Adapted From: Erickson and others, 1987, "Submerged islands" in Down to Earth: AIMS Education Foundation, Fresno, California

Purpose: Students will create a two-dimensional contour map of a submerged area from a simulated survey using sounding techniques.

Suggested Student Grouping: Two students per group.

Framework Integration: Framework: Scale and Structure.
Science Skills: Contouring; Map interpretation.
Related disciplines: Astronomy (navigation, star charts); History (naval warfare); Oceanography (history of exploration, topography of the ocean floor).

Related Activities:
Building a Topographic Model; A Model of Sea-floor Spreading.

In the activity Building a Topographic Model, students created a three-dimensional model of a landform above sea level. But what about all the landforms below sea level? We now know that mountain ranges and canyons on the ocean floor are as spectacular as any on land. There are submarine topographic features comparable in scale to the Grand Canyon, the Rocky Mountains, the mesas of the Southwest, and the Great Plains of the Midwest (Duxbury and Duxbury, 1989). Even though they are covered by water, these landforms can be shown on maps like those showing their terrestrial counterparts, and most students have probably seen the National Geographic's maps of the ocean floors.

Bathymetric maps are topographic maps of landforms that are submerged beneath the ocean or a lake. Mapping the bottom of a body of water presents some special problems because the bottom is not visible. Several techniques are currently used to collect depth data for mapping. When mapping a small, shallow lake it is possible to go out and measure the depth of selected locations with a weight tied to a rope which is lowered over the side of the boat. A long pole can also be marked with depth intervals and used to make soundings. To collect depth data, one must systematically row the boat around the lake and lower the rope over the side of the boat or drop the stick until it reaches the bottom. The sites where depths are measured are marked on a map, and the depth data are used to draw the contour lines for the lake bottom. Because of the vast size and depth of the Earth's oceans, it is not feasible to use this procedure. As a result, systematic mapping of the ocean floors began much later than terrestrial mapping.

It wasn't until the sextant and accurate chronometers were invented that latitude and longitude could be established on the seas out of sight of land. The invention of the sextant in 1731 by John Hadley enabled ships' navigators to establish their position relative to visible stars and constellations. An accurate timepiece recorded the exact times when the angles between the ship and the stars were fixed, thus allowing a precise latitude and longitude to be determined. These inventions allowed Captain James Cook to chart newly discovered lands in the South Pacific in the 1700s.
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The first scientific expedition to seriously attempt to measure the ocean's depths was the voyage of the British ship H.M.S.Challenger (1872-1876). The voyage of theChallenger lasted nearly three and a half years and logged 68,890 miles (Duxbury and Duxbury, 1991). Weighted wires were lowered over the side of the ship to measure depths out of sight of the land. The Challenger took soundings at 361 sites, collected samples of deep-sea water, made measurements at various depths, conducted studies of ocean circulation, and collected thousands of biological specimens and sediment samples from the ocean floor (Duxbury and Duxbury, 1991). Measuring ocean depth continued in this laborious fashion until the mid 1920's. This method of measuring depths was also used for lakes.

The first use of sonar (Sound Navigational Ranging) to locate underwater features was aboard the German research ship Meteor. Sonar devices use echoes from the ocean floor (or other underwater sound emitters such as a ship's hull) to measure ocean depth.The Meteor made 13 Atlantic crossings between 1925 and 1927, mapping the ocean bottom continuously. Sonar systems became highly accurate during World War II, when submarine warfare raged in both the Atlantic and Pacific oceans. Deep-ocean trenches in the western Pacific provided havens for Japanese submarines because they were too deep for sonar to penetrate. The use of sonar to map the sea floor following World War II dispelled any lingering notions that the Earth's ocean floors were featureless plains. Scientists mapped oceanic ridges, submerged islands, plateaus, and trenches — discoveries which helped with the understanding of plate tectonics. In the late 1960's the National Geographic Society published maps showing the topography of the ocean floors, allowing the general public to see the underwater part of the planet for the first time.

Today, oceanographers continue to use advanced sonar systems for exploration and detailed mapping of the sea floor. A side-scanning sonar device developed by British scientists is able to map wide bands to the sides of the vessel that tows it, rather than simply obtaining depth data from directly beneath the scanner. This device, which is called GLORIA (Geologic Long-Range Inclined Asdic), has produced very detailed maps of the continental margin adjacent to North America. Another type of sonar, multibeam sonar, emits signals of differing frequencies. This allows for detailed mapping of ocean bottom features in three dimensions. In 1993, GLORIA scientists announced that the largest cluster of volcanoes on earth, encompassing an area the size of Washington state, had been discovered in the Pacific Ocean near the East Pacific Rise. Remarkable new submarine landforms continue to be revealed as the technology for submarine mapping improves.

Many examples of submerged mountains and hills have been discovered on the sea floor. Many of these features are underwater volcanoes formed in locations where magma rises from beneath the oceanic crust to erupt on the sea floor (Fig. 1).
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Figure 1. Volcanic origin of submarine mountains. Dark cross hatched pattern represents coral. Stippled areas around volcanoes represent sediment.

Figure 1

Some of these cones may grow to rise above sea level, forming islands like the Hawaiian chain or the Philippine Islands. After active volcanic eruptions cease, an island may be eroded by wave action until it is first partly submerged and finally completely submerged to form a seamount (Fig 1). Coral reefs growing around the subsiding volcano may form an atoll. Guyots are seamounts with flat tops. Of course, not all seamounts began as islands; some were never tall enough to rise above sea level.

