G205: PLATE TECTONICS The goal of today’s lab is to learn how to recognize plate boundaries on a map and calculate
rates of plate movement. For today’s lab, please refer to pages 39-72 in your lab Manual (9th
1 million years = 106 years 1 million millimeters = 1 km 1000 mm = 1 meter 1000 m = 1 km
Part 1: The Origin of Magma
Examine the pressure-temperature (P-T) diagram for mantle peridotite in Figure below
(10th Figure 2.8 p. 53; 9th Figure 2.7 p. 41), and located point X. This point represents a mass of
peridotite buried 80 km underground.
1. According to the continental geothermal gradient, rocks buried at 80 km beneath a
continent would normally be heated to what temperature? °C
2. According to the oceanic geothermal gradient, rocks buried at 80 km beneath a continent
would normally be heated to what temperature? °C
3. About what temperature would peridotite at a depth of 80 km need to be subjected to in
order to be 100% liquid magma? °C
Part 2: Using Earthquakes to Identify Plate Boundaries (10th p. 64; 9th p. 47)
Earthquakes generally occur along plate boundaries from the shifting and movement of
plates. On the picture below, draw the plate boundaries by tracing the earthquakes on the
map. Then label the East Pacific Rise, Galapagos Rise, Chile Rise, the Cocos Plate, the Nazca
Plate, the Pacific Plate, the Caribbean Plate, and the South American Plate (Refer to your lab
manual: 11th Figure 2.1 p. 39; 10th Figure 2.5 p. 48; 9th Figure 2.3 p. 36).
Plot the locations of earthquake foci (depth of earthquake vs. its location east or west of the
trench) on the cross section on the next page. Draw a line representing the likely upper
surface of the subducting plate.
1. What kind of plate boundary is shown in your cross section (Circle your answer)?
Convergent / Divergent / Transform
2. At what depth does magma probably originate above the plate? km. How can you
tell (Hint: think about the relationship between magma and volcanic activity)?
Part 3: Analysis of Atlantic Seafloor Spreading (10th p. 63; 9th p. 49)
1. Did the sea floor spread apart at exactly the same rate on both sides of the Mid-
Atlantic ridge? How can you tell?
2. How far apart are points B and C today, in kilometer? km
3. 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.
Average Rate: km/my
4. Convert your answer from km per millions years to meters (m) per year. Show your
Average rate: m/yr
4. How many millions of years AND in what geologic period of time were Africa and
North America part of the same continent? Show your work. (Use the answer from
Question 2 to help solve this).
Millions of Years: my
5. How far in meters have Africa and North America moved apart since the United States
formed in 1776? (Use your answer in Question 3 to help solve this). Show your work.
Part 4. The San Andreas Transform-Boundary Plate Motions For this part, you will be completing just part B of Activity 2.7 in your lab manual. I would suggest first browsing through part A, however, so that you know what is being shown on the map. Address part B of this activity in the space below.
B. The map for part B of this exercise shows Global Positioning System (GPS) stations and observations of the former Southern California Integrated GPS Network (SCIGN), now operated by the United State Geological Survey, NASA Jet Propulsion Laboratory. The length of the arrows indicates the actual direction and rate that bedrock beneath the reference point is moving in mm/yr.
1. Notice that both plates are moving northwest here. Estimate in cm/year how
much faster the Pacific Plate is moving than the North American Plate. ________mm/yr
2. Consider your answer above and the relative motion of the plates along the San
Andreas Fault. Which of the strike-slip fault types below is most like the San Andreas Fault, left-lateral or right-lateral? _________________________