1

Geology 2 – Physical Geology Lab

Lab #2 – Interpreting Tectonic and Bathymetric Maps

Student Outcomes:

Be able to understand and use basic map information, including scale, contour interval, legend, etc.

Be able to recognize tectonic features, including mid-oceanic ridges, seamounts, trenches, and plate

boundaries on a bathymetric map

Be able to identify the three types of tectonic plate boundaries using a bathymetric map

Understand the processes occurring at the three types of plate boundaries

Be able to determine the direction of tectonic plate motion by interpreting seafloor features on a bathymetric

map

Create a cross-section using a bathymetric map

Understand and use contour lines and contour intervals on a map

Recognize and identify topographic features associated with the San Andreas and San Gregorio Faults

Be able to identify seafloor features such as continental shelf, continental slope, continental rise, submarine

canyons, and seamounts on a bathymetric map.

Develop a beginning understanding of how our world works so the student can make more accurate

assessments of presented information about our world and become responsible leaders and voters.

Materials:

Map of “Measured and Estimated Seafloor Topography” from World Data Center Map of “Monterey Bay

National Marine Sanctuary” from Monterey Bay Sanctuary Foundation, 1999.

Both of the previous maps are on reserve at the MPC library and available through the class website.

Wall map of the World

Various other bathymetric maps and resources available during lab

Plate tectonic map of the world

Globe

Important: In the first parts of this lab, we want you to rely primarily upon the map titled “Measured and Estimated

Seafloor Topography.” We encourage you to look at all the other maps, but answer the questions based on what

you can see on this map.

Bathymetry Map Orientation

1) Using the bathymetric map “Measured and Estimated Seafloor Topography,” locate the colored depth scale

at the top. What color represents the deepest areas of the ocean?

Lightest purple

2) What depth is represented by green? (Hint: Interpolate to the nearest 500 meters)

3500 m

3) What is the average depth of the oceans abyssal plains?

5500 to 6000 m

4) How would you distinguish ocean floor that is 6,000 meters deep from ocean floor that is 7,000 meters

deep?

6000 m deep is dark purple; 7000 m deep is light purple

2

Atlantic Ocean

1) Name at least one place where the Mid-Atlantic Ridge is exposed above sea level. Use the map behind the

screen, or any other lab resources, to find place names.

Iceland; Azores

2) Where are the deep, broad abyssal plains of the Atlantic Ocean?

a. Describe their location using a relationship to a continent or other widely recognized feature (i.e., “east

of South America but west of the Mid-Atlantic Ridge).

West of the Mid-Atlantic Ridge and east of both the North American continent and the South American

continent. There are also some nice abyssal plains east of the Mid-Atlantic Ridge and west of both north

Africa and central Africa.

b. How did you recognize the abyssal plain? What are its defining features?

Deep, featureless, flat, and covering a wide extent of hundreds or thousands of square kilometers.

3) Where are the deepest areas of the Atlantic Ocean? (Hint: Remember the deepest bathymetry is shown by a

light purple color, not deep purple or blue.)

a. Describe their location using the name of the nearby island chain. See map behind screen for names.

These are deep-sea trenches located near the Caribbean and near the South Sandwich Islands.

b. What is the tectonic setting of these deepest areas?

Convergent margins; deep-sea trenches mark the location of convergent margins.

4) Is the east coast of South America at a plate boundary or in the middle of a plate? How do you know?

The east coast of South America is in the middle of a plate. The reason is because there are no bathymetric

features associated with plate margins observed near the east coast of South America. Examples of these

kinds of bathymetric features would be mid-ocean ridges, volcanoes, deep-sea trenches, or transform faults

(recognized by straight lines). None of these are observed near the east coast of South America.

Indian Ocean

1) Is Madagascar part of the African Plate? Why or why not? (Hint: do you see any plate margin features

between Madagascar and continental Africa?)

Yes. Only shallow water separates Madagascar from the African continent, and there is no evidence of a

plate boundary (i.e., trench, mid-ocean ridge, transform features) between them.

2) Is Madagascar part of the African continent? Why? Or why not? (Hint: Continental boundaries are defined

by the edge of the continental shelf.)

No. Deep water separates Madagascar from the African continent. In other words, continuous continental

shelf does not occur between Africa an Madagascar

3) India and Australia are both part of the Indo-Australian Plate. Examine the portion of this plate northwest

of Australia.

What seafloor features are found along the northeast

boundary of this portion of the Plate?

Deep-sea Trenches, +/- volcanoes

What kind of tectonic boundary is this?

Convergent

4) Examine the southwestern boundary of the Indo-Australian Plate.

What seafloor features are found along the

southwest boundary of this portion of the Plate?

A mid-ocean ridge

What kind of tectonic boundary is this?

Divergent

5) Considering your observations in answering the previous two questions, what direction do you think the

Indo-Australian plate moving? Explain your conclusion.

The plate is moving north or northeast, towards the convergent margin and away from the divergent

margin.

3

Pacific Ocean

1) Western Boundaries of the Pacific Plate

a. What seafloor features do you find along the western boundaries of the Pacific Plate?

Deep-sea trenches and chains of explosive volcanoes

b. What type of tectonic boundary is this?

Convergent boundary

2) Southeastern boundary of the Pacific Plate

a. What seafloor features do you find along the southeastern boundary of the Pacific Plate?

