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# Seafloor Spreading Activity

Derived from The Race Is On...with Seafloor Spreading! . JOIDES Resolution Retrieved 12 Dec 2014.

## Overview

1. The map shows three lithospheric plates: Pacific plate, Cocos plate, and Nazca plate. Locate these on a world map of the lithospheric plates .
2. The black double lines shown in the map indicate the East Pacific Rise (EPR) spreading center. The gray-scale areas represent the paleomagnetic anomalies ("stripes").
3. The anomalies are numbered 1 to 4.
4. The ages of these anomalies are shown in the key at the top of the map.
5. A scale of 500 kilometers is shown at the bottom of the map .
6. There are two transect lines drawn on the map. Line "A" is for the Cocos-Nazca plate rift. Line "B" is for the Pacific-Nazca plates.

## Directions

1. Measure and record the length (in cm) of the map's 500 km scale ______________.
2. For each transect line on the map:
1. Measure the distance (cm) from the East Pacific Rise (EPR) across each numbered magnetic anomaly ("stripe") . Record in the data tables.
2. Calculate actual width (km) of each anomaly by multiplying the measured width by the conversion factor from step one. Record in the data tables.
• Anomaly width (km) = (500km / length of scale) · measured map width of anomaly (cm)
• If the 500 km scale bar measured 2.7 cm and the measured map width of anomaly 1 on transect line A was 0.9cm :
• Anomaly 1 width = (500 km /2.7 cm ) · 0.9cm = 167 km
3. Convert the width from km to cm. Record in the tables.
4. Record the maximum age for each magnetic anomaly. Use the oldest value given in the key for each anomaly's age. For example, for anomaly 3 use 15 Ma.
5. Calculate the spreading rates in centimeters/year (cm/yr).
 Step Measurement Anomaly 1 Anomaly 2 Anomaly 3 Anomaly 4 2a. Measured map distance from EPR across anomaly (cm) 2b. Calculate actual anomaly width (km) 2c. Convert actual anomaly width to cm 2d. Maximum Age (years) 2e. Average Seafloor Spreading Half Rate (cm/yr)

 Step Measurement Anomaly 1 Anomaly 2 Anomaly 3 Anomaly 4 2a. Measured map distance from EPR across anomaly (cm) 2b. Calculate actual anomaly width (km) 2c. Convert actual anomaly width to cm 2d. Maximum Age (years) 2e. Average Seafloor Spreading Half Rate (cm/yr)

## Questions

1. Are areas of seafloor spreading along a divergent or a convergent plate boundary? Why?

2. What anomaly number represents the youngest seafloor? What is its age range? What anomaly number represents the oldest seafloor? What is its age range?

3. Explain what has happened to the magnetic anomalies located at the Clipperton FZ (fracture zone).

4. Between which two plates is the rate of seafloor spreading greater?

5. As these plates continue to form, what tectonic process must be occurring along the Mexico and Central American coasts?

6. Are the spreading rates uniform along a plate-to-plate boundary? Does your data support your answer