Plate Tectonics Lab Seafloor spreading and plate tectonics are major concepts in geology. In 1915 Alfred Wegener suggested that todays continents had originally been part of a “supercontinent” called Pangaea which had, at some point in geologic time, fragmented into pieces (the continents) and gradually drifted to their present positions. Very few additional technical studies were undertaken to explain or refine this concept of continental drift until the early 1950s. Scientists like Harry Hess, J. Tuzo Wilson, Robert Dietz, Patrick Hurley, Fredrick Vine, and others began to synthesize the extensive oceanic geological data on a global scale to evaluate further the viewpoint current at that time that the oceanic crust was still closing and opening. Abundant new hydrographic and geophysical data derived from studies of ocean ridges, ocean trenches, earth geomagnetism, earthquakes and the like were amassed and analyzed utilizing computer synthesis. Thus, many geologists began to reinterpret Wegener's ideas in terms of a dynamic earth on which surface plates composed of continental and ocean-floor rock move as large units. The figure below is a composite illustration of the modern theories of plate tectonics showing three major concepts.

The rigid upper layer of the earth is known as the lithosphere. The lithosphere includes the continental and oceanic crust and the upper part of the mantle. Because of stresses set up, apparently by movement of mantle material, the lithosphere is often broken by a system of fractures which occur in specific zones called plate boundaries These plate boundaries are outlined by the locations of volcanoes and earthquake epicenters (see Fig. 6.1). Modern plate tectonics theory postulates that plate boundaries can be included in one of three categories. The first type of plate boundary is the diverging boundary also known as a spreading center, and is exemplified by the East Pacific Rise and the Mid-Atlantic Ridge. Impelled by the movement of heated mantle material upwelling from below the lithosphere, the older lithospheric rock is spread apart and newly-generated volcanic material fills the fracture at the spreading center As new ocean-floor rock is created at the mid-ocean ridges, there must be a compensating destruction of old ocean-floor crust so the earth doesn't constantly expand. This destruction of earth's old crust appears to happen in the areas where it is subducted in to ocean trenches. A

Plate Tectonics Lab subduction zone is an area where a cold slab of seafloor is forced back into the mantle beneath another plate. Plates are moving toward each other at this, the second type of plate boundary, known as a converging boundary. Geothermal heat and friction increase the temperature of the down-going plate and create magma, which erupts at the surface as a chain of andesitic volcanic islands or andesitic volcanoes along a continental margin (e.g., the Andes of South America). Continental collisions produce mountain chains like the Hymalayas where the continents are sutured together. Earthquakes occur at various depths along the subducted plate. The third common type of plate boundary is formed in areas where lithospheric plates slip horizontally past each other and is called a transform fault. Transform faults are formed either in the ocean or on a continent by a system of large lithosphere firactures that occur essentially perpendicular to a spreading center. The transform faults are developed as a consequence of differential spreading rates caused by the curvature of the earth’s spherical surface. The San Andreas Fault system in California is an example of a transform fault connecting the East Pacific Rise in the Gulf of California to the Juan de Fuca Rise off the coast of Oregon and Washington. Rates of plate movement are slow, to be sure. They are calculated in terms of centimeters per year. Two of the exercises in this lab will allow you to calculate the rate of movement of two different plates and compare those rates. Exercise 1: Seafloor spreading in the South Atlantic Alfred Wegener had plenty of evidence supporting his theory of continental drift but he could not come up with a mechanism that explained what he saw. In this exercise you will look at the evidence, play with the way the continents fit together and calculate the rate that the continents have spread apart. Procedure: Cut out the continents of South America and Africa found on the last page. Take care to include the continental shelf area when you cut them out. Next, take the cutouts and lay them on top of the map on the following page and slide them back and forth noting the direction and rotation needed to move the continents from their joined position (in the center) to their present positions. Also take note of the structural trends, tillite patterns and continental shelf material with respect to how they line up when the continents are together.

Plate Tectonics Lab

Fig.1

1. In the 1960’s Bullard postulated that the continental shelves should be included as part of the continent. After fitting the continent cutouts on the overlay map, decide whether Bullard’s or Wegener’s approach to continental fit works better. Explain your answer using the information in the lab.

