GG101 The Sea Floor

GEOLOGY/GEOPHYSICS 101 Program 5

THE SEA FLOOR

Hello. In 1912, a German meteorologist named Alfred Wegener detailed a theory that would provide the key to a major revolution to understanding Earth.

As part of his continental drift hypothesis, Wegener predicted that a key to understanding the Earth would lie in discoveries that he made on the ocean floor. Despite many years of rejections by geologists, it turns out that Wegener was right. The theory of plate tectonics did, indeed, arise from the revelations discovered on sea floor features.

The theory of plate tectonics incorporates Wegener's theory of continental drift but also a more modern theory called "Seafloor Spreading." It shows how sea floor processes are intimately linked with geological processes that occur on the continents.

Now, before the 1940s, it was generally thought that the sea floor was mostly flat and featureless. Mapping the sea floor is very difficult and time consuming and not very accurate. Sonar was invented in the 1920s to help locate submarines but also to help in mapping the ocean floor. When maps of the sea floor began to emerge in the 1960s, they revealed a very rich structure of the ocean floor.

We find many undersea volcanoes, deep trenches 30 thousand feet deep, an undersea mountain range 80 thousand kilometers long that's as tall as any on Earth and stretches around the Earth like a seam on a giant baseball. We find large fractures on it extending outward from the mid-ocean ridges and flat shelves surrounding the continents, which are cut with deep canyons. Yes, the broad, flat, featureless plains are there, but it's not those that are the interesting features. It's the other things that were located in the mapping.

Before we actually start the lesson, I want to remind you of the text assignment.

For this lesson, there are quite a few learning objectives. I'll summarize them and elaborate more on some of them after the video, but for now, I'd just like to tell you what the objectives are.

On the test, you may be given the picture and asked to identify these various features. In addition to this, we need to describe the typical features of submarine canyons and understand the current theories on how these canyons are formed.

But before we actually start today's lesson, I want to remind you that we're already half way to the first exam. The first exam will come at the end of Lesson 6 after Chapters 1 to 4, and I hope you're not getting too far behind, and I also have to remind you that you can't succeed in this course by just watching the videos.

The videos are designed to supplement the text and the study guide, and you have to put a little time into it outside of just watching the videos, and, if you are falling behind, now's a good time to get caught up.

 

 

Plate tectonics is what we call in Science a unifying theory, and it's just a model, which incorporates many facts from many broad different areas. In order for Science to progress, there has to be eventually some unifying theory, and in Physics unifying was the theories of motion and gravitation, which allowed Sir Isaac Newton to predict the motions of the planets and understand how things behave in a gravitational field like here on the Earth.

In biology, the unifying model, the unifying theory was theories of evolution and genetics, which help us to understand how organisms evolve, how they reproduce, and how traits are passed on from one generation to the other.

In chemistry, the unifying model is the atomic theory.

In geology, the model is plate tectonics.

The sea floor is one of the most difficult places in the universe to study. The sampling the sounding techniques require large amounts of time, and, not only that, but you miss all but the largest features. Try to imagine that you want to sample the sea floor, or, better yet, suppose you only want to find the depth of the ocean. Well, the easiest way to do that is to be out there on a boat and throw something over the side tied to a rope, wait till it hits the bottom, and then pull the rope up and measure how much rope. Well, the problem with this is obvious, I think. Number 1, if you are measuring, let's say, the average depth of the ocean, about four thousand meters, a little over 12 thousand feet. You throw something over the side and wait for it to hit the bottom. It may take three or four hours for it to fall to the bottom. Well, in the meantime, you sit there and wait, and wait, and wait, until eventually it hits the bottom, but even then, you're not sure that the rope is hanging straight down. If the ship has drifted and the rope is at an angle, then the length of the rope is actually longer than the sea floor. And, of course, in order to find out how deep the ocean is, you have to not just get it down there but you have to pull it back up. Even with electronic winches on modern ships, it takes another three to four hours to pull the rope back up again, so to get one sample, one point, on the ocean bottom requires a ship and a crew for a whole day.

Now, to make a map requires thousands of such points, especially to make a map of something as large as the Pacific Ocean may require tens or even hundreds of thousands of such points. So, the point of all this is that even a simple thing like making a map of the sea floor requires many, many hours of time.

Now, with modern inventions, such as the echo sounder, which I'll come back to in a minute, it makes it a little bit easier to draw the maps. Now, we might also note here that people were actually on the surface of the moon before they walked on the deep sea floor.

