Bathymetry and Sea Floor Topography

CBGS Marine Science

Sara Beam

Bathymetry is the study of the depth of the oceans and the topography of

the sea floor

• Sea Floor topography is a function of the plate tectonic processes shaping the earth’s surface.

• At spreading centers we find oceanic ridges

• At subduction zones we find the greatest ocean depths- trenches.

Ocean depths have corresponding ecological zonation

Cross sectional view of the Atlantic passive margin

• Coastal plain--Continental shelf--Continental shelf---shelf break---slope---rise---Ocean floor

This is an image of a the spreading center

called the Mid-Atlantic ridge

This is a picture of the pillow basalts that come out of the spreading center.

So how fast is the sea floor spreading ?

• A theoretical model of the formation of magnetic striping. New oceanic crust forming continuously at the crest of the mid-ocean ridge cools and becomes increasingly older as it moves away from the ridge crest with seafloor spreading : a. the spreading ridge about 5 million years ago; b. about 2 to 3 million years ago; and c. present-day. Kios and Tilling, 1996, Dynamic Earth, USGS

The African Rift Valley is a new spreading center being born!

(extends from Lebanon in the north to Mozambique in the south)

California’s Central Valley and the San Andres fault are NOT spreading centers; the

SA is a transform fault, where NA plate is sliding along the Pacific plate

The Peru Chile trench is easily seen here

This is an area of intense subduction where the Nazca plate is being detroyed and is pushing up the Andes mountains in the process.

Note the very narrow continental shelf at the active coastal margin, wide shelf at passive margin

• The Aleutians and the Islands of Japan are two island arcs formed by the subduction of oceanic crust- this is why there is so much seismic activity in Alaska and Japan !!

• (and tsunamis in the Pacific!)

Seamounts are submerged volcanoes more than 1000m above sea floor

• Seamounts can stand alone or more often in island arcs or chains- like the Hawaiian-Emperor Chain

Coral atolls are the visible remnants of volcanic islands

The Hawaiian Islands are formed by a “hot spot”

• Hot spots are formed when magma wells up from deep in the mantle and breaks through the ocean crust, resulting in island chains. This magma is different from island arc because it comes from deep in the Earth’s interior.

• In the old days, ships had to go out and make manual soundings with lines and lead weights OR in this map, they ran transect lines of the coast with early sonar and plotted them on navigational charts like this

• Bathymetric images can clearly outline the features on the continental shelf and show us the deep holes, shoals, the channels.

• mouth of Bay of Fundy

• Northeast Channel

• Stellwagen/Georges Banks

Submarine canyons form during the active history of a

plate margin

• Monterey Canyon is a major conduit of terrigenous sediment from land out to sea

• Hatteras Canyon may be a drowned river valley from lower sea level

**Note the narrow vs. wide continental shelves

Active coast- narrow shelf

Passive coast- wide shelf

Sonar Methods• Multibeam

• sidescan

• This image of the sea floor was created using multibeam bathymetric sonar

• bumps and dips in the ocean surface are caused by minute variations in the earth's gravitational field. For example the extra gravitational attraction due to a massive mountain on the ocean floor attracts water toward it causing a local bump in the ocean surface; a typical undersea volcano is 2000 m tall and has a radius of about 20 km. This bump cannot be seen with the naked eye because the slope of the ocean surface is very low. These tiny bumps and dips in the geoid height can be measured using a very accurate radar mounted on a satellite

Gravity= G m1*m2 d2

So we can use sonar to map sea floor bathymetry, but how can we find out

what’s going on down there ?

• Sonar technology sees hard rocks.

• Submersibles and cameras can only look at pinpoint locations in the deep sea…

• How can we study life on the bottom of the ocean in the deepest places effectively ?

• How do we find hydrothermal vents ?

Cross sectional view of sea floor spreading.

Hydrothermal vents occur at spreading centers and the breaks in the crust are faults where adjustments to the crust occur.

http://www.onr.navy.mil/Focus/ocean/habitats/vents1.htm

• http://www.youtube.com/watch?v=rFHtVRKoaUM&feature=related

The Discovery of HTV’s is one of the greatest modern discoveries

which may help to explain the origins of life on Earth.

Extremophiles

• http://video.nationalgeographic.com/video/player/environment/habitats-environment/oceans/hydrothermal-vents.html

Two Modes of Primary Production on Earth

Chemotrophs are organisms that obtain energy by the oxidation of electron donating

molecules in their environments.

One of the greatest mysteries yet to be solved…

• How do these vent organisms colonize new hydrothermal vents ?

• Vents are ephemeral, they turn on and off, they may only persist for a short time (evolutionarily)

• How do organisms spread there colonists over thousands of miles in the deep ocean ??????