National Park of American Samoa 

Geologic Resource Evaluation Report 

Natural Resource Report NPS/NRPC/GRD/NRR—2008/025 

National Park Service 

U.S. Department of the Interior 

Natural Resource Program Center 
THIS PAGE: THIS PAGE: 

Surgeonfish and butterflyfish on coral reefs at National Surgeonfish and butterflyfish on coral reefs at National 

Park of American Samoa

ON THE COVER: ON THE COVER: 

Fringing coral reefs at Ofu Island in American Samoa

Photos by: Peter Craig, Nati Photos by: Peter Craig, National Park of American Samoa
National Park of American Samoa 

Geologic Resource Evaluation Report 

Natural Resource Report NPS/NRPC/GRD/NRR—2008/025 

Geologic Resources Division 

Natural Resource Program Center 

P.O. Box 25287 

Denver, Colorado 80225 

February 2008 

U.S. Department of the Interior 

Washington, D.C.
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gre_publications) on the internet, or by sending a request to the address on the back cover. 

Please cite this publication as: 

Thornberry- Ehrlich, T. 2008. National Park of American Samoa Geologic Resource 

Evaluation Report. Natural Resource Report NPS/NRPC/GRD/NRR—2008/025. National 

Park Service, Denver, Colorado. 

NPS D- 21, February 2008 
 NPSA Geologic Resource Evaluation Report iii

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 
 NPS Geologic Resources Division iv 

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
 NPSA Geologic Resource Evaluation Report 1

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 

Park Service unit south of the equator. It comprises parts 

of four separate islands and harbors lush rainforests, 

steep terrain, diverse coral communities, lagoons, and 

beaches. Habitats at the park include shoreline wetlands, 

cloud forests, and littoral forests. 

Geology is fundamental to the creation of the islands and 

to the environments of American Samoa. Geology 

influences surface water flow, and it contributes to 

climate, weather, hydrology, and topography. Geologic 

units and structures provide the framework for the 

craggy, eroded volcanic island peaks blanketed by 

rainforests, for beaches, lagoons, and pristine reef areas 

in the South Pacific Ocean. 

The islands of American Samoa rise thousands of feet 

from the Pacific Ocean floor as a series of broad shield 

volcanoes. These volcanoes formed as the Pacific plate 

moved over a stationary hotspot deep in the earth. 

Alternating lava flows and airfall (pyroclastic) debris that 

hardened into tuff, volcanic breccia, lapilli, and cinder 

layers form these volcanoes. Although the initial 

volcanism that created these islands has ceased, the 

tectonically complex setting of American Samoa—near 

the junction of the Pacific and Australian plates—means 

that volcanoes may again become active on any Samoan 

island. 

Although the islands of American Samoa are geologically 

young (<2 Ma), after active volcanism ceased intense 

weathering and erosion produced today’s extremely 

steep topography. Wind, water, and slope processes 

carved through the deeply weathered volcanoes to 

produce steep slopes, amphitheater valleys, beaches and 

volcanic soils that now support rainforests. Fringing 

several parts of the islands of Tutuila, Ofu, Olosega, and 

Ta’u are coral reefs of great biodiversity and resilience. 

Samoa has been occupied by humans for more than 

3,000 years. Early inhabitants may have used the islands 

as a base for exploring and settling Polynesia. Since that 

time, a human presence and concomitant development 

has steadily increased in American Samoa, especially on 

the island of Tutuila. Given the limited natural resources 

available to island inhabitants, the sustainability of this 

development is now in question. 

Because tourism has become a major industry, it is 

increasingly important to maintain island beaches and 

reefs in a natural state. Optimal beach management can 

be hindered by riprap, jetties, and other nearshore 

structures adjacent to park lands. Loss of sand to erosion 

and sand mining reduces sand supply to beaches, which 

are shrinking. Geologic hazards such as debris flows and 

landslides threaten the growing populated areas. The 

following four topics of concern to park management 

have geological underpinnings: 

• Ocean temperatures and reef health. Surface water 

temperatures in the South Pacific are rising. Coral 

species in American Samoa are the current subject of 

much study because some species are able to 

withstand extreme temperatures. However, slight 

increases in ocean temperature can cause other corals 

to “bleach” or expel their symbiotic zooxanthellae 

algae and, if the high temperature is prolonged, coral 

polyps can die. 

Excess nutrients and pollutants also harm the reefs. 

Transplanting resistant coral species to vulnerable 

areas may be a way to preserve the reefs at American 

Samoa and elsewhere throughout the Pacific. 

• Sea level rise. Sea level is likely to rise in the near 

future. Global climate changes are warming and 

expanding surface waters within the South Pacific 

Ocean Basin, and melting ice caps shed water into the 

oceans. Rising sea level may force changes in the reefs, 

park infrastructure, and park management. 

• Increased human development. The increasing 

population of American Samoa is placing demands on 

the available space, building materials, fresh water, and 

natural areas of the islands. It also increases air and 

water pollution. Because private lands surround 

several park units within National Park of American 

Samoa, cooperation between resource managers at the 

park and adjacent landowners is essential in reducing 

potential harm to park resources. 

• Seismicity and mass wasting. American Samoa lies in a 

tectonically active area. The Pacific plate subducts 

beneath the Australian plate along the KermadecTonga trench. At the northern end of this trench, the 

plate is being torn laterally along an east- westtrending transform zone. This region produces many 

seismic events each year. Although most are small, 

even small earthquakes can trigger landslides, 

submarine debris flows (potentially leading to 

tsunamis), and other mass wasting along the steep 

island slopes. Seasonal rains and cyclones cause wind 

and wave damage and create flashfloods and mass 

wasting. Slope vulnerability assessments and geologic 

hazards maps are needed data sets for park resource 

management. 
 NPS Geologic Resources Division 2 

Introduction 

The following section briefly describes the National Park Service Geologic Resource 

Evaluation Program and the regional geologic setting of National Park of American 

Samoa. 

Purpose of the Geologic Resource Evaluation Program 

The Geologic Resource Evaluation (GRE) Program is 

one of 12 inventories funded under the NPS Natural 

Resource Challenge designed to enhance baseline 

information available to park managers. The program 

carries out the geologic component of the inventory 

effort from the development of digital geologic maps to 

providing park staff with a geologic report tailored to a 

park’s specific geologic resource issues. The Geologic 

Resources Division of the Natural Resource Program 

Center administers this program. The GRE team relies 

heavily on partnerships with the U.S. Geological Survey, 

Colorado State University, state surveys, and others in 

developing GRE products. 

The goal of the GRE Program is to increase 

understanding of the geologic processes at work in parks 

and provide sound geologic information for use in park 

decision making. Sound park stewardship relies on 

understanding natural resources and their role in the 

ecosystem. Geology is the foundation of park 

ecosystems. The compilation and use of natural resource 

information by park managers is called for in section 204 

of the National Parks Omnibus Management Act of 1998 

and in NPS- 75, Natural Resources Inventory and 

Monitoring Guideline. 

To realize this goal, the GRE team is systematically 

working towards providing each of the identified 270 

natural area parks with a geologic scoping meeting, a 

digital geologic map, and a geologic report. These 

products support the stewardship of park resources and 

are designed for non- geoscientists. During scoping 

meetings the GRE team brings together park staff and 

geologic experts to review available geologic maps and 

discuss specific geologic issues, features, and processes. 

Scoping meetings are usually held for individual parks 

and on occasion for an entire Vital Signs Monitoring 

Network. The GRE mapping team converts the geologic 

maps identified for park use at the scoping meeting into 

digital geologic data in accordance with their innovative 

Geographic Information Systems (GIS) Data Model. 

These digital data sets bring an exciting interactive 

dimension to traditional paper maps by providing 

geologic data for use in park GIS and facilitating the 

incorporation of geologic considerations into a wide 

range of resource management applications. The newest 

maps come complete with interactive help files. As a 

companion to the digital geologic maps, the GRE team 

prepares a park- specific geologic report that aids in use 

of the maps and provides park managers with an 

overview of park geology and geologic resource 

management issues. 

For additional information regarding the content of this 

report and up to date GRE contact information please 

refer to the Geologic Resource Evaluation Web site 

(http://www2.nature.nps.gov/geology/inventory/). 

Geographic Setting 

The National Park of American Samoa is the only 

National Park Service unit in the southern hemisphere. It 

is located in the South Pacific more than 4,200 

kilometers (km) (2,600 miles [mi]) southwest of Hawaii 

and 2,900 km (1,800 mi) northwest of New Zealand, near 

the International Date Line (between 13° and 15° south 

latitude). The park was authorized on October 31, 1988, 

and since the land is non- federal a 50- year lease was 

signed on September 9, 1993. The park covers 9,000 acres 

(2,500 acres submerged) in the Samoa Archipelago on 

four volcanic islands (of seven in American Samoa): T’au, 

Ofu, Olesega, and Tutuila (fig. 1). The islands are remote, 

isolated, and undeveloped. 

Geologic Setting 

The Samoa Archipelago is part of a physiographic area 

known as Polynesia, a triangular area bounded by the 

Hawaiian Islands, New Zealand, and Easter Island (fig. 

2). Within Polynesia, many volcanic island chains 

intersect. The western edge of the Samoan chain 

terminates in a region known as the northern Melanesian 

borderland. This complex region contains island arcs 

(such as Tonga and Lau Islands), Peggy Ridge, and other 

volcanic features such as the Samoa Ridge near the 

convergent margin of the Pacific plate and the Fiji 

Plateau, west of the Tonga trench (fig.3). The northern 

boundary between the Fiji Plateau and the Pacific plate is 

a transverse boundary which is being sheared in a westto- east trend north of the Tonga trench (also called the 

Kermadec- Tonga trench). The Tonga trench is a westdipping subduction zone located less than 200 km (120 

mi) south of Samoa. This extensive crustal intersection is 

7,000 to 9,000 meters (m) (23,000 to 29,500 feet [ft]) 

below the ocean floor. 

Much like the islands of Hawai’i, the Samoan Islands 

were formed as a result of a crustal plate (the Pacific 

plate) moving slowly over a stationary hotspot. Hotspots 

are localized areas where hot mantle material wells up 

towards the surface of the earth. Crustal regions over 

these hotspots may experience volcanic eruptions and 

geothermal activity, such as the volcanic activity at 

Yellowstone National Park. As volcanic material is 

extruded or ejected over the hotspot, masses of lava may 

accumulate on the ocean floor. 
 NPSA Geologic Resource Evaluation Report 3

A 3,444 - meter (11,300 ft) high submerged, active volcano 

is located 43 km (27 mi) east of the island of Ta’u. This 

feature (named Vailulu’u) still lies some 610 m (2,000 ft) 

below the ocean’s surface. Eventually, if enough lava 

accumulates, the lava pile will break the ocean surface to 

become an island. Once subaerially exposed, the new 

island is susceptible to weathering and erosion. Thus, the 

longer an island is exposed, the more heavily eroded it 

becomes. 

In the Samoa Archipelago, the Pacific plate is moving 

westward across a hotspot; thus, the easternmost 

Samoan Islands are the youngest, least eroded islands of 

the chain. The entire Samoan chain stretches east to west 

along the crest of the Samoa Ridge for more than 485 km 

(300 mi) of high volcanic islands, atolls, seamounts, and 

submerged reef banks. The western islands compose the 

Independent State of Samoa (formerly Western Samoa) 

and are the largest, oldest, and most heavily weathered 

islands of the archipelago. The American Samoa islands 

are approximately 1 million years younger than those of 

the Independent State of Samoa. These islands are host 

to developed fringing coral reefs and climax wet, tropical 

forests. 

Tutuila Island 

The long, narrow island of Tutuila is the largest of the 

seven islands that compose American Samoa (fig. 4A). 

Volcanic activity is responsible for the shape and depth 

of Pago Pago harbor, one of the largest natural harbors in 

the south Pacific. The harbor, which formed in a 

collapsed caldera that cuts deeply into the south- central 

coast of Tutuila, almost divides the island in two. 

A steep ridge runs from east to west along the 32 km (20 

mi) length of the island. This ridge is interrupted by 

summits such as Matafao Peak (the tallest at 653 m, [2,142 

ft]), North Pioa Mountain, and Mount Alava, which 

mark the southern boundary of the park area on the 

island (fig. 5). Approximately 200 closely spaced streams 

create numerous steep slopes and gorges separated by 

narrow ridges. Drowned valleys form embayments such 

as Pago Pago, Fagaitua, Afono, Fagasa, Vatia, and Nua 

Seetaga Bays. Prominent craters on Tutuila include 

Oloava, Fogama’a, and Fagatele craters. Approximately 

1.6 km (1 mi) southeast of the southern tip of Tutuila is 

the tiny island of Aunu’u. The island has scenic beaches 

and an extinct crater that contains Pala Lake, a unique 

expanse of fiery red quicksand. 

Ta’u, Ofu, and Olosega Islands (Manu’a Islands) 

The remaining three islands of National Park of 

American Samoa are T’au, Ofu, and Olosega islands, 

which are collectively referred to as the Manu’a Islands. 

This group of islands lies 100 km (60 mi) east of Tutuila. 

The tallest peak in American Samoa, Lata Mountain (a 

shield volcano), rises 966 m (3,170 ft) above Ta’u. The 

southeastern half of Ta’u lies within the national park. 

The island has numerous volcanic cones, thin lava flows, 

and tuff beds. The southern shore of the island contains 

sea cliffs rising more than 610 m (2,000 ft) above the 

ocean. Ofu has a sandy beach area and a pristine coral 

reef; a bridge connects it to Olosega. These two small 

islands are the remnants of an eroded large volcano that 

may have once been a double caldera. Lava flows, 

pyroclastic beds, and dikes characterize the volcanic 

rocks exposed on the slopes of the islands. The 

shorelines present steep cliffs more than 100 m (330 ft) 

high on the east side of Olosega and on the west side of 

Ofu (figs. 4B, 4C ). 

