Plate Tectonics

LAB 1: PLATE TECTONICS Lab Structure Yes – meet as a class at the start of your scheduled lab Synchronous lab work time Asynchronous lab work Yes Lab group meeting Yes – meet after you have completed Lab 1 Quiz Yes – on Blackboard Recommended additional work Yes – complete Google Earth Tutorial before Lab 1 Pencil, pencil crayons, ruler, printed Plate Boundaries Required materials Map Learning Objectives After carefully reading this section, completing the exercises within it, and answering the questions at the end, you should be able to: • Explain the difference between oceanic and continental crust. • Describe the motion of plates at divergent, convergent, and transform boundaries. • Characterize divergent, convergent, and transform plate boundaries by their associated geological features and processes. • Describe how mantle plumes and resulting hot spot volcanoes can be used to determine the direction of plate motion. • Understand the historical contributions of geoscientists who proposed the theory of plate tectonics. Key Terms • Volcanology • Lithosphere • Tectonic plate • Bathymetry • Continental crust • Divergent • Topography • Oceanic crust • Convergent • Geochronology • Mantle • Transform • Mantle plume • Asthenosphere • Seismology • Hot spot Lab 1: Plate Tectonics | 29 1.1 Discovering Plate Tectonics As we discovered in the introduction to this lab manual, plate tectonics is the model or theory that we use to understand how our planet works. More specifically it is a model that explains the origins of continents and oceans, folded rocks and mountain ranges, igneous and metamorphic rocks, earthquakes, volcanoes, and continental drift. Plate tectonics was first proposed just over 100 years ago, but did not become an accepted part of geology until about 50 years ago. It took 50 years for this theory to be accepted for a few reasons. First, it was a true revolution in thinking about Earth, and that was difficult for many established geologists to accept. Second, there was a political gulf between the main proponent of the theory Alfred Wegener (from Germany) and the geological establishment of the day, which was mostly centred in Britain and the United States. Third, the evidence and understanding of Earth that would have supported plate tectonic theory simply didn’t exist until the middle of the 20th century. Before we delve into the details of plate tectonics, we need to examine some historical context to understand how this theory developed over the course of the 20th century. Most of the key evidence for plate tectonics came from studying the ocean floor. Before 1900, we knew virtually nothing about the bathymetry and geology of the oceans. By the end of the 1960s, we had detailed maps of the topography of the ocean floors, a clear picture of the geology of ocean floor sediments and the solid rocks underneath them, and almost as much information about the geophysical nature of ocean rocks as of continental rocks. Some of the most influential discoveries by geoscientists in the 20th century that have shaped modern plate tectonic theory are briefly summarized below. The 1920s to 1950s Development of acoustic depth sounders to map the ocean floor (Figure 1.1.1) leads to discovery of major mountain chains in all of the world’s oceans. During and after World War II, there was a well- organized campaign to study the oceans, and by 1959, sufficient bathymetric data had been collected to produce detailed maps of all the oceans (Figure 1.1.2). The important physical features of the ocean Figure 1.1.1: Depiction of a ship-borne acoustic depth sounder. floor are: The instrument emits a sound (black arcs) that bounces off the seafloor and returns to the surface (white arcs). The travel • Extensive linear ridges (commonly in the cen- time is proportional to the water depth. tral parts of the oceans) with water depths in the order of 2,000 to 3,000 m (Figure 1.1.2, inset a) • Fracture zones perpendicular to the ridges (inset a) • Deep-ocean plains at depths of 5,000 to 6,000 m (insets a and d) • Relatively flat and shallow continental shelves with depths under 500 m (inset b) • Deep trenches (up to 11,000 m deep), most near the continents (inset c) • Seamounts and chains of seamounts (inset d) 30 | 1.1 Discovering Plate Tectonics Figure 1.1.2: Ocean floor bathymetry (and continental topography). Inset (a): the mid-Atlantic ridge, (b): the Newfoundland continental shelf, (c): the Nazca trench adjacent to South America, and (d): the Hawaiian Island chain. With developments of networks of seismograph stations in the 1950s, it became possible to plot the loca- tions and depths of both major and minor earthquakes with great accuracy. It was found that there is a remarkable correspondence between earthquakes and both the mid-ocean ridges and the deep ocean trenches. In 1954 Gutenberg and Richter showed that the