Lab #3 - Plate Tectonics(latest)- Anna-2
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2-1 Name: Lab Period: Lab 2: Plate Tectonics Objective: Students will be introduced to the theory of plate tectonics and different styles of plate margins and interactions. Introduction The planet can be divided into discrete layers based on material properties, including the inner and outer core, mesosphere, asthenosphere, and lithosphere. The lithosphere (crust and uppermost mantle) behaves rigidly, “floating” on the more plastic asthenosphere. This rigid exterior layer does not form an unbroken shell around the planet but is divided into discrete regions called tectonic plates. The boundaries between these plates do not necessarily correspond with the boundaries of continents: tectonic plates may contain oceanic crust, continental crust, or combinations of oceanic and continental crust. Our understanding of the movements and interactions of these plates –
the theory of plate tectonics –
provides a framework that helps to explain many of our geologic observations including locations of mountains, volcanoes, earthquakes, different rock types and many others. Plate Tectonics Tectonic plates move in different directions and at different speeds, driven by circulation of the underlying material, which in turn is convecting as part of the long-term release of heat from the planet’s interior. Because Earth's size remains constant, as these plates move, they must interact with each other at their margins. There are three possible types of plate margins. First, there are divergent margins, where plates move away from each other and new crustal material is created between them. This new crust, formed by magma rising from the underlying mantle, is generally oceanic-type crust and forms mid-ocean ridges (between oceanic crustal plates) or rift zones (between continental crustal plates). Second, there are convergent margins, where plates come together (or collide). In most cases, this collision forces the more dense plate under the less dense plate in a subduction zone. However, when two continental crustal plates collide, they are both too buoyant to subduct and instead deform, crumpling to form high Anna Do
3
2-2 mountain ranges like the Himalayas. Finally, there are transform margins, where plates slide past each other. These margins are basically large strike slip faults, such as the San Andreas Fault in California. The direction and speed of plate motion can be characterized in two ways: relative motion and absolute motion. Relative motion measures the velocity of the movement of one plate relative to another moving plate. Rates of relative motion across mid-ocean ridges are often measured using the magnetic reversals recorded in the oceanic crust. Since the ages of these magnetic reversals have been determined by radiometric dating, we can measure the distance between the ridge and a known (dated) magnetic reversal and calculate a rate of spreading. This rate is called a half spreading rate, since it only represents the growth on one side of the ridge, and can be doubled to determine the full spreading rate. Absolute motion measures the velocity of the movement of one plate relative to a fixed, stationary reference point deep in Earth’s interior. Absolute motion is often measured using hot spots since they are areas of igneous activity with sources in the deep mantle, below the drifting plates. The location of a hot spot is apparently fixed, perhaps being tied to some sort of feature at the core-mantle boundary, and does not change as plates move above it, so the surface expression of the hot spot is a line of volcanoes that increase in age away from the hot spot. The distance, direction, and age of a volcano with respect to its hot spot allow us to calculate the absolute speed and direction of plate movement. Part 1. Tectonic Structures Use the large map in the hallway to answer the following questions. 1. Study the long mountain chain running North-South along the center of the Atlantic Ocean. a. What is this topographic feature? b. What type of plate boundary is this? c. What type of volcanic rock is produced here? The topographic feature running North-South along the center of the Atlantic Ocean is the Mid-Atlantic ridge.
The Mid-Atlantic ridge is a divergent plate boundary where new ocean floor is formed as the plates spread apart.
The Mid-Atlantic ridge produces basaltic volcanoes.
2-3 d. Note that the main ridge is offset by perpendicular transform faults. Sketch such an offset between two spreading segments and indicate the relative motions of the plates. Where does strike-slip motion occur? 2. Find the Peru-Chile trench. a. What type of margin does this indicate? b. What continental topographic feature runs parallel to this margin? c. Notice the absence of a trench along the Atlantic coast of South America. Explain the reason for this difference, including the difference in volcanic activity along the two coastlines. Is this also true for North America? 3. The Aleutian Islands are an example of island-arc volcanism. Name another group of islands in the Pacific that appears to have been formed this way (is adjacent to a trench). 4. What type of margin (convergent, divergent, or transform) is most common around the edge of the Pacific Ocean? 5. In light of your answer to the previous question, is the Pacific Ocean growing wider or narrower from east to west? Can you tell this unambiguously? Does it help to consider what is happening (or not happening) at the edges of the Atlantic Ocean? Strike-slip motion occurs at transform faults related to mid-ocean ridges combined with seafloor spreading segments, forming a zigzag pattern. The Peru-Chile trench is a result of a convergent boundary, where the oceanic Nazca Plate is subducted beneath the continental South American Plate in a subduction zone.
The Andes Mountains on the west coast of South America is a topographic feature that runs parallel to the Peru-Chile trench.
The Atlantic coasts of both North & South America don’t have trenches & volcanoes because they are passive continental margins and far away from any plate boundary. The Pacific coasts of the Americas are both active continental margins located on plate boundaries, while the Pacific coast of South America is a convergent plate boundary. These plate boundaries create the trenches and the large volcanoes.