Ancient Samoan History 

The word Samoa is derived from the native Samoan 

name, sa ia moa. This name refers to a legend describing 

the volcanic origin of the land itself. As described in the 

legend, the rocks “cried to the Earth,” and the Earth 

became pregnant. Salevao, the god of rocks, observed 

motion in the moa, or center of the Earth. A child was 

born and named sa ia Moa, from the place where it was 

seen moving. Salevao said he would become loose stones, 

and that everything which grew would be sa ia Moa or 

“sacred to Moa.” This sacred name is abbreviated as 

Samoa (Keating 1992). 

Evidence suggests that people have lived on the islands 

for more than 3,500 years. An archaeological site (the 

To’aga archaeological sequence) on Ofu dates ~3,000 

years ago (Dickinson 2001). Samoa may have been the 

base of settlement expeditions throughout Polynesia. 

Adzes (a primitive stone tool) made from Tutuila basalts 

have been found throughout the Pacific. 

Inland quarries, streambed tool fabrication areas, and 

agricultural sites dating back 1,000 years have been 

recognized on Tutuila (Addison 2002; Arthur 2002). 

Fine- grained, dense basalt, suitable for tool making, is 

not a common resource on most Pacific islands. For this 

reason, it appears the basalt of Tutuila was prized and 

widely traded (Arthur 2002). 
 NPS Geologic Resources Division 4 

Figure 1. Location of American Samoa with islands containing NPS lands highlighted in purple. Original map at 

http://www.lib.utexas.edu/maps/australia/samoa_islands_2002.gif (accessed May, 10, 2006). Graphic by Trista L. Thornberry-Ehrlich 

(Colorado State University). 
 NPSA Geologic Resource Evaluation Report 5

Figure 2. Location of Samoa Islands in the South Pacific. Graphic courtesy of the National Park Service at 

http://www.lib.utexas.edu/maps/american_samoa.html (accessed March 17, 2007). 
 NPS Geologic Resources Division 6 

Figure 3. Features within the Melanesian Borderland. Graphic adapted from figure 1 from Turner and Hawkesworth (1998) by Trista L. 

Thornberry-Ehrlich (Colorado State University). 
 NPSA Geologic Resource Evaluation Report 7

Figure 4A. Bathymetry surrounding the island of Tutuila at 5 m resolution. Data from National Oceanic and Atmospheric Administration 

Coral Reef Ecosystem Division. Graphic downloaded from http://www.soest.hawaii.edu/pibhmc/pibhmc_AmSamoa.htm (accessed 

March 17, 2006). 
 NPS Geologic Resources Division 8 

Figure 4B. Bathymetry surrounding the islands of Ofu-Olosega at 5 m resolution. Data from National Oceanic and Atmospheric 

Administration Coral Reef Ecosystem Division. Graphic downloaded from http://www.soest.hawaii.edu/pibhmc/pibhmc_AmSamoa.htm 

(accessed March 17, 2006). 
 NPSA Geologic Resource Evaluation Report 9

Figure 4C. Bathymetry surrounding the island of T’au at 10 m resolution. Data from National Oceanic and Atmospheric Administration 

Coral Reef Ecosystem Division. Graphic downloaded from http://www.soest.hawaii.edu/pibhmc/pibhmc_AmSamoa.htm (accessed 

March 17, 2006). 
 NPS Geologic Resources Division 10 

Figure 5. Shallow water areas around American Samoa and locations mentioned in the text. Information from National Oceanic and 

Atmospheric Administration (2007); graphic by Trista L. Thornberry-Ehrlich (Colorado State University). 
 NPSA Geologic Resource Evaluation Report 11

Geologic Issues 

A Geologic Resource Evaluation scoping session for National Park of American Samoa 

was held on March 20- 21, 2003, to discuss geologic resources, address the status of 

geologic mapping, and assess resource management issues and needs. The following 

section synthesizes the scoping results, in particular those issues that may require 

attention from resource managers. 

This section discusses the management of natural 

resources. The most critical topics are listed first. 

Potential research projects and other topics of scientific 

interest are presented at the end of this section. 

Reef and Ecosystem Health 

Coral Reefs 

Coral reefs are an important resource at National Park of 

American Samoa. The reefs are an integral part of 

Samoan culture and a source of food for local villages. 

They protect shorelines from wave action, especially 

during seasonal storms (NOAA 2007). In addition, the 

reefs attract visitors and researchers from around the 

world. Most of the beach sediment fringing the islands is 

derived from the adjacent coral reefs (Richmond 1995). 

The high geothermal gradient in the Samoan region and 

active volcanism produces a large hydrothermal vent 

system currently active at the Vailulu’u seamount 

(Staudigel et al. 2004). This and other smaller vents 

associated with rejuvenated volcanism may have farreaching consequences for the park. Although these 

vents are located outside the park, tides and oceanic 

currents disperse warmer, mineral- laden (dense) vent 

water around the area. The higher- than- usual oceanwater turbidity, density, and temperature may harm the 

park’s coral reefs. At this time (2007), it is unclear 

whether venting has in fact harmed the shallow reefs 

(Bruce Richmond, written comm. 2007). 

Sea level is likely to rise in the near future. Global climate 

changes are warming and expanding surface waters 

within the South Pacific Ocean Basin, and melting ice 

caps shed water into the oceans. Rising sea level may 

force changes in the reefs. American Samoa corals are 

renowned for their resilience to extreme environmental 

stress. The corals have survived four hurricanes in the 

past 18 years, a predatory starfish invasion (Acanthaster

planci), and high temperatures (NOAA 2007). 

Although resilient, Samoan corals are not indestructible. 

Slight increases in ocean temperature (1- 2 days at 

temperature elevations of 3- 4°F or several weeks of 

temperature elevations of only 1- 2°F) can cause the 

corals to “bleach” or turn pure white. 

Plant- like cells (zooxanthellae) living in coral tissues are 

released at higher temperatures depriving the coral of 

symbiotic nutrient exchange. If the zooxanthellae are not 

recovered, the coral will die. Once coral die, they can 

become covered with slimy mats of green algae (Craig 

2006). Algae overgrowth can stunt coral recovery efforts, 

which are already slow. 

Human- induced changes also threaten the current state 

of the reefs. Fishing pressure has reduced the overall 

health of the reef ecosystem. Rapid population growth 

threatens the reef environment with coastal alterations, 

loss of wetlands, soil erosion, coastal sedimentation, 

solid and hazardous waste disposal, water contamination 

and air pollution (NOAA 2007). 

The coral reefs of National Park of American Samoa can 

serve as an indicator of changes in the seas of the region. 

According to estimates, approximately 30% of the 

world’s coral reefs are severely damaged and as much as 

60% may perish by the year 2030 (Piniak et al. 2004). At 

American Samoa, temperatures in some pools reach 

35.5°C and can fluctuate as much as 6°C degrees in a 

single day. Such high temperature fluctuations would kill 

most corals. Factors associated with this resiliency 

include acclimatization, chemical properties, types of 

symbiotic zooxanthellae, adaptation, evolution of larvae, 

and susceptibility to disease. Careful study of these 

factors is necessary in order to apply the results to other 

reefs (Piniak et al. 2004). 

Baseline studies and inventories followed by careful 

monitoring of reef conditions are vital to reef 

management and to the understanding of coral reef 

response to ocean changes. 

Backreef Moats 

Shallow lagoonal areas (backreef moats) are on the 

landward side of several fringing reefs of American 

Samoa. These moats may be typical of fringing reef 

zones—shoreline, lagoonal moat (with or without patch 

reef), reef flat, reef crest (area exposed at low tide), and 

the upper and lower reef slopes. Moats typically range 

from 1 m (3 ft) to about 10 m (30 ft) deep and may contain 

rocky substrates in addition to the predominantly sandy 

bottom. Where hard substrate exists, coral growth can be 

elaborate, and patch reefs may extend in mushroomshaped bulbs from the sea floor (Kirda 1998). 

Backreef moat areas are important as natural buffers 

along the shoreline of the Samoa Islands. They collect 

sediment, support coral colonies, and act as nursery 

areas for local fisheries. Existing backreef areas need 
 NPS Geologic Resources Division 12 

protection from human influences and degradation 

within the National Park of American Samoa. Formation 

of further backreef moats by creation of offshore ridges 

may extend these unique and invaluable habitats. 

On the island Ofu, the backreef moat supports a diverse 

coral community including reef- building corals. A 

continuous reef crest separates and protects this area 

from the open ocean. It contains several interconnected 

pools about 1- 2 m (3- 6 ft) deep. No streams enter the 

moat so it is only slightly influenced by adjacent land. 

Temperatures in the moat frequently are more than 3°C 

above the average summer water temperature, 32- 34°C. 

As part of the protected National Park of American 

Samoa, this moat offers research opportunities 

uncompromised by most human influences (Smith 

2004). 

Ecosystem Conservation 

Ecosystems in National Park of American Samoa range 

from lagoon, coral reef, and littoral strand to tropical 

rain forest to montane, ridge top, mountaintop scrub, 

and cloud forest (www.nps.gov/npsa). All of these 

ecosystems are built on geologic substrates and by 

geologic processes. Those processes influence how 

ecosystems respond to climate change, seasonal storms 

and droughts, and human intervention. The isolation of 

the islands means biodiversity is somewhat limited. 

Thirty- two percent of the local plant species are 

endemic however, endemic marine species have yet to be 

investigated (Craig 2006). These geographically isolated 

ecosystems yield abundant opportunities to study global 

changes in climate, ocean conditions, and air quality in a 

relatively undisturbed environment. 

Inventory, Monitoring, and Research Needs for Reef and 

Ecosystem Health 

• Complete baseline inventories and characterize 

present reef conditions. Locate areas targeted for 

monitoring of ocean change and the consequences of 

any change. 

• Monitor sea level changes and ocean temperatures at 

various locations around coral reefs. Note changes in 

water turbidity and density. 

• Cooperate as part of the Coral Reef Advisory Group in 

NOAA’s Coral Reef Ecosystem Research Plan 

http://coris.noaa.gov/activities/coral_research_plan/p

dfs/american_samoa.pdf (accessed March 17, 2007). 

• Measure amounts of dissolved solids in ocean water 

and correlate with oceanic currents to determine 

potential source areas (such as active hydrothermal 

centers, submarine volcanoes). 

• Continue to cooperate with multidisciplinary studies 

of coral persistence in American Samoa (Piniak et al. 

2004). 

• Support plans to responsibly manage village- based 

fisheries. 

• Support legislation that strengthens penalties for 

malicious coral bleaching (fishing with chlorine 

bleach), dumping waste in waterways, releasing sewage 

from piggeries onto the reefs, or improper land- use 

activities that release excess sediments. 

• Identify, protect, and nurture naturally resistant corals 

for possible use in re- seeding areas affected by coral 

bleaching. 

• Continue to support backreef moat characterization 

studies that involve transplanting corals into backreef 

areas most likely to sustain long- term growth (Smith 

2004). 

• Research the possibility of building onto fringing reefs 

or creating artificial reefs to expand backreef moat 

habitat. 

• Investigate placing restrictions on access to especially 

fragile and sensitive backreef moat areas. 

• Promote studies that emphasize the role of geology as 

the foundation for the ecosystems at National Park of 

American Samoa. 

• Create interpretive programs and exhibits highlighting 

the role of geology in ecosystem development. 

Water and Energy Resources 

A population growth in American Samoa of 2.1 % per 

year strains the natural resources available on these 

isolated islands. Chief among these resources is fresh, 

potable water for drinking and municipal use. 

Freshwater from precipitation that is neither discharged 

by streams, evaporated, nor used by plants is stored in 

the subsurface to recharge groundwater. Many of the 

volcanic rocks that form American Samoa are highly 

permeable and fractured and have a low hydraulic 

gradient; however, volcanic dikes may act as 

impermeable barriers to groundwater movement. 

Near the coast, lighter freshwater floats on top of a 

saltwater aquifer. The fresh water percolating into the 

ground depresses the salt water beneath an island 

forming a profile that has the appearance of a lens 

referred to as the Ghyben- Herzberg lens. For every foot 

of fresh water above sea level on dry land, an estimated 12 

m (40 ft) of fresh water lies below sea level floating in the 

lens. The division between fresh and salt water is not 

sharp, and coastal areas near the lens contain brackish 

water unsuitable for drinking (Keating 1992). 

Because the islands of American Samoa are underlain by 

many rock types, hydrogeologic properties can change in 

a short distance. Younger lavas in general are more 

permeable than older volcanic rocks and alluvial 

deposits. Wells in younger, permeable lavas sustained 

yields of up to 300 gallons per minute (Keating 1992). On 

American Samoa, volcanic rocks outside of calderas 

locally produce water. However, volcanic rocks within 

calderas are not productive because of their limited 

subaerial collection area. Zones of local permeability and 

productivity are associated with fractures, faults, 

volcanic ejecta, and boundaries between distinct flow 

units (Keating 1992). 
 NPSA Geologic Resource Evaluation Report 13

It is estimated that 23 million gallons per day of 

groundwater recharges the aquifer in the highly 

permeable volcanic rocks of the Leone peninsula (fig. 6) 

(Keating 1992). Given the spatial variation in 

permeability, even productive wells may have limited 

recharge potential. Thus, it is essential for park resource 

management to understand the nature of the 

hydrogeologic system at American Samoa at a fine scale. 

Human activities continue to degrade air and water 

quality at National Park of American Samoa. Preindustrial contaminants generally were limited to dust 

and to natural volcanic gases. Today, lead is of major 

concern. Lead is carried to the islands as industrially 

produced carbonaceous aerosols that are recycled as sea 

spray salt, and then deposited on plant leaf surfaces. 

These lead- rich aerosols are then reintroduced into the 

air as plant aerosols (Settle and Patterson 1991). 

As human development increases in American Samoa, 

importing goods, supplies, and energy sources will 

become more expensive. A new airport at Ofu is 

requiring massive amounts of fill dirt. Increased 

development also increases the need for rocks, cement, 

aggregate, lumber, and other building materials. 

