(48).Sea floor.

Let's go to sea!

To understand what came next, we need to go to sea aboard a research vessel. From the photo you can probably tell that a research vessel is no cruise ship. It's a lot smaller, and community spaces are filled with science labs, not swimming pools. The food ranges from barely edible to tasty and filling, but is rarely sumptuous. But with a research vessel we can gather data to explore the seafloor. Let’s go on one now!

Life at Sea

We’ll go out on the research vessel (R/V in ship-speak) Atlantis, owned by the US Navy and operated by the Woods Hole Oceanographic Institution for the oceanographic community.

The Atlantis has six science labs and storage spaces, precise navigation systems, seafloor-mapping sonar and satellite communications. Most importantly, the ship has all of the heavy equipment necessary to deploy and operate Alvin, the manned researchsubmersible.

The ship has 24 bunks available for scientists, including two for the chief scientists. The majority of these bunks are below waterline, which makes for good sleeping in the daytime. Ship time is really expensive research, so vessels operate all night and so do the scientists. Your “watch”, as your time on duty is called, may be 12-4, 4-8 or 8-12 – that’s AM and PM. Alternately, if you’re on the team doing a lot of diving in Alvin, you may just be up during the day. If you’re mostly doing operations that don’t involve Alvin, you may just be up at night. For safety reasons, Alvin is deployed and recovered only in daylight.

[Figure 1]

Alvin is deployed from the stern of the R/V Atlantis.

Scientists come from all over to meet a research ship in a port. An oceanographer these days doesn’t need to be near the ocean, he or she just needs to have access to an airport!

Let’s begin this cruise in Woods Hole, Massachusetts, Atlantis’ home port. Our first voyage will be out to the Mid-Atlantic Ridge. Transit time to the research site can take days. By doing this virtually, we don’t have to spend days in transit to our research site, and we don’t have to get seasick!

As we head to the site, we will run the echo sounder. Let’s see what we can find!

Echo Sounding

The people who first mapped the seafloor were aboard military vessels during World War II. As stated in the Earth as a Planet chapter, echo sounders used sound waves to search for submarines, but also produced a map of seafloor depths. Depth sounding continued in earnest after the war. Scientists pieced together the ocean depths to produce bathymetric maps of the seafloor. During WWII and in the decade or so later, echo sounders had only one beam, so they just returned a line showing the depth beneath the ship. Later echo sounders sent out multiple beams and could create a bathymetric map of the seafloor below.

We will run a multi-beam echo sounder as we go from Woods Hole out to the Mid-Atlantic Ridge.

Features of the Seafloor

Although they expected an expanse of flat, featureless plains, scientists were shocked to find tremendous features like mountain ranges, rifts, and trenches. This work continues on oceanographic research vessels as they sail across the seas today. The map in the Figure belowis a modern map with data from several decades.

The major features of the ocean basins and their colors on the map in Figurebelow include:

mid-ocean ridges: these features rise up high above the deep seafloor as a long chain of mountains, e.g. the light blue gash in middle of Atlantic Ocean.

rift zones: in the middle of the mid-ocean ridges is a rift zone that is lower in elevation than the mountains surrounding it.

deep sea 

trenches: these features are found at the edges of continents or in the sea near chains of active volcanoes, e.g. the very deepest blue, off of western South America.

abyssal plains: these features are flat areas, although many are dotted with volcanic mountains, e.g. consistent blue off of southeastern South America.

See if you can identify each of these features in Figure below.

[Figure 2]

A modern map of the southeastern Pacific and Atlantic Oceans.

When they first observed these bathymetric maps, scientists wondered what had formed these features. It turns out that they were crucial for fitting together ideas about seafloor spreading.

Continental Margin

As we have seen, the ocean floor is not flat: mid-ocean ridges, deep sea trenches, and other features all rise sharply above or plunge deeply below the abyssal plains. In fact, Earth’s tallest mountain is Mauna Kea volcano, which rises 10,203 m (33,476 ft.)meters) from the Pacific Ocean floor to become one of the volcanic mountains of Hawaii. The deepest canyon is also on the ocean floor, the Challenger Deep in the Marianas Trench, 10,916 m (35,814 ft).

The continental margin is the transition from the land to the deep sea or, geologically speaking, from continental crust to oceanic crust. More than one-quarter of the ocean basin is continental margin. (Figure below).

[Figure 3]

The continental margin is divided into the continental shelf, continental slope, and continental rise, based on the steepness of the slope.

Summary

Much of what went into developing plate tectonics theory involved work done at sea.

Echo sounders used to search for enemy submarines during World War II allowed scientists to piece together bathymetric maps of the seafloor. Multi-beam sounders work on research vessels today.

These maps revealed amazing features like mid—ocean ridges, deep sea trenches, and abyssal plains.