Philippines, Mariana Islands, Kermadec Islands Convergent boundaries
The Pacific ocean is getting narrower because the Atlantic Ocean is wider as the mid-ocean ridge creates new oceanic crust that is incorporated into the oceanic plates. This pushes the Pacific Ocean closer together with more oceanic plate being subducted.
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2-4 Part 2. Plate Motions As we have discussed, the velocities of tectonic plate movements have 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 ship-board 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 deep-seated 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.
2-5 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. The extreme southeast end of each chain is marked by active volcanoes. (Base image courtesy of the USGS) Tuamoto Group Austral Group
2-6 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 (4.89 Ma) (5.1 Ma) 3.7 Ma 2.6 Ma 1.9 Ma 1.76 Ma (1.28 Ma) (1 Ma) (0.5 to 0 Ma) (1.3 Ma) (0.75 Ma) (Image courtesy of the USGS) (Base image courtesy of the USGS)
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2-7 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 Google Earth and the Hawaii .kmz file. 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? ____________ (Image courtesy of the USGS) 600
122.7
12.27
614 cm in 50 years
2-8 Question #5
In what direction is the Pacific Plate traveling? Explain. _________________ This next activity determines the average rate that the Pacific Plate has moved over the last 65 million years. Seamount or Island Distance (km) Age Suiko 4,860 65 Koko 3,752 48 Midway 2,432 28 Necker 1,058 10 Kauai 519 5 Question #6
Determine the average rate of plate motion over a much longer time period. Notice that the island of Midway is quite a distance away from the current Hawaiian hot spot. Using the same procedure as above, calculate the average rate of plate motion on this longer timescale. As determined from your work, this rate is __________________ kilometers / million years. Convert your answer to cm/yr (centimeters / year): __________________ (Image courtesy of the USGS) Because the Hawaiian Islands are progressively older toward the Northwest and younger toward the Southeast, the Pacific Plate is traveling in Northwest direction.
86.9
8.69 cm/yr
2-9 Question #7
Has the Pacific Plate been moving slower or faster over the last 5 million years than it has in the past? The rate (and direction!) of plate movement can vary over time. Explain why this might happen. ______________________ Question #8
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. The Galapagos Islands
are part of another volcanic island chain formed by passing over a hot spot. Image courtesy of the USGS. A, Marquesas; B, Society; C, Samoa; D, Eastern Caroline; E, Pitcairn; F, Guadalupe; G, Galapagos; H, Easter; I, Juan Fernandez; J, Iceland; K, Ascension; L, Madeira; M, Canary; N, Azores; O, Cape Verde; P, Fernando de Noronha; Q, Lord Howe; R, Martin Vaz; S, St. Helena; T, Tristan da Cunha; U, Reunion; V, Comores; W, Bouvet; and X, Kerguelen. Because in 65 ma, the average rate of plate motion is 7.52 cm/yr while in 4.89 ma, the rate is 11.84 cm/yr, the Pacific Plate has been moving faster over the last 5 million years. The rate and movement of a lithospheric slab will change the rate and direction of plate movement as the increase or decrease resistance in the path of convection occurs due to the type of convergent collisions.
T = 72.53 million years
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2-10 Question #9
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 below. Question #10
Knowing the direction of plate movement, which of these two islands would you think is the younger and which is the older? Why? _____________________ (Image courtesy of the USGS) Nazca
Eastern
Fernandina is the youngest island and San Cristobal is the oldest island. Because The Galapagos Islands moves with Nazca plate, which is traveling in the Southeast direction over the hot spot, the older islands are found in the southeast.
2-11 Question #11
Isla San Cristóbal, the nearest to the mainland, is approximately four million years old and composed of eroded, rounded cones, while Isla Fernandina is considered to be one of the most active volcanoes in the world and is erupting today. Using the Google Earth and the Galapagos .kmz file, the distance between these two islands is approximately ___________ km. Question #12
How fast is this plate moving? Show your work. Give your answer in centimeters per year. _____________ Question #13
How long will it take before the Galapagos Islands to go down the trench off of South America's west coast? (You will need to use Google Earth to determine the distance) _________________ (Image courtesy of NASA) 243
T= d/r = 24,300,000/4,000,000 = 6.075 cm/yr
T = d/r = 100,000,000/ 6.075 = 16,460,905 years
Distance = 1,000 km
2-12 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 630,000 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. Image above courtesy of the USGS In actuality, the hot spot is stationary. It is the North American plate which is moving across it. How fast is the plate moving? We can apply the same method as before in order to calculate
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2-13 this rate. Study the map above or below to determine the distance from the current hot spot to the 12.5 million year old Bruneau-Jarbridge Caldera in southern Idaho. Question #14
From the Yellowstone Hot Spot calculate how fast is the North American plate moving. Show your work. Give your answer in centimeters per year. _____________ Question #15
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? _________________ r = 3.8 cm per year
We expect the hot spot to be in South West in the next 12.5 million years because the Caldera forming eruptions are older toward the South West, so the North American plate is moving towards South West over a hot spot.