Geothermal energy may reduce the islands’ reliance on 

imported petroleum products. Geothermal energy is 

relatively nonpolluting and can be developed in small 

areas (Olson 1987). The geothermal gradient around 

Samoa is relatively high, and a hydrothermal plume in the 

Pacific Ocean may be economically feasible to exploit 

(Lupton et al. 2004). 

Inventory, Monitoring, and Research Needs for Water and 

Energy Resources 

• Monitor air quality (in particular, lead and particulate 

matter) and correlate with local volcanic activity to 

separate fluctuations from background levels; initiate a 

record of local air change. 

• Investigate alternative energy sources, such as 

geothermal, wind, and solar, for use at park facilities 

(Olson 1987). 

Geologic Hazards 

The particular combination of tectonic setting and 

climate in American Samoa creates several geologic 

hazards: earthquakes, volcanic eruptions, tsunamis, slope 

failures (rapid mass wasting), cyclones, drought and 

flooding. 

Earthquakes 

Seismic events are common in Samoa and the potential 

for future earthquakes is high. The largest earthquake 

recorded in Samoa (since recording began in 1917) was a 

magnitude 8.3 on the Richter scale in 1917. A magnitude 

8.0 earthquake occurred in 1995 (Tavoi et al 2001). 

Average events are magnitude 4.5–5. Most larger 

earthquakes occur to the southwest and are associated 

with the Tonga arc (fig. 3) (Tavoi et al 2001; Leavasa 

2004). 

Near Vailulu’u, active volcanism produces as many as 40 

small earthquakes in a single day (Konter et al. 2002; 

Konter et al. 2004) as brittle rock fails during 

redistribution of magma movement and in the magma 

chambers below the seamount (Konter et al. 2004). 

Subduction of the Pacific plate into the Tonga trench 

(figs. 17, 19) has led to high seismicity and deep 

earthquakes. Another system of seismic activity, trending 

east- west through Fiji west to the New Hebrides 

(Vanuatu), also contributes to the earthquake potential 

in the area. Earthquakes east and north of the trench are 

at shallower depths (less than 70 km, 43 mi) and are 

usually the result of internal deformation of the Pacific 

plate or subsurface movement of magma accompanying 

volcanism (Hill and Tiffin 1993; Leavasa 2004). In the 

area south of American Samoa, the Pacific plate is cut by 

small- scale normal and thrust faults striking northsouth and dipping westward. This orientation is in 

response to the bending of the plate into the Tonga 

trench and to longitudinal compressive stresses within 

the plate. In addition, the tearing of the Pacific plate, 

associated with the junction of the subduction zone and 

the transverse boundary north of the trench, causes 

earthquakes in the area (Hill and Tiffin 1993). 

Volcanic hazards 

The Samoa Islands are part of a volcanic archipelago 

located along the crest of the Samoa Ridge overlying the 

stationary Samoa hotspot. In general, isotopic dating of 

dredge basalts from along the entire trend of the Samoan 

lineament indicates that Samoan volcanism is older to 

the west (Hart et al. 2004). The trend in dates is 

consistent with the velocity of the Pacific plate 

movement listed below (Hart et al. 2004). However, as 

discussed below in the Geologic History section, a 

strictly plume- driven hotspot model is not necessarily 

consistent with the volcanic activity of the Samoan 

Islands. On the westernmost and oldest (~5 million years 

old) Samoan island of Savai’i, voluminous young volcanic 

rock indicates rejuvenated volcanic activity whereas on 

Upolu, the second oldest island, there is little to no 

rejuvenated volcanic activity (Hart et al. 2004; 

McDougall 1985). 

Rejuvenated volcanic activity along the Samoan Ridge 

may be explained by the unique tectonic setting of the 

Samoan archipelago close to the junction of the Pacific 

plate and the Australian- Indian plate (Hart et al. 2004). 

At this point, the Pacific plate is tearing laterally in a 

pattern propagating eastward at the northern end of the 

Tonga arc (Hart et al. 2004). The nature of this boundary 

produces earthquakes, as described above, and may also 

be responsible for some renewed volcanic activity in the 

area, including that on Savai’i (Keating 1992; Hart et al. 

2004). Crustal tearing may facilitate volcanic 

rejuvenation extraneous to the Samoan hotspot. 

Volcanic activity and associated hazards are still a very 

real possibility at National Park of American Samoa. 

Resource management at National Park of American 

Samoa can assume future volcanism may occur in areas 
 NPS Geologic Resources Division 14 

distant from the present hotspot location beneath 

Vailulu’u. As recently as 1866, subaqueous eruptions and 

earthquakes spewed clouds of pumice, volcanic ash, and 

steam for several months in the Manu’a islands (Keating 

1992; Craig 2006). In 1905, lava flows from several large 

eruptions destroyed villages in Savai’i, Samoa (Craig 

2006). Recent volcanic events such as these pose hazards 

of lava flow destruction, emission of noxious gases and 

fumes, mantling ash falls and pyroclastic flows, and 

direct blast damage. 

Mass wasting 

The flanks of Samoan volcanoes are thickly mantled by 

coarse debris flows, especially offshore. Both large and 

small slump blocks are common in these locations 

(Keating and Karogodina 1990). In sea floor images taken 

in western Samoa, debris avalanche deposits extend from 

the island slope onto the adjacent abyssal plains, 

covering areas of more than 20,000 km2

 (7,700 mi2) (Hill 

and Tiffin 1993). This debris may have been transported 

as sheet flows, semi- coherent blocks, or chaotic flows 

(Keating et al. 2000). Triggered by seismicity associated 

with volcanism or the local tectonic setting near the 

Kermadec- Tonga trench (fig. 3), catastrophic failure of 

these submarine debris slopes (perhaps triggered by a 

tectonic shift in the seafloor) could displace the water 

column enough to produce a tsunami. 

In addition to submarine debris flows, mass wasting on 

land produces catastrophic landslides, rockfalls, and 

slumps in most coastal and upland areas of American 

Samoa. Severe rockfalls have killed people on Tutuila. 

Torrential tropical rains and cyclones increase the 

potential for flash floods and mass wasting on island 

slopes. Slopes underlain by weakly cemented or 

unconsolidated volcanic ejecta or by heterogeneous 

layers such as basalt flows, tuffs, and lapilli tend to fail 

when undercut. Moreover, cyclones usually coincide 

with elevated water levels owing to storm surge and 

winds that affect all coasts of the islands (Richmond 

1995). These conditions, when combined with the seismic 

potential for the area, create a considerable safety hazard 

for National Park of American Samoa visitors, staff, and 

facilities. 

Inventory, Monitoring, and Research Needs Geologic 

Hazards 

• Set up a GIS database that incorporates geology, 

meteorology, seafloor mapping, seismic monitoring, 

volcanic monitoring, bathymetry, physiography, 

engineering, geomorphology, population distribution, 

and land- use patterns; this information can be used to 

assess the risks of a given hazard in specific geographic 

areas of the park (Shorten 2001). 

• Refer to the Pacific Disaster Center website for hazard 

maps, vulnerability assessments, and mitigation plans 

for geologic hazards at American Samoa 

http://www.pdc.org/iweb/pdchome.html (accessed 

March 17, 2007). 

• Improve seismic hazard maps and analyses at a finer 

scale, especially those pertaining to park facilities, 

future development, and visitor- use areas (McCue 

1999). 

• Cooperate with the U.S. Geological Survey, the 

Ministry of Agriculture, Forestry, Fisheries, and 

Meteorology, and the Global Seismological Network 

to improve seismic monitoring throughout Samoa, to 

standardize seismic monitoring, and to increase the 

number of seismograph recording stations (Leavasa 

2004). 

• Identify areas susceptible to mass wasting and debris 

flows, and map landslide scars. Monitor slide- prone 

areas and measure distance of slope movement, visible 

cracks, slumps, and scars. 

Coastal Erosion and Beach Loss 

Beaches 

Sand beaches are present only in limited areas of the 

coastline of Tutuila. Coastal plains, which are underlain 

by a mixture of terrigenous and marine sediments and 

support beaches, wetlands, stream valleys, alluvial fans, 

and mangrove forests, are separated by headlands of 

volcanic rocks (Richmond 1995). Beaches fringe 

approximately 20% of the island’s coastline. Beach 

features include pocket beaches, barrier spits, tombolos, 

and small flood- deltas at the mouths of stream valleys 

(Richmond 1995). Beaches are an important and limited 

natural resource to American Samoa and are not 

desirable sources for sand and gravel. Sand mining on 

coastal beaches would severely degrade the resource and 

affect the sediment budget for the entire coastal system. 

The sands of the shoreline are present as loose, fine to 

coarse grains and as moderately lithified beach rock. 

Sediment type depends on the material available and the 

energy regime associated with deposition. Higher energy 

conditions during deposition yield coarser sediment 

deposits (Richmond 1995). 

The composition of Tutuila sand is primarily calcareous 

fragments of coral, shells, and algal mat remnants 

(Keating 1992). In most areas of American Samoa, 

lagoonal sediments are fine- grained and friable. Lagoons 

help to protect the coast during storms (Lewis 1989). 

Coastal deposits contain sand and gravel eroded from 

Holocene basalts and carbonaceous sediments. These 

deposits are present only locally at the ends of lava fields 

in alluvial deltaic settings near coastal lagoons. These 

areas may support seagrass beds that act as nurseries for 

reef fisheries (Lewis 1989) but are too fragile for mineral 

extraction. 

Because of the isolation of American Samoa, 

construction materials are both expensive and difficult to 

import. Thus, careful management is essential to protect 

the islands’ indigenous resources. Colluvium and 

alluvium add silt, clay, loam, silty clay, boulders, gravels, 

and sand to the natural resources of American Samoa 

(Keating 1992). Crushed dense volcanic rock, such as that 

at the Pago caldera complex, is suitable for concrete 
 NPSA Geologic Resource Evaluation Report 15

aggregate and road material. Cinder deposits are also 

used for road surfaces. These deposits are found at the 

Futiga and Mapusaga cones. 

Coastal Engineering 

The islands of American Samoa are slowly subsiding into 

the sea. Because these are now extinct volcanoes, 

subsidence occurs due to the lack of thermal uplift 

associated with active volcanism, deflation of vacated 

magma chambers below the islands, and the weight of 

the mass of volcanic rock on the ocean floor (see 

Volcanoes and Volcanism section below). As an island 

subsides, coastal, low- lying areas are slowly submerged, 

changing shoreline dynamics. 

Structures intended to reduce erosion along one stretch 

of beach often cause beach loss further along the 

direction of longshore transport. For instance, the 

majority of villages and island infrastructure are located 

on the coastal plains of Tutuila. Where erosion threatens 

villages, buildings, or roads, engineering structures such 

as riprap, revetments, groins, or seawalls are typically 

constructed to prevent coastal erosion (Richmond 1995). 

Riprap is composed of large, loose blocks (concrete, 

rock, asphalt or other resistant material) placed to 

stabilize and armor a coastline susceptible to erosion. 

Riprap is placed below slopes and along sandy beaches to 

reduce sediment movement and to decrease erosion and 

undercutting of the coastline. 

Shoreline armoring has severely reduced the amount of 

beach available for traditional recreational uses and has 

degraded the natural state of Tutuila beaches. The 

shoreline is prevented from normal migration in 

response to storm surges and cyclones (Richmond 1995). 

Water that ponds behind the riprap allows sand to 

accumulate, whereas water scours the ocean- facing side, 

which is starved of sand and thus preferentially erodes. 

The more extensive the riprap structure, the more the 

erosional effect further down (down- current) the 

shoreline. 

This erosion, combined with island subsidence, sand 

extraction, and structures designed to buffer against 

waves, is causing significant coastal disturbance in the 

park and throughout the Samoa Islands (Richmond 

1992a). 

Inventory, Monitoring, and Research Needs for Coastsal 

Erosion and Beach Loss 

• Refer to U.S. Army Corps of Engineers shoreline 

inventories from 1980 and 1989, which describe the 

developed shoreline emphasizing engineering 

structures and erosion. Incorporate this information 

into the park GIS. 

• Research recycled sand alternatives including crushed 

building material (such as old concrete or asphalt). 

• Focus an interpretive program on the origin, 

distribution, and movement of sand supply. 

• Discourage the use of beach sand as construction 

material. 

• Cooperate with local developers to reduce the amount 

of riprap in areas adjacent to park beaches. 

• Create education programs that depict the 

consequences of riprap along beaches. Highlight the 

natural processes of sand supply and movement, 

longshore drift, and erosion. 

• Research alternatives to riprap that will stabilize 

surrounding beach areas for adequate recreation. 
 NPS Geologic Resources Division 16 

Geologic Features and Processes 

This section describes the most prominent and distinctive geologic features and processes 

in National Park of American Samoa. 

Volcanoes and Volcanism 

The volcanic features at National Park of American 

Samoa range from broad, gently sloping, eroded shield 

volcanoes to steep, slightly weathered, cinder- cone 

deposits. Many craters are present throughout the 

islands (Keating 1992). Samoan shield volcanoes erupt 

from fissures and along the flanks, rather than 

exclusively from summit craters and most are lava flows 

rather than airfall debris (Hawkins 1987) 

On Tutuila, the largest (approximately 32 km (20 mi) long 

and 9 km (5.5 mi) wide) and oldest island in the park, the 

landscape is dominated by the eroded Pago shield 

volcano. This volcano’s maximum elevation is 653 m 

(2,142 ft) at Matafao Peak (fig. 6). Based on the dip of 

post- caldera lavas, the mountain once may have risen as 

high as 1,200 m (3,937 ft) above present sea level (Keating 

1992). Rugged terrain with steep, narrow valleys and 

deeply embayed coastlines characterize the island. 

Several overlapping basaltic shield volcanoes underlie 

the island along a rift zone that strikes N. 70° E. at an 

angle to the overall west- east trend of the Samoan chain. 

Among the larger volcanic centers on the island are 

Olomoana, Alofau, Pago, and Taputapu (fig. 6). Most 

lavas are present as shallow dipping flows 1 to 10 m (3 to 

33 ft) thick composed of olivine basalt and hawaiite 

(McDougall 1985). 