In fact, we know much more about the moon than we know about the deep sea floor. In many ways, it's easier to go to the moon than it is go to great depths in the ocean because, after all, if you're on the moon in the absence of atmosphere, the pressure suits have to provide oxygen and a living environment for the people inside them, but they only have to be able to withstand one atmosphere of pressure; in other words, normal Earth's atmosphere, but the bottom of the ocean with the atmospheric where the water pressure may be tens of thousands of times of that of Earth's surface, we have to be able to withstand those large pressures, and not only that, but water is corrosive.

The vacuum of the moon is not corrosive, so any device that's designed to go to the bottom of the ocean has to be able to withstand not only the pressure but also the corrosiveness of the water.

Another thing that makes it difficult to study the sea floor is that we can see planets, and stars, and the moon, and the sun, and so forth, through visible light. In the case of the moon, the light is reflected. In the case of the sun and stars, the light is emitted, so we can study them directly with telescopes or with a spectroscope.

We can't see the bottom of the ocean.

There are many more methods available now then there were earlier this century. In fact, beginning in about the 1960s it became possible to employ a wide variety of different techniques, which include echo sounding, and remote sensing, and robot and manned submersibles, and, also, deep sea drilling. You'll see more about each of these methods in the video, 1 and I'll come back and talk about them some more after the video.

But the gist of all this is that the sea floor was very little understood until the 1960s. With the invention of the echosounder, it became possible to get a much more detailed picture of the ocean floor, and, in fact, features which were previously unimagined anywhere on earth were found to be quite common on the sea floor.

Most notable of these features is the mid-ocean ridge system. It's taller than most continental mountains, rises two to three kilometers above the ocean floor and maybe kilometers. At the center is a great rift, a double peak with a great rift. Also, we find on the ocean floor deep sea trenches, undersea volcanoes and vast areas of abyssal plain. It's not only the presence of the features but also the location of the features in relation to each other and in relation to the continents, which really lead us to the global theory of plate tectonics.

It explains sea floor structures, as well as geologic structures that occur on the continents. Well, the theory of plate tectonics, then, is a unifying theory, which links sea floor processes and features to continental processes and features and links all this into the Earth's interior. So with all this in mind, let's watch the video.

Major funding for "Earth Revealed" was provided by the Annenberg CPB Project.

One of the most extraordinary moments in human history occurred when astronauts first set foot on the moon, and, yet, on the surface of our own planet there lie tracts of territory where no one has every walked. Only a very small group of people has even glimpsed this place firsthand. It's not too surprising that humans have not actually walked on the floor of the deep ocean when we consider the extreme conditions that exist in that environment.

Here on the Earth's surface we live in sunlight and warmth, but the deep sea floor is in absolute darkness with temperatures close to freezing. A person standing on the floor of the open ocean would also be exposed to the tremendous weight of the overlying water. This column of water which is over three and a half kilometers high exerts a pressure hundreds of times greater than atmospheric pressure here on Earth. A person my size, for example, would be exposed to a total force of about 9 million kilograms. That's about a hundred times the weight of this entire ship.

A force like that would instantly crush my body to a lump about the size of a soccer ball, but the fact that scientists don't actually walk on the floor of the deep ocean doesn't mean we know nothing about it. Using research submersibles, which are specially designed to withstand the tremendous pressures, scientists have examined portions of the deep ocean floor in person by peering through windows one-half meter in thickness. Each window is made of a single flawless quartz crystal. It is mounted in a titanium metal pressure hall one meter thick, but most of what we know about this remote and alien environment has not been learned by personal observation. It's been obtained by using indirect techniques.

The high technology of sea floor exploration has produced a wide variety of remote sampling tools and marine robots that act as the eyes, and the ears, and the hands of the scientists who study the deep ocean floor. Studying the ocean floor using these remote techniques has produced a vast wealth of information, and the more we learn about it, the more intriguing it seems.

In fact, the deep sea floor has not only played a prominent role in the evolution of our planet. It remains the most tectonically vigorous region on Earth. We call our world "Planet Earth." A more apt name might be "Planet Ocean" since the oceans cover more than 70 percent of the globe's surface. This vast amount of water probably grew from the condensation of volcanic steam and from the melting of ice in comets which struck the primeval Earth.

From the beginning, the oceans have contained more than just water. Carbon dioxide, chlorine, and sulfur dioxide, which are volcanic gases, dissolve easily in water. Sodium, magnesium, and many other elements weathered from rocks on land continuously pour into the sea from the mouths of rivers. These chemicals make oceans salty.

If the surface of Earth were completely level, an ocean would cover the entire planet, but the surface is made up of two contrasting materials. Light granitic rock floats high forming the world's continents. The heavier basaltic rock tends to sink forming the world's deep ocean basins. The oceans average between three and four kilometers deep; however, compared to the total diameter of Earth, the oceans are but a patchy film of liquid covering the surface.