ocean-ridge earthquakes were all relatively shallow, and confirmed what had first been shown by Benioff in the 1930s — that earthquakes in the vicinity of ocean trenches were both shallow and deep, but that the deeper ones were situated progressively farther inland from the trenches (Figure 1.1.3). Figure 1.1.3: Cross-section through the Aleutian subduction zone with a depiction of the increasing depth of earthquakes “inshore” from the trench. [Image Description] Another key component in the case for plate tectonics came from studies of remnant magnetism in the rocks that make up the ocean crust. Usually, remnant magnetism is caused by the presence of ferrous (unox- 1.1 Discovering Plate Tectonics | 31 idized) iron on the seafloor, often from a volcanic rock containing grains of magnetite (Fe3O4), a highly magnetic mineral. As the mineral magnetite crystallizes from magma, it becomes magnetized with an orientation parallel to that of Earth’s magnetic field at that time. Rocks like basalt, which cool from a high temperature and commonly have relatively high levels of magnetite (up to 1 or 2%), are particularly susceptible to being magnetized in this way, but even sediments and sedimentary rocks, as long as they have small amounts of magnetite, will take on remnant magnetism because the magnetite grains gradually become reoriented fol- lowing deposition. In the 1950s, scientists from the Scripps Oceano- graphic Institute in California persuaded the U.S. Coast Guard to include magnetometer readings on one of their expeditions to study ocean floor topography. The first comprehensive seafloor paleomagnetic data set was compiled in 1958 for an area off the coast of B.C. and Washington State. This survey revealed a bewildering pattern of low and high magnetic intensity in seafloor rocks (Figure 1.1.4). When the data were first plotted on a map in 1961, nobody understood them — not even the scientists who collected them. But these paleomagnetic data became a key piece of evidence for seafloor spreading. Figure 1.1.4: Pattern of seafloor magnetism off of the west coast of British Columbia and Washington. The 1960s In 1960, Harold Hess, a widely respected geologist from Princeton University, advanced a theory with many of the elements that we now accept as plate tectonics. Hess proposed that: • new sea floor was generated from mantle material at the ocean ridges, • old sea floor was dragged down at the ocean trenches and re-incorporated into the mantle, • plate tectonics are driven by mantle convection currents, rising at the ridges and descending at the trenches (Figure 1.1.5), • the less-dense continental crust did not descend with oceanic crust into trenches, but that colliding land masses were thrust up to form mountains. Hess’s theory formed the basis for our ideas on seafloor spreading and continental drift, but it did not deal with the concept that the crust is made up of specific plates. Although the Hess model was not roundly criticized, it was not widely accepted (espe- cially in the U.S.), partly because it was not well supported by hard evidence. Figure 1.1.5: A representation of Harold Hess’s model for In 1963, J. Tuzo Wilson of the University of seafloor spreading and subduction. Toronto proposed the idea of a mantle plume or hot spot—a place where hot mantle material rises in a stationary and semi-permanent plume, and affects the overlying crust. He based this hypothesis partly on the distribution of the Hawaiian and Emperor Seamount island chains in the Pacific Ocean (Figure 1.1.6). The volcanic rock making up these islands gets 32 | 1.1 Discovering Plate Tectonics progressively younger toward the southeast, culminating with the island of Hawaii itself, which consists of rock that is almost all younger than 1 Ma. Wilson suggested that a stationary plume of hot upwelling mantle material is the source of the Hawaiian volcanism, and that the ocean crust of the Pacific Plate is moving toward the northwest over this hot spot. Near the Midway Islands, the chain takes a pronounced change in direction, from northwest-southeast for the Hawaiian Islands and to nearly north-south for the Emperor Seamounts. This change is widely ascribed to a change in direction of the Pacific Plate moving over the stationary mantle plume, but a more plausible explanation is that the Hawaiian mantle plume has not actually been stationary throughout its history, and in fact moved at least 2,000 km south over the period between 81 and 45 Ma.1 In addition to his contributions on mantle plumes and plate motion, Wilson also introduced the idea that the crust can be divided into a series of rigid plates in a 1965 paper, and thus he is responsible for the term plate tectonics. Figure 1.1.6: The ages of the Hawaiian Islands and the Emperor Seamounts in relation to the location of the Hawaiian mantle plume.

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