FOR SEA Institute of Marine Science
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FOR SEA Grades 9-12
Physical Science
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If I Can't See It, How Do I Know It's There?

Lesson by Gene Williamson, Whitford Intermediate School
Beaverton, Oregon

Key Concepts
  1. It is possible to know what something looks like without seeing it.

  2. Most models of the ocean floor are vertically exagerated.

  3. Topographic maps can show a three dimensional surface on a flat sheet of paper.

  4. Land forms on the sea floor are similar to land forms on the continents.

  5. Many factors affect the accuracy of scientific investigations.

Oceanographers have discovered the shape of the ocean floor by measuring the depth of the ocean in many places. Early tools included lead weights, lowered on marked ropes or cables to the ocean floor. From such depth readings, scientists gradually built a picture of the ocean floor they could not see. These methods were very slow and eventually were replaced by sonar systems which bounced sound waves off the bottom.

Today, sophisticated side-scan sonar and satellite data are fed into computers that are giving us the most detailed pictures of the ocean floor ever obtained. All of the methods allow us to "see" the bottom of the ocean.

Every good model of the ocean floor shows all features as both taller and steeper than they actually are. This occurs for a very practical reason. If the model or sketch were prepared to exact scale, it would need to be very large. Vertical exaggeration is the price we pay to get the model down to a workable size.

Additional background information is found in the preceding activity "The Ocean Floor".

For each pair of students:
  • 1 shoe box model, sealed with masking tape, and with a grid of holes on top

  • 1 bamboo probe, or the equivalent, to be used in sounding the model

  • 2 copies of all student pages

  • plenty of 1/4" quadrille graph paper

  • tag board or old manila folders for mounting cut-out graphs

  • large (11"x17" min.) sheets of drawing paper or tag board for topographic maps

  • tape for fastening graphs to tag board

Teaching Hints
"If I Can't See It, How Do I Know It's There?" provides students with an experience close to the real process of sounding the ocean floor. Each pair of students builds a model of the seafloor inside a shoebox. They trade completed and covered shoeboxes and use skewers as probes to measure someone else's shoebox creation. The students use the data they collect to map the shoebox ocean floor without actually seeing what is at the bottom of the shoebox.

This three-part activity assumes that students have a working knowledge of the topography of the sea floor that goes beyond having done the activity titled "The Ocean Floor". Students should know the size and shape of the sea floor features listed here, and should understand their location with regard to the three major divisions of the ocean floor discussed in the previous activity. The FOR SEA grade 8 curriculum, Ocean Studies, Ocean Issues, contains additional activities dealing with ocean basin topography.

Building a model of the sea floor in a shoe box is a way to acquire and demonstrate an understanding of the structure of the sea floor. Have ocean bottom maps, models or globes available so students may pattern their boxes after real ocean floor features and patterns. A small map of a fictional ocean floor is included in this lesson. Students may choose from the following features in building their shoe box models of the sea floor:

  • abyssal hills
  • abyssal plains
  • atoll
  • bay
  • continental rise
  • continental shelf
  • continental slope
  • fan
  • fracture zone
  • gulf
  • guyot
  • island arc
  • mid-ocean ridge
  • rift valley
  • seamount
  • sumbarine canyon
  • trench
It is certainly not necessary to use all items on this long list. Students should have at least 10 objects from which to choose as they build their models. Note that models with considerable relief will be easier to map.

You may find it necessary to build some shoe box models both to demonstrate different construction techniques and to use in class if you do not get enough acceptable models from the students.

The following hints will help assure the mechanic part of this activity goes smoothly.

  1. Building the shoe box model is a very important part of this lab. Each pair of students will need to secure a sturdy shoe box complete with top. The top will need to have small holes (a compass point works well for this) punched every centimeter to provide a grid through which the ocean model may be probed. A centimeter grid template is included with this activity. The requirements for an acceptable student model, as well as some aids and suggestions for constructing the model are included in the student pages.

    Let students share ideas and ask questions about the process. They may use any materials that fulfill the requirements outlined in the student pages. Successful models have been constructed out of everything from plywood to LegosTM to home-made play dough.

  2. Demonstrate every step. Do not assume that students will know what to do, but do be ready to accept student suggestions for making things go faster and easier. Most of this activity has been developed by students with an instructor wise enough to keep his ears open and his mouth shut.

  3. Bamboo skewers are available at most grocery stores and virtually all Asian markets. They make ideal probes. Students seem to have the most trouble with the concept of "to the nearest half centimeter" when reading the probe. They will either round off to the nearest centimeter, or will try to read to the nearest millimeter.

