PLATE TECTONICS EXERCISE (Modified from North Seattle Community College online exercise) Introduction: As discussed in our textbook, the speed at which tectonic plates move has been calculated in several ways over the last 40 years. Recent satellite technology allows us to determine precise distances and changes in distances on extremely fine scales of time and space. We can determine current plate movements for most areas of the globe. In order to calculate AVERAGE plate movements over longer periods of time, we must rely on other methods. The record of magnetic field reversals in oceanic lava flows can be converted to a time clock by matching the pattern of magnetic changes in these rocks with the same pattern (radiometrically dated) in basalts erupted on land. By using magnetometers on ships, scientists do not need to dive down and collect seafloor specimens to test and to date; instead the magnetic record on the seafloor can be determined from simple shipboard measurements. By knowing times and distances, we can then calculate how fast the seafloor has spread away from divergent plate boundaries. Another major means of calculating plate velocities is tracking the volcanic "footprints" of hot spots as tectonic plates move across them. We assume that a "hot spot" originates from a relatively stationary source deep within the Earth's mantle. As plates move, these deepseated plumes "burn" new spots on the plates. These spots might be volcanic islands in the ocean or volcanic landforms on continents. The hot mantle plume is like a lit match. Hold a piece of paper over it and it will begin to burn a hole in that paper. Move the paper slowly and the match will burn a series of holes in the paper, the oldest "burn being the one farthest away from the match. The "age" of the burns and their distance from the match can tell us how fast the paper has moved over it. P.S. DON'T TRY THIS AT HOME. The Hawaiian Islands, the Galapagos Islands, and Yellowstone National Park are examples of "hot spots." In this activity we will use readily available information for each of these geologic paradises in order to estimate how fast three specific tectonic plates have moved over the time of the last millions of years. Part I: Hawaiian Islands Hot Spot While visiting Hawaii in the 1960's, Tuzo Wilson, one of the founders of the theory of plate tectonics, noticed some interesting features about ocean islands. On a map of the Pacific basin, he found three linear chains of volcanoes and submarine volcanoes (seamounts). As shown below, these are the (1) Hawaii Islands -Emperor Seamounts; (2) The Pitcairn Island - Tuamotu Group; and (3) the Macdonald Seamount - Austral Group. Notice that the eastern most island or seamount of each chain is volcanically active.
As we can see, although separated by thousands of miles, the three linear chains are parallel to each other. Of the three, the Hawaii-Emperor seamount chain was the most well known. Wilson reviewed the reports that had been published on these island chains and recorded the age of each island in the Hawaiian chain. An interesting pattern emerged. For each chain, the islands become progressively younger to the southeast. Active volcanoes mark the extreme southeast end of each chain. Wilson proposed that the Hawaiian Islands formed successively over a common source of magma called a hot spot. The Island of Hawaii is currently located above the hot spot. Hot, solid rock rises to the hot spot from greater depths (see the sketch below). Due to the lower pressure at the shallower depth, the rock begins to melt, forming magma. The magma rises through the Pacific Plate to supply the active volcanoes. The older islands were once located above the stationary hot spot but were carried away as the Pacific Plate drifted to the northwest. Image Source: Eruptions of Hawaiian Volcanoes: Past, Present, and Future: U.S. Geological Survey General Interest Publication.
Activity I: Directions: Use the map and the following information to determine the rate of motion of the Pacific Plate over the Hawaiian hot spot. The volcano that formed the Island of Niihau is 4.89 million years old. Rate is the distance traveled over a period of time. The distance traveled is equal to the distance from the present location of the hotspot (southeast Hawaii) to Niihau. Time is the age of the island. Question #1 Start by measuring the distance from southeast Hawaii to Niihau. Use the scale on the map. The distance is km. Question #2 To determine the average rate of motion for the Pacific Plate, divide the distance to Niihau by the age of the island. The rate of plate movement is km/ma (kilometers per millions of years). Question #3 Convert your answer to cm/yr (centimeters per year). The rate of Pacific Plate movement is centimeters per year. Question #4 Using this rate, how far will the Pacific Plate move in 50 years? Question #5 Repeat the exercise above using the island of Molokai instead. From the distance and age of this island, the rate of Pacific Plate movement is centimeters per year.
