Ocean floor notes
Earth Science
The Ocean floor and it's four main ocean basins
The ocean floor, earth has been nicknamed the blue planet because over 71% of its surface is covered in oceans. You should remember these four main ocean basins. The northernmost is the Arctic Ocean, the largest is the Pacific Ocean. We live closest to the Atlantic Ocean and the fourth ocean basin is the Indian Ocean. Of these four, the Pacific is the largest and deepest, the Atlantic is about half the size of the Pacific, the Indian is just slightly smaller than the Atlantic, and the Arctic is the smallest and most shallow ocean. The symmetry is the measuring of the ocean floor and charting the shape of the ocean floor. And another word for shape of the ocean floor is topography.
Oceanographers can map the ocean floor using several different methods. One of those is sonar, sonar stands for sound, navigation, and ranging. And involves sitting soundwaves through the water, which travel through the water until they hit something solid and bounce back to the boat. Since water always sound always travels through water at 1500 m/s, oceanographers can tell how deep the solid ground is because of how long it takes the soundwaves to return. They plot one point at a time and then chart this out to infer the shape of the land below them. You can imagine that plotting one point at a time is a very slow way to map an area of the ocean. So oceanographers can also use multibeam sonar, which measures a section of the ocean, floor at a time. And they can add to their knowledge of the ocean floor by studying satellites. Satellites can measure the elevation of the surface of the water on the ocean.
The surface water mimics the ocean floor topography. In other words, the shape of the land on the ocean floor. So in places where there are mountains and ridges, the water above there is slightly elevated. While the water above canyons entrenches shows a slight depression. Satellites enable oceanographers to map larger sections of the earth. Scientists can also study the ocean floor using submersibles, which are underwater crafts designed for deep sea research. They load the submersibles with various tools so they can test the water and the sediments at the bottom of the ocean. Some submersibles carry thermometers to measure temperature of the water, the submersibles contain cameras and lights to take pictures. They measured the water pressure at various depths, and they can collect water and sediment samples to bring back to the surface to study closer. From these tools, we have learned that the ocean floor is not flat, but it has many different features.
Some similar to features we see on the continents. We're going to start our discussion of the ocean floor features by working our way from the continent outward into the ocean. If you started to walk from the continent to the ocean, you would first be walking on the continental shelf. Notice in this picture, it shows a continent, the continental shelf is an area that has a very gentle slope and notice the water depths here isn't as great as it is in the deep ocean. The continental shelf is important because it contains mineral deposits like sand and gravel, which humans can use for resources in different ways. And we also find large reservoirs of oil and gas in the continental shelf area. We'll talk about these more later. The continental shelf is further very important because 90% of all the living things in the ocean are concentrated in this continental shelf area. That's because the organisms can have access to the nutrients that accumulate at the bottom of the continental shelf while still being able to receive sunlight because the continental shelf is the only area where the sunlight can penetrate through to the ocean floor since it is so shallow. So these are very important fishing grounds. We can continue out in the water.
You would notice that the land drops off steeply. We call this the continental slope. In some parts of the continental slope, there are submarine canyons which are carved when mud and sand on the continental shelf becomes disturbed and that heavy sediment rich water scrapes along the side of the continental slope to form the submarine canyon. At the bottom of the continental slope, we have another gradual decline into the deep ocean floor, so this section of the picture shows the continental rise. So again, in this picture, you have the continent, the continental shelf, the continental slope is steep, continental rise is more gradual again until we finally reach the deep ocean bottom. The flattest parts of the ocean floor are called the abyssal plains. These are actually some of the flattest land on earth because so much sediment accumulates here. It settles down to the bottom of the ocean, and fills in depressions, and so forth to make very flat land.
Down on the abyssal plains, and in the deep parts of the ocean, all the water is very cold, except for a couple places we'll talk about in a minute. It's also very dark, remember sunlight can penetrate through the shallow water of the continental shelf, but it doesn't reach very far at all into the deep water, and it's very quiet along the bottom of the ocean. Other important features are deep sea trenches, which are long near creases in the ocean floor. They are created when one plate collides with another plate at a convergent plate boundary. We'll talk more about plate boundaries when you study geology. You can see this is an artist representation of what a deep sea trench might look like. Again, the deep parts of the ocean are very cold, dark, and quiet.
This graph represents the height of Mount Everest, the highest elevation on earth's continent, and it shows you that some trenches are as deep as Mount Everest is high. The deepest trench is the Mariana trench, its 11,022 meters below sea level. Seamounts are also created at convergent plate boundaries. They are volcanic volcanos and their peak does not reach the oceans surface. If the volcano grows out of the water, then it's a volcanic island. A geo is a once-active volcano that hasn't erupted for thousands of years or longer. The top part of the volcano has been there so long that it's been eroded to a flat surface. A mid-ocean ridge is a huge chain of underwater mountains. They are created at divergent boundaries, and you'll see a picture of this in the next slide.
This shows the places where you can find mid-ocean ridges as the name hints they form at the centers of ocean basins. This picture shows two tectonic plates. They are moving away from each other. So this leaves a gap between the plates, magma and these thinner sphere rises up to fill in that gap. When the magma hits that cold ocean water it solidifies forming new press, but notice that the magma has also pushed the edges of these two plates upward. That gives height to the mid ocean ridge. So the rising magma forms new crust and this causes the sea floor to expand because both of these plates are moving away from each other and new crust is being made to fill in that gap. So the sea floor is spreading, the Atlantic Ocean is getting larger.
The mid-Atlantic ridge is the largest of these mid-ocean bridges. Again, here's a picture of all the features we just talked about. And again, the sea floor is important to humans because we get lots of resources. We can mine sand and gravel from the sea floor to add to our recreational beaches. And a couple of slides, I'll show you how sand is constantly being moved along the shore. So if you have a nice pretty beach in front of your hotel over time that sand gets moved or eroded by the movement of the ocean waves. So we can mine sand from further out in the sea to replenish our recreational beaches.
And of course, we make concrete and our roads cement asphalt using gravel again much of that can be mined from the sea floor. We also get a lot of salt from the sea, so some of the salt we eat comes from the sea and we could also use it for DIC of roads. The ocean is also an excellent place to find oil and natural gas deposits. These fuel sources were once microscopic marine organisms and over millions of years, these microscopic marine organisms that had been buried undergo chemical reactions to form a fuel source that we can burn and use to heat our homes, eat water, or make gasoline for our automobiles.