In the last lesson we discussed water and how much there is of it. But what is beneath it? What is on the ocean floor? Remember that in the early 1800s they had no idea how deep the oceans were, much less what was on the bottom. But they were certainly curious. You might also remember that in the mid-1800s it was being taught that the seafloor was a relatively featureless landscape consisting of mud and sand. After all, for many reasons, this was logical. A logical thought being considered factual science – does that sound familiar? (The Nature of Science Lesson 3).
The obvious next step would be to test this hypothesis, but how? This was an activity in the Nature of Science Lesson 4, what technology would be needed to test this?
Well, of course you are going to need a ship – enter the HMS Challenger.
You will need a crew to operate this ship – enter the crew if the HMS Challenger.
You will need the technology to measure depth and obtain samples from the bottom.
These had to be developed, nothing existed at that time. What they came up with was a platform extending off one side of the Challenger. On this platform they would have a steam driven winch to lower equipment and enough cable to do it. You wonder how much cable they knew to bring – but it worked. A grab device was developed to “grab” a chunk of the seafloor. They also had a dredge to drag across the bottom to collect marine life (if any) and this could grab sediment samples as well.
Then we have to determine where, and how many samples are we going to take (n=)? Their cruise would circle the globe sampling all the seas and the entire trip would take five years to complete. It set sail in 1872.
What did they learn?
First, they discovered the seafloor is not featureless. There were numerous mountain ranges across the globe. We know that many are volcanic. Canyons, trenches, and large flat abyssal plains were charted. The deepest point they found was large trench, which they called the Mariana’s Trench (because they were near the Mariana’s Islands in the South Pacific) and the deepest point was about 36,000 feet, a place we now call the Challenger Deep. They discovered the seafloor has many of the features we find on land. Canyons, mesas, rifts, mountains, volcanos, and we have more recently discovered high saline lakes.
Second, they discovered that though there are many types of rock on the seafloor, it is dominated by basalt. Basalt is an igneous rock (formed by volcanic activity) like granite but is denser.
But how accurate were the measurements taken by Challenger? Could we develop a technology that would do this better?
Enter the Germans and their Meteor Expedition in 1925.
This cruise followed the footsteps of the Challenger but with a new technology – the echo sounder. This new technology used sound echoes bouncing off the seafloor to more accurately map them. With this they were able to develop the ocean floor maps we can find on the internet today.
Today we have visited the seafloor with cameras, robots, and even manned submersibles getting more and more information each time.
It was during the time of the Meteor that another German, Alfred Wegener, made observations while studying climate in Greenland that would change the way we look at the ocean floor, and the planet. He discovered fossils in the Greenland landscape that suggested it was once much warmer there. What logical explanation could you make about tropical and subtropical fossils in such cold place? There were two possibilities in his mind. 1) the entire planet was warm at one time, or 2) Greenland had moved closer to the poles.
Wegener noticed that, while looking at a map of the world, the shapes of the continents could fit together. The east coast of South America fits into the curve of the west coast of Africa. Florida lies right on Morocco. Australia fits into India, etc. He proposed that the continents were once all together in a giant land mass he called Pangea and that many colder continents, including Antarctica, were more tropical in location than they are now. He called this idea the theory of continental drift.
Interesting observations and interesting explanation, but is it true?
As you might guess, the established scientific community at the time thought the idea of “continental drift” was nuts. Continents don’t “drift” across the surface of the seafloor? They need more evidence than fitting map pieces together. They would need to develop a test/experiment that would support this crazy idea. Wegener returned to Greenland several times testing new technologies and looking for evidence to support his theory. He died in the frozen ice trying to do just that.
Though the majority of the scientists of his time did not believe this theory, some did and continued the work. It was not until the 1950s, when technology had improved, that testing began to showed Wegener might be correct. There was evidence such as how the continents DID fit together, and certain endemic plants and animals found on the western coast of Africa were also found on the eastern coast of South America, but there were measurements of movement.
