Part I – Bathymetry profiles and GeoMapApp

Plot topographic and bathymetric (ocean depth) to relate large-scale features on Earth’s surface to lithospheric plates, earthquakes, and volcanoes. This exercise also provides experience with graphing with GeoMapApp and concepts of scale.

A bathymetric profile provides a "skyline view" of the sea floor in which hills are seen as rises and valleys as depressions. For a graphical profile to illustrate the true shape of the sea floor, a ratio of 1:1 for vertical and horizontal distances must be the same or have a ratio of 1:1. This means that one unit on the vertical scale is the same distance as one unit on the horizontal scale. However, the slopes of ocean features (i.e., relief) are generally so small that it is difficult to see sea floor features. Typical Atlantic Ocean basin features are only a few kilometers high, while the basin itself extends laterally for thousands of kilometers. If the profile were displayed with a 1: 1 ratio on a regular sheet of paper, it would appear as a flat line.

In order to illustrate the details of the sea floor relief, the depth scale of an ocean profile is vertically exaggerated (stretched) relative to the horizontal scale. Vertical exaggeration causes distortion in the shapes of the bathymetric features that are being illustrated with the amount of distortion increasing with the amount of exaggeration. As vertical exaggeration increases on a profile, hills appear to be higher, valleys deeper and the slopes between them become much steeper. Slopes, that in reality are gentle, will look steep; steep slopes will appear to be precipitous. For example, when the vertical scale has been stretched four times relative to the horizontal scale, we have a vertical exaggeration (VE) of 4.

The vertical exaggeration (VE) associated with a general profile is found by dividing the horizontal scale by the vertical scale according to:

VE = Horizontal Scale/Vertical Scale

Example: VE= (1”= 100ft)/(l”= 25ft)= 4

PROFILING EARTH’S SURFACE USING GEOMAPAPP

PURPOSE

• To relate large-scale features on Earth’s surface to lithospheric plates, earthquakes, and volcanoes.

• To introduce GeoMapApp, an easy-to-use mapping program focused on marine geology and geophysics.

OVERVIEW

First, follow the directions given below to draw a cross section showing elevation along the line (B-B’) using GeoMapApp. After printing out your cross section, add the characteristics specified in (1-5).

INSTRUCTIONS

Download GeoMapApp onto your computer or laptop. Go to

http://www.geomapapp.org/index.htm and follow the instructions for downloading.

After downloading the program, double-click on the GeoMapApp.jar icon to run the program. Choose the Mercator map.

Inside GeoMapApp, click on the Distance/Profile tool at the top of the screen:

You may need to patiently wait while GeoMapApp opens the profiling tool. When the program is ready, refer to the world map pictured in this exercise. To draw the B-B’ line in GeoMapApp, place your cursor in the appropriate location on the Pacific Plate. Click on the mouse as you drag the cursor over just north of Madagascar. When you first do this, the line might be drawn as a great circle. In the profile tool, click the box specifying that you would like to draw the cross section along a straight line:

Next, save the profile to your computer and then print it out.

Using a pencil, add the following characteristics to your topographic profile. For some of these, you will need to refer to outside sources such as your textbook. For others, you may discover tools or databases within GeoMapApp to help you.

Profile 1 Questions:

★Q1.1: Identify and label the positions of the following features on the crosssection:

a. Plate boundaries and their types (e.g., label the East Pacific Rise and indicate that it is a mid-ocean ridge)

b. Lithospheric plates and their names

c. Continental margins and their types (passive or active)

★Q1.2: Put a series of X’s within the cross-section to represent zones where significant earthquake activity might be expected to occur. Be sure to place the X’s at the appropriate depths.

★Q1.3: Put Δ‘s on the surface of the cross-section to illustrate where you would expect volcanic activity to occur.

Atlantic Ocean Basin

Now produce a bathymetric profile across the Atlantic from N. America (~75°W,40°N) to N. Africa (~14°W,20°N). Try to include only a little bit of the land above sea level.

Profile 2 Questions:

★Q1.4: On the graph, label the following features of the ocean basin: continental slope, abyssal plain, mid-Atlantic ridge.

★Q1.5: What is the approximate ocean depth at the mid-Atlantic ridge and the relief of the ridge (difference in depths, or elevations, between the top of the ridge and the adjacent, relatively flat ocean bottom)?

Ocean depth at mid-Atlantic ridge __________

Relief (difference in ocean depth between the ridge and the abyssal plain) of the mid-Atlantic ridge __________

★Q1.6: What is the approximate slope of the west flank of the mid-Atlantic Ridge (measure the difference in ocean depth along the profile between about 2200 km and 3200 km distance and divide by the difference in distance, 3200-2200 or 1000 km). Be sure that both measurements, differences in depth, and distance, are in the same units, either km or m. The resulting number will be the slope expressed as a ratio. The slope can also be given as a percent or as an angle.

Approximate slope of the west side of the mid-Atlantic ridge __________.

Produce a bathymetric profile across the Mariana Trench region and include both the volcanic arc ridge along with the farther back remnant arc.

Profile Questions:

★Q1.7: On the bathymetric profile, label the trench, the active volcanic arc, and the remnant volcanic arc.

★Q1.8: Under the profile, sketch what you think is happening in the crust and upper mantle (i.e., what are the plates doing here?).

Data Link