Laboratory #7: Plate Tectonics

Transcription

1 Materials Needed: 1. Pencil 2. Colored Pencils 3. Metric/Standard Ruler 4. Calculator 5. Tracing Paper Laboratory #7: Plate Tectonics Plate Tectonics The Earth is composed of layers. At the center is a nickel and iron core, which is separated into a solid inner core and a liquid outer core. Next is the mantle, which is contains the rigid mesosphere, the plastic and ductile asthenosphere, and the rigid upper mantle. The crust and lithosphere are a part of the upper mantle. Plate tectonics has been an accepted theory since the 1960 s. According to this theory, the crust of the Earth is composed of plates that move over the asthenosphere. There are two basic types of plates: heavy, thin and dense oceanic plates, which are primarily composed of basalt; and thick, lighter continental plates, which are comprised of silicate rocks. There are three types of plate boundaries. Divergent plate boundaries (also called spreading centers) occur where two plates are moving away from one another. Convergent plate boundaries exist where two plates are moving towards one another. Convergent plate boundaries are classified based upon the type of crustal plate involved: ocean-ocean, oceancontinent, and continent-continent. Finally, transform plate boundaries are located where two plates are sliding past one another. The largest plate is the Pacific plate, followed by the African plate, Eurasian plate, Australian-Indian plate, Antarctic plate, North American plate, and South American plate. Smaller plates include the Nazca plate, Philippine plate, Caribbean plate, Cocos plate, and Juan de Fuca plate.

2 Most continental shorelines do not coincide with plate boundaries (one important exception is the west coast of South America). Nor are most shorelines zones of earthquake activity. For this reason, continental shorelines are referred to as "passive margins," places that may have been plate boundaries in the past, but are no longer active. The east coast of North America is a passive margin, as are the east, south and west coasts of Africa, northern and Western Europe, and others. The eastern boundary of the North American plate is in the middle of the Atlantic Ocean, where there is a belt of active seismicity. Thus the North American plate is made up of both continent and ocean. In fact, most plates consist of both continental and oceanic material. There are several entirely oceanic plates (Nazca, Cocos), but no entirely continental plates (with the possible exception of the Arabian peninsula; it depends on the criteria one uses to define individual plates and how much ocean is required to be considered "ocean"). The fact that continents are included as part of plates made of both continent and ocean suggests that the continents do not move independently of the oceans as Wegener envisioned, but rather that continent and ocean move together as part of a single plate. Thus from a geodynamical perspective, a "plate" appears to be a more fundamental unit than a continent or ocean.

3 Laboratory #7: Plate Tectonics Part I: Crustal Plates and Plate Boundaries 1. Figure 3 shows the major crustal plates and plate boundaries of Earth. a. Color all of the divergent plate boundaries red, subduction zones blue, continentcontinent convergent boundaries green, and transform faults yellow. b. Shade in areas with chains of volcanoes using a blue pencil. c. Make a key for the colors on your map. d. Next to each plate boundary, place a set of arrows to indicate the relative rate of movement for major plates. 2. How many plates are there? 3. W hich plate is totally surrounded by a convergent plate boundary? 4. Which plate(s) do not contain significant areas of continental landmasses? 5. In general, do continental shorelines coincide with plate boundaries? 6. Are there any plates that have both continents and oceans? If so, name them. 7. List the plates that are bounded in part by: The Mid-Atlantic Ridge: The East Pacific Rise: 8. What major island mass lies on the axis of the Mid-Atlantic Ridge?

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5 Laboratory #7: Plate Tectonics Part II: Movement of the Volcanoes in the Hawaiian Ridge over the Hawaiian Hot Spot. The premise behind plate tectonics is that the crustal plates are moving with respect to one another over geologic time. The rates of movement of crustal plates can be determined by using data from the plate margins along the mid-ocean ridges, where the amount of movement can be measured. To measure the movement of two adjacent crustal plates along the margins of a divergent plate boundary, two things must be known: 1) two points on adjacent diverging plates that were once at the same geographic coordinates but have since moved away from each other over a known distance; 2) the time required for the two points to move from their original coincident positions to their present positions. Determining the age in actual years of the two points requires knowing the age of the rocks in that location. As new crust forms on either side of a spreading center, this rock becomes magnetized according to the polarity of the Earth at the time of the rocks formation. Thus, if the absolute age of the magnetic anomaly is known, then the age of the rocks in that location are also known. The map of below shows part of the Hawaiian Ridge and the absolute dates of lava in bold black numbers that millions before the present. Hawaii contains an active volcano, Mauna Loa, so the lava from it is zero years old. The lava on Nihoa Island is 7 million years old. Thus, according to the hot spot hypothesis, Nihoa Island was once an active volcano standing where Hawaii stands today. The Lab: Figure the rate in centimeters per year using the distances from Hawaii to each of the three dated lavas on the map located on the next page. Make your distance measurements from the center of the zero on Hawaii to the center of each of the boldface numbers on the ridge.

