bathymetric evidence for seafloor spreading

life at sea

Well 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 research submersible. 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 - thats AM and PM. Alternately, if youre on the team doing a lot of diving in Alvin, you may just be up during the day. If youre mostly doing operations that dont involve Alvin, you may just be up at night. For safety reasons, Alvin is deployed and recovered only in daylight. 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 doesnt need to be near the ocean, he or she just needs to have access to an airport! Lets 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 dont have to spend days in transit to our research site, and we dont have to get seasick! As we head to the site, we will run the echo sounder. Lets 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.

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, Earths 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 1.3). Click image to the left or use the URL below. URL:

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 1.2 is a modern map with data from several decades. The major features of the ocean basins and their colors on the map in Figure 1.2 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 1.2. 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.


instructional diagrams

No diagram descriptions associated with this lesson


very flat areas that make up much of the ocean floor are

a) plateaus

b) trenches

-->  c) abyssal plains

d) mid-ocean ridges

on a bathymetric map where shallower depths are lighter blue and deeper depths are darker blue

-->  a) the light blue gashes are mid-ocean ridges and the dark thin lines are the trenches.

b) the mid-ocean ridges are dark blue lines and the trenches are light blue arcs.

c) the entire ocean floor appears as a deep blue.

d) the deepest depths are along the mid-ocean ridges and the shallowest depths are along the abyssal plains.

longest chain of mountains in the world.

a) abyssal plains

b) trenches

c) continental margins

-->  d) mid-ocean ridges

to map seafloor bathymetry, a research vessel uses an echo sounder, which

-->  a) uses a sound beam to determine the depth to the seafloor.

b) takes photos of the seafloor and then creates a map from them.

c) carries a crew of three scientists and pilots to map the seafloor.

d) is dropped to the seafloor on a line with the length of the line that is submerged indicating seafloor depth in that area.

the study of underwater depth of lake or ocean floor.

a) geomorphology

b) climatology

-->  c) bathymetry

d) glaciology

from the bottom of the deepest trench to the top of the highest mountain, the relief of the pacific ocean basin totals nearly 70,000 feet.

-->  a) true

b) false

this is the transition from the land to the deep sea.

a) continental drift

b) continental shelf

c) continental rise

-->  d) continental margin

earths tallest mountain from base to top is

a) mt. everest

-->  b) mauna kea

c) mt. chimborazo

d) mt. mckinley

the continental shelf is made up of the continental margin, continental slope and continental rise.

a) true

-->  b) false

the deepest canyon in the ocean floor.

a) hellenic trench

b) philippine trench

-->  c) marianas trench

d) japan trench

diagram questions

No diagram questions associated with this lesson