Time Frame - 1 hour

Advanced Preparation

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 the 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 the recently active spreading center (Gizo Ridge) is now subducting beneath the New Georgia Islands generating tectonic uplift.

The presenter/teacher will review the following information before doing this exercise.

• Geologic setting
• Barrier reefs and atolls
• Coral biology
• Dating with C-14

Materials

• Maps showing the location and regional geologic setting of the New Georgia Islands
• 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)
• Software (Excel) or graph paper

Procedure

1. Use the Seismic Eruption software to plot the locations of volcanoes and earthquakes in the SW Pacific. Point out New Georgia Islands.
2. Participants/students will use radiocarbon dates and elevation above current sea level of raised coral reefs to calculate rates of tectonic uplift corresponding to the locations sampled.
3. They will (a) plot the results on the map with the locations, and (b) draw contour lines using an appropriate interval. The results will reveal different rates of tectonic uplift.
4. Participants/students will develop a conceptual model to explain the pattern of uplift for the last 10,000 years that emerges from the data.