6 Laboratory #7: Plate Tectonics 1. From Hawaii (0 m.y.) to Nihoa Island (7 m.y.) 2. From Nihoa Island (7 m.y.) to just west of Necker Island (10 m.y.) 3. From just west of Necker Island (10 m.y.) to Midway Island (20 m.y.) 4. Do the rates of movement based on the three dates indicate that the movement has been constant or variable? Map of the Hawaiian Ridge in the Pacific Ocean. Contour interval = 1000 meters Scale: 1 cm = 170 km.

7 Laboratory #7: Plate Tectonics Part III: Restoration of the South Atlantic Coastline 50 Million Years before Present Given the evidence of spreading along the Mid-Atlantic Ridge, it can be deduced that the Africa and South America plates are moving away from each other carrying the continents of Africa and South America with them. By using the pattern of magnetic lineations shown on the map on the next page, it is possible to reverse the spreading process and restore the positions of the African and South American coastlines to a time when a particular set of magnetic lineations was being formed on the Mid-Atlantic Ridge. For the purposes of this exercise we win use anomaly number 21, which, according to the magnetic lineation time scale page 51, was formed 49.6 or roughly 50 million years ago. Proceed as follows. 1. On the map on the South Atlantic Ocean, draw a red line over each of the magnetic lineations of anomaly number 21 on the South American side of the Mid-Atlantic Ridge. Connect the segments of the number 21 anomaly with a red line drawn along the fracture zones against which they terminate. Start with the point where anomaly 21 touches the Ascension F. Z. Follow Anomaly 21 with your red pencil southward until it reaches the Bode Verde F. Z., then along the Bode Verde F. Z. westward to the northern end of the next fracture zone. Continue until you have reached the southernmost fracture zone on the map. 2. Attach a piece of tracing paper over the map with tape or paper clips, and repeat the process described above for Anomaly 21 on the African side of the Mid-Atlantic Ridge. Draw this line in red pencil on the tracing paper. 3. With the tracing paper still in place, trace the coastlines of Africa and South America on the tracing paper with black pencil. Also, trace on the tracing paper the boundaries of map and the 20 South latitude line in black pencil. 4. Detach the tracing paper and slide it toward South America until the red line on the tracing paper matches the red line on the map. When the two lines are matched as closely as possible, hold the tracing paper in place and trace the coastline of South America in red pencil on the tracing paper. Trace also the 20 South line on the tracing paper in red pencil. 5. The map you have constructed on the tracing paper shows the Mid-Atlantic Ridge as it existed when magnetic anomaly 21 was being formed. Your tracing paper also shows the relative positions of segments of the coastlines of Africa in black pencil and South America in red pencil as they were approximately 50 million years ago. This reconstruction is based on the assumption that the continents of Africa and South America were fixed to their respective plates during the spreading process over the past 50 million years. The continents moved with respect to each other because the tectonic plates to which they were attached moved as spreading continued along the Mid-Atlantic Ridge. What is the evidence that the movement of the two plates was not strictly in an east-west direction? 6. Was the earth's magnetic field normal or reversed at the time represented by your map on the tracing paper?

8 Laboratory #7: Plate Tectonics Map of the South Atlantic Ocean showing part of the Mid-Atlantic Ridge in black bars, with east-west fracture zones, and selected m agnetic anom alies. The ages of the num bered anom alies or m agnetic lineations can be determ ined from the m agnetic tim e scale in the next page. (From Magnetic Lineations of the W orld s Ocean Basins, Copyright 1985 the Am erican Association of Petroleum Geologists.

9 Marine magnetic anomalies. (A) The black line shows positive anomalies as recorded by a magnetometer towed behind a ship. In the cross section of the oceanic crust, positive anomalies are drawn as black bars and negative anomalies are drawn as white bars. (B) Perspective view of magnetic anomalies shows that they are parallel to the rift valley and symmetric about the ridge crest. Magnetic Time Scale. Magnetic anomaly identification numbers are given on the top and absolute time scale in millions of years (Ma) is given on the bottom. Normal magnetic anomalies are shown in black, and reversed polarity is shown in white. The absolute age of the anomalies and their polarities can be read directly from the chart. For example, anomaly 4 is 7 million years old, and it is a positive anomaly.