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Table of Contents Table of Contents List of Figures .............................................................................................................. iv Executive Summary ...................................................................................................... 1 Introduction ................................................................................................................... 2 Purpose of the Geologic Resource Evaluation Program ............................................................................................2 Geographic Setting.....................................................................................................................................................2 Geologic Setting .........................................................................................................................................................2 Ancient Samoan History .............................................................................................................................................3 Geologic Issues........................................................................................................... 11 Reef and Ecosystem Health .....................................................................................................................................11 Water and Energy Resources...................................................................................................................................12 Geologic Hazards .....................................................................................................................................................13 Coastal Erosion and Beach Loss..............................................................................................................................14 Geologic Features and Processes............................................................................. 16 Volcanoes and Volcanism ........................................................................................................................................16 Coral Reefs and Sea Level Change .........................................................................................................................16 Weathering and Erosion ...........................................................................................................................................17 Map Unit Properties .................................................................................................... 23 Map Unit Properties Table - Tutuila Map Units.........................................................................................................24 Map Unit Properties Table - Tau Map Units .............................................................................................................25 Map Unit Properties Table - Ofu and Olosega Map Units.........................................................................................26 Geologic History.......................................................................................................... 27 Glossary....................................................................................................................... 37 References................................................................................................................... 39 Appendix A: Geologic Map Graphic .......................................................................... 42 Attachment 1: Geologic Resource Evaluation Products CD NPSA Geologic Resource Evaluation Report iii List of Figures Figure 1. Location of American Samoa. .........................................................................................................................4 Figure 2. Location of Samoa Islands in the South Pacific ..............................................................................................5 Figure 3. Features within the Melanesian Borderland. ...................................................................................................6 Figure 4A. Bathymetry surrounding the island of Tutuila................................................................................................7 Figure 4B. Bathymetry surrounding the islands of Ofu-Olosega. ...................................................................................8 Figure 4C. Bathymetry surrounding the island of T’au ...................................................................................................9 Figure 5. Shallow water areas around American Samoa .............................................................................................10 Figure 6: Map showing features mentioned in the text.................................................................................................18 Figure 7: Development of a volcanic island chain over a stationary hotspot ................................................................19 Figure 8: Simplified stages of hotspot island volcanism ...............................................................................................20 Figure 9: Pocillopora eydouxi Antler Coral ...................................................................................................................21 Figure 10: Acropora abrobanoides...............................................................................................................................21 Figure 11: Goniopora fruiticosa....................................................................................................................................22 Figure 12: Gardineroseris planulata.............................................................................................................................22 Figure 13. Geologic time scale.....................................................................................................................................30 Figure 14. Generalized location of plates in the Pacific basin ......................................................................................31 Figure 15. Geographic map of the current tectonic plates............................................................................................32 Figure 16. Location of hotspots across the South Pacific.............................................................................................33 Figure 17. Bathymetry of the Samoan seamounts .......................................................................................................34 Figure 18. Samoa Islands atop the Pacific plate in the vicinity of the Tonga trench.....................................................35 Figure 19. Evolution of the positions of the Tonga trench, seamounts, and Vitiaz lineament.......................................36 iv NPS Geologic Resources Division Executive Summary This report accompanies the digital geologic map for National Park of American Samoa in American Samoa, which the Geologic Resources Division produced in collaboration with its partners. It contains information relevant to resource management and scientific research. National Park of American Samoa is the only National and sand mining reduces sand supply to beaches, which Park Service unit south of the equator. It comprises parts are shrinking. Geologic hazards such as debris flows and of four separate islands and harbors lush rainforests, landslides threaten the growing populated areas. The steep terrain, diverse coral communities, lagoons, and following four topics of concern to park management beaches. Habitats at the park include shoreline wetlands, have geological underpinnings: cloud forests, and littoral forests. • Ocean temperatures and reef health. Surface water Geology is fundamental to the creation of the islands and temperatures in the South Pacific are rising. Coral to the environments of American Samoa. Geology species in American Samoa are the current subject of influences surface water flow, and it contributes to much study because some species are able to climate, weather, hydrology, and topography. Geologic withstand extreme temperatures. However, slight units and structures provide the framework for the increases in ocean temperature can cause other corals craggy, eroded volcanic island peaks blanketed by to “bleach” or expel their symbiotic zooxanthellae rainforests, for beaches, lagoons, and pristine reef areas algae and, if the high temperature is prolonged, coral in the South Pacific Ocean. polyps can die. The islands of American Samoa rise thousands of feet Excess nutrients and pollutants also harm the reefs. from the Pacific Ocean floor as a series of broad shield Transplanting resistant coral species to vulnerable areas may be a way to preserve the reefs at American volcanoes. These volcanoes formed as the Pacific plate moved over a stationary hotspot deep in the earth. Samoa and elsewhere throughout the Pacific. Alternating lava flows and airfall (pyroclastic) debris that • Sea level rise. Sea level is likely to rise in the near hardened into tuff, volcanic breccia, lapilli, and cinder future. Global climate changes are warming and layers form these volcanoes. Although the initial expanding surface waters within the South Pacific volcanism that created these islands has ceased, the Ocean Basin, and melting ice caps shed water into the tectonically complex setting of American Samoa—near oceans. Rising sea level may force changes in the reefs, the junction of the Pacific and Australian plates—means park infrastructure, and park management. that
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