2. Based on fig. 2, why should the continental shelves be considered to be part of the

continents?

Plate Tectonics Lab

Fig.2

3. If the distance between South America and Africa is 4,828 km presently, and 120 million years ago (Early Cretaceous) they were together (see fig.1), determine the spreading rate between the continents in cm/yr. (Show your work) Note: 1 km. = 100,000 cm.

4. The graph on the right shows the age of the bedrock at various distances from the Mid-Atlantic ridge. Determine the spreading rate according to this graph (select any point along the line then compute: distance/time)

__________________ cm/yr

5. How does your answer to question 4 compare with your answer in question 3? Explain why this is so.

6. When you moved your continents back to the pre-drift position, was the movement parallel to the lines of latitude? In discussing your answer, explain their motion using direction and rotation ( east, west, northeast, clockwise, counter clockwise etc.)

7. List all the evidence you can that supports the theory that the continents were once

connected? (at least 4 pieces)

Plate Tectonics Lab

Exercise 2: Hotspots

There are places on the earth far from plate boundaries where the internal heat in the Earth is so great, that it pushes up magma and creates a chain of volcanic islands as the plate moves over the hotspot. The Hawaiian Islands is such a chain. In the following exercise, you will determine the approximate rate of movement of the Pacific plate bases on the age and position of the island’s bedrock.

Plate Tectonics Lab

Island Age of island

in M.Y. Distance in km. (cm x 10-5)

Measured distance in cm. from map.

Distance in centimeters (17x106 x cm.)

Difference in age

Rate of movement cm/yr

Hawaii

0.0 xxx xxx xxx xxx xxx

Maui

1.0 Hawaii- Maui

Molokai

1.6 Molokai-Maui

Oahu

2.5 Oahu-Molokai

Kauai

4.7 Kauai-Oahu

Nihoa

7.0 Nihoa- Kauai

Midway

20.0 Midway- Nihoa

8. What is the average rate of movement of the Pacific plate? ______ cm/yr.

Plate Tectonics Lab 9. Between which years was the plate movement most rapid? __________ 10. Between which years was the plate movement slowest? __________ 11. Which plates seem to be moving faster, the Pacific or Atlantic? ________

Exercise 3: Converging Boundaries and Subduction Zones As stated previously, if ocean floors are spreading in areas of mid-ocean ridges, they must be converging in other areas, if the surface area of the earth is to stay constant. Apparently, deep ocean trenches represent zones of subduction where ocean-floor rock sinks into the mantle. One line of evidence for this theory lies in the distribution of earthquakes of intermediate and deep focal depths. Earthquakes occur only in rigid crustal rock material, not in plastic or molten mantle or core material.

Most earthquakes are the shallow focus type that occurs within the upper 70 kilometers of the earth (within the lithosphere). Only near trenches do earthquakes of intermediate (70 to 350 kilometers) focal depth occur. This fact would indicate that something solid and rigid is being forced deep into the mantle in these areas. Since the earthquakes are occurring within this subducted plate, it can be profiled by locating earthquakes and their various focal depths relative to the position of the trench. The following map shows an area in the western Pacific called the Izu Trench, located between the Mariana Trench and the Japan Trench. Plotted on this figure are the epicenters of recent earthquakes, with different symbols for each of the three types of focal depths. A line

of cross section, A-A', is indicated. Data Assume that you manage a seismic monitoring station set up in the Benin Island group to the south and east of the Izu Trench (see map). Over a week-long period, you record 24 significant earthquakes in the Izu Trench (along cross section A-A') and calculate their focal depths and

Plate Tectonics Lab relative distances from your recording station. These data are compiled in the following table.

12. What is the approximate angle of descent for the sinking slab? __________ 13. a. Name the over-riding (upper) plate involved in this boundary: _______________________ b. Name the subducting plate (lower) involved in the boundary: _______________________ 14. Explain how you arrived at your answer to question 13. 15. Could the subducting plate be the over-riding plate? Explain your answer. 16. Compare the angle of descent of the plate at the three trenches. Which is the steepest? Shallowest?

Plate Tectonics Lab

Plate Tectonics Lab Plate Tectonics Lab