Name:

Lab Partner(s):

Lab day and time:

TA:

(Due at the start of discussion section, one week after being handed out)

The ocean floor can be subdivided into two main parts based on depth: (1) continental margins, and (2) deep ocean basins. Continental margins can be further subdivided into two main parts: (2a) a shallow, nearly flat continental shelf close to shore, and (2b) a more steeply-dipping continental slope the separates shallow and deep parts of the ocean. Deep submarine canyons are sometimes cut into the continental shelf and slope, perpendicular to the shelf edge. One of these canyons cuts through Monterey Bay (Fig. 1), running perpendicular to the shoreline just off Moss Landing (halfway between Santa Cruz and Monterey). The deep ocean basins are not completely smooth, but contain massive mid-ocean ridges (submerged volcanic mountain ranges) and deep-sea trenches (the Mariana Trench which is about 11 km deep). Between these features, there are broad, relatively flat regions called abyssal plains. Also, seamounts and guyots, which are individual volcanic mountains, pepper the deep sea floor throughout the ocean basins. Your text describes these features and others in detail, and we discuss their origins and significance in class.

Oceanographers describe the shape of the sea floor in terms of its bathymetry, variations in depth below sea level. This is equivalent to discussing topography on land. On both cases, values are usually referenced to sea level. Bathymetry can be represented on maps in numerous ways, including contour lines, simple shading, color coding, and texture. In addition, cross sections through topographic features illustrate changes in depth. In this assignment, you will look at several different kinds of maps showing sea floor bathymetry and construct a cross section of the Monterey Canyon using a bathymetric contour map.

Look at the world map of the ocean basins (see your text, p. 112-113). On this map, bathymetry is represented by color, with dark blue corresponding to deep water, and light blue corresponding to shallow water

1a. Sketch a rough cross section across the sea floor feature that runs from 100°W to 120°W (longitude) at 20° S (latitude) showing whether the feature is concave (like a "") or convex (line an ""). Don't worry about getting the depths exactly right, just sketch the general shape.

b. Give the full name of this feature (look in the text book, p. 516-517).

2. Sketch a rough cross section across the sea floor feature just south of the Aleutian Island chain showing whether the feature is concave (like a "") or convex (line an "").

3. Look at the continental shelves off both eastern and western North America. What is the major difference between the shapes of these shelves?

A contour map shows the three-dimensional features of the Earth's surface in two dimensions. Topography (on land) and bathymetry (at sea) can both be shown with contours. Contour lines used in a bathymetric map are also called isobaths. Contours are lines that connect points of equal elevation. Thus if you were walking on a mountain (or on the bottom of the ocean) and stayed on a contour, you would not climb higher or lower as you walked. If a contour is labeled 100 m, then all points on the contour have an elevation (for a topographic map) or a depth (for a bathymetric map) of 100 m. The contour interval is the difference between two adjacent contour lines. For example, if there is one contour showing depth of 100 m and the next contour shows a depth of 150 m, then the contour interval is 150-100 = 50 m. Some maps use more than one contour interval so that they can show very detailed features that occur in one area, and coarser features that occur in other areas.

1. Figure 1 is a bathymetric map of southern Monterey Bay. There are two different contour intervals used on this map. The contours are labeled in meters below sea level. Unit conversions are listed in Appendix 1 at the back of your textbook.

a. What two contour intervals are used on this map? Give your answer in units of meters.

What are the two contour intervals in units of feet?

b. At what depth does the contour interval change?

What is this depth in feet?

2. Not all contours are labeled, but you can figure out what depths unlabeled contours represent by counting the number of lines between labeled contours.

a. List the water depths (in both meters and feet) for the contours labeled a to f on Figure 1. Make sure you label the units for each answer, or no credit will be given.

Depth (meters) Depth (feet)

a.

b.

c.

d.

e.

f.

b. Estimate the water depths for the points labeled g to j on Fig. 1. These points are located between adjacent contours.

Depth (meters) Depth (feet)

g.

h.

i.

j.

3. The relative spacing of contour lines on a bathymetric map can be used to interpret the shape of the sea floor. What does the spacing of contours indicate about the slope of the seafloor in these three cases:

closely-spaced contours:

widely-spaced contours:

change in contour spacing:

A cross section drawn from a bathymetric map is a plot of depth on an X-Y graph, where the X-axis is distance along the profile, and the Y-axis is depth. The cross-section shows you what a slice through the bathymetric map would look like.

4a. Look at Fig. 2 and sketch a rough cross across Monterey Bay, from Monterey to Santa Cruz (B-B'). Don't worry about getting the depths exactly right, just show the basic shape of the profile. Mark the North and South ends of your cross section.

4b. Name the primary feature in the middle of the cross section.

4c. Approximately how far offshore from Moss Landing does this feature extend? Give your answer in kilometers and nautical miles.

5. Now draw an accurate cross section showing the bathymetry of the sea floor along line A-A' from Fig. 1. This is the same feature you should have sketched in Question 4, but this time, use the labeled graph paper provided with this problem set.

The best way to complete this cross section is to use a ruler to find the distance along the transect at which you cross each contour, then mark each of these locations with a dot on the graph paper. When you are done plotting all the dots, connect them with lines. Be sure to use all of the contour lines along profile A-A'. Ask your neighbor or TA if you need help.

6. The continental shelf is a part of the continental landmass that is submerged below sea level. During periods when most of the continents are free of ice (like now), sea level is relative high, and the parts of the continental shelves that are under water are relatively wide. When sea level was lower, some of the shelf was exposed, and the submerged part was narrower. The edge of the continental shelf, called the shelf break, is marked by an abrupt increase in the slope (steepness) of the sea floor. The shelf break occurs at an average depth of 150 meters below sea level most places in the world.

a. Look at Fig. 2. Determine the distance (in both kilometers and nautical miles) from the shoreline of the town of Davenport to the shelf break directly south of Davenport.

b. Determine the distance (in both kilometers and nautical miles) to the shelf break directly west of Point Lobos.

Extra credit: The cross section you constructed in Problem 5 does not give the true shape of the canyon because the vertical scale is in units of meters and the horizontal scale is in units of kilometers. Choosing two different scales was necessary because the canyon is much wider than it is deep. As a result your cross section is vertically exaggerated, making the canyon walls look steeper than they are in reality. The horizontal scale for the cross section is 1 cm = 1 km. The vertical scale is 1 cm = 50 m. What is the vertical exaggeration of your cross section? Express your answer in the form: vertical exaggeration = 40X or 20X or 15X, etc.