LAB: PLATE TECTONICS GOAL: Calculate rates of plate movement

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1 Name LAB: PLATE TECTONICS GOAL: Calculate rates of plate movement Rates of plate movement are calculated by measuring the distance rocks have moved since they formed. Rates are expressed in cm per year (cm/yr). Rate = distance divided by time. Three types of Plate Boundaries that are used to calculate rates of plate movement are: 1. Sketch: 2. Sketch: 3. Sketch: 1. Calculate rates at a spreading ridge (mid ocean ridge). A. The map below shows the ages of seafloor basalt between North America and Africa a. Draw a red line on the map to show the exact location of the divergent plate boundary between North America and Africa b. Draw two blue lines on the map to show the exact position of two different transform fault plate boundaries. B. Notice that points B and C were together 145 million years ago, but did the sea floor spread apart at exactly the same rate on both sides of the mid ocean ridge? How can you tell?

2 C. How far apart are points B and C today in kilometers? km a. Calculate the average rate, in km per million years, that points B and C have moved apart over the past 145 million years. Show your work. b. Convert your answer above from km per million years to mm per year. D. Based on your answer in C.a. above, how many millions of years ago and in what geologic period of time were Africa and North America part of the same continent? Show your work. E. Based on your answer in C.b. above, how far in meters have Africa and North America moved apart since the United States was formed in 1776? 2. Calculate rates at a Hot spot (mid ocean ridge). A. The next figure shows the distribution of the Hawaiian Islands chain and Emperor Seamount chain. The numbers indicate age in millions of years old or ago (Ma), obtained from the basaltic igneous rock of which each island is composed. a. In general, how is the Emperor Seamount chain related to the Hawaiian Island chain? b. What was the rate in centimeters per year (cm/yr) and direction of plate motion of the 2300km long Emperor Seamount chain from 20 to 40 Ma? c. What was the rate in centimeters per year (cm/yr) and direction of plate motion in the Hawaiian region from 4.7 to 1.6 Ma? d. What was the rate in centimeters per year (cm/yr) and direction of plate motion from 1.6 Ma to the present time? B. Based on your work above, explain how the direction and rate of the Pacific Plate movement changed over the past 60 million years.

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4 2. Movement along the San Andreas Fault A. The two bodies of the Oligocene volcanic rocks (about 23.5 million years old) located along either side of the San Andreas Fault were once one body of rock, but they have been separated by displacements along the fault. Note that half arrows have been placed along the sides of the fault to show relative plate motion along the transform plate boundary here. a. Assuming that theses rocks began separating soon after they formed, what is the average rate of fault displacement in centimeters per year (cm/yr)? Show your work.

5 b. An average displacement of about 5 m (16 ft) along the San Andreas Fault was associated with the devastating 1906 San Francisco earthquake that killed people and destroyed properties. Assume all displacements along the fault was produced by Earth motions of the same magnitude, how often must such earthquakes have occurred in order to account for the total displacement? Show your work. B. The above map shows some GPS reference stations and observations from the JPL-NASA GPS Time Series. The length of the arrows indicates absolute plate motion, the direction and rate that the plate is mobbing in mm/yr at the GPS station (which is attached to the bedrock of the plate). a. Notice that both plates are mobbing northwest here. Estimate in cm/yr how much faster the Pacific Plate is mobbing than the North American Plate. b. Add half arrows along the San Andreas Fault to show the relative movement between the two plates. C. What is the difference between absolute plate motion and relative plate motion?