Dikes of similar composition that strike parallel to the 

Samoan chain are common in the Pago volcano. Today, 

only the northern half of the Pago shield remains; the 

rest was removed during the formation of Pago caldera 

(now drowned and forms Pago Pago Harbor). Other 

shield volcanoes erupted at about the same time as the 

Pago volcano. The Olomoana and Taputapu volcanoes 

were built on eastern and western extensions, 

respectively, of the same rift zone. Recent reactivation of 

volcanism at Tutuila produced the cinder cones, ash, and 

tuff of the Leone volcanic rocks. These rocks dominate a 

broad plain in the southwestern portion of the island 

(McDougall 1985), the only place on Tutuila where cones 

are concentrated (Keating 1992). 

Volcanic cones are present at Tauga Point on the 

northwest tip of the island of Ofu. At Lemaga Point on 

the southeast tip of Olosega (fig. 6), another volcanic 

cone is exposed. The oldest rocks on Ofu- Olosega, the 

Asaga Formation, form volcanic breccia cones. Several 

large and small craters are scattered across the landscape 

of Ta’u. Red Lake, on Aunu’u Island (not part of the 

park) is a crater lake—a drowned volcanic crater 

(Keating 1992). 

The volcanoes of American Samoa are considered classic 

examples of the plume- driven, age- progressive hotspot 

island chain (fig. 7), in part because of the 

geomorphology of the islands. The oldest volcanic 

islands are also the most weathered and eroded (fig. 8). 

The youthful, uneroded shield landscape at Ta’u is in 

sharp contrast to the deeply incised valleys of Tutuila 

(Hart et al. 2004). Recent studies show that island 

formation was more complex than simply a hotspot 

source of volcanism. The islands appear too limited in 

geographic scope and lack the oceanic basaltic plateaus 

necessary to strictly fit the deep- mantle plume, longlived hotspot model (Clouard and Bonneville 2001). 

Although the overriding trend is for volcanic islands in 

Samoa to increase in age from east to west, rejuvenated 

volcanism on older islands (such as Savai’I and Upolu) in 

the chain are putting this classic model into question 

(Hawkins 1987). Other hotspot island chains have 

intersecting lineaments that further complicate the 

hotspot model including the Tuvalu lineament and the 

lineaments from the Cook- Austral or Louisville hotspots 

(see fig. 16 below). The location of the Samoa Islands just 

north of the Kermadec- Tonga trench (fig. 3), where it 

swings westwards towards Fiji, may help explain the 

volcanic rejuvenation and complexity of island 

formation (Hawkins 1987). 

Coral Reefs and Sea Level Change 

Coral reefs surround all islands of National Park of 

American Samoa. Corals thrive in warm, clear water, and 

framework- building corals grow at depths between the 

low- water level and no more than 20 m (66 ft) 

(Richmond 1995). Around Tutuila, the reefs display 

considerable spatial variation in the amount of 

development and distance to the island’s shoreline. This 

variation may indicate the presence of a drowned reef 

forming a shelf around certain parts of the island at 58–72 

m (190–236 ft) depth (Keating 1992). Off Tutuila, a 

narrow reef structure is formed by the coalescence of the 

fringing reef and barrier reef. This feature is only 10.5 m 

(34 ft) across (fig. 4A) (Keating 1992). Fringing reefs 

consist of a reef flat extending seaward from the 

shoreline and terminating in a reef crest beyond which a 

steep reef face forms the seaward margin (Richmond 

1995). Reef development is limited around Ofu, Olosega, 

and Ta’u (Keating 1992), because submarine slopes 

around the islands are steep. Water reaches depths of 610 

m (2,000 ft) within 0.8–3 km (0.5–2 mi) offshore (fig. 4BC). 
 NPSA Geologic Resource Evaluation Report 17

Corals are important to National Park of American 

Samoa for at least two reasons. First, they protect 

adjacent coastlines (Bruce Richmond, written comm. 

2007). Coral organisms secrete carbonate minerals from 

their cells in successive layers and build widespread coral 

“rocks” that slowly add to island area (1.3–7.6 centimeters 

(cm)/year, 0.5–3.6 inches (in.)/year). These carbonate 

minerals form a solid structure that buffers the shoreline 

from large waves and storms. Coral and other reef 

organisms also supply much of the nonvolcanic sediment 

at American Samoa (Richmond 1995). Coral is used for 

building material and adds to the sands in the lagoons 

and beaches (www.nps.gov/npsa). 

Coral reefs are also important because of their high 

degree of biodiversity. The number of coral species 

represented in the reefs of National Park of American 

Samoa is vast. Families may contain many species, such 

as Zoanthidae, Alcyoniidae, Actinodiscidae, 

Acroporidae, Pocilliporidae, Poritidae, Agariciidae, 

Siderastreidae, Fungiidae, Oculinidae, Pectiniidae, 

Mussidae, Faviidae, Merulinidae, Helioporidae, and 

Milleporidae (figs. 9–12). National Park of American 

Samoa maintains a coral inventory website 

(http://www.nps.gov/npsa/NPSAcorl/corlnamA.htm) 

(last accessed January 9, 2008) that is regularly revised as 

new species are documented. 

Effective resource management at American Samoa 

National Park requires an understanding of global 

climate change. A small rise in sea level could drown the 

coral reefs and coastal plains of the islands. Previous sea 

level positions are recorded on the islands by drowned 

reefs, truncated ridges, wave- cut benches, and cliffs 

(Keating 1992). On Upolu (fig. 1), a positive sediment 

budget allowed mangrove swamps to develop; these 

swamps and larger fringing and barrier reefs may 

indicate a small relative sea- level lowering since the past 

700- 1,000 years (Richmond 1992a; Goodwin and 

Grossman 2003). 

Understanding the history of sea level changes, especially 

those during the most recent (Holocene) epoch, is 

important for constraining geodynamic theories of 

crustal flexure and plate tectonics, interpreting the 

environments of early human occupation sites, and 

predicting future environmental conditions, especially as 

they pertain to coral reefs and ecosystem health 

(Dickinson 2001). The overall pattern of Holocene sea 

level for the equatorial Pacific indicates a middle to late 

Holocene highstand (1–3 m above present average value) 

(Grossman et al. 1998). Subsequently, sea level lowered to 

present elevations. 

In the Samoa Islands this drop in sea level stranded 

wave- cut benches, beach rock, caves, and coral reefs) on 

dry land (Dickinson 2001). This drop in sea level may also 

have coincided with early human migrations into island 

groups of the northwestern and southwestern Pacific 

Ocean Basin. The early settlers would have taken 

advantage of newly available coastal environments 

created by lowered sea level (Dickinson 2001). 

Weathering and Erosion 

The current landscape at National Park of American 

Samoa illustrates the contrasting forces of island building 

by volcanism and reef formation and the persistent 

forces of weathering and erosion that lower island 

elevation and reduce their size. On Tutuila, the Pago 

volcanic rocks that form the main caldera show 

amphitheatre- headed canyons and deep V- shaped 

valleys. Deeply incised canyons and valleys are also 

common in the Taputapu volcanic rocks. Erosion of the 

Alofau and Olomoana volcanic rocks is less pervasive. In 

these areas, only limited canyons and shallow gullies 

exist. The youngest rocks on Tutuila, those of the Leone 

volcanic rocks, possess weakly developed drainage 

patterns and little soil (Ollier 1988; Keating 1992). 

Amphitheatre- headed canyons commonly form in 

response to local springs (Ollier 1988). Springs develop in 

volcanic rocks of high permeability and readily available 

groundwater. The flow from springs locally erode and 

undercut valley sides roughly along the elevation of the 

local water table. Streams are concentrated at the valley 

head, and landslides and mass wasting reduce the valley 

sides in short bursts. Relatively flat valley floors are 

commonly mantled with talus deposits. The intersection 

of the valley and the adjacent plateau is typically an 

abrupt cliff (Ollier 1988). 

Perhaps the most dramatic erosional features at National 

Park of American Samoa are the numerous sea cliffs. 

These cliffs typically develop on a coast that lacks a 

buffering coral reef and thus is exposed to direct attack 

by waves. Such cliffs line the Leone peninsula on Tutuila 

and encircle the islands of Ofu and Olosega (Keating 

1992). Erosional features at American Samoa such as seacut caves, benches, and ancient shorelines record 

variation in sea level during the past million years. 

Benches on Tutuila are 3–6 m (10–20 ft) above the 

present high tide level. Openings in sea- cut caves at 

Mataae Point and Fagaitua (fig. 6) are now 5 m (16 ft) 

above high tide (Keating 1992). 

 
 NPS Geologic Resources Division 18 

Figure 6: Map showing features mentioned in the text. Information from National Oceanic and Atmospheric Administration (2007), and 

Keating (1992); graphic by Trista L. Thornberry-Ehrlich (Colorado State University). 
 NPSA Geologic Resource Evaluation Report 19

Figure 7: Development of a volcanic island chain over a stationary hotspot in the mantle and lower lithosphere. Graphic by Trista L. 

Thornberry-Ehrlich (Colorado State University). 
 NPS Geologic Resources Division 20 

Figure 8: Simplified stages of hotspot island volcanism. After volcanism ceases, erosion and subsidence slowly lower the island into 

the sea. Graphic by Trista L. Thornberry-Ehrlich (Colorado State University) after figure 29 in Keating, 1992. 
 NPSA Geologic Resource Evaluation Report 21

Figure 9: Pocillopora eydouxi Antler Coral. Photograph by Eva DiDonato (National Park Service). 

Figure 10: Acropora abrobanoides. Photograph by Eva DiDonato (National Park Service). 
 NPS Geologic Resources Division 22 

Figure 11: Goniopora fruiticosa. Photograph by Eva DiDonato (National Park Service). 

Figure 12: Gardineroseris planulata. Photograph by Eva DiDonato (National Park Service). 
 NPSA Geologic Resource Evaluation Report 23

Map Unit Properties 

This section identifies characteristics of map units that appear on the Geologic Resource 

Evaluation digital geologic map of National Park of American Samoa. The 

accompanying table is highly generalized and is provided for background purposes only. 

Ground- disturbing activities should not be permitted or denied on the basis of 

information in this table. More detailed map unit descriptions can be found in the help 

files that accompany the digital geologic map or by contacting the National Park Service 

Geologic Resources Division. 

Mapping of National Park of American Samoa is divided 

into three distinct island groups: Tutuila, Ta’u, and Ofu–

Olosega. These islands sit on the Samoa Ridge, which is 

composed of thin- bedded pahoehoe basalt flows (or 

pillow lavas, if extruded under water). These flows built 

platforms 3- 5 km (2- 3 mi) high on the ocean floor that 

may rise above sea level (Wingert and Pereira 1981). The 

islands are composed almost entirely of volcanic rocks 

and their weathering products. 

On Tutuila, the oldest rocks are from the Masefau dike 

complexes containing flows cut by basaltic dikes and 

talus breccias. Younger volcanic rocks include the 

Olomoana and, Alofau volcanic rocks, the Pago volcanic 

series, the Taputapu volcanic rocks, the ‘Aunu’u tuff, and 

the Leone volcanic rocks. These eruptive rocks are from 

distinct volcanic episodes separated by quiescent 

erosional periods. Most of these units contain basalt, 

olivine basalt, picrite- basalt, and hawaiite flows; less 

common are volcanic breccias, cinder cone deposits, 

volcanic tuffs, and ash deposits (Wingert and Pereira 

1981). The youngest map units on Tutuila are the loose 

calcareous beach sands, slope talus, and fluvial alluvium 

on the shores and valley floors of the island. 

The Manu’a Islands (T’au, Ofu, and Olosega) are located 

along the crest of the easternmost portion of the Samoa 

Ridge (figs. 3, 19). The volcanic units of Ta’u contain, 

from oldest to youngest, the Lata, Tunoa, Luatele, and 

Fiti’uta Formations. 

The geologic units on T’au are thick flows of a’a and 

pahoehoe nonporphyritic basalt, olivine basalt, and 

picrite- basalt. Deposits associated with caldera collapse 

include ponded lavas and pyroclastic deposits (Wingert 

and Pereira 1981). 

On Ofu and Olosega, the oldest unit (the Asaga 

Formation) is composed of volcanic breccia, and 

composite and tuff cones. The Tuafanua Formation 

contains olivine basalt, basalt, picrite- basalt, and 

hawaiite flows with interbedded ash, tuff, and breccia 

beds. The Nu’u Formation contains lapilli tuff and 

hawaiite and olivine basalt flows. All Manu’a Islands are 

covered in part by younger, unconsolidated surficial 

units including noncalcareous alluvium, slope talus, 

marsh, and stream deposits as well as modern calcareous 

beach sand and beach rock (Wingert and Pereira 1981). 

The following pages present a tabular view of the 

stratigraphic column and an itemized list of features of 

each map unit. This table specifies several properties of 

each unit in the stratigraphic column, such as name and 

map symbol, description, resistance to erosion, suitability 

for development, hazards, potential paleontologic 

resources, cultural resources and mineral occurrence, , 

recreational use, habitat and geologic significance. 
Map Unit Properties Table – Tutuila Map Units 

Age Map Unit (Symbol) Unit Description Erosion Resistance Suitability for Development Hazards Paleontologic Resources Cultural Res

Recent Sedimentary Rocks (Ra) Unit consists of loose calcareous beach sand along coastal areas and may include slope talus at the foot of valley walls and river deposited alluvium on valley floors; units are a maximum of 60 m thick. Very low Development not recommended on this highly permeable, erodable, and fragile unit. Mass wasting (landslides, rockfalls, slumps, slope creep, etc.); extreme beach erosion Fragments of foraminifera, algae, shells, and corals Sediments may c

pre- historic artiflocal populations indigenous peop

Recent Leone Volcanic: Leone Volcanic (Rll); stony ash(Rla); cinder cone (Rlc); lithic/vitric (Rlt) 

Rll contains olivine pahoehoe basalt flows. Rla contains deposits of stony ash along the source fissure zone. Rlc consists of cinder cone deposits localized at the upper end of the source fissure. Rlt contains at least 2 thick tuff 

beds from Vailoatai, Fagatele, and Fogãma’a Craters with 

material from Fogãma’a Crater atop the Fagatele Crater volcanic deposits. 

