"EXPLORING OCEAN FEATURES"
bathymetric mapping: the measurement of depths of water in oceans, seas, and lakes
Sea floors have a variety of features including smooth gradual slopes, mountain ranges, volcanoes, trenches, and sea mounts. On the east coast of the United States the continental shelf generally extends in a gentle slope under the ocean, however the West Coast, especially off of Southern California, is quite different. Here the near shore land under the sea contains deep submarine canyons, underwater mountain ranges, deep trenches and valleys. The submarine canyons look like canyons cut by rivers on land....which they once were! Although Catalina Island is only 21 miles across the San Pedro Channel from the Southern California coast, deep trenches and canyons in the Channel might reach 490 fathoms (2940 feet)...or over a half-mile deep! The underwater topography of this area is complex and fascinating. Since most students think of a simple sloping sandy bottom under the ocean, it is helpful for them to understand the variety of land forms possible, in terms of geological and geographical knowledge, as well as potential habitat variations.
Measuring and mapping the land under water is called bathymetric mapping. Prior to the 1920's oceanographers measured the depth of the ocean using long lines with weights attached that were marked at regular intervals (meters or feet) with knots. The lines were lowered into the ocean until the weight touched the bottom and the depth was noted by the knot mark. Currently, oceanographers use sonar, or sound waves, to measure the ocean bottom. Sound waves are sent from the bottom of a ship toward the ocean floor. By measuring the time it takes the sound to return to the ship (received by a recording device), the ocean depth can be calculated because the speed at which sound travels through water is known (1,454 meters per second). In the following activity the student will do some simplified versions of bathymetric mapping using "depth lines", as well as some simple topographical mapping.