Plate Tectonics: Recycling the Seafloor A Classroom Activity for Ocean Gazing Episode #1: An Internet portal into the ocean

Written by: Lisa Ayers Lawrence, Virginia Sea Grant, Virginia Institute of Marine Science

Grade Level: 8-12 continents’ edges seem to fit together like puzzle pieces. Over the centuries, scientists Lesson Time: 45 min. and naturalists speculated on this, and in 1912 German meteorologist Alfred Wegener Materials Required proposed his theory of continental drift. world map (1), Northeastern Pacific Supported by the fit of the continents and quarterly map (1), Eastern equatorial the presence of matching fossils and Pacific regional map (1), North Atlantic geologic structures on different continents, regional map (1), USGS map (1) the continental drift theory contends that all the continents were connected in one Summary supercontinent called Pangea, which split Outline the location of plate boundaries apart into Laurasia and Gondwanaland 200 using the National Oceanic and million years ago. Over time, these Atmospheric Administration’s Acoustic continents split and drifted apart forming Monitoring Program’s underwater the earth as we know it today. earthquake data. With advances in technology, especially Objectives during World War II, more clues to the earth’s geologic history were unearthed. ü Classify the earth’s layers and types of Scientists were better able to map and study plate boundaries. the ocean floor and discovered the presence ü Predict locations of plate boundaries. of an underwater mountain range chain ü Interpret relationships between plate called the mid-ocean ridge system and boundaries and seismic activity. deep-sea trenches. They also discovered that the ocean floor is not as old as expected Vocabulary and that the magnetic properties of the Continental drift, Plate tectonics, ore, ocean sediment alternate in a matching Mantle, Crust, Plate boundaries, Divergent, pattern on either side of a mid-ocean ridge. Convergent, Transform, Boundary zones These discoveries led geologist Harry Hess to develop his seafloor spreading theory. Introduction According to this theory, the mid-ocean Since the days of Leonardo da Vinci, people ridges are spots where the earth’s crust is have pondered over the fact that the weak, allowing magma from the earth’s

mantle to come up to the surface and create Data Activity new seafloor. Deep-sea trenches are areas In this activity students will use data from where old seafloor is being driven back into underwater earthquakes to deduce the the mantle. Further research found that location of plate boundaries. earthquakes and volcanic activity are concentrated near ridges and trenches. The 1. Divide class into pairs or small groups. clues all led to the modern day theory of Print a copy of the world map for each plate tectonics. group. To understand plate tectonics, we must first 2. Access the National Oceanic and look at the make-up of the earth. The earth Atmospheric Administration’s Acoustic is divided into three main layers: Monitoring Program’s Ocean Seismicity data. In the lower left corner, click on • core - made of dense metals, the and print out the following (to enlarge inner core is solid and the outer core maps click directly on the image): is liquid ü Northeastern Pacific quarterly map • mantle - a thick layer of semi-solid ü Eastern equatorial Pacific regional molten rock surrounding the core map • crust - the thin, hard outer layer ü North Atlantic regional map broken into different slabs or plates 3. Using the location of seismic activity (underwater earthquakes) from the The semi-solid mantle moves in a circular NOAA data, ask your students to outline motion, pulling the overlying crustal plates on the world map where they believe the with it. The mantle’s circular motion comes relevant plate boundaries are. When from two sources. First, heat from within they are finished, compare to the USGS’s the earth warms the mantle material, map of the plates and discuss. Did the causing it to rise toward the crust. Near the students’ plate boundaries align with crust, the mantle material cools and sinks those of the USGS? Ask them to finish down toward the core. The second force sketching and label the relevant plates. impacts the mantle at deep-sea trenches. Were there areas with earthquakes but The heavy crustal plate is pulled by gravity no plate boundaries? down into the mantle dragging mantle material with it (this is called “slab pull”). Related Resources Due to this convection motion in the Geological oceanography, Physical mantle, the large plates covering the earth’s oceanography surface are constantly moving. The boundaries between these moving plates References are very active areas. The plate boundaries can be one of four types: “Historical perspective.” This Dynamic Earth: The Story of Plate Tectonics. United States Geological Survey. January 13, 2009 • divergent - moving away from one another (mid-ocean ridges) http://pubs.usgs.gov/gip/dynamic/historic • convergent - moving toward one al.html another (trenches) • transform - slipping past one “Developing the theory.” This Dynamic another (faults) Earth: The Story of Plate Tectonics. United • boundary zones - complex areas States Geological Survey. May 5, 1999 with several interacting plates

Plate Tectonics: Recycling the Seafloor Page 2 http://pubs.usgs.gov/gip/dynamic/develop http://pubs.usgs.gov/gip/dynamic/slabs.ht ing.html ml

“Mid-ocean ridge, the baseball seam.” This Sources Dynamic Earth: The Story of Plate To access an online version of this activity, Tectonics. United States Geological Survey. you can go to the following URL: May 5, 1999 http://www2.vims.edu/bridge/DATA.cfm? http://pubs.usgs.gov/gip/dynamic/baseball Bridge_Location=archive0902.html .html The related podcast episode for this activity “Harry Hammond Hess: Spreading the can be found by going to the podcast section seafloor.” This Dynamic Earth: The Story of of www.oceangazing.org Plate Tectonics. United States Geological Survey. May 5, 1999 http://pubs.usgs.gov/gip/dynamic/HHH.ht ml

“Some unanswered questions.” This Dynamic Earth: The Story of Plate Tectonics. United States Geological Survey. May 5, 1999 http://pubs.usgs.gov/gip/dynamic/unansw ered.html

“Understanding plate motions.” This Dynamic Earth: The Story of Plate Tectonics. United States Geological Survey. May 5, 1999 http://pubs.usgs.gov/gip/dynamic/underst anding.html

“World Map.” Holt Rinehart Winston World Atlas. http://go.hrw.com/atlas/norm_map/world .gif

“Acoustics Monitoring Program: Ocean Seismicity.” Vents Program. National Oceanic and Atmospheric Administration (NOAA). http://www.pmel.noaa.gov/vents/acoustics /seismicity/seismicity.html

“Moving slabs.” This Dynamic Earth: The Story of Plate Tectonics. United States Geological Survey. November 25, 2008

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