  4. Do not expect all students to complete all parts. Some will be better at one part than another. Let them teach each other. Since they should be working in pairs, you need to decide whether they will pair themselves or you will assign pairs.

  5. Analysis of an unknown shoe box develops the ability to make careful observations and measurements, organize data, display data in new ways, and make inferences based on the data collected.

  6. Topographic mapping is a new skill for most students. It can be a great deal of fun, but it can also be a trying and, therefore, a learning experience.

    A practice topographic map is included to help you and the students learn this new skill. To avoid having students deal with unnecessary lines in their topographic maps, use the 0.5 inch grid provided. Have students place the grid behind their map paper and then copy their data sheet by placing the numbers at the intersections of the grid. Have them write the data in hard, sharp pencil. The numbers should be written as small and as lightly as possible. This will facilitate not only the construction of the map, but also the removal of the data numbers when the map is finished.

  7. You may involve students in any of the phases of the project you and they have time for and think are worthwhile. They may be involved in the relatively simple building of the ocean floor models, probing through the lids of the shoeboxes to collect data, constructing graphs and three-dimensional models of their data, and in the more difficult construction of topographic maps of the shoebox floors. Do not string the activity out too long. Here is a suggested schedule:
1 day - Describe or demonstrate the building of models and requirements for an acceptable model. Plan to collect the models one week after they are assigned. You will need overnight to seal up boxes and put identifying numbers on each.

2 days - Hand out boxes and have students collect data.

3 days - Students graph data, cut out graphs, and prepare 3-D models.

1 day - Students should prepare grids for topographic maps.

1 day - Give instructions on topographic mapping and do demonstration. Use an overhead projector to go through mapping process on step at a time. This may take more than one day when you are starting out.

3 days - The students work on topographic maps. They will need a great deal of help and encouragement. You will need many aspirin.

1 day - Students report findings, write conclusions and when completed projects are submitted, open the shoe box. They will want to open the box earlier than this. Be strong. Remember that our first objective is to teach students to see without looking.

This is twelve class periods if everything goes smoothly. Should you choose to skip topographic mapping, the abbreviated activity can be completed in about eight class periods.
Key Words
  • calibrate - to adjust or systematically standardize the graduations on any quantitative measurement instrument
  • grid - any set of intersecting parallel lines, a piece of graph paper is an example
  • interval - in this case, the space between any two marks on a measuring instrument
  • probe - in this activity, a bamboo stick which is used to measure the depth at each "station"
  • side-scan sonar - a type of sonar which is capable of taking depth data for a wide swath of the sea floor beneath a research vessel
  • station - the position of a research vessel at the time data is collected
  • vertical exageration - an effect caused by using different scales for vertical and horizontal distances in models of the ocean floor, it makes models of features steeper than they are in reality
  1. Feed depth data into any of a number of computer graphing programs to draw the cross-section maps. More sophisticated programs could be used to show three dimensional views.

  2. Have students color the topographic maps to show relief in much the same way that computer generated maps of the sea floor are drawn. Colors represent the range of depths between adjacent lines, and should be added only after the topographic map is completed satisfactorily. This works best if the colors become darker as the water gets deeper. For example, shading from white to pastel blue to blue to navy blue works well.

  3. Have students use the library or other research facility to find out how modern oceanographers are using satellites to measure ocean depths. It is a first class "science fiction" story of satellites, lasers, and computers. We have come a long way from the lead weight and rope.

Answer Key
Analysis Questions
  1. Answers will vary, but most students will see distinct similarities.

  2. Answers will vary, but in general smaller objects are harder to locate and identify. Also objects near the edge of the box may have distorted data caused by the probe contacting the sides of the box.

  3. The most obvious ways to improve accuracy would be to use more accurate measurement and to use more stations and gather data at smaller intervals. This is the advantage of sonar which can collect continuous data along the ship's track.

  4. Modifications might include a hollow boring tip or some type of sticky substance attached to the point of the probe. Accept any reasonable answer.

  5. The measurement of depth with rope and lead weight was slow and exceedingly tedious. Sonar allowed for virtually unattended and constant measurement of the water depth under the hull. The accuracy and the availability of maps increased dramatically with the introduction of sonar.

  6. Trawl fishermen are interested in detecting environments that fish like to frequent, and they are also interested in not allowing expensive nets to become entangled in unseen hazards.

  7. The captains of ocean going vessels seldom worried about the depth of the water under their hulls until they arrived "close upon the land", at which time an accurate understanding of water depth became a matter of life and death. Also, given the tedious nature of depth measurement, there were very few measurements being made in deep water. Magellan, on his final voyage around the world (the ship made it, Magellan did not) used every available foot of line and still could not reach the bottom of the Pacific.

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