Moderate 

Highly porous; may be unsuitable for waste treatment facility and unstable for building foundations 

Ash and cinder cone deposits are unstable on slopes and pose mass wasting hazards 

Plant and animal casts possible, fragments of drowned coral reef 

Humans may havwitnessed erupti

QUATERNARY - 

Recent ‘Aunu’u Tuff (Rat) Unit comprises ‘Aunu’u Island of lithic to vitric tuff deposits. Unit is more than 200 m thick. Moderate 

Highly porous; may be unsuitable for waste treatment facility and unstable for building foundations 

Contains sharp, glassy fragments; may pose mass wasting hazards 

Plant and animal casts possible 

Volcanic glass mprovided tool ma

PLIOCENE – 

EARLIEST 

PLEISTOCENE Trachyte Plugs and Dikes: Trachyte Plugs and Dikes (Pt); pumice deposits (Ptp) Pt consists of 642 m of dense, cream- colored trachyte present in plugs and dikes. Plugs are weathered and exposed as conspicuous eroded bulbous domes which are younger than the surrounding volcanic rocks. Ptp is pumice deposits (highly porous lava) associated with the Vatia Plug. Pt high; Ptp moderate Pumice rich areas should be avoided for waste treatment facility development and trails Rockfall and landslide hazards if undercut on a slope None Topographic hig

have been attractindigenous peoprituals and settlerstrategic position

Taputapu Volcanics 

(Po) 

Undifferentiated olivine basalts (2 to 15 m thick), dipping 5°- 10° from a rift zone parallel to the Samoan Ridge interlayered with thin- bedded cinder cone deposits, dikes, and thin vitric tuff beds. In some areas the volcanics are capped by thicker flows of porphyritic and nonporphyritic olivine poor basalts. Some red vitric tuff and cinders present locally. 

Moderate 

Abundance of unconsolidated or loosely consolidated cinder cone deposits may render unit unstable for foundations, avoid heavily fractured basalts 

Fractured basalt flows, and cinder cone deposits are unstable on slopes and pose mass wasting hazards 

None Volcanic glass m

provided tool ma

Pago Volcanic Series: Pago Volcanic Series (Ppe); andesite/basalt flows (Ppi); lithic/vitric tuff (Ppt) 

The series comprised of extra- caldera volcanics of the largest shield forming Tutuila: upper member - basaltic and andesitic flows with associated cones, dikes, and trachyte plugs; lower member predates caldera formation of primitive basalt flows, and associated cones and dikes (1 to 15 m wide). Pisolitic beds present locally. Ppt is confined to within the 10- km wide Pago Caldera and consists of interbedded lithic to vitric tuffs. Flows are as much as 150 m thick. 

Ppe moderate to high; Ppi and Ppt moderate 

Heterogeneity of unit may render unit unstable for foundations; avoid development if highly fractured and/or degraded 

Rockfall and landslide hazards if undercut on a slope, especially if highly fractured or heterogeneous 

None None docum

Alofau Volcanics (Pa) 

Thin- bedded pahoehoe basalt flows, some of which are primitive olivine- bearing associated with cones, vitric tuff beds, and numerous cross cutting dikes; unit forms a shield- shaped dome.; total thickness more than 962 m. 

Moderate 

Suitable for most development unless highly fractured or undercut on a slope. 

Rockfall and landslide hazards if highly fractured and undercut on a slope 

None None docum

Olomoana Volcanics 

(Pol) 

Areally restricted thin bedded primitive olivine basalt flows dipping 10° north; flows cap andesite deposits and associated cones, vitric tuff beds, and volcanic plugs. 

Locally trachyte lavas form domes. Some flows are 

interbedded with palagonitized vitric tuffs; thickness as much as 322 m. 

Moderate to high; low to moderate for loose pumice rich material. 

Unit suitable for most development unless highly fractured or undercut on a slope. 

Rockfall and landslide hazards if tuffs are weathered out below more resistant trachyte on slopes 

None Pumice may have 

used as tool mate

PLIOCENE – EARLIEST PLEISTOCENE 

Masefau Dike 

Complex (Pm) 

Narrow basaltic dikes (a few cm to 2 m wide) and associated intraformational slope talus breccia cut older thin, basaltic flows; dikes are vesicular and platy with local amygdaloidal textures. 

Moderate to high where dikes are coarsely crystalline 

Avoid areas where flows are underlain by slope talus breccia as instability may occur. 

Rockfall and landslide hazards if undercut on slope 

None None docum
Map Unit Properties Table – Tau Map Units 

Age Map Unit (Symbol) Unit Description Erosion Resistance Suitability for Development Hazards Paleontologic Resources Cultural Resources M

Spe

RECENT Calcareous Sediments (Qb) Modern beach deposits consisting of unconsolidated fragments of shells and other marine organisms. May be present as coquina or beachrock. Very low to low Development not recommended on this highly permeable, erodable, and fragile unit. Extreme beach erosion associated with this unit Coral and shell fragments Sediments may contain pre- historic artifacts from local populations of indigenous peoples CoRECENT Noncalcareous Sediments: alluvium (Qa); marshes (Qm) Units consist of noncalcareous alluvium, slope talus, and stream deposits. Marsh deposits are fine- grained with organic material and mud, located behind constructional beaches. Very low Development not recommended on this highly permeable, erodable, and fragile unit. Mass wasting hazards (landslides, rockfalls, slumps, slope creep, etc.) associated with this unit Plant and animal debris Sediments may contain pre- historic artifacts from local populations of indigenous peoples N

docu

RECENT Fiti’uta Formation: lava flows (Qfl); cinder cone (Qfc) Qfl consists of basalt and olivine basalt lava flows that form a bench at Fiti’uta. Qfc is associated with Qfl as cinder cone deposits interrupting the sequence of basaltic lava flows. Moderate 

Where abundant cinder cone deposits exist, unit is highly porous and might prove unsuitable for waste treatment facility development

Bench of basalt may be susceptible to rockfall and landslides if undercut or sitting atop weathered cinder cone deposits on a slope. 

Coral blocks 

interbedded in tuff 

layers, plant and 

animal casts 

possible 

Humans may have witnessed eruptions 

Dunit

flows 

dunite 

and co

QUATERNARY 

RECENT Faleãsao Formation (Qft) Unit contains undifferentiated tuff complex of palagonitized vitric- crystal lapilli tuff, volcanic breccia, and occasional, roughly horizontal lava flows from at least three cone centers at Faleãsao, To’a, and Fa’asamene Coves. Moderate 

Unit is highly porous and might prove unsuitable for waste treatment facility development, may be unstable for building foundations 

Ash, tuff, and cinder cone deposits are unstable on slopes and pose mass wasting hazards. Some units also contain sharp, glassy fragments 

Plant and animal 

casts possible 

Humans may have witnessed eruptions 

Volc

Luatele Formation: pahoehoe flows (Qlp); ponded lava flows (Qlc) 

Qlp contains thin pahoehoe lava flows of olivine basalt and picrite- basalt. Unit forms the Luatele shield on the northeastern portion of the island of Tau. Qlc contain 300 m thick ponded lavas from flows which partly fill local depressions in the caldera. 

Moderate 

Unit is suitable for most development unless highly fractured or undercut on a slope. 

Rockfall and landslide hazards associated with this unit if undercut on a slope 

None None documented O

phe

Tûnoa Formation: lava flows (Qte); cinder cone (Qtec); volcanic deposits (Qtc); cinder cone (Qtcc) 

Qtec contains cinder cone deposits from the Tunoa shield which is comprised of basalt and olivine basalt lava flows on the northwestern portion of Tau. Qtc is composed of volcanic deposits of red vitric and crystal ash, lapilli tuff, and associated olivine basalt lava flows, these units and associated cinder cone deposits partly fill the depression formed during the collapse of the volcanic shield. 

Moderate 

Heterogeneity of unit as well as abundance of unconsolidated or loosely consolidated cinder cone and lapilli deposits may render unit unstable for foundations 

Ash and cinder cone deposits are unstable on slopes and pose mass wasting hazards. Some units also contain sharp, glassy fragments 

None None documented 

O

phen

volca

lap

cry

Lata Formation: postcaldera (Qle); postcaldera cinder cone (Qlec); post- caldera lava flows (Qlel); inra- caldera (Qli); intra- caldera cinder cone (Qlic); intra- caldera lava flows (Qlil) 

Formation is comprised of olivine basalts, picrite- basalts, basalts, and hawaiites present as flows, cinder cone deposits, and other eruptions associated with the caldera collapse on Tau. 

Moderate 

Unit is suitable for most development unless highly fractured, rich in cinder cone deposits or undercut on a slope. 

Ash and cinder cone deposits as well as fractured basalt flows are unstable on slopes and pose mass wasting hazards 

None None documented O

phe

PLIOCENE 

Lata Formation: precaldera (Tle) 

The pre- caldera member consists of interlayered lava flows of olivine basalt, picritebasalt, basalt, feldspar- phyric basalt, and 

hawaiite. Occasional tuff beds are interlayered locally. 

Moderate 

Unit is suitable for most development unless highly fractured or undercut on a slope. 

Rockfall and landslide hazards associated with this unit if undercut on a slope 

None 

Phenocrysts may have provided trade materials 

Feld

o

phe
Map Unit Properties Table – Ofu and Olosega Map Units 

Age Map Unit (Symbol) Unit Description Erosion Resistance Suitability for Development Hazards Paleontologic Resources Cultural Resources Mineral Specimens 

RECENT Calcareous Sediments (Qb) Modern beach deposits consisting of unconsolidated fragments of shells and other marine organisms. May be present as coquina or beachrock. Very low to low Development not recommended on this highly permeable, erodable, and fragile unit. Extreme beach erosion associated with this unit Coral and shell fragments 

Sediments may contain pre- historic artifacts from local populations documenting 3,000 years of occupation of indigenous peoples 

Coquina 

RECENT Noncalcareous Sediments: alluvium (Qa); marshes (Qm) 

Units consist of noncalcareous alluvium, slope talus, and stream deposits. Marsh deposits are finegrained with organic material and mud, located behind constructional beaches. 

Very low 

Development not recommended on this highly permeable, erodable, and fragile unit. 

Mass wasting hazards (landslides, rockfalls, slumps, slope creep, etc.) associated with this unit 

Plant and animal 

debris 

Sediments may contain pre- historic artifacts from 3,000 years of uninterrupted occupation of indigenous peoples 

None 

documented 

RECENT - 

PLEISTOCENE Nu’u Formation: tuff (Qnt); lava flows (Qnl) Qnt is present as young palagonitized lapilli tuff forming Nu’utele and Nu’usilaelae islets. Qnl is present as flows of hawaiite and olivine basalt, unit also fills former eroded stream valleys on western Ofu. Moderate Unit is highly porous and might prove unsuitable for waste treatment facility development, may be unstable for building foundations Ash and cinder cone deposits are unstable on slopes and pose mass wasting hazards. Some units also contain sharp, glassy fragments Plant and animal casts possible Humans may have witnessed eruptions from 3,000 years Bp Ejected lapilli 

PLIOCENE Tuafanua Formation: A’ofa shield (Ttae); Sili shield (Tts); A’ofa caldera volcanic deposits (Ttai) 

Ttae is composed of coalescing shields (A’ofa) of pahoehoe and a’a olivine basalt, basalt, picrite- basalt, and hawaiite flows dipping 10- 20 degrees with interlayered ash, tuff, and breccia beds. Shields and the Fatuaga breccia cone are cut and intruded by numerous igneous dikes locally. Similarly, the Sili shield is comprised of coalescing layers of olivine basalt, picrite- basalt, basalt, and hawaiite flows with interbedded ash, tuff, and breccia layers. Ttai contains thick, ponded flows of olivine basalt, hawaiite, and ankaramite locally within the A’ofa Caldera deposits. 

Moderate 

Heterogeneity of unit as well as abundance of unconsolidated or loosely consolidated cinder cone and lapilli deposits may render unit unstable for foundations 

Mass wasting hazards (landslides, rockfalls, slumps, slope creep, etc.) associated with this unit especially where flows are present above more easily eroded ash, tuff, and breccia layers 

None None documented 

Olivine 

phenocrysts, 

ankaramite 

PLIOCENE 

Asaga Formation: Fatuaga Point breccia cone (Tafb); Fatuaga Point plug (Tafi); Toaga composite cone (Tat); Lemaga Point tuff cone (Tam); Samoi tuff cone (Tas); Taugã point cinder cone (Tac) 

Tafb is volcanic breccia in a cone formation. Tafi is a volcanic plug associated with the volcanic flows at Fatuaga Point. Tat contains various volcanic ejecta in a composite cone. Tam is comprised of interlayered tuffs in a cone at Lemaga Point. Tas is also a tuff cone located at the west end of Samoi. Tac is composed of a series of older cinder cones oriented along the regional rift zone. 

Tafi is moderate to high, Tafb, Tam, Tat, Tas, and Tac are moderate 

Volcanic ejecta may be too permeable for waste treatment facility development and unconsolidated cone deposits may be unstable for building foundations 

Ash and cinder cone deposits are unstable on slopes and pose mass wasting hazards. Some units also contain sharp, glassy fragments 

None None documented Volcanic ejecta 
 NPSA Geologic Resource Evaluation Report 27

Geologic History 

This section describes the rocks and unconsolidated deposits that appear on the digital 

geologic map of National Park of American Samoa, the environment in which those 

units were deposited, and the timing of geologic events that created the present 

landscape. 

