The Ocean Bottom

Most of the ocean floor is relatively inaccessible to humans. On average, the ocean floor is covered by 12,000 feet of water. Currently, we have less than 10% of the ocean bottom mapped using high resolution sonar. But what do we know about the ocean bottom? First, it’s wildly complex. All of the kinds of features you would expect to see on land; mountains, canyons, plains, plateaus are all present on the ocean floor, but in wildly exaggerated forms. The tallest peaks, the deepest canyons, the longest mountain ranges, the flattest plains on Earth are all under the ocean.

The Continental Shelf

Let’s start from land. The areas of the ocean that are shallow and close to land are known as the continental shelf. Most of the continental shelf is less than 200 feet deep. Widths of continental shelves vary widely. Passive (non-geologically active) margins tend to have broad shelves and gently sloping continental slopes. Broad continental shelves can be found spanning the Bering Sea, the South China Sea, the Gulf Coast of Florida, and New England. Active margins tend to have narrow shelves and steep continental slopes. Active margins make up nearly all of the coastlines of the Pacific Ocean, including the West Coast of the United States, and the South Coast of Alaska. Check this out to learn more about active and passive margins.

The continental shelf is where humans who venture out into the ocean and draw resources from it, spend most of their time. The continental shelves are strongly influenced by land. Deep-sea upwelling currents rise up to the shelves, it mixes with coastal runoff making the continental shelf rich in nutrients. When mixed with abundant light, life thrives on the continental shelf.

The Continental Slope

At the edge of the continental shelf at an average depth of 460 ft or 140 meters there is a steep drop off known as the continental slope or shelf break. In just a few kilometers water depth may drop to depth of 12-15,000 feet or 4-5,000 meters.

Abyssal Plains

In most of the ocean, once you reach the bottom of the shelf break you would find yourself in a vast, flat, plain. With an average depth of 10-12,000 feet Abyssal Plains are deep. Covering 70% of the ocean floor, abyssal plains are also the largest habitat on earth. The Abyssal Plains represent some of the least mapped areas of the planet. We don’t know that much about them. What we do know is that much of the plains, are incredibly flat, like the kind of flat that would make Kansas look hilly. However, even these flat plains are broken up by interesting features such as seamounts and guyots.

Seamounts and Guyots

The oceans abyssal plains are peppered with hills and mountains. By definition, seamounts are geological structures that rise more than 1,000 meters (3,300 feet) above the surrounding the seafloor. Some seamounts may rise up to 5,000 meters (16,400 feet), nearly reaching the ocean surface. Seamounts are areas of high biological activity. Much like mountains on land can create complex weather patterns, seamounts can created complex current patterns that influence what lives on and above them.

In the Earth’s geological history, some undersea volcanos did break the ocean’s surface. They were islands. They were subjected to waves and weather, and had their exposed surfaces eroded. The weight of those islands causes them to sink down into the seafloor. What remained was a flatted seamount called a guyot.

Deep Sea Trenches

Certainly one of the most dramatic features of the ocean floor is the deep sea trenches that are found at the edges of geologically active margins. On our undersea trip from Seward, the Aleutian Trench would be just to our west as we came down off the shelf break. Trenches are the deepest places on Earth. The Mariana Trench in the southwestern Pacific, not far from the Island of Guam, is 36,201 feet deep at its deepest point. That is almost seven miles below the surface of the ocean.

We will discuss this in more detail below, but the Earth’s crust is broken up into large pieces or plates. Oceanic crust tends to be thiner and denser than continental crust. As plates move against each other oceanic crust is pushed underneath the continental crust. Deep sea trenches are found where oceanic plates are sub-ducted beneath continental plates.

Mid-Ocean Ridges

It would be a very long trip from Seward, but the Mid-Ocean ridge system stretches across 65,000 kilometers of the ocean bottom. It is the longest mountain range on Earth. This mountain ranges are what geologists refer to as spreading zones. They represent cracks in the Earth’s crust where magma pushes up to form new crust. Over time, this material builds up forming mountains. Forces within the mantle for new crust outward. We now know this leads to process called sea-floor spreading.