Scientists have identified several different regions of the sea floor. Most of us have had first hand experience with the shallowest and most accessible area, the Continental Shelf. Every time we go wading in the ocean, we're walking at the landward edge of a Continental Shelf. Shelves cover one-sixths of Earth's surface, sloping gently down from shorelines at an angle of about one-tenth of a degree. Worldwide, shelves average 70 kilometers in width, but in some regions, notably the Pacific Coast, shelves can be as narrow as two kilometers. If we started at the beach and went toward the deep ocean, we would see first, the Continental Shelf, and then, usually, there's a quiet distinct break in slope, and it's suddenly steep.

That's really the geologic edge of the continent, so it's shallow at about 200 meters depth, though it varies from place to place. Suddenly, its deep, and then it drops all the way to four or six kilometers depth to the deep ocean, so it's very tall scarf from the edge of the Continental Shelf to the deep ocean. That's called the Continental Slope.

Then, when you get to the base of the slope, there's a huge long shallow slope where all the debris that's coming off the continent is deposited, and so it's a very gradual slope that's called the Continental Rise.

If you were coming towards the continent, you'd slowly rise up it, and all the debris that's coming out of all the rivers and off of the slope by a lot of different processes, all end up in a big pile, slowly getting thinner and finer as you go away from the base of the slope.

In many places, the Continental Shelves and Slopes are furrowed by deep V-shaped valleys known as "Submarine Canyons." Where the shelf is wide, as on the Atlantic Coast of the United States, these canyons begin far out from shore near the outer edge of the shelf. On the narrow continental shelves of the Pacific Coast, the heads of some canyons lie close to the surf zone.

They've been a mystery for a long time, why they're there and how they're cut so deep, and they're also very interesting because an awful lot of the sediment that comes off the continent and onto the ocean floor goes down those canyons and deposits onto the ocean floor and makes huge stands of sediment at the foot of the slope. Beyond the Continental Rise lies what is, quite literally, the flattest region on earth, the Abyssal Plain.

This landscape is formed as layer upon layer of sediment settles on the ocean floor. This material known as "pelagic" sediment, consists of organic as well as inorganic matter. "Pelagic" sedimentation is not restricted to the abyssal plain but occurs throughout the oceans. The pelagic sediments are made of the debris that falls out of the water column and just, it's like a snowfall. It falls on top of all the topography, and they're made of the microscopic shells of organisms that are in the plankton, and, also, all the clay in dust that blows out to sea in the air and then falls down through or that is stirred up in the water. These sediments come down very gently and drape over the topography, so it saves all the mountains and hills structured even though it softens them up just like a snow fall would.

Farther out along the abyssal plain, the sea floor bedrock becomes shallower, and the sedimentary cover becomes thinner exposing lava hills, which are actually the tops of volcanoes. Thousands of these volcanoes are scattered across the deepocean floor. The larger ones are called "Seamounts." In fact, underneath the veneer of sedimentary deposits the entire surface of the sea floor is volcanic in origin although few signs of any recent eruptions are visible, except at mid-ocean ridges. Ridges are broad mountainous features rising some two kilometers high above the surrounding sea floor and stretching up to 1,500 kilometers wide. The ridges are connected in a single system snaking across the undersea landscape from one ocean to another, winding around the globe like the seam on a baseball. Unlike the crest of a continental mountain belt, which usually consists of a series of tall peaks, the crest of a mid-ocean ridge is occupied by a cliff-rimmed valley extending along much of its length.

The walls and floor of this valley are split by numerous fissures. It is in this fissured valley that the most frequent volcanic action occurs on the ocean floor in the form of undersea lava flows.

This volcanic activity is the key to a phenomenon that long puzzled geologists. There is a great disparity between the age of the Continental Crust and that of the Oceanic crust. The rock underlying the continents is as much as four billion years old, and yet the oldest rock found beneath the ocean dates back only about 200 million years. The fact that there is no ocean crust on Earth older than 200 million years raises an intriguing question. Does this mean that oceans didn't exist on Earth during the preceding 4.4 billion years of Earth history? Clearly, this can't be true.

Fossils of organisms that lived in the sea are found in rocks older than 200 million years on every continent on Earth. In fact, fossils of multicellular marine animals range in age back at least 670 million years, and evidence of colonial marine algae, called "Stromatolites" have been found in the oldest sedimentary rocks on Earth. Discovered in Northwest Australia, these sedimentary rocks and the stromatolites they contain are definitely of marine origin and range in age between 2.8 and 3.5 billion years. If, then, oceans did exist throughout most of Earth history, what has happened to all of the old ocean crust.