In geologic terms, the rock units in National Park of 

American Samoa are young. Volcanism created most of 

these rocks beginning 23 million years ago (Ma)—

compared to more than 4 billion years of Earth’s history 

(fig. 13). Even so, the geologic setting and history of the 

Samoan archipelago is complex. Knowing how the 

islands formed is vital to understanding the current 

landscape and to predicting potential future geologic 

events. 

In the late Paleozoic, all continental landmasses joined to 

form one large supercontinent called Pangaea (fig. 13). 

During this time, mountain ranges formed by active 

continental collision. A huge water body, the 

Panthallassic Ocean, surrounded Pangaea. This water 

body had persisted in some form since the late 

Proterozoic Era (about 570 Ma), when it appeared after a 

previous supercontinent, Rodinia, broke apart (Fossil 

Museum 2002). 

The supercontinent Pangaea began to break apart early 

in the Triassic Period. Pangaea split into a northern 

continent called Laurasia and a southern continent 

called Gondwana. Further rifting divided Laurasia into 

the North American and Eurasian continents, whereas 

Gondwana eventually separated into the continents of 

South America, Africa, Australia, and Antarctica (Condie 

and Sloan 1998). Continental rifting opened new oceans 

such as the Atlantic Basin between the Americas and 

Europe and Africa. The Indian Ocean Basin formed 

between Africa, Antarctica, and Australia. Rifting 

continued throughout the Mesozoic (Fossil Museum 

2002). The oceanic crust of the Panthallassic Ocean Basin 

was also changing during this time. 

Approximately 125 Ma (early to middle Cretaceous), 

evidence suggests that a massive increase in volcanic 

activity in the western Pacific Ocean Basin produced 

large volcanic plateaus above several large mantle 

plumes. This activity was concurrent with a rapid 

increase in sea- floor- spreading rates. Rates increased 

by 50% to 100% and remained high until the late 

Cretaceous (Condie and Sloan 1998). This event 

correlates with rising sea level, global climate change 

(warming), and several extinction events in the middle 

Cretaceous. 

The present Pacific plate fills most of the North Pacific 

Ocean Basin, but this relation was not always so. The 

Pacific plate, on which the Samoa Archipelago is located, 

is relatively young in geologic terms. In the Cretaceous, 

several plates existed within the basin, likely derived 

from the partitioning of the Panthalassic Ocean upon the 

breakup of Pangaea. 

The Pacific plate started as a small central plate 

surrounded by the Aluk plate to the south, the Farallon 

plate to the east, and the Kula plate to the north (fig. 14) 

(Condie and Sloan 1998; University of California Santa 

Barbara 2006). Separated by mid- ocean ridges, the plates 

surrounding the Pacific plate began moving away from it. 

During the middle Tertiary, the surrounding plates were 

mostly assimilated into the earth’s crust by subduction. 

Oceanic crust is denser than continental crust, so in a 

collision between the two, the oceanic crust tends to sink 

(subduct) beneath the continental crust. This subduction 

generates heat as the plate sinks into the upper mantle. 

The oceanic crust melts and rises to the surface often 

forming a volcanic arc above the melting plate, in effect 

recycling the oceanic crust material. 

The Kula plate plunged beneath the northeast Asian 

subduction zone, possibly coincident with the opening of 

the Sea of Japan. A remnant of this plate remains as an 

inactive area of the Bering Sea. The Farallon plate 

subducted beneath North and South America resulting 

in the Sevier- Laramide orogenic event and the eventual 

formation of the San Andreas boundary. Remnants of 

this plate include the Juan de Fuca plate off the coast of 

the Cascade volcanic chain in Oregon and Washington, 

the Cocos plate in the eastern Pacific off the coast of 

central America, and the Nazca plate subducting beneath 

South America (Condie and Sloan 1998). During this 

time, the Pacific plate enlarged by seafloor spreading to 

nearly fill the north Pacific Basin. It now is moving slowly 

northward and westward—away from the East Pacific 

Rise spreading center and towards the subduction zones 

bordering the Australian- Indian plate, the Philippine 

plate, the Eurasian plate and the Aleutian Islands of the 

North American plate (fig. 15)(University of California 

Santa Barbara 2006). 

The Pacific plate covers a considerable portion of the 

earth’s crust, and is the largest tectonic plate on the 

planet today. Throughout the Pacific Basin are linear 

chains of volcanic islands and seamounts (submerged 

volcanoes). Many of these chains progress in age from 

one end to the other (fig. 7). As does The linear trend of 

the Samoa Islands records the movement of the Pacific 

plate over a stationary hotspot in the upper mantle. 

Other such spots across the basin are: the HawaiianEmperor seamount chain, the Caroline, Marquesas, 

Society, Pitcairn, Austral, and Easter hotspots (fig.16) 

(Condie and Sloan 1998). Several of these hotspots are 
 NPS Geologic Resources Division 28 

part of a feature referred to as the South Pacific isotopic 

and thermal mantle anomaly. This anomaly, an area of 

possible mantle upwelling, supports at least eight major 

hotspots. The western terminus of the anomaly is just 

west of Samoa and the eastern edge is near the Easter 

Island hotspot (Koppers et al. 1998). Other criteria limit 

classic hotspots to those active areas displaying extensive 

oceanic basaltic plateaus and long- lived volcanic activity. 

These criteria exclude the Samoan hotspot as well as 

limit the number of spots considered to be deep- mantle 

plumes in the Pacific to three (Clouard and Bonneville 

2001). 

Hotspots form in response to rising plumes of very high 

temperature material from the lower mantle, just above 

the core- mantle interface. These plumes are thought to 

form as a result of localized thermal disturbances in the 

molten core of the earth. A portion of the core cools, 

causing the overlying mantle to heat up and rise owing to 

its decreased density. Once the plumes reach the upper 

levels of the mantle (~200 km (125 mi) deep), the lower 

pressure causes the material to melt. If this molten 

material (magma) finds a way to the outer crust, it may 

erupt and produce a series of volcanoes that decrease in 

age toward the plume (hotspot) (Condie and Sloan 1998). 

The Samoa Islands are part of a volcanic archipelago that 

is located along the crest of the Samoa Ridge overlying 

the Samoa hotspot. This ridge extends more than 485 km 

(300 mi) from the Rose Atoll, westward towards Savai’i 

and beyond (fig. 1) (Wingert and Pereira 1981; Hart et al. 

2004). The volcanic- lineament model at Samoa is not 

necessarily consistent with a strictly plume- driven 

hotspot model. On the westernmost and oldest (~5 

million years old) Samoan island of Savai’i, voluminous 

young volcanic rock has sparked debate over the validity 

of the plume- driven hotspot model to the Samoan 

Archipelago (Hart et al. 2004). Upolu, the second oldest 

island, is dated at 2.8–1.7 Ma with little to no rejuvenated 

volcanic activity (McDougall 1985). 

Isotopic dating of dredge basalts from four seamounts 

and submarine banks from along the entire trend of the 

Samoan lineament (some 1,300 km (800 mi) west of the 

island of Savai’i) indicates that Samoan volcanism is 

older to the west (fig. 17). The oldest rocks are more than 

23 Ma at Alexa Bank, some 1,690 km (1,050 mi) west of 

Vailulu’u, the current hotspot location (Hart et al. 2004). 

The next- oldest date is 11.1 Ma at Combe seamount 

(some 940 km (584 mi) west of Vailulu’u). These and 

other age dates are in keeping with the velocity of the 

Pacific plate movement listed below (Hart et al. 2004). 

Rejuvenated volcanism on Savai’i and other areas, such 

as Lalla Rookh, may be explained by the unique tectonic 

setting of the Samoan archipelago (Hart et al. 2004). 

Samoa is close to the junction of the Pacific plate and the 

Australian- Indian plate. Currently at Samoa, the plate is 

moving N. 63° W. at 71 mm/year (2.8 in./year). The 

Australian plate, New Caledonia, and the Fiji platform 

are converging in an approximate N. 30° E. direction 

(Hart et al. 2004). At this point, the Pacific plate is tearing 

laterally in a pattern propagating eastward at the 

northern end of the Tonga arc (fig. 18) (Hart et al. 2004). 

Convergence of lithospheric plates produces a suite of 

geologic features such as volcanic arcs, fore- arc basins, 

deep- sea trenches, high relief (>600 m [>1,900 ft]) 

associated normal and reverse faults, and outer trench 

swells (also called oceanic rises) (Coulbourn et al. 1989; 

Hill and Tiffin 1993). 

The nature of this boundary produces earthquakes 

associated with two structural features: (1) the partial 

subduction of the Pacific plate along the Tonga trench 

(figs. 17, 18) some 160 km (100 mi) south of Samoa and (2) 

the transform fault boundary between the northern edge 

of the Australian- Indian plate and the Pacific plate 

(bounding the Tonga trench (figs. 17, 19)) (Hart et al. 

2004; Craig 2006). The crustal tearing at this junction 

may also be responsible for some renewed volcanic 

activity in the area, including that on Savai’i, and the 

formation of the Vitiaz lineament (figs. 17, 19). The 

lineament is a tectonically inactive structure composed 

of a series of west- northwest to east- southeast and eastnortheast to west- southwest trending segments and a 

belt of seamounts and ridges in front of large volcanic 

massifs (Pelletier and Auzende 1995; Hart et al. 2004). 

Renewed volcanism may relate to crustal interactions 

between the Pacific plate and the eastward migration of 

the Tonga trench corner (fig. 19) (Keating 1992). That 

trench corner migrates by >190 mm/year (7.4 in./year). 

Hart et al. (2004) suggest that the Vitiaz lineament is the 

trace of the eastward motion of this tear. Tearing may 

facilitate volcanic rejuvenation unrelated to the Samoan 

hotspot. If this is so, resource management at National 

Park of American Samoa can assume future volcanism 

may occur in areas distant from the present hotspot 

location. 

Volcanic activity on the oldest island of National Park of 

American Samoa, Tutuila, lasted a relatively short time, 

some 0.6 million years (my), during the early Pleistocene 

(fig. 13). The major shield volcano of Pago was built 

between 1.54 and 1.28 Ma (Walker and Eyre 1995). Lavas 

forming that volcano erupted at the same time as Alofau 

and Olomoana volcanic rocks. The collapse of Pago’s 

caldera occurred approximately 1.27 Ma. Minor caldera 

filling and emplacement of trachyte lava bodies ended 

about 1.03 Ma (McDougall 1985; Walker and Eyre 1995). 

Tutuila’s long axis is oriented east- northeast, in contrast 

with the west- northwest orientation of rift zones and 

island elongation elsewhere in Samoa (including the 

islands of Manu’a). Feeder dikes present on Tutuila also 

show this east- northeast orientation. This difference is 

enigmatic and may suggest that the crustal offset 

associated with concealed faults (related to Samoa’s 

location north of the Tonga trench and the Vitiaz 

lineament) has shifted volcanism during the formation of 

Tutuila (Walker and Eyre 1995). 

The Manu’a Islands lie approximately 115 km (71 mi) east 

of Tutuila. These islands were formed by later volcanism 
 NPSA Geologic Resource Evaluation Report 29

associated with the migration of the Cretaceous- age 

crust of the Pacific plate over the stationary Samoa 

hotspot. Some dates report volcanism on Ofu and 

Olesega at less than 0.3 Ma and less than 0.1 Ma for the 

island of Ta’u. From these dates the average historic rate 

of migration of the volcanic center from Tutuila to 

Manu’a is about 100 mm/year (4 in./year) (McDougall 

1985). 

Ta’u is the easternmost Samoan island; however, 

volcanic activity associated with the stationary hotspot 

beneath the mobile Pacific plate has not stopped along 

the Samoan chain. A 3,444 - meter (11,300 ft) high 

submerged, active volcano is located 43 km (27 mi) east 

of Ta’u. This feature (named Vailulu’u) still lies some 610 

m (2,000 ft) below the ocean’s surface. It is isolated from 

Ta’u and has an enclosed crater. If volcanism persists at 

Vailulu’u, the volcano may rise above the surface of the 

ocean in the next 1,000 years (Lippsett 2001). 

Volcanic activity is still a very real possibility at National 

Park of American Samoa. Tutuila Island was formed 

during a rapid series of eruptions only 1.5 Ma. In the 

Manu’a islands, subaqueous eruptions and earthquakes 

spewed clouds of pumice, volcanic ash, and steam for 

several months from September to November in 1866 

(Keating 1992; Craig 2006). 

The currently active submarine volcano, Vailulu’u, is 

building layers of basaltic lava and venting hydrothermal, 

mineral- laden water towards the ocean’s surface and 

may in the future become the next Samoan island (Hart 

et al. 2004; Staudigel et al. 2004). As recently as 1905, lava 

flows from several large eruptions destroyed villages in 

Savai’i, Samoa (Craig 2006). 

During periods of volcanic quiescence, the basalts, tuffs, 

breccias, cinder cones, and ash deposits are exposed to 

intense weathering and erosion in the wet Samoan 

climate. Landforms thus produced on the islands include 

amphitheatre valleys, steep- sided stream valleys, 

dissected volcanic plateaus, alternating valley and ridge 

topography, small- scale gullies, isolated plateau 

remnants, talus slope deposits, levee deposits, sea cliffs 

and benches (Ollier 1988). Ocean waves continuously 

pound the shorelines, carrying away sands and gravels 

eroded by the islands’ rivers. Coral reefs rim the islands 

and contribute carbonate sediments to the islands’ 

beaches. 