A Mid-ocean ridge

b. What type of tectonic boundary is this?

A Divergent Boundary

3) Considering your answers to the previous two questions, what direction do you think the Pacific Plate is

moving? Explain your rationale.

The Pacific Plate is moving towards the northwest, away from the divergent boundaries and towards the

convergent boundaries. The motion is roughly parallel to the orientation of the Hawaiian Seamount Chain.

4) Locate the Hawaiian-Emperor Seamount Chain.

a. Notice the bend in the chain. How might this bend have formed? The age of the rocks at the bend is 43

million years old.

The direction that the Pacific Plate was moving changed 43 million years ago

b. Why do you think the seamounts become progressively deeper to the north and west of Hawaii?

Seamounts become denser as they cool, causing them to subside. The older seamounts are progressively

cooler, and therefore deeper, with distance from the source hotspot.

5) Where in the Pacific Ocean would you expect to find the oldest oceanic crust? Why?

At a subduction zone farthest away from the mid-ocean ridge where the new crust is being formed. After

new crust forms at a mid-ocean ridge, plate tectonics moves it away from the ridge. Thus, those locations

farthest away from the ridge would have the oldest oceanic crust. These would be found immediately

adjacent to a dee-sea trench, where the old crust is about to be subducted.

6) Locate the Marshall Islands and

the Louisville Ridge in the

southwest Pacific on the map at

the end of the lab worksheet.

Notice how the islands and the

ridge seem to connect with one

another. What do you think

happened to the missing islands in

between? What feature do you see

on the bathymetric map that

supports your guess?

The missing islands were

subducted down a subduction

zone. The deep-sea trench

oriented roughly north-south

supports this idea.

4

7) Notice the chain of islands extending north from New Zealand. Why might you expect to find active

volcanoes on these islands? What kind of volcanoes would you expect them to be? Explain using what you

know about plate boundaries.

Active volcanism is characteristic of subduction zones and convergent margins. Dee-sea trenches are

characteristic of convergent margins. Therefore, the volcanoes would be explosive -- strato-volcanoes with

steep sides.

Cross-sections

Using the bathymetric and topographic maps in lab, draw cross-sections along the lines indicated on the map at

the end of the worksheet. Use the lines A-B, C-D, and E-F provided on the next page to draw your cross-

sections. (Note that cross-section C-D goes right through Hawaii.) In your cross-sections, be sure to include

and label the following features, if present:

Lithosphere

Asthenosphere

Crust

Approximate topography and bathymetry

Sea level

Sites of active volcanism

Hot spots

Trenches

Convergent margins

Divergent margins

Transform margins

Mid-ocean ridges

See next page

5

6

Monterey Bay Area

Refer to the map of the Monterey Bay National Marine Sanctuary put out by the Monterey Bay Sanctuary

Foundation (1999) to answer the questions in this section.

1) Ask your instructor to show you the San Andreas Fault (SAF) on the map.

a. Draw the location of the SAF on the map on the next page.

See map on following page; eastern fault is the SAF.

b. In the northernmost part of the map, which bay marks the course of the SAF?

Tomales Bay; Covered by the SAF on the map on following page.

c. Near what famous landmark is the SAF located offshore rather than onshore?

Golden Gate

d. Near what highway does the SAF cross the Pajaro River near Watsonville?

Highways 101 or 156 are both acceptable

e. In the southern portion of the map, which highway follows the SAF most closely?

Highway 25

f. What kind of fault is the San Andreas Fault?

A transform fault; it is so big that if forms a transform margin between the North American and

Pacific Plates. Right lateral or strike-slip are also acceptable answers

2) Note the light blue lines labeled with numbers in the offshore portion of the map. These lines are called

contour lines. A contour line is a line joining points of equal elevation, or, in this case, of equal depth. The

contour interval is the vertical distance between contour lines.

a. Referring to the map legend, in what unit is the depth recorded on this map?

meters

b. What is the contour interval on this map close to shore?

20 m

c. What is the contour interval on this map far from shore?

200 m

3) The Monterey Bay National Marine Sanctuary map shows several seafloor features exceptionally well,

including examples of the continental shelf, continental slope, continental rise, submarine canyons, and

seamounts. Locate and observe examples of each of these features on the map in lab.

See map on following page, where the labels are but one example of these features shown on this map.

4) Find the Monterey Fan in the southwestern region of the map.

a. If you drilled a hole here, what kind of material would you expect to find?

Sediment

b. How would it have gotten there?

Turbidity flows or currents moving down the submarine canyon or over the continental slope

c. What could be the source?

Erosion of the coast ranges

5) How deep is the Monterey Canyon near the intersection with the Carmel Canyon in meters and in feet?

(One meter = 3.28 feet)

2000 m = 6560 feet

6) Ask your instructor to show you the trace of the San Gregorio Fault south of San Francisco to the Santa

Lucia Range. Draw the location of the San Gregorio Fault in the image on a following page.

See map on following page; western fault is the San Gregorio Fault.

Remember the bathymetric and the Marine Sanctuary maps are available for your use at the reserve desk at the

library. The bathymetric map is also available on the class website.

7

Continental

Shelf

Continental

Rise

Continental

Slope

Submarine

Canyon

Seamount