically stable; not much change happens over long periods of time; whereas, active continental margins are found on the leading edges of continents, where the continents are distorted.
It's evident that the sediment must accumulate on the continental margins for long periods of time; in fact, we can do a little bit of calculation. The average sedimentation rate on continental margins is around 25 centimeters or about 10 inches per year; that's about the length of my hand.
Did I say per year? Twenty-five centimeters per thousand years. It makes a little bit of difference. At this rate it takes about 40 million years to accumulate 10, 000 meters of sediment. We might also note here that nowhere is the ocean 10, 000 meters deep, so in order for these sediments to be deposited underwater, there must also be an isostatic adjustment taking place; in other words, the weight of the sediment depresses the continental margin due to the extra load of the sediment.
We might note here that this 40 million year period is really quite short in geologic terms although it's very long in human terms, and we might also note that the 40 million years to accumulate this 10, 000 meter thick layer of sediments is about the same amount of time that it takes a mountain range to erode.
So we have an older mountain range on the continent giving up sediments to the sea floor, and those sediments then later become compressed and a new mountain range added or accreted to the edge of the continent. It's probably caused by changing plate motions.
Now, keep in mind that the rising convection currents are rather chaotic, and as spreading centers change location, as continents bump into each other, as ocean plates stall and move at different speeds, a passive continental margin may eventually change into an active continental margin. The resulting compressive forces at the new active margin deform sediments, change them into sedimentary rock, and if they're subducted deep enough, may even melt them.
Various different things happen as the rocks are subducted depending upon the rock type and the rate at which the process happens, the temperature, and how deep the rocks are subducted. We'll examine the nature of these forces and the types of deformation that rocks undergo in the next lesson.
The evidence shows us that some mountain ranges conform and reform several times. The Appalachians, for example, show at least three separate episodes of mountain formation, which represents successive openings and closing of the Atlantic Ocean; in other words, before the supercontinent of Pangea broke up 200 million years ago, the Appalachians had already been formed, and the new rift opened up along a different seam from the old rift; in fact, the seam never opens in exactly the same place, and parts of the Northeastern United States, which are now attached to the North American Continent actually belong to the Continent of Africa, or in some cases, the Continent of Europe.
We also note that the structures of the Appalachian mountains: the folds, and the faults, and the metamorphic belts, and so forth, continue on the other side of the Atlantic. There are mountain ranges with the same structures that are found in Ireland, and Scotland, and Greenland, and in Scandinavia, and this continuation of the structures was noted by Wegener in his Continental Drift Theory; in fact, he used this as one of the lines of evidence to suggest that the continents might have at one time been connected in the supercontinent.
We see that the Appalachians and other mountain ranges may have rivaled the Himalayas in size more than once. This is difficult to imagine because on our paltry scale of human existence, mountains seem to be an everlasting feature, and, in fact, the changes in a given mountain range that have happened during the time humans have been on the Earth are really insignificant.
It's safe to say that even the Appalachians looks pretty much the way they do now when the first people arrived on the North American Continent as much as 50, 60, 70 thousand years ago. All of this is part of our difficulty in visualizing geologic time. When we try to compare geologic time scales with human time scales, our time really is insignificant in relation to the age of the Earth.
This just points to the fact that all Earth processes, including mountain building and erosion, are slow but relentless, and like all geologic processes, as slow as they are, over a long enough period of time, they can produce tremendously large results.
You see, the way that this mountain building process works is a little bit like scraping peanut butter and jelly off of a knife onto a piece of bread. The peanut butter and jelly get deformed and mixed around and stick to the edge of the bread. Now, of course, the actual process is somewhat more complicated than this, not only because of the large time scale, but because rocks are really a little more complicated than peanut butter and jelly.
But sediments, old sea floor pieces of continents, or whatever else happens to be drifting along with the lithospheric plates are deformed as they're incorporated into active margins at subduction zones.
One good example of this is ophiolites. You may remember that ophiolites are fragments of the upper mantle and crust, which involve the gabbros, the sheeted dikes, and the pillow of basalts.
The Island of Cypress, as noted in the last lesson, is one of these pieces of ophiolites, which was rafted for some distance over from a spreading center along the lithospheric plate, which is now embedded, in this case, in oceanic crust, which protrudes above sea level to become an island.
There are other examples of ophiolites, which are incorporated into the structures of folded mountain ranges on several continents. We may also find simply fragments of oceanic crust. In the Cascade Mountains and on the Olympic Peninsula in the western part of Washington State, we find mixed in with sedimentary sequences of various types, pieces of basaltic oceanic crust that seem somehow really out of place with the continental rocks. We also find evidence for pieces of broken continents being incorporated into the mountain ranges.
These have just recently been discovered, recently being in the last 10 or 15 years and had been called "terranes."
Studies of the West Coast of the United States revealed that along the newly emergent coastal ranges we find rock types that don't seem to correspond to one another. You find a piece of one type of rock next to a piece of different kind of rock, and it's not clear exactly the relationship under which these different types of rocks formed until you incorporate the plate tectonics model.
If we look at the world map today, we see next to some of the major continents, small pieces of continental fragments. One example, the islands of New Zealand. Now, these happen to correspond to a subduction zone, and there are active andesite volcanoes on the island of New Zealand, but it's a small crustal fragment, and another example is the island of Madagascar off the east coast of Africa.