A volcano’s own mass causes it to sink and subside into 

the oceanic crust of the Pacific plate (fig. 8) (Ollier 1988; 

Craig 2006). Samoan volcanoes are subsiding at various 

rates (average at Tutuila is ~0.03 mm/year [0.001 

inches/year]) (Dickinson 2001). Submarine mass wasting, 

landslides, and debris flows carry material from the 

shoreline, down the slopes of the islands to spread onto 

the deep sea floor. These mass movements are an 

important influence on the lowering of the Samoan 

Islands (fig. 8) and the development of the overall ocean 

island volcanic complex (Keating et al. 2000). 
 NPS Geologic Resources Division 30 

Figure 13. Geologic time scale; adapted from U.S. Geological Survey (http://www.usgs.gov) and Condie and Sloan (1998). Red lines indicate 

major unconformities between eras. Included are major events in life history and global tectonic events that affected the South Pacific 

region. Absolute ages in millions of years. 
 NPSA Geologic Resource Evaluation Report 31

Figure 14. Generalized location of plates in the Pacific basin during the middle Cretaceous. Graphic is by Trista L. Thornberry-Ehrlich 

(Colorado State University). 
 NPS Geologic Resources Division 32 

Figure 15. Geographic map of the current tectonic plates. Graphic is from www.wikipedia.org (accessed August 20, 2007). 
 NPSA Geologic Resource Evaluation Report 33

Figure 16. Location of hotspots across the South Pacific. Graphic by Trista L. Thornberry-Ehrlich (Colorado State University) adapted from 

figure 2 in Clouard and Bonneville (2001). 
 NPS Geologic Resources Division 34 

Figure 17. Bathymetry of the Samoan seamounts (some named) relative to the subaerial islands including Ta’u, Tutuila, and Upolu. Note the 

location of the Tonga trench, Vailulu’u, Lalla Rookh, and the Vitiaz lineament, a tectonic structure mentioned in the Geologic History section 

of this report. Graphic is from Hart et al. (2004). 
 NPSA Geologic Resource Evaluation Report 35

Figure 18. Samoa Islands atop the Pacific plate in the vicinity of the Tonga trench. Note the location of the crustal tear just south of the 

Samoan Islands. Not to scale. Graphic by Trista L. Thornberry-Ehrlich (Colorado State University) adapted from figure 3 in Hawkins (1987). 
 NPS Geologic Resources Division 36 

Figure 19. Evolution of the positions of the Tonga trench, seamounts, and Vitiaz lineament relative to Fiji and the Samoan Island archipelago 

from ~4 Ma to present. Diagram is only roughly to scale. Graphic is by Trista L. Thornberry-Ehrlich (Colorado State University), based on 

information from Hart et al. (2004), figure 9. 
 NPSA Geologic Resource Evaluation Report 37

Glossary 

This glossary contains brief definitions of technical geologic terms used in this report. Not all 

geologic terms used are referenced. For more detailed definitions or to find terms not listed 

here please visit http://wrgis.wr.usgs.gov/docs/parks/misc/glossarya.html. 

alluvium. Stream- deposited sediment that is generally 

rounded, sorted, and stratified. 

aquifer. Rock or sediment that are sufficiently porous, 

permeable, and saturated to be useful as a source of 

water. 

ash (volcanic). Fine pyroclastic material ejected from a 

volcano (also see tuff). 

basin (structural). A doubly- plunging syncline in which 

rocks dip inward from all sides (also see dome). 

basin (sedimentary). Any depression, from continental 

to local scales, into which sediments are deposited. 

beach. A gently sloping shoreline covered with sediment, 

often formed by action of waves and tides. 

bedrock geology. The geology of underlying solid rock 

as it would appear with the sediment, soil, and 

vegetative cover stripped away. 

breccia (volcanic). A coarse- grained, generally unsorted 

volcanic rock consisting of partially welded angular 

fragments of ejecta such as tuff or ash. 

calcareous. A rock or sediment containing calcium 

carbonate. 

caldera. A large bowl- or cone- shaped summit 

depression in a volcano formed by explosion or 

collapse 

clastic. Rock or sediment made of fragments or preexisting rocks. 

conglomerate. A coarse- grained sedimentary rock with 

clasts larger than 2 mm in a fine- grained matrix. 

continental crust. The type of crustal rocks underlying 

the continents and continental shelves; having a 

thickness of 25- 60 km (16- 37 mi) and a density of 

approximately 2.7 grams per cubic centimeter. 

continental drift. The concept that continents have 

shifted in position over Earth (see and use ‘plate 

tectonics’). 

convergent margin. A plate boundary where two 

tectonic plates are moving together (such a boundary 

will be a zone of subduction or obduction). 

cross section. A graphical interpretation of geology, 

structure, and/or stratigraphy in the third (vertical) 

dimension based on mapped and measured geological 

extents and attitudes depicted in an oriented vertical 

plane. 

crust. The outermost compositional shell of Earth, 10- 40 

km (6- 25 mi) thick, consisting predominantly of 

relatively low- density silicate minerals (also see 

oceanic crust and continental crust). 

debris flow. A rapid and often sudden flow or slide of 

rock and soil material involving a wide range of types 

and sizes. 

deformation. A general term for the process of faulting, 

folding, shearing, extension, or compression of rocks 

as a result of various Earth forces. 

dike. A tabular, discordant igneous intrusion. 

divergent boundary. A tectonic plate boundary where 

the plates are moving apart (such a boundary will form 

a spreading ridge or continental rift zone). 

fault. A subplanar break in rock along which relative 

movement occurs between the two sides. 

fracture. Irregular surface of breakage in a mineral; also 

any break in a rock, such as a crack, joint, or fault 

hawaiite. A type of volcanic rock with a potash:soda 

value of less than 1:2, a moderate to high color index, 

and a modal composition that includes essential 

andesine and accessory olivine. 

hot spot. A volcanic center, 100 to 200 km across and 

persistent for at least a few tens of millions of years, 

that is thought to be the surface expression of a rising 

plume of hot mantle material. 

igneous. Refers to a rock or mineral that originated from 

molten material; one of the three main classes or rocks: 

igneous, metamorphic, and sedimentary. 

intrusion. A body of igneous rock that invades older 

rock. The invading rock may be a plastic solid or 

magma that pushes its way into the older rock. 

island arc. A line or arc of volcanic islands formed over 

and parallel to a subduction zone. 

landslide. Any process or landform resulting from rapid 

mass movement under relatively dry conditions. 

lapilli. Pyroclastics in the general size range of 2 to 64 

mm. 

lava. Magma that has been extruded out onto Earth’s 

surface, both molten and solidified. 

lineament. Any relatively straight surface feature that can 

be identified via observation, mapping, or remote 

sensing, often representing tectonic features. 

lithology. The description of a rock or rock unit, 

especially the texture, composition, and structure of 

sedimentary rocks. 

lithosphere. The relatively rigid outmost shell of Earth’s 

structure, 50 to 100 km (31 to 62 mi) thick, that 

encompasses the crust and uppermost mantle. 

magma. Molten rock generated within Earth that is the 

parent of igneous rocks. 

mantle. The zone of Earth’s interior between crust and 

core. 

normal fault. A dip- slip fault in which the hanging wall 

moves down relative to the footwall. 

oceanic crust. Earth’s crust formed at spreading ridges 

that underlies the ocean basins. Oceanic crust is 6- 7 

km (3- 4 mi) thick and generally of basaltic 

composition. 

outer trench swell. A subtle ridge on the seafloor near an 

oceanic trench formed where a subducting plate 

begins to flex and fault into the trench. 
 NPS Geologic Resources Division 38 

Pangaea. A theoretical, single supercontinent that 

existed during the Permian and Triassic Periods (also 

see Laurasia and Gondwana). 

permeability. A measure of the ease or rate that fluids 

move through rocks or sediments. 

picrite. Olivine rich basalt. 

plate tectonics. The theory that the lithosphere is broken 

up into a series of rigid plates that move over Earth’s 

surface above a more fluid aesthenosphere. 

plume. A persistent pipelike body of hot material moving 

upward from the earth’s mantle into the crust. 

pluton. A body of intrusive igneous rock. 

plutonic. Describes igneous rock intruded and 

crystallized at some depth in Earth. 

porphyritic. An igneous rock characteristic wherein the 

rock contains conspicuously large crystals in a finegrained groundmass. 

pyroclastic. Pertaining to clastic rock material formed by 

volcanic explosion or aerial expulsion from a volcanic 

vent; also pertaining to rock texture of explosive 

origin. 

recharge. Infiltration processes that replenish 

groundwater. 

reverse fault. A contractional, high angle (>45o), dip- slip 

fault in which the hanging wall moves up relative to the 

footwall (also see thrust fault). 

slump. A generally large, coherent mass movement with 

a concave- up failure surface and subsequent 

backward rotation relative to the slope. 

spring. A site where water flows out at the surface due to 

the water table intersecting the ground surface. 

strike. The compass direction of the line of intersection 

that an inclined surface makes with a horizontal plane. 

strike-slip fault. A fault with measurable offset where the 

relative movement is parallel to the strike of the fault. 

subduction zone. A convergent plate boundary where 

oceanic lithosphere descends beneath a continental or 

oceanic plate and is carried down into the mantle. 

subsidence. The gradual sinking or depression of part of 

Earth’s surface. 

talus. Rock fragments, usually coarse and angular, lying 

at the base of a cliff or steep slope from which they 

have been derived. 

tectonics. The geological study of the broad structural 

architecture and deformational processes of the 

lithosphere and aesthenosphere (also see structural 

geology). 

thrust fault. A contractional, dip- slip fault with a 

shallowly dipping fault surface (<45o

) where the 

hanging wall moves up and over relative to the 

footwall. 

tombolo. A bar or barrier that connects an island with the 

mainland or with another island. 

topography. The general morphology of Earth’s surface 

including relief and location of natural and 

anthropogenic features. 

trace. The exposed intersection of a fault or lineation 

with Earth’s surface. 

trachyte. A group of fine- grained, generally porphyritic, 

extrusive rocks having alkali feldspar and minor mafic 

minerals. 

transgression. Landward migration of the sea due to a 

relative rise in sea level. 

trend. The direction or azimuth of elongation or a linear 

geological feature. 

tuff. Generally fine- grained, igneous rock formed of 

consolidated volcanic ash. 

vent. An opening at the surface of Earth where volcanic 

materials emerge. 

volcanic. Related to volcanoes; describes igneous rock 

crystallized at or near Earth’s surface, such as a lava. 

water table. The upper surface of the saturated (phreatic) 

zone. 

weathering. The set of physical, chemical, and biological 

processes by which rock is broken down in place. 
 NPSA Geologic Resource Evaluation Report 39

References 

This section lists references cited in this report as well as a general bibliography that may 

be of use to resource managers. A more complete geologic bibliography is available from 

the National Park Service Geologic Resources Division. 

Anonymous. 1998. South Pacific Islands. Mining Annual 

Review 1998: 125. 

Addison, David J. 2002. Traditional Samoan use of the 

Tutuila Island Landscape. In 83rd annual meeting of 

the Pacific Division, AAAS; program with abstracts ed. 

Christianson, Roger G. Proceedings of the Pacific 

Division, American Association for the Advancement 

of Science 21 (1): 29. 

Arthur, Riley A. 2002. Ancient quarries of Tutuila. In 

83rd annual meeting of the Pacific Division, AAAS; 

program with abstracts ed. Christianson, Roger G. 

Proceedings of the Pacific Division, American 

Association for the Advancement of Science 21 (1): 29. 

Aston, James. 1997. South Pacific regional environment 

programme; overview of climate change activities 

within the South Pacific regional environment 

programme. In International workshop; Planning for 

climate change through integrated coastal 

management; Volume 2, Country and regional reports, 

eds. Morrison, Steve, Katie Ries: 88- 89. 

Bruce, J.G. 1988. Soil moisture and soil temperature 

regimes. In Soil taxonomy and fertility in the South 

Pacific; proceedings of the XVth international forum 

on Soil taxonomy and agrotechnology transfer, eds. 

Asghar, M., T.J. Davidson, R.J. Morrison, R. J.: 37- 45. 

Chase, T. E., W.R. Normark, P. Wilde, Christina E. 

Gutmacher, B.A. Seekins, K.E. Lund, S.C. Vath, S. C. 

1998. Marine topography of offshore Tutuila Island, 

Samoa Islands. Map and Chart Series Geological 

Society of America MCH068 

Clouard, Valerie; Alain Bonneville. 2001. How many 

Pacific hotspots are fed by deep- mantle plumes? 

Geology (Boulder) 29 (8): 695- 698. 

Condie, K.C., Sloan, R.E. 1998. Origin and Evolution of 

the Earth, Principles of Historical Geology. PrenticeHall, Inc. 

Corporate United Nations. 1995. Guidebook to water 

resources, use and management in Asia and the Pacific; 

volume one, Water resources and water use. 

Corporate United Nations, Economic and Social 

Commission for Asia and the Pacific, Bangkok, 

Thailand Water Resources Series 74. 

Coulbourn, William T., Peter J. Hill, Douglas D. 

Bergersen. 1989. Machias Seamount, western Samoa; 

sediment remobilization, tectonic dismemberment and 

subduction of a guyot. Geo- Marine Letters 9 (3): 119-

125. 

Craig, P. 2006. Natural History Guide to American 

Samoa. National Park of American Samoa. 

http://www.nps.gov/npsa/book/index.htm (accessed 

April 29, 2006). 

Davies, H., P. Bani, P. Black, L. Smith, E. Garaebiti, P. 

Rodda, P. 2005. Oceania (including Fiji, PNG and 

Solomons). In Encyclopedia of geology; Volume 4., 

eds. Selley, Richard C., L. Robin M. Cocks, Ian R. 

Plimer. Elsevier Academic Press: Oxford, United 

Kingdom: 109- 122. 

Dickinson, William R. 2001. Paleoshoreline record of 

relative Holocene sea levels on Pacific islands. EarthScience Reviews 55 (3- 4): 191- 234. 

Fossil Museum. 2002. Geological History. 

http://www.fossilmuseum.net/GeologicalHistory.htm 

(accessed May 3, 2006). 

Goodwin, Ian D., Eric E. Grossman. 2003. Middle to late 

Holocene coastal evolution along the south coast of 

Upolu Island, Samoa. Marine Geology 202 (1- 2): 1- 16. 

Grossman, E.E., C.H. Fletcher, III, B.M. Richmond. 1998. 

The Holocene sea- level highstand in the equatorial 

Pacific: analysis of the insular paleosea- level database. 

Coral Reefs 17: 309- 327. 