Oceanographic cruises to the mid-Atlantic Ridge is where this began. The mid-Atlantic Ridge is the longest mountain range on the planet, literally running from the Arctic to the Antarctic beneath the Atlantic Ocean. It is a rift valley, similar to the rift valley in east Africa. A rift valley is a mountain range that has a rift (or crack) running down the middle of it. This rift is a crack where magma from the earth’s interior pushes up and is released onto the earth’s surface. In some locations, this release is explosive, as with numerous large volcanic eruptions. In others it is more of gentle oozing of material. As magma breaks the surface and meets the atmosphere it chemically reacts with it and becomes lava. This lava oozes to each side of the crack and cools forming igneous rock. The idea is that this new lava material could be pushing the older igneous rock further away from the rift valley, but could it be pushing the continentals further away as well. Was there evidence of this?
These scientific cruises planned to test this. They needed to obtain rock material right along the rift and then they would systematically collect additional rock further away. They would repeat this sampling along transects running from the north to the south of the mid-Atlantic Ridge and on both sides of it. They made some interesting discoveries. First, using radio-isotope dating, the rock does get older as you move away from the valley. It also got older at the same rate on both sides of the valley, and as you moved north to south. This evidence suggest that the earth is literally splitting as these rift valleys. But they also discovered that the earth’s magnetic field flips every 25,000 years or so. More study had to go into magnetic reversal, how it happens, and what impact that would have on life, but it also brings up a question… If this rock is being shoved away from the valley, where is it going? It eventually has to hit something.
Here is another way science works. As this team was working on studying whether rock actually moves, others were working on other pieces of the story. Each team will publish what they learn in scientific papers and attend conferences together to find what each is learning. This information can enhance or change the direction of each other’s studies. Here were some connections made.
Using sound, they had determined what the interior of the earth was like. Sound changes speed and direction as it moves through materials of different densities. It will also change the frequency of the sound waves moving through them. Producing sounds on one side of the planet, and receiving on the other, found that there is a solid interior core. There is a more gaseous-liquid outer core. The core is surrounded by a thick layer of molten rock we call the mantle. All of this covered by a thin shell of solid rock we call the crust. Think of it like an egg. There is the solid core (egg yolk), the molten mantle (the egg white), and the thin crust (the eggshell).
The new theory is that the core heats the interior of the earth. The intense gravitational forces keep it from expanding but this also generates intense energy that expands into the mantle. Just as hot air rises, cools, and descends – so does the mantle. This circular movement of the mantle pushes on the crust, which is broken into sections called plates. The boundary between these plates are fault zones, where earthquake and volcanic activity are high. Along the rift valleys the seafloor is splitting, moving away from the fault and pushing plates. Those plates collide with other plates. In some cases, one plate slides beneath the other and back into the mantle. The opposing plate is shifted upwards, forming mountains and volcanos – these are called subduction zones. Some plates slide past each other, going in opposite directions, like the San Andreas fault in California. The seafloor is mostly basalt, an igneous rock denser than granite. Granite makes up the continents. The continents ride higher on the mantle due to its lower density and slides across the surface of the earth. This idea is called plate tectonics and there is a lot of scientific support for it.
What we see today are granite-based continents, some of which are below sea level. The portion of the granite continent below sea level is known as the continental shelf. This shelf can end very near the shore or extend hundreds of miles offshore. At the edge of the shelf is the continental slope, which drops to the seafloor. Some portions of the shelf and slope have deep sea canyons. The seafloor will have fault lines with underwater seamounts and volcanos. There are deep sea vents where hot gases from the mantle escape into the sea. Here there are smokers and chimneys releasing hot gases into the sea at around 400 F. Some of the gases fuel a deep-sea ecosystem based in a process called chemosynthesis (instead of photosynthesis). And there is a lot more to discover. Some have said we know more about the moon than we do the ocean floor, and that may be true. There is certainly more to do there.
Using the map, you developed in Nature of Science Lesson 1, or a new one, label the locations of the following seafloor geological features. With this you should be able to see the individual plates. Go a step further and label whether this fault is a site for seafloor spreading, or subduction, or a location where two plates are colliding and moving upwards together, or a plate boundary where the plates are sliding past each other.
Aleutian Trench East Pacific Rise
Java Trench Marianas Trench
Mid-Atlantic Ridge Puerto Rican Trench
San Andreas Fault Tonga Trench
You can find this by searching for the major fault lines of the Earth in your favorite search engine. Click images and you will discover the names of the plates, where earthquakes happen, and even some photos of these from beneath the sea and on continents.
Search for an image of the seafloor on the internet and have the kids create a model of the seafloor using Play-Do.