Lab 3 Answers

13 Pages

Earth, Space Science and Engineering
Course Code
ESSE 1010
Alireza Panahi

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Name:_____ ANSWERS __________________ Lab. Session:_______ Student #______58 MARKS IN TOTAL_______ YORK UNIVERSITY DEPARTMENT OF EARTHAND SPACE SCIENCE & ENGINEERING Fall 2010 EATS 1010 3.0 Laboratory Exercise #4 PLATE TECTONICS This lab is divided into three parts, each dealing with a different aspect of plate tectonics. Read the lab over beforehand and bring a pencil, metric ruler and either your textbook or your class notes to the laboratory.Allow yourself about one hour of time for each exercise. Instructions are given for each part. Follow the procedures as outlined and fill in your answers to all the questions on the work sheets provided. Hand in your completed lab before the end of the session. Be sure your name is on the front page and also on each of your answer sheets (in case some of the pages come apart). PART 1. ISLAND ARC/OCEAN TRENCH EARTHQUAKES [12 MARKS] Introduction In the western and northern Pacific Ocean there are many arc-shaped groups of islands. The Aleutians, the Marianas, Japan and Indonesia are all examples. There are also island arcs in the Caribbean and South Atlantic. Island arcs have two important characteristics: (1) they all have deep ocean trenches nearby and (2) they are areas of volcanism and intense earthquake activity. The location in the Earth where an earthquake occurs is called its focus. Geophysicists have discovered that most of the world's intermediate and deep-focus earthquakes (below 100 km) occur near island arc/trench systems. For many years there was great curiosity about the unusual character of island arcs. There was no explanation for them prior to the Theory of Plate Tectonics. It proposed that if oceanic plates are created at a mid-ocean ridge, there must be some place where the far ends of the plates dive back down into the mantle. This occurs in the vicinity of island arcs producing the trench as well as volcanic and earthquake activity. Earthquake studies in such regions provide evidence of subduction of the diving plate. Objective In this exercise you will: 1) plot and interpret an earthquake focus profile, and 2) explain the seismic characteristics of a subduction zone. Procedure 1. The table below lists the locations of earthquakes that have occurred in the Marianas Trench area. Plot the location of each on the Earthquake Focus Profile provided as page 4. 2. Study your earthquake focus profile and answer the questions on page 3 in the spaces provided. Earthquakes of the Marianas TrenchArea Horizontal Horizontal Horizontal Depth (km) Distance from Depth (km) Distance from Depth (km) D i s t a n c e from Trench (km) Trench (km) Trench (km) 0 16 216 264 516 456 0 50 232 254 550 500 0 72 250 264 580 450 0 96 300 360 580 464 32 168 350 364 600 480 66 132 350 400 600 508 83 168 383 408 600 524 83 184 416 432 624 508 116 232 448 448 624 524 116 248 472 416 650 532 150 200 472 432 667 548 150 260 500 450 200 216 500 458 200 248 516 432 Page3 Questions 2.What is the relation between the depth of earthquakes and their distance from the Tonga Trench? EARTHQUAKE DEPTHS INCREASE AS DISTANCE FROM THE TRENCH INCREASES. [1 MARK] 3. Draw a line through the points on your profile. Describe the appearance of the line: LINE DESCENDS UNIFORMLY FROM THE TONGATRENCH TO ADEPTH OF ALMOST 700 KM [ 1 MARK] 4.Give an explanation for the distribution of the earthquake foci. Does the shape of the earthquake focus pattern support the theory of the plate tectonic concepts that you learned in class. Explain. ACCORDING TO PLATE TECTONICS, COLD RIGID PLATES ARE SUBDUCTED INTO THE MANTLE AT SUBDUCTION ZONES, MARKED BY AN OCEAN TRENCH AT THE UPPER SURFACE. [1 MARK] THE PLATES ARE BRITTLE AND FRACTURE, PRODUCING THE PATTERN OF EARTHQUAKES IN THE PLOT. [ 1 MARK] THE PATTERN OF EARTHQUAKES IN THE PLOT SUPPORTS THIS IDEA. [1 MARK] 5.What surface feature(s) result(s) from the melting of the sinking plate and/or mantle immediately above the sinking plate? VOLCANOES (or ISLAND ARCS) (or VOLCANIC ISLANDS) [1 MARK] 6.As the lithosphere plate dives into the mantle, it begins to warm and soften. At what depth is the plate no longer brittle enough to support earthquakes? __~700 km (> 667 km is OK) [1 MARK] Page4 Page5 PART 2: HOT SPOTS AND PLUMES [26 MARKS] Introduction Hot spots are zones where unusually high heat flow has been recorded at the Earth's surface. Several dozen are known. These are often found well within the tectonic plates, as for example, the Hawaiian Islands. They are isolated and not associated with spreading centres or subduction zones. The cause of hot spots is believed to be plumes of abnormally hot rock welling up from deep within the mantle. Although the origin of these mantle plumes is uncertain, these sites often experience spectacular volcanic activity. The volcanoes of Hawaii are a famous example. Here, eruptions of Mauna Loa have occurred recently. Northwest of Hawaii a north-trending row of volcanic peaks is found on the ocean floor far below sea level. They are called the Emperor Seamounts. Though the Hawaiian Islands and the Emperor Seamounts meet at an angle, they appear to be part of a single, continuous chain of volcanoes. It is interesting to note that of the entire chain only the island of Hawaii and Loihi (which is located to the southeast of Hawaii) are volcanically active. These islands are located farthest to the southeast and have the highest heat flow. The unusual arrangement of these volcanic islands and seamounts could be explained by the movement of the Pacific Ocean floor over a hot spot. If so, this evidence would provide a verification for the theory of plate tectonics and sea floor spreading. Objective In this exercise you will: 7.Explain how a mantle plume may produce a chain of volcanoes 8.Understand how the direction of movement of a tectonic plate can be determined 9.Describe how a change in plate-movement direction can be detected 10.Compute ocean floor spreading rates, and 11.Explain how hot spots help to verify plate tectonics theory. Procedure Use the map on page 6 and a metric ruler to determine the distance from the island of Hawaii to each island or seamount listed below. Make all measurements along the trend of the island chain. Fill in the distances on the chart and answer the questions on page 7. Page6 Page7 Distance (km) Date of Last Eruption (Millions of Years ago) Hawaii 0 Recent West Maui 150 1.3 West Molokai 212 1.8 Waianae (Oahu) 294 3.5 Kauai 396 5.6 Nihoa 573 ±10% is OK 7.5 Necker 792 10.0 Midway 1775 17.7 Kanmu 2546 38.0 Koko 2758 46.0
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