Hart, S.R., M. Coetzee, R.K. Workman, J. Blusztajn, 

K.T.M. Johnson, J.M. Sinton, B. Steinberger, J.W. 

Hawkins. 2004. Genesis of the Western Samoa 

seamount province: age, geochemical fingerprint and 

tectonics. Earth and Planetary Science Letters 227: 37-

56. 

Hawkins, A. Brian. 1987. Samoa; significance of its 

geological setting. In Papers presented at the first 

Sino- British geological conference on geotechnical 

engineering and hazard assessment in neotectonic 

terrains, eds. Hung, Ju- Jiang, Louis S. Teng, Louis S.. 

Chung Kuo Ti Chih Hsueh Hui Chuan Kan = Memoir 

of the Geological Society of China 9: 159- 174 
 NPS Geologic Resources Division 40 

Hekinian, R. J.L. Cheminee, J. Dubois, P. Stoffers, S. 

Scott, C. Guivel, D. Garbe- Schoenberg, C. Devey, B. 

Bourdon, K. Lackschewitz, G. McMurtry, E. Le 

Drezen. 2003. The Pitcairn Hotspot in the South 

Pacific; distribution and composition of submarine 

volcanic sequences. Journal of Volcanology and 

Geothermal Research 121 (3- 4): 219- 245 

Hill, B. R. 1996. Western Pacific Islands wetland 

resources. In National water summary on wetland 

resources, compilers Fretwell, Judy D., John S. 

Williams, Phillip J. Redman, Phillip. U. S. Geological 

Survey Water- Supply Paper W2425: 405- 410. 

Hill, P. J., D.L. Tiffin. 1993. Geology, sediment patterns, 

and widespread deformation on the sea floor off 

Western Samoa revealed by wide- swath imagery. In 

Southwest Pacific, ed. Tiffin, D. L. Geo- Marine 

Letters 13 (2): 116- 125. 

Izuka, Scot K. 1999. Hydrogeologic interpretations from 

available ground- water data, Tutuila, American 

Samoa. Water- Resources Investigations - U. S. 

Geological Survey: WRI 99- 4064 

Izuka, Scot K. 1999. Summary of ground- water data for 

Tutuila and Aunuu, American Samoa, for October 1987 

through September 1997. Open- File Report - U. S. 

Geological Survey: OF 99- 0252 

Keating, Barbara H. 1992. Geology of the Samoan Islands. 

In Geology and offshore mineral resources of the 

central Pacific basin, eds. Keating, B.H., B.R. Bolton, 

127- 178. New York: Springer. 

Keating, Barbara H., Charles E. Helsley, Irina 

Karogodina. 2000. Sonar studies of submarine mass 

wasting and volcanic structures off Savaii Island, 

Samoa. In Landslides and tsunamis, eds. Keating, 

Barbara H., Christopher F. Waythomas, Alastair G. 

Dawson. Pure and Applied Geophysics 157 (6- 8): 1285-

1313. 

Keating, Barbara H., Irina Karogodina. 1990. Mass 

wasting of the slopes of Savaii, western Samoa. Eos, 

Transactions, American Geophysical Union 71 (43): 

1623. 

Kirda, Mike. 1998. Reef Zones. 

http://www.reefs.org/library/talklog/m_kirda_062198.

html (accessed May 5, 2006). 

Konter, J. G., H. Staudigel, S.R. Hart, P.M. Shearer. 2004. 

Seafloor seismic monitoring of an active submarine 

volcano; local seismicity at Vailulu'u Seamount, 

Samoa. Geochemistry, Geophysics, Geosystems - G 

super 3 5 (6). 

Konter, Jasper, Huber Staudigel, Stan Hart. 2002. Seismic 

activity at Vailulu'u, Samoa's youngest volcano. Eos, 

Transactions, American Geophysical Union 83 (47): 

1048. 

Koppers, Anthony A. P., Hubert Staudigel, Jan R. 

Wijbrans, Malcolm S. Pringle. 1998. The Magellan 

Seamount Trail; implications for Cretaceous hotspot 

volcanism and absolute Pacific Plate motion. Earth and 

Planetary Science Letters 163 (1- 4): 53- 68. 

Leavasa, Malaefatu. 2004. Seismic observation in Samoa 

2003. Bulletin of the International Institute of 

Seismology and Earthquake Engineering 38: 83- 89. 

Lewis, K.B. 1989. Lagoonal sand and gravel surveys, 

Western Samoa. Geological Society of New Zealand 

Miscellaneous Publication 43: 59. 

Lippsett, Laurence. 2001. Voyage to Vailulu'u. Woods 

Hole Currents 9 (1): 8- 15. 

Lupton, John E., Douglas G. Pyle, William J. Jenkins, 

Ronald R. Greene, Leigh J. Evans. 2004, Evidence for 

an extensive hydrothermal plume in the Tonga- Fiji 

region of the South Pacific. Geochemistry, Geophysics, 

Geosystems - G super 5 (1). 

McCue, Kevin. 1999. Seismic hazard mapping in 

Australia, the Southwest Pacific and Southeast Asia. In 

The Global Seismic Hazard Assessment Program 

(GSHAP) 1992- 1999, ed. Giardini, Domenico. Annali 

di Geofisica 42 (6): 1191- 1198. 

McDougall, Ian. 1985. Age and evolution of the volcanoes 

of Tutuila, American Samoa. Pacific Science 39 (4): 311-

320. 

Millen, David Walter. 2000. Tectonics of the 

northernmost Tonga subduction zone and a 

comparative study of the tectonics of subduction zone 

terminations. Doctoral dissertation: Indiana University 

at Bloomington, Bloomington, IN. 

National Park Service (NPS). 2007. National Park of 

American Samoa. www.nps.gov/npsa (accessed May 5, 

2006). 

NOAA. 2007. NOAA’s Coral Reef Ecosystem Research 

Plan for Fiscal Years 2007 to 2011. National Oceanic 

and Atmospheric Administration 

http://coris.noaa.gov/activities/coral_research_plan/ 

(accessed March 17, 2007). 

Ollier, C. D. 1998. A national park survey in western 

Samoa, terrain classification on tropical volcanoes. 

Zeitschrift fuer Geomorphologie. Supplementband 68: 

103- 124 

Olson, Harry J. 1987. Geology and geothermal potential 

of the islands of Kermadec, Tonga, Samoa, Fiji, 

Vanuatu and Santa Cruz in the Southwest Pacific 

Ocean. Transactions - Geothermal Resources Council 

11: 285- 291. 
 NPSA Geologic Resource Evaluation Report 41

Pelletier, Bernard, Jean- Marie Auzende. 1995. Geometry 

and structure of the Vitiaz trench lineament (SW 

Pacific). Marine Geophysical Researches 18 (2- 4): 305-

335. 

Pendleton, Elizabeth A., Robert E. Thieler, Jeffress S. 

Williams, S. 2005. Coastal vulnerability assessment of 

National Park of American Samoa (NPSA) to sea- level 

rise. Open- File Report - U. S. Geological Survey: OF 

2005- 1055. 

Peterson, Frank L. 1993. Hydrogeology of volcanic 

oceanic islands. In Selected papers on Environmental 

hydrogeology from the 29th international geological 

congress (I.G.C.), ed. Sakura, Yasuo Hydrogeology, 

Selected Papers 4: 163- 171. 

Piniak, Greg, Charles Birkeland, Ginger Garrison. 2004. 

Persistence of Coral Reefs Under Extreme 

Environmental Stress in American Samoa. U.S. 

Geological Survey, Soundwaves, 

http://soundwaves.usgs.gov/2004/10/SW200310-

225dpi.pdf (accessed May 5, 2006). 

Richmond, Bruce M. 1992a. Coastal geology of Upolu, 

western Samoa. In Geology and offshore mineral 

resources of the central Pacific basin, eds. Keating, 

B.H., B.R. Bolton, 101- 125. New York: Springer. 

Richmond, Bruce Mark. 1992b. Holocene 

geomorphology and reef history of islands in the South 

and Central Pacific. Doctoral dissertation: University 

of California at Santa Cruz: Santa Cruz, CA. 

Richmond, B.M. 1995. Tutuila, American Samoa coastal 

resource inventory sites: reconnaissance shoreline 

geology. U.S. Geological Survey, Open- File Report: 

OF 95- 512. 

Settle, Dorothy M., Clair C. Patterson. 1991. Eolian inputs 

of lead to the South Pacific via rain and dry deposition 

from industrial and natural sources. In Stable isotope 

geochemistry; a tribute to Samuel Epstein, eds. Taylor, 

Hugh P., Jr. James R. O'Neil, James R., Isaac R. Kaplan. 

Special Publication - Geochemical Society 3: 285- 294. 

Shorten, Graham G. 2001. All- hazards risk assessment in 

Pacific cities. In Geoinformation and GIS application 

for the urban areas of East and Southeast Asia; 

proceedings, eds. Wakita, Koji, Yusaku Taguchi. 

Technical Bulletin - Coordinating Committee for 

Coastal and Offshore Geoscience Programmes in East 

and Southeast Asia 30: 157- 163 

Smith, Lance W. 2004. Influence of water motion on 

resistance of corals to high temperatures: Evidence 

from a field transplant experiment. Hawai’I 

Cooperative Fishery Research Unit, Zoology 

Department, University of Hawai’I at Manoa, HI. 

http://www.hawaii.edu/coral/Okinawa%20Proceeding

s%20manuscript.pdf (accessed May 5, 2006). 

Staudigel, H., S.R. Hart, A.A.P. Koppers, C. Constable, R. 

Workman, M. Kurz, E.T. Baker. 2004. Hydrothermal 

venting at Vailulu'u Seamount; the smoking end of the 

Samoan chain. Geochemistry, Geophysics, 

Geosystems - G super 5 (2). 

Tavoi, Samuelu, Sitivi K. Wright, Jr., Talia Lameko. 2001. 

Seismic observation in Samoa. Bulletin of the 

International Institute of Seismology and Earthquake 

Engineering 35: 181- 185. 

Turner, S. Hawkesworth, C. 1998. Using geochemistry to 

map mantle flow beneath the Lau Basin. Geology 26 

(11): 1019- 1022. 

University of California Santa Barbara. 2006. Pacific 

Hemisphere Plate, 80 Ma to Present. University of 

California Santa Barbara, Department of Geological 

Sciences, Educational Multimedia Visualization 

Center. http://emvc.geol.ucsb.edu/index.htm 

(accessed May 2, 2006). 

Walker, George P. L., Paul R. Eyre. 1995. Dike complexes 

in American Samoa. Journal of Volcanology and 

Geothermal Research 69 (3- 4): 241- 254. 

Westlake, S. 1998. Samoa coastal hazard mapping. In 

Proceedings of the natural hazards management 

workshop, compilers Johnston, D. M., P.A. Kingsbury. 

Institute of Geological & Nuclear Sciences 

Information Series: Lower Hutt, New Zealand. 

Wheeler, Christopher Waldo. 2000. Stratigraphy, 

petrology, and geochemistry of a mid- oceanic 

carbonate platform; Niue Island, South Pacific. 

Doctoral dissertation: Louisiana State University, 

Baton Rouge, LA. 

Wingert, E.A., J.M. Pereira. 1981. A coastal zone 

management atlas of American Samoa. University of 

Hawaii Cartography Laboratory, GSA Bulletin 55: 13-

17. 

Wong, Michael F. 1995. Analysis of streamflow 

characteristics for streams on the island of Tutuila, 

American Samoa. Water- Resources Investigations - 

U. S. Geological Survey WRI 95- 4185. 

Wright, Dawn J., Brian T. Donahue, David F. Naar. 2002. 

Seafloor mapping and GIS coordination at America's 

remotest national marine sanctuary (American 

Samoa). In Undersea with GIS, ed. Wright, Dawn J. 

Environmental Systems Research Institute Press: 

Redlands, CA: 33- 63. 

 
 NPS Geologic Resources Division 42 

Appendix A: Geologic Map Graphic 

The following page is a preview or snapshot of the geologic map for National Park of 

American Samoa. For a poster- size PDF of this map or for digital geologic map data, 

please see the included CD or visit the Geologic Resource Evaluation publications Web 

page (http://www2.nature.nps.gov/geology/inventory/gre_publications). 

The Department of the Interior protects and manages the nation’s natural resources and cultural heritage; provides scientific and other 

information about those resources; and honors its special responsibilities to American Indians, Alaska Natives, and affiliated Island 

Communities. 

National Park of American Samoa 

Geologic Resource Evaluation Report 

Natural Resource Report NPS/NRPC/GRD/NRR—2008/025 

NPS D-21, February 2008 

National Park Service 

Director • Mary A. Bomar 

Natural Resource Stewardship and Science 

Acting Associate Director • Mary Foley, Chief Scientist of the Northeast Region 

Natural Resource Program Center 

The Natural Resource Program Center (NRPC) is the core of the NPS Natural Resource Stewardship and 

Science Directorate. The Center Director is located in Fort Collins, with staff located principally in 

Lakewood and Fort Collins, Colorado and in Washington, D.C. The NRPC has five divisions: Air 

Resources Division, Biological Resource Management Division, Environmental Quality Division, Geologic 

Resources Division, and Water Resources Division. NRPC also includes three offices: The Office of 

Education and Outreach, the Office of Inventory, Monitoring and Evaluation, and the Office of Natural 

Resource Information Systems. In addition, Natural Resource Web Management and Partnership 

Coordination are cross- cutting disciplines under the Center Director. The multidisciplinary staff of NRPC 

is dedicated to resolving park resource management challenges originating in and outside units of the 

national park system. 

Geologic Resources Division 

Acting Chief • John Vimont 

Planning Evaluation and Permits Branch Chief • Carol McCoy 

Credits 

Author • Trista Thornberry- Ehrlich

Review • Bruce Richmond 

Editing • Mary- Margaret Coates, Sid Covington, and Melanie Ransmeier 

Digital Map Production • Jim Chappell, Stephanie O'Meara, and Josiah Engblom 

Map Layout Design • Georgia Hybels
Geