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Using Data from Fossil Corals (Uplift Corals) to Understand Tectonic Processes

Student worksheet

Name: ______

Date: ______

Background

Barrier reef islands surrounding the coastlines of the New Georgia Islands represent a stage in the sequence of atoll formation proposed by Charles Darwin in 1842. Barrier reefs and atolls form as coral reefs anchored to volcanic islands grow upward to keep pace with actual rising sea level, or apparent sea level rise due to subsidence of the islands. Emergent barrier reefs also exist on the New Georgia Islands reaching up to 25 meters above sea level. Their presence indicates the interruption of Darwin’s predicted subsidence sequence in the formation of atolls by tectonic uplift, which has raised these coral reefs above sea level to create islands capped by a series of coral reef terraces. Indeed, earthquakes provide evidence that a recently active spreading center, the Gizo Ridge, is now subducting beneath the New Georgia Islands generating tectonic uplift.

Materials

  • Map showing the location of the New Georgia Islands and sampling locations.
  • Data set (sampling locations, elevation above sea level, C-14 age data for coral reefs sampled at about 30 locations in the New Georgia Islands)
  • Computer - PC or Mac (provided)
  • Vellum/tracing paper/transparency sheet for each student. Note that if transparencies are used, then students will need special transparency markers.

Procedure

  • Use Alan Jones’ Seismic Eruption software program ( installed on the computers in the classroom to see the locations of volcanoes and earthquakes in the SW Pacific and explore the geologic setting of the Solomon Islands.
  • Using the data on the data sheet, calculate rates of tectonic uplift in mm/year for the locations sampled. To do this you will use the elevation above current sea level of the raised coral terraces and radiocarbon dates corresponding to the time at which the reefs were lifted out of the ocean—an event that killed them since corals are marine organisms that can only survive in the ocean.
  • Plot the uplift rates that you have calculated for each sampling location on a sheet of vellum, tracing paper or a transparency placed over the map in your packet. You may draw contour lines using an appropriate interval, or color code uplift rates using an appropriate range (0 – 0.9 mm/yr, 1.0 – 1.9 mm/yr, etc.). Determine whether the data you’ve plotted on the map reveal different rates of tectonic uplift for the New Georgia Islands.

Note: Instead of the two methods above, you may develop other creative ways of depicting the data appropriate to the task. However, if you choose to create contour maps, be sure to follow the basic rules of contouring.

  • Develop a conceptual model to explain the pattern of uplift for the last 10,000 years that emerges from the data and share your results with the class.
  • Discuss the model presentations. The questions below are intended to guide the discussion.
  1. Where is uplift most pronounced?
  2. Where is uplift the least pronounced?
  3. What is causing the uplift? Make drawings to depict what you think is happening and label the plates and the features.
  4. Notice that there is a difference in the amount of displacement (uplift) as you cross the New Georgia Islands from east to west. Why?
  5. The data in this activity cover the past 10,000 years (Holocene epoch). Can we assume that sea level remained constant during this time span?
  6. There are fossil reefs that date back to older than 100,000 years in the New Georgia Islands. Could we use C-14 to date these reefs?

Summative Assessment

Taylor, Mann and colleagues calculated rates of tectonic uplift by combining elevation data with Carbon-14 dating of emergent reef terraces. By dating distinct notches in the reef terraces, the researchers were also able to document the times of individual earthquake events and amount of uplift associated with each earthquake. Their results are published in the following articles:

Mann, P., F. W. Taylor, M. B. Lagoe, and A. Quarles (1998), Accelerating late Quaternary uplift of the New Georgia Island Group (Solomon island arc) in response to subduction of the recently active Woodlark spreading center and Coleman Seamount, Tectonophysics, 295, 259 – 306.

F. W. Taylor, F.W., Paul Mann, M. G. Bevis, R. L. Edwards, Hai Cheng, Kirsten B. Cutler, S. C. Gray, G. S. Burr, J. W. Beck, David A. Phillips, G. Cabioch, and J. Recy (2005), Rapid forearc uplift and subsidence caused by impinging bathymetric features: Examples from the New Hebrides and Solomon arcs, Tectonics, Vol. 24, TC6005,doi:10.1029/ 2004TC001650.

a.Read these papers and write a report comparing the scientists’ models and conclusions with the different models and conclusions presented by the class groups.

B. Today, geophysicists continue to monitor plate motion in many different parts of the world using the U.S. Global Positioning System (GPS). Currently, there are 24 GPS satellites, each traveling in a 12-hour, circular orbit 20,200 kilometers above Earth. The satellites are positioned so that six are observable nearly 100% of the time from any point on Earth. The GPS satellites transmit ranging codes on two-radio frequency carriers at L-band frequencies, which can be detected by ground-based GPS receivers that researchers set up in the field. Special ground-based stations perform satellite monitoring that permits the locations of GPS receivers to be determined with a high degree of accuracy. Researchers are able to study the motions of tectonic plates, displacements associated with earthquakes, sea level fluctuations, and Earth orientations by monitoring very precisely the change in the receiver locations over time. With the technology currently available, an accuracy of a few millimeters can be achieved.

Write a report on the EarthScope Plate Boundary Observatory and its role in the larger EarthScope experiment.

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DATA SHEET

Island and Sampled Localitya / Lat.
(S) / Long.
(E) / Height
(above living
coral, in m) / Height
(above SL
in mm) / Ageb
(yr B.P.) / Uplift Rate
(mm/yr)
Rendova and Tetapare Islands
6. Asovo Point Peninsula / 8°35′ / 157°20′ / 6.5 / 6500 / 2630.00
6. Asovo Point Peninsula / 8°35′ / 157°20′ / 18.0 / 18000 / 4910.00
6. Asovo Point Peninsula / 8°35′ / 157°20′ / 18.1 / 18100 / 6690.00
10. South Rava Point / 8°41′ / 157°19′ / 1.6 / 1600 / 5170.00
7. Rano Village / 8°38′ / 157°17′ / 28.0 / 28000 / 5430.00
8. South Mbarora Bay / 8°40′ / 157°18′ / 11.9 / 11900 / 5340.00
8. South Mbarora Bay / 8°40′ / 157°18′ / 1.7 / 1700 / 600.00
8. South Mbarora Bay / 8°40′ / 157°18′ / 15.7 / 15700 / 4845.00
8. South Mbarora Bay / 8°40′ / 157°18′ / 22.1 / 22100 / 5520.00
12. W of Kioroso River / 8°43′ / 157°37′ / 46.8 / 46750 / 7430.00
Ranongga Island
14. Kukuri Point / 8°01′ / 156°36′ / 11.9 / 11900 / 3620.00
15.N of Konggu Village / 32.0 / 32000 / 7025.00
16. Pienuna Village / 14.0 / 14000 / 4355.00
18. S of Kolomali Village / 8°01′ / 156°35′ / 1.4 / 1430 / 470.00
18. S of Kolomali Village / 8°01′ / 156°35′ / 1.6 / 1630 / 590.00
New Georgia, Vangunu and Parara Islands
26. Rereghana Island / 8°18′ / 157°23′ / 0.4 / 420 / 180.00
47. Hotoanivena Island / 8°44′ / 158°04′ / 0.8 / 750 / 6430.00
57. Viru Harbor / 8°29′ / 157°35′ / 6.5 / 6500 / 5500.00
57. Viru Harbor / 8°29′ / 157°35′ / 4.1 / 4100 / 5570.00
39. West Maitu Island / 8°34′ / 158°08′ / 2.0 / 2000 / 4120.00
33. Ramata Island / 2.6 / 2610 / 6100.00
25. Roviana Island / 8°22′ / 157°17′ / 3.0 / 3000 / 6220.00
27. Ararosa Pass / 8°22′ / 157°32′ / 3.6 / 3600 / 6430.00
29. Vululi Point North / 8°03′ / 157°17′ / 1.6 / 1630 / 720.00
29. Vululi Point North / 8°03′ / 157°17′ / 4.9 / 4900 / 6810.00
Kolombangara Island
63. Ruvi Bay / 8°04′ / 156°57′ / 1.0 / 1000 / 2250.00
61. Unnamed islet S of Vila Point / 8°07′ / 157°3′ / 3.3 / 3260 / 4330.00
Ghizo Island
67. Logha Island / 8°02′ / 156°47′ / 1.0 / 1000 / 5340.00
Simbo Island
70. Nusasimbo Island / 8°07′ / 156°32′ / 1.0 / 1000 / 5490.00
VellaLavella Island
72. Maravari Village / 7°50′ / 156°41′ / 1.0 / 1000 / 6180.00
77. Paramata / 7°44′ / 156°32′ / 1.0 / 1000 / 870.00
73. Northeast Lambulambu / 7°41′ / 156°47′ / 1.0 / 1000 / 6490.00

Site TXESS Revolution. Last Revised May 2011

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