If you imagine these continental fragments embedded in the lithospheric plates, they may be rafted great distances across the Earth's surface, where they eventually will come in contact with a continent and be scrunched up against the edge of the continent and partially subducted. Keep in mind that the continental fragments themselves can't really be subducted because they're lighter. They're lighter in density, and they basically float on top of the oceanic crust and the mantle, so they can't be carried down completely and subducted, so the West Coast of the United States is made of many of these terranes.
As many as 25 or 30 different terranes have been identified, some of them having moved as much as 10, 000 miles across the Pacific Ocean from the Southern Hemisphere. The rocks that make up these terranes, the continental fragments, are distorted and blended with the other folded rocks that form from the orogenic process of distorting of sediments.
Also note here that descending plate motions create intense heat and pressure. Now, this pressure and the heat increases with depth. On one hand, the Earth's temperature naturally increases as you go deeper; on the other hand, some of the energy of the movement of the plates, the kinetic energy of the plates is translated into heat, which causes the temperature to rise even more than it would otherwise.
It is a problem with geologists, or maybe we shouldn't say it's a problem; should say it's been discussed how these cold descending basaltic plates can generate heat in this way, but it's one of those minor problems with the Theory of Plate Tectonics that needs to be worked out.
Because of the difference in heat and pressure as you go deeper in the Earth, different effects happen, and as we'll see in the next lesson, the way in which rocks respond to forces depends upon several factors, which include how strong the forces are, what the temperature of the rock is, and the time period over which the forces operate.
For now we can just note that rocks near the surface tend to be brittle; that means that when they're subjected to forces, they tend to crack and break to cause earthquakes, so we find most of the earthquakes associated with the active plate margins either near the surface or down to a fairly limited depth.
As the sediments and assorted pieces are buried a little deeper, we find that the heat lithifies the rocks; that is, changes them into different forms and causes them to fold. Rocks which are subjected to high temperatures tend to bend rather than break, so the folding of the rocks then takes place with some fracturing but mostly in what we call a plastic deformation. At yet greater depths, the atoms that make up the individual minerals in the rocks can actually rearrange themselves without melting.
This is what happens when regional metamorphism occurs. "Regional metamorphism" means metamorphism, the changing of the rocks, over a fairly large region. Here the rocks are drastically changed as new minerals form from the minerals in the original rock and leave a signature for us to examine later to give us clues about the depth at which these transformations occurred.
At yet greater depth, we find that the rocks actually begin to melt. The first material that forms as the rock melts tends to consist of the lighter elements like silica and aluminum, and I'll remind you that if these terms seem a little strange right now, we'll come back and learn more details about the minerals, and rocks, and atoms, and so forth, in later lessons.
The first material to melt is much lighter and tends to rise and placing itself as igneous intrusions or batholiths. The heavier material tends to sink into the mantle, or it's reincorporated. The plates lose their identity at about 700 kilometers depths.
Okay, there are many effects of mountain building which are of interest to geologists. This actually represents a continuing differentiation of the Earth. The process of "orogeny" tends to concentrate the lighter elements in the crust when recycles the heavy elements back into the mantle, and this, overall, increases the layering of the Earth.
It's apparent that this process must have begun as soon as there was oceanic crust; in other words, in the very early stages of the Earth's formation, or at least it must have begun as soon as plate tectonic movements started, and since we believe that plate tectonics is driven by convection currents, there is every reason to believe that this process has been going on basically since the time that the Earth's crust solidified.
This, of course, adds to the layering of the crust and is involved in this process of separation into mantle, crust and core. We see the results of these ancient orogenic processes in the distorted rocks of the cratons and the continental shields. It's almost as if the continents grow from the center outwards.
Now, the first continents that formed must have formed from similar processes as we see happening in more modern mountain ranges; the difference, of course, the first continents that formed had to rely upon interactions between oceanic plates. As the video showed, there are significant differences in the way mountain ranges form, depending upon whether the colliding plates carry continents or not. On one hand, you can have two plates collide, which only carry oceanic crust, or one or the other of the plates, either the moving one or the stationery one can carry continental fragments or continents, or both of the plates may carry continents as happened in the formation of the Himalayas when the Continent of India moving across from the southern ocean collided with the Continent of Asia, so the continents then grow outward from the center, so that we find the oldest rocks in the continents near the center.
In contrast with the oceanic crust, we find the oldest rocks in the ocean near the edges of the oceans. This is due to the fact that the two processes although complimentary are really quite different.
Oceanic crust is created at the center and moves away so it becomes older as you move away from the ridge. Continental crust, on the other hand, is formed in a core. When the upper mountain range is eroded away, the core is left behind, but those sediments are placed offshore where they're crumpled, and squeezed, and deformed against the edges. When that mountain range is eroded, those sediments are deposited offshore where they're eventually crumpled and added to the edge of the new continent.
This is all part of a rock cycle where rocks are transformed from one form into the other from igneous sediment to metamorphic rocks. It's all part of a cycle of uplift, erosion, deposition, deformation, metamorphism, and melting, which creates new land to be eroded.
It's within this new land, the uplift of land, that we find the rich in variety of environments on the Earth's surface. It causes things like streams, ground water, and landscape development, all of which are things which geologists would like to understand. Earth is a dynamic planet; it's geologically alive; it's continually changing, driven on one hand by the massive internal heat engine which manifests itself in plate tectonics, and, on the other hand, the surface is acted on by the energy of water, ice, and wind, which is extracted from the external heat engine, the sun.
Next time we'll do Lesson 8, "Earth Structures, " so before then you should read Chapter 6, pages 123 to 145 and be sure to understand the difference between "elastic, " plastic, " and "brittle deformation."
Well, that's it. Study hard. I'll see you next time.