Analysis of cores taken from submarine rock reveals that the seabed becomes steadily older with increasing distance from the mid-ocean ridges. This research indicates that the sea floor originates and grows outward from the ridges. Geologists call this process "Sea Floor Spreading." The fissures atop a mid-ocean ridge are conduits through which fresh lava emerges. The lava hardens but soon the crust is pulled open forming new fissures and allowing more lava to emerge. Repeated continuously over millions of years, this process forms new sea floor. As the sea floor is pulled away from the ridge, it becomes cooler and denser.

Eventually, it becomes so cold and dense, it can sink back into the mantle against the edge of a nearby continent or smaller ocean basin. This sinking of oceanic crust called "Subduction" explains the absence of very ancient sea floor. Quite simply, it disappears back into the Earth. Subduction begins at enormous underwater trenches, some of them several times deeper than the Grand Canyon. Because of the great depth, marine geologists have had to come up with a host of ingenious ways of exploring the deep sea floor.

The primary tool used by Earth scientists to study the ocean floor is a research vessel like this one, outfitted with a variety of ocean graphic sampling instruments. Mounted on the stern of the vessel, is this A-frame, which is a hydraulically movable rack used to lift and deploy the instruments. The oceanographic sampling instruments are tethered to the vessel with this steel cable wound around a revolving drum. Scientists can take a bite of sediment or rock from the ocean bottom using a sampling instrument like this camel-grab. It takes sediment samples very quickly but only of the upper few centimeters of ocean bottom.

Oftentimes, an undisturbed sample of the deeper layers is required to examine variations in the accumulated sediment on the ocean bottom with time. This box core takes an entire column of sediment, which later can be split open and the individual layers analyzed like pages in a book. But marine geologists need more than physical samples to understand the sea floor. It's like trying to understand an entire city by only visiting certain buildings and street corners.

By examining the shape and global distribution of submarine land forms and determining their rock composition as well, geologists have been able to link the processes which create these land forms with the formation of the ocean crust itself. Most topographic mapping techniques used in seafloor exploration are based on the principal of sound waves bounced off the sea bottom, a technique called "Echo Sounding." First developed for the military detection of enemy submarines in the 1920s, echo sounding devices emit a pulse of sound toward the ocean bottom and record the amount of time it takes for that sound to bounce off the bottom and back to the device. By converting echo time into water depth, an echo sounding device draws a two dimensional profile of the ocean floor beneath the research vessel.

In recent years, however, marine geologists have employed more sophisticated acoustical techniques to map the topography of the sea floor. These robotically controlled imaging devices can map an area of the ocean floor tens of kilometers wide in a single pass. This three dimensional view of the ocean bottom can be clearer, more detailed, and even more valuable than the view of ocean floor from the window of a research submersible. One such imaging device is currently being used to map the territorial waters of the United States. In 1983, the Federal Government expanded this region to include all terrain within 200 miles of American shores.

The U. S. Geological Survey was assigned the task of mapping this enormous new acquisition for possible future development of resources. At the core of this effort is a a sophisticated echo sounding technology called "Gloria." Gloria enables broad areas of the sea floor on either side of a ship to quickly be scanned. Unlike traditional sonar, which only provides a profile of the sea floor directly beneath the ship. Gloria itself is towed behind the ship, trailing 200 meters away, some 50 meters beneath the surface. Gloria changed our impression of the sea floor by at one time letting us view the entire sea floor, rather than viewing small patches beneath the ship, many, many tracks going every which way, and trying to interpolate between those tracks. Now with Gloria we can see the entire surface. We can see where the seamounts are. We can see exactly where the channels are, where the meanders are. We can see where the canyons are, the gullies. We know now the locations of all these features down to very small scale features. In that respect, Gloria was a complete revelation.

To accurately map an area of ocean floor using Gloria technology requires very precise navigation. The longitudes and latitudes of specific sites are determined with the assistance of earth orbiting satellites in communication with the research vessel. This technique is termed "GPS," the Global Positioning System." Navigation is the most important of all the data we collect. If we can't locate the image or a bit of the image exactly where it is on the surface of the sea floor on the map, then we haven't really done our job. Someone must be able to go back after identifying something on a Gloria mosaic, go back to that latitude and longitude and find that image piece on the sea floor, so we use global positioning. We use GPS navigation, and it is the one thing that we pay more attention to on board the ship and after the cruise than anything else. It's the cornerstone for everything we do.

At this stage, however, there is still no image to look at. The Gloria data exists only as digitized bits of information in a computer, which must be processed and enhanced to create images showing the topography of the sea floor. Ultimately, other valuable information will come from Gloria work. Depths may be accurately determined for locations far away from the research vessel and using characteristics of reflected sonic energy, it may even be possible to determine the type of material making up the sea bed. The individual images are pieced together to create a large mosaic view of the sea floor.