Question #6 Is this rate different than the rate calculated using Niihau? What might be one good reason why the rate would be different? Question #7 What direction is the Pacific Plate traveling? Explain. Activity II: This next activity determines the average rate that the Pacific Plate has moved over the last 65 million years. The ages of the islands and seamounts increase with distance away from the Hawaiian hot spot. This table shows these ages and distances for islands and seamounts in the Hawaiian - Emperor chain. Seamount or Island Distance (km) Age ------ ------------- ----- Suiko 4,860 65 Koko 3,758 48 Midway 2,432 28 Necker 1,058 10 Kauai 519 5 Question #1 Plot this data on the graph provided. Once the points are plotted on the graph, use a ruler to draw a straight line that starts at the origin and most closely goes near all the data points (this is called a "best-fit" line). Determine the slope of the line (pick any point on the line and divide the distance value by the time value). This slope is equal to the average rate of plate motion. As determined from your work, this rate is kilometers / million years. Question #2 Convert your answer to cm/yr (centimeters / year):
Question #3 Has the Pacific Plate been moving slower or faster over the last 5 million years than it has in the past? Explain. Question #4 The trajectory of plate motion points toward Hokkaido on the northern part of the Japanese Island chain, 6,300 km (3,900 mi) away. A subduction zone offshore of Japan consumes the Pacific plate, which is partly melted to create the volcanoes of Japan. If the "Plate Tectonic Express" operates without change, the Big Island of Hawaii will be headed down the Japanese trench. How long will it take Hawaii to reach Japan? Show your work. Part II: Galapagos Islands The Galapagos Islands are part of another volcanic island chain formed by passing over a hot spot. Use the World map in our classroom and the map of the tectonic plates on page 68 or 98 to determine on which tectonic plate the Galapagos Islands are found. Question #1 The Galapagos Islands are found on the plate which is traveling in the direction. Find the islands of San Cristobal and Fernandina on a map of the Galapagos Islands. Question #2 Knowing the direction of plate movement, which of these two islands would you think is the younger and which is the older? Why?
The Plate Tectonic story of the Galapagos is nicely presented in the following article. From the information in this article, we can calculate the rate of movement for this particular tectonic plate. Plate Tectonics and the Formation of the Galapagos Islands by Dr. Robert Rothman But Darwin only had part of the answer. A more complete answer to the origin of the Galápagos could not be had until after 1958, when continental drift, or plate tectonics, was discovered. We now understand that the surface of the earth is divided into massive tectonic plates which slowly drift across the globe. The formation of the Galápagos is intimately tied to the history of the Nazca plate, on which they lie. The Galápagos are located on the very northern edge of the Nazca plate, which is bounded by the Cocos (north), the Pacific (west), the South American (east), and the Antarctic (south) plates (see map). The Nazca plate itself is currently drifting south, away from the Cocos plate, and east, away from the Pacific plate. Since the net direction of drift is southeast, the Nazca plate is colliding with the South American plate. At the point of collision, the South American plate, which is made of light continental crust, is riding up over the Nazca plate, which is made of dense oceanic crust. This type of plate interaction is called subduction. As the Nazca plate is forced into the mantle, it melts and its melt products work their way up to the surface to form volcanoes. The land is further raised by the crumpling effect as the western edge of the continent rides up over the descending plate. The result of all of this is the Andes, a young, highly volcanic, rapidly growing mountain chain. This same movement of the Nazca plate is responsible for producing the cluster of volcanic islands we call Galápagos. There is a large body of geophysical evidence for the existence of enormous plumes of hot mantle material that originate near the earth's core and rise all the way to the crust. These plumes seem to be stable over many millions of years. and with time, they burn through the crust to form an underwater volcano which may eventually grow big enough to become an island.. But, because the crustal plate is in constant motion, the island will eventually move off of the hot spot. thereby making room for a second volcanic island. And a third, and a fourth... Thus are archipelagos like the Galápagos formed. Islands farthest from the hot spot are older and more eroded while islands near or on the hot spot are younger and steeper. Thus Isla San Cristóbal, the nearest to the mainland, is approximately four million years old and composed of eroded, rounded cones, while Isla Fernandina dates at less than 7000 years and is considered to be one of the most active volcanoes in the world. Recently former Galápagos islands, now submerged, have been discovered between Isla San Cristóbal and the mainland. This discovery may double the age of the islands. Indeed, several million years from now the present islands may likewise sink beneath the waves only to be replaced by a new set of Galápagos Islands. Who can imagine what course further evolution will take?
Question #3 By reading the article above, we find that the ages of the islands of San Cristobal and Fernandina are and. Question #4 Using the map above, the distance between these two islands is km. Question #5 How fast is this plate moving? Show your work. Give your answer in centimeters per year. Part III: Yellowstone National Park Hot spots may occur on continental lithosphere as well as oceanic lithosphere. For example, Yellowstone National Park is a huge volcanic caldera (collapsed summit of a volcanic cone) which we believe had a culminating eruption some 600,000 years ago. This is only the latest in a series of major caldera-forming eruptions that have traveled across the Pacific Northwest during the last 16 million years. In fact, we can track the movement of this still-active volcanic hot spot as it has shifted from Oregon through Idaho (creating its Snake River Plain Volcanic Province) into Wyoming. Check out the large map attached to view this track In actuality, the hot spot is stationary. It is the North American plate that is moving across it. How fast is the plate moving? We can apply the same method as before in order to calculate this rate. Study the attached map to determine the distance from the current hot spot to the 12.5 million year old Bruneau-Jarbridge Caldera in southern Idaho. Question #1 From the Yellowstone Hot Spot calculation, how fast is the North American plate moving? Show your work. Give your answer in centimeters per year. Question #2 In what direction is the North American plate moving? Explain how the Yellowstone hot spot shows this? Where do we expect the hot spot to be in another 12.5 million years?