Lab 3 Activity
Lab 3 Activity: Using Data to Investigate Plate Boundary Properties
Overview
In this lab you will use a variety of maps and data sets to help you understand how we know where plate boundaries are on Earth, and how we know what types of boundaries are present.
A. Spreading Rates: How Fast Is the Atlantic Ocean Getting Wider?
The spreading rate of a mid-ocean ridge refers to how quickly it gets wider. This is usually expressed in units of cm per year. For this exercise you will use the maps in Figure 2.5, 2.6, and 2.7 to make quantitative and qualitative observations about the spreading rate along the Mid-Atlantic Ridge.
How to Use Figures 2.6 and 2.7
Figures 2.6 and 2.7 show the age of the ocean floor (i.e., how long ago a particular region of ocean crust formed at the Mid-Atlantic Ridge before being pushed to either side). They are colour coded, but also include age contour lines (black) in 5 million year increments. Every 30 million years is marked with a heavier black line. You can use the colours to tell the age, or you can use the age contours. If you’re not sure which age to use for the questions that follow, go with the age represented by the nearest contour line. You can also use the spacing of the age contours (the width of the colour bands) to compare the rate of spreading from one area to the next. The more rapidly crust formed during an interval, the wider that age band will be. If crust formed more slowly, the age band for that interval will be narrower.
A.1. Spreading rate of the Mid-Atlantic Ridge between Recife and Douala
a) On the print version of Figure 2.5 in your lab handout, use a ruler to draw a line between Recife and Douala. Measure the length of the line, then use the scale at the bottom of the map to determine the distance in km.
Measured distance: ____________ cm
Map distance: ____________ km Hint: How many cm is 1000 km on the map scale?
Map distance converted to cm (you need this for the calculation): _____________ cm Remember: there are 1000 m in every km, and 100 cm in every m.
b) How long has the Atlantic Ocean been spreading between Recife and Douala? Hint: In Figure 2.B, how old is the oldest ocean crust between Recife and Douala?
Spreading started ____________________________ years ago.
c) Calculate the spreading rate using the formula below, and show your work. Remember that the rate should be in units of cm/year.
A.2. Why is the Atlantic Ocean different widths at different locations?
The distance between Recife and Douala is short enough that you can use a straight line to measure between them without having to worry about the curvature of the Earth introducing too much of an error. At other locations, the Atlantic Ocean is too wide to do this without considering Earth’s curvature. Fortunately we can use Figures 2.4 and 2.5 to draw some qualitative conclusions to help understand spreading rates in the Atlantic Ocean.
a) Examine the age contours between Jacksonville and Dakhla (Figure 2.6). Does a