On an overlay placed across the mosaic, the geologists trace the important features which will assist them in making a final map. This overlay will also help in describing the geology of this newly explored undersea terrain. The atlas compiled from Gloria's maps and images is expected to have many applications. The people that are interested in Gloria data vary from oil company geologists that want to know either where features are, the geometry of certain features, which we can provide them.

Academic scientists want to know possibly an area that they're interested in. If we've imaged that area, they know now where to focus all their energy and all their money to look at the process that they happen to be interested in. The Military, I'm certain is interested in Gloria data. It's like any base map. It's a reconnaissance of the sea floor, so it provides the basis for beginning to understand everything that's there.

Some researchers are trying to understand where valuable geological resources are located on the sea floor. In a world of diminishing raw materials, the hope of exploiting the oceans for their resources has persisted for decades. At present, the only geologic resources being tapped from the sea floor are oil and gas, which occur within the shallow continental shelves. But in recent years, exploration has revealed many other resources. At the mid-ocean ridges, for example, copper, zinc, iron, gold, and other metals are being actively deposited from hydrothermal vents.

On the deep sea floor, nodules of valuable manganese ore are scattered by the millions. Manganese nodules are not composed purely of manganese. Only about 20 percent of a nodule contains manganese. Another 15 percent or so may contain iron with traces of cobalt, nickel, and other metals, but most of the nodules are made up of calcium carbonate and volcanic fragments. Where the manganese comes from has been a mystery, a problem for science. Two possibilities are that the manganese is extracted from waters washing off the continents. Ore from exhalations through undersea hot springs in the mid-ocean ridges. The mid-ocean ridge origin is favored, but how the manganese is transferred through the water to very slowly accumulate as these nodules nucleate in the sea floor is in detail unknown.

But the technology required to harvest these undersea resources is forming, and as with offshore oil drilling, there are serious environmental concerns. Companies have experimented with dredging the ocean bottom for manganese nodules using giant suction devices. This has been successful enough to excite some commercial interest. For example, in Hawaiian waters, harvesting ores off the mid-ocean ridge probably requires more care and is prohibitively expensive.

Then there are environmental concerns. For example, if a mining operation were to ever start on a mid-ocean ridge, it could not help but destroy some of the fragile and spectacular ecosystem, which makes its habitat there. In addition to its economic applications, the use of sonar technology has helped scientists answer some profound questions about the nature and growth of the sea floor. For example, mapping shows how the positions of mid-ocean ridges change over time and how the structures of ridges vary with different rates of spreading. Despite the usefulness of indirect means of studying the sea floor, more direct observations must sometimes be made. Samples are dredges to examine the make up of the sea bed, and cameras operated by remote control are sent to the bottom of the ocean. You can imagine trying to take a picture it's really hard because it's pitch black, and so you have to have the lighting, you have to bring your own lighting and turn it on and off.

Then, some of the more glamorous ways to study the ocean floor are to actually go in a small submarine. I've taken 12 dives to the deep ocean. It's like going to Mars; it's so different. But that's very frustrating too. It's like trying to do Geology in a jeep at midnight with dirty windows with your headlights without ever being able to get out of the jeep. It's very, very frustrating business.

It's lucky for us that the oceanic crust and processing in the ocean are relatively simple compared to the land, or we would never figure it out at all. But even with these limitations, submersibles and remote cameras have provided a wealth of information about the sea floor.

Unique life forms thrive in the ocean environment from the deep sea floor to the shallow continental shelves. Many organisms live suspended in the water throughout most or all of their lives. Others inhabit the ocean floor burrowing into its soft sediment or scurrying across the silty bottom. Life has been found in many unexpected places.

One of the most spectacular discoveries occurred when submersibles first explored the active volcanic region atop the mid-ocean ridges. The most exciting adventure that we've had in the Submarine Alvin was the discovery and exploration of hot vents on the sea floor. They range form just a little bit warm to outrageously hot, 350 degrees centigrade. You know, ordinarily water would boil at 100 degrees centigrade, but the pressure's so high there that it doesn't boil, but it comes out extremely hot. When it's a little bit cooler, there are lots of strange animals that live around there that have never been seen anywhere else, including really long tube worms with bright red flesh at the end. There are crabs running all round eating everything else. Giant claims like this with red flesh. All those animals are living on the chemical energy that's in the water of the vents. They have special bacteria that use the hydrogen sulfide; that's gas that poisons us if we get a tiny whiff. They're using it and using it for its chemical energy and passing that down the food chain, so it's a totally different ecological situation then we're used to. It's so exciting to see a living thing in such an alien environment. They're really bizarre; really bizarre.