How We Came to Know the Seafloor

During the World War II, navy ships carried echo sounders used to locate enemy submarines. An echo sounder is a type of sonar that send sounds down into the water below. As those sounds bounce off of hard surfaces, the echo sounder measures the time it takes and calculates a depth. Echo sounders, not only could detect submarines, they also took measurements of ocean bottom topography. When scientists were able to piece together the data from these echo sounders they used it to create the first maps of the ocean bottom. This type of map is known as a bathymetric map and is similar to a topographic map of the land surface.

It is important to note that before the invention of high resolution sonar, all that we knew about the bottom of the ocean came from dropping lead-weighted lines to measure depth or heavy dredges to bring up bottom samples. It was like trying to decipher what was in your gift wrapped birthday present by poking holes in the package and measuring the depth of the penetration. Prior to these first bathymetric maps, the ocean floor was hidden to us and was wrongfully assumed to be relatively flat and boring.

The bathymetric maps that were created from this new data were far more detailed that maps of the past and they revealed surprising features.  The map pictured above was painstakingly created by Marie Tharp using data from Bruce Heezen. It’s worth noting that at the time, women were not allowed on oceanographic vessels but Tharp took thousands of sonar readings and detail by detail hand drew the ocean floor.

Years before, explorer and meteorologist Alfred Wegener had devolved the theory that the Earth’s continents once fit together in a giant super continent called Pangea and that over time they had drifted apart.  Anyone can see that the continents of Africa and South American fit together like puzzle pieces.  At the time he published them, Wegener’s ideas were not widely accepted. He lacked a mechanism to explain how continents moved. Yet these new bathymetric maps showed the continents also matched up against features under the oceans. In the 1960’s and 70’s researchers were also piecing together the mechanism Wegener had lacked.

Evidence for Seafloor Spreading

Scientists in the 19th century discovered something very weird about the Earth’s magnetic field. Periodically the magnetic poles flip-flop. North becomes south and south becomes north.  This discovery was made by scientists using magnetometers, devices that measure the direction and strength of a magnetic field frozen in the magnetic crystals of rocks.

Scientists discovered that the normal and reversed magnetic polarity of seafloor basalts creates stripes across the ocean floor. There is one long stripe with normal polarity, next to one long stripe with reversed polarity and so on across the ocean bottom. On either side of mid-ocean ridge, these stripes form mirror images of each other. The magnetic polarity maps also show that the magnetic stripes end abruptly at the edges of continents, which are sometimes lined by a deep sea trench .

As hot magma emerges at the mid-ocean ridges it forms new seafloor. When the lava cools, its magnetite crystals take on the current magnetic polarity. The polarity is locked in when the lava solidifies and the magnetite crystals are trapped in position. Reversals show up as magnetic stripes on opposite sides of the ridge axis. As more lava erupts, it pushes the seafloor that is at the ridge horizontally away from ridge axis. This continues as the formation of new seafloor forces older seafloor to move horizontally away from the ridge axis.

Ocean rock, in general, is far younger than rock found on the continents. The oldest ocean rock is around 180 million years old while the oldest continental crust is around 4 billion years old However, when comparing ocean rocks to each other, a clear pattern emerges. The youngest rocks are found in the mid-ocean ridges.  These rocks represent the cooled magma of recent undersea volcanic eruptions. As one goes towards the continents, on either side of the ridge, the ages of the rock increase symmetrically on both sides of the ridge.

Sediments confirm the theory

Scientists also discovered that there are virtually no sediments on the seafloor near the mid-ocean ridges.  However, sediment layers grow increasingly thick in both directions away from the ridges. The rate of sediment accumulation on the ocean floor can be measured by sediment traps and it provided a timeline that matched well to the age of ocean floor rocks.

It is the creation and destruction of oceanic crust, then, that is the mechanism for large continents move over time. Rather than drifting across the oceans, the Earth’s continents are riding on a conveyor belt of oceanic crust that takes them around the planet’s surface.