A bathymetric map is made just like a topographic map. Once the depth data is plotted on a grid, a line (contour line) is drawn connecting points of the same depth. Once the contour lines are drawn, a two-dimensional picture of the ocean floor or lake bottom becomes visible. (See Background Information in the activity Building a Topographic Model for more information on contour lines, contour intervals, and topographic maps.)

1) cardboard box (can be of varying size); a shoe box would work.
2) simple wooden or clay model of a seamount or guyot.
3) cm square graph paper (two sheets per group). Note: use of 1 cm square paper will take more time, but will produce a more detailed map.
4) pre-drilled lid or wooden board for the top of the box.
5) wooden dowel — must be able to slide through hole in grid and reach the bottom of the cardboard box.

1) Make simple models of various shapes to represent submarine landforms.
2) Tape model securely to the bottom of a cardboard box .
3) Place a lid with pre-drilled holes on top of the shoe box. Tape the lid in place. Suggestion: Try using a peg board with already drilled holes; available at hardware stores.
4) Place the grid paper over the pre-drilled top of the box and secure in place. The grid paper on top of the model does not need to be replaced at the end of each class session. The first group of students will need to find the holes, which may take a little bit of extra time. One centimeter grid paper will produce a precise map, but it will also require more time for the students to complete the activity.
5) Each lab station should have a cardboard box with a pre-drilled lid and a model inside, and metric ruler.
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1) In this activity, students will collect depth data as if they were using sonar.
2) The top of the box represents the water surface.
3) Students will slide a stick into predetermined holes on the grid at one centimeter intervals until the stick hits the model in the box.
4) Students will measure, in millimeters, the distance from the "surface water mark" to the end of the stick. This distance will represent the depth of that point.
5) Students will then record that depth on a separate sheet of grid paper which is identical to the grid paper on top of the box.
6) After students have plotted all of the points on the grid, they will connect all the points of equal depth.
7) A figure will emerge representing a submarine landform. However, at this point in the activity, the numbers will appear to be backwards. Because the numbers on the grid represent the distance between the top of the island and the water surface, the top of the island will be the smallest number; conversely, the ocean bottom will be the largest number.
8) Allow students to compare the map that they constructed with the actual model to help them visualize the model represented by the map.

Modifications: Make models of deep-sea trenches and submarine canyons.
Extensions: Have students use the library to write a report about the history of ocean exploration and oceanography, from pre-historic times to the latest technologies.

Duxbury, A.C., and Duxbury, A.B., 1991, An Introduction to the World's Oceans (3rd. ed.): Wm. C. Brown Publishers, Dubuque, Iowa, 446 p.

Erickson, S., Gregg, D., Helling, F., King, M.W., and Starkweather, J., 1987, Submerged Islands in Down to Earth, AIMS Education Foundation, Fresno, CA, p. 24-26.

See Additional References and Resources in A Model of Sea-Floor Spreading for information about how to obtain maps of the ocean floor and books, slides, and videos relating to the sea floor.

A related activity — Erickson, S., Gregg, D., Helling, F., King, M.W., and Starkweather, J., 1987, Mapping the Ocean Floor in Down to Earth, AIMS Education Foundation, Fresno, CA, p. 27-29.

atoll — a ring-shaped coral reef surrounding a lagoon and surrounded by open ocean. An atoll forms around a volcanic island, which subsequently sinks below sea level.
bathymetric map — a map showing the topography of the floor of the ocean or other body of water using contours connecting points of equal depth.
chronometer — a portable clock that measures time with great accuracy.
guyot — a submerged seamount with a flat top.
seamount — an isolated peak of volcanic origin that rises at least 1000 meters above the floor of the ocean.
sextant — an instrument for measuring the angular distances of celestial bodies; used in the open ocean to determine longitude and latitude.
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In our discussion of plate tectonics and earth movement, we have learned about the formation of submarine landforms. In today's laboratory investigation, you will be creating a two dimensional map of underwater hills or mountains.

The purpose of this activity is to create a two-dimensional contoured map from a simulated, underwater, three dimensional model using sounding techniques.

— cardboard box with holes in the lid.
— ruler
— model of underwater landform
— two sheets of 1 or 2 cm square graph paper
— masking tape
— wooden stick (sonar stick)

1) Working with your partner, place the wooden stick through each of the holes on the grid paper until you hit bottom.
2) When you "hit bottom", use the ruler to measure the depth in millimeters. Record the depth in the same location on a separate piece of graph paper in millimeters.
3) Mark the stick with a pen or pencil to show the various depths so that you won't have to measure the depth every time you move to another location on the grid.
4) Continue taking soundings until you have finished the grid.
5) Connect points of equal depth to develop the two dimensional map of the submerged landform. Contour lines run through points of equal depth.
6) Round your contour lines slightly to a more real appearance.
7) Open the top of the box containing the "underwater landform" and compare the model to your two-dimensional map.
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For the questions below, assume that each millimeter of your "sounding stick" represents one meter of actual depth in the real ocean.

1) How does the model compare with your map?
2) What is the maximum depth of your "ocean "
How does this compare with the depths of real oceans?
3) How far below the ocean's surface is the tallest part of your island?
4) How could you have made your map better?
5) How can people use maps that show the features on the ocean bottom?