These weird and wondrous denisons of the deep are the only examples of life which get their entire supply of energy and warmth from the interior of the Earth itself. They live kilometers beneath the ocean surface in a world as alien as any encountered in outer space.

The vast commitment of people and technology focused on understanding the sea floor is driven by two fundamental human motives.

The sea floor also plays a vital function in determining global climate. In understanding its function helps us to better address such environmental problems as the Greenhouse Effect, changing global climate, and rising sea level. The sea floor makes up over two-thirds of the Earth's surface, and until recently, its vast submerged landscape remained virtually unknown, but now as we begin to know and understand this strange dark world, we're learning new facts about the way our planet behaves and discovering new opportunities to improve the future of all humankind.

Major funding for "Earth Revealed" was provided by the Annenberg CPB Project.

What a remarkable planet. Now, I really liked this video.

Even though we have to understand that we can't really show the features of the ocean floor, we have to look at models, either maps, or diagrams, or figures of various kinds. Even if we drained all the water off the Earth, we still wouldn't get a real good picture of these various features. There are several things to note in this lesson, and I'll spend some time with you elaborating on some of these.

Don't forget to look at the pictures and the diagrams in the textbook. These are especially important in understanding this material.

So we see that the sea floor has provided us with important clues to understanding planetary processes, and we see that these processes on the sea floor are intricately linked to processes on the continents. The big surprise was the size and variety of features on the sea floor where formerly it was thought to be flat and featureless.

Another big surprise is that the sea floor is much younger than the continents. Now, in fact, the oldest rocks on the sea floor are only about 200 million years old. Does this mean that there's only been an ocean for 200 million years? Not likely because early organisms for which we have records in the fossil record go back at least 3 billion years, so if the oldest rocks on the ocean floor are only 200 million years old, this is only about four percent of the age of the Earth; in other words, there's been enough time to make all of the ocean floor that we have about 23 times in Earth's history.

When we compare this with the ages of the rocks on the continents, we find that the continental rocks are about 3.6 billion years old. In other words, they go back about three-quarters of the Earth's history. What can we learn from all this? If the ocean floor rocks are much younger than the continental rocks, that must mean that the ocean floor is continuously being created and destroyed.

In fact, it's created at the mid-ocean ridges and destroyed at the trenches through this process called "Subduction." We also note that the sea floor is composed of different material than the continents. There are two kinds of rocks that make up much of the Earth's crust. The oceanic crust is basaltic. It's erupted through mid-ocean ridges, and the ophiolites, which we learned about in the last lesson are thought to be pieces of this ridge, which has been brought above sea level and embedded in the continental crust, which shows the deep structure of the ridge system. We also see that hot spot in isolated volcanoes that form seamounts and aseismic ridges like the Hawaiian Islands are also very common, and that the largest part of the sea floor is covered with both pelagic and terriginous sediment, pelagic sediment coming from organisms within the sea whose remains settled to the bottom and become sediment when they die, and, also, terriginous material carried into the oceans from streams and distributed throughout the ocean by currents and so on.

A contrast to continental crust is granitic or andesitic. As we mentioned last time, the lighter granite essentially floats on the more dense oceanic crust than mantle leaving a large continental block, the edges of which are the continental shelves, which are now submerged, and that these continental shelves are cut by large submarine canyons. We also note that andestic volcanoes are found near subduction zones associated with the subduction zones. Okay. The video showed many techniques to study the sea floor. These include echo sounding, heat flow, magnetism, bottom sampling, and coring, and drilling. I want to talk a minute about submarine canyons. Many of these submarine canyons found on the continental shelves are huge features. Some of them, in fact, are actually larger than the Grand Canyon.

Many of them are associated with large rivers but empty into the ocean near the submarine canyons, but many are not, and from what we understand about stream processes, the way streams erode and carve valleys, these submarine canyons are too deep to be cut by normal stream processes. Normally, a stream, such as we usually understand them, cannot erode below sea level, and the floors of some of these submarine canyons are several thousand meters below sea level. Now, some people think that the actual submarine canyons may have been started on land on the continental shelf.

During the last glacial period when sea level was up to 300 or 400 feet lower than it is now. If that's the case, then the submarine canyons as they were formed on the continental shelf provide a place for sediment to be channeled. The modern theories now suggest that the submarine canyons are cut and maintained by turbidity current. The word "turbidity" comes from the word turbid, which means muddy. Turbidity currents are great masses of sediment laden in water, which rush down the submarine canyons. They have great erosive power, and they carry much sediment to the sea floor, and when they reach the base of the submarine canyons, they spread out into large fans, which are called "alluvial fans," which when the fans coalesce or come together, form the Continental Rise.

One of the distinctive features of turbidity currents is that they leave a particular type of sediment called "turbidites," and these turbidites display a particular sedimentary structure, which can help us to understand. This sedimentary structure is called "graded bedding."

I'll show you some examples of graded bedding. In fact, here on the desk in front of me, I have a couple of examples. Do you notice inside the tube, first of all, there's water, and at the bottom, there's a whole series of sediments. At the bottom, the sediments are much more course grained, and as we move up toward the top, the sediments become finer and finer grained until right at the very top is this band of red material. This red material is very fine grained clay sized sediments, which take a long time to settle. In fact, you can see the cloudiness of the water above. There are still many of these colloidal size or submicroscopic size clay particles that are still settling out, and, in fact, it will take them quite a bit of time to settle. There have been examples of water collected from the Mississippi River where the clay particles take years to settle to the bottom of a container this size. What I'd like to do is to show you how this works.

Basically, a turbidity current, remember, is a mixture or a slurry of particles of all different size, and what causes the graded bedding is that when particles settle, the largest particles settle faster through the water, and, by the way, this also explains why we find large amounts of pelagic sediments consisting of clay. The clay that's introduced the water by rivers near the shoreline simply take months, or sometimes years, or even thousands of years to settle all the way to the bottom.

What I want to do is to shake this one, and we can watch the sedimentation happening. What we're going to see is that very quickly the finer particles will settle out at the bottom, and as time passes, the finer sediments will settle, and the top of it will probably remain cloudy for quite some time.

Well, I'm going to give this a little shake to stir it up pretty well. Okay. The table is a little wet here, so there it's all shaken up. You can see now the slurry of material that settles to the bottom, and already the coarser grained materials are beginning to settle. You can see that the finer materials are beginning to clear, but the column itself still contains quite a bit of the clay.

In fact, if you compare the one I just shook to the one that's been sitting for awhile, you can see that the one that's been sitting for awhile is significantly clear. You also notice that the red band, which occurs on the one that's been sitting for awhile hasn't yet accumulated because all the red material, the fine grained clay, is slowly settling on the top.

The study of turbidity currents helps us in many ways because when we find turbidity currents it almost guarantees that based on the principle of uniformitarianism; that is, the present is the key to the past, it virtually guarantees that if sediments are found, which display this graded bedding that they were deposited in this method by a turbidity current.

Okay, let's turn our attention now to Benioff zones. Benioff zones are regions of intense earthquake activity, which are found on the continental side of trenches, and they're associated with subduction zones that converge at plate boundaries. The significance to the Benioff zone is that earthquakes get deeper toward the continent along the Benioff zone. Now, what does this have to do with anything? Well, earthquakes result from movements of pieces of the Earth. The descending earthquake zone that we call the "Benioff zone" points to the fact there are movements taking place, and that the subducted lithospheric plate is being basically crammed down into the interior of the Earth, eventually to be reincorporated into the mantle at about 700 kilometers.

I say 700 kilometers because it's at that depth where the earthquake activity virtually disappears along the Benioff zone, and it's thought that this represents the depth where the lithospheric rocks simply become too plastic to fracture and cause earthquakes. We might note here that andesitic volcanism is also associated with the trenches. The rock that we call "andesitic" was named after the Andes Mountains in South America where it was first discovered, and this mountain range is associated with a subduction zone and a trench off the Western Coast of South America.

In general, volcanoes on the continental side of the trenches are very different in character from the volcanoes that occur on the oceanic side of the trenches; in fact, they're different enough that we characterize them as oceanic or basaltic volcanoes, on one hand, and continental or andesitic volcanoes on the other hand.

We will learn more about volcanoes in a separate lesson later on, but for now we need to understand that there is a distinct difference, and that there are basically two very different types of volcanoes that have different rock types and different properties. The andesitic volcanoes often form and arc shaped group of volcanic islands on the continental side of the trench called an "Island Arc," and sometimes the island arcs are found on the continents embedded in mountain ranges, the best example of this being the Andes, and, once again, we'll learn more about folded mountain ranges and how this relates to plate tectonics as we move along in the course.

Okay. Another thing we need to distinguish is between seamounts, guyots, atolls, and aseismic ridges. Well, let me give a little bit of a definition. Seamounts are undersea volcanoes, essentially large undersea volcanoes. Seamounts are generally defined to be more than one kilometer or 1,000 meters above the surrounding ocean floor, but we're not going to get picky about the size. Let's just say that seamounts are large undersea volcanoes. Many of these seamounts are active; that is, they still produce lava during undersea eruptions, but some are not active.

By contract, a guyot is a flat-topped seamount, and it's thought that the guyots were eroded flat by waves near the ocean surface before they sank to the present depth. They're now deeply submerged; in fact, the tops of many guyots are several hundreds or thousands of meters below the surface, far too deep for any wave action to have occurred, so they must have sunk.

In the next lesson, we'll get a sense of how this sinking takes place.

Aseismic ridges are chains of islands, seamounts, guyots, and atolls, which have little or no earthquake activity. The word "seismic" means earthquake, so "aseismic" means no earthquakes. There are several examples of these in all oceans but especially in the Pacific, and if you look at the map on page 57 in the textbook, you can see lots of examples of these aseismic ridges. Several examples, the Hawaiian Islands, on which we're sitting right now is one of the best examples, but, also, the Line Islands and the Tuamotos, and several other examples. Compared to mid-ocean ridges, which also have numerous shallow earthquakes and basaltic volcanism, the aseismic ridges have basaltic volcanoes but little or seismic activity.

Another thing now to turn our attention to is coral reefs. Reefs are basically ridges of coral algae and other calcareous organisms, which extract a mineral called "calcium carbonate" from seawater and use it to form their shells and houses and so forth. Coral reefs can only form in water of certain salinity, in water of high enough temperature down to the depth where light penetrates. This is a good example of uniformitarianism because when we find coral reefs in ancient sediments we know that the conditions must have been right for corals to grow. Coral reefs are common in ancient sedimentary rocks. The most common example is the Permian reef in Western Texas, which contains extensive petroleum deposits. Back in the 1840s, a young scientist named Charles Darwin was selected to be the chief scientist on an around-the-world voyage, which was designed to collect information about the Southern Hemisphere. The ship was called the HMS Beagle, and as a result of this trip, Darwin made observations, which he used then to come up with the Theory of Evolution and Natural Selection. He did by looking at exotic life forms on Pacific Islands and noting that newer islands have life forms, which are radically different from those on the adjacent continents, but even if Darwin wasn't famous for his Theory of Evolution, he'd be famous for his Theory of Atoll Development.

Atolls are islands, usually in the Pacific Ocean, which are basically simply coral reefs. They're usually ring shaped or horseshoe shaped or "U" shaped with various gaps between the various pieces of the coral reef enclosing a fairly shallow lagoon that's comprised of pieces of coral reef and so forth. Now, Darwin was a very astute observer. Not only that, but before Darwin left on the voyage of the Beagle, he had discovered a copy of a textbook by Charles Lyell, an introductory textbook on Geology, and Charles Lyell was one of the first people to recognize the vast expanse of geologic time, and it was Darwin who made the connection that in order for atolls to evolve from islands and also for various life forms to evolve, large amounts of geologic time were necessary. Through Darwin's powers of observation, along with this new concept of the immensity of geologic time, allowed these theories to come about. Darwin observed that there were more atolls in the Western Pacific Ocean and in the Eastern, and he suggested that atolls are formed by the sinking of a volcanic island as coral grows upwards. Now, understand that when coral grows, it grows upwards. Not upwards, not downwards, but upwards, so Darwin suggested that all coral reefs start out first of all fringing an island. He called these "fringing reefs" that surround an island, and as the island sinks, the coral grows straight upward, but because the island is sinking, the shoreline moves inland, so that eventually a barrier reef is formed, and as the island sinks, the barrier reef simply means that the reef doesn't actually touch the island, but between the reef and the island is a lagoon. The lagoon gets filled with coral debris, and shells, and so forth, which are broken off of the outer ridge of the coral reef by waves and deposited.

In the third stage, the island has sunk completely. Darwin calls this stage an "Atoll." In this case, the island has almost really disappeared, leaves only the reef, which is roughly circular in outline with gaps. In the center of the coral reef is a shallow lagoon. Darwin was amazingly astute, as I mentioned earlier, to be able to recognize this gradual change. He offered no explanation for why the islands might sink. It seemed apparent that the islands must be sinking, but he had no explanation.

We'll find in plate tectonics a ready-made explanation for the sinking of the islands. Also, the age relationships of the Hawaiian Islands also support this theory. We might consider a "guyot" to be basically an atoll without the growth of coral, a sinking caused by the same processes that we'll look at the next time.

Okay. Well, next time, we'll look at Lesson 5, "Birth of a Theory" where we'll detail Wegener's Theory of Continental Drift and look at the developments of the Theory of Plate Tectonics.

So for next time, you should read the text assignment. This Chapter 4, it actually could split into two lessons, so for next time we'll read pages 65 to 79. Don't forget to study the photographs and diagrams carefully and follow the plan in the study guide before you watch the video. I think that's it for this time.

Study hard, but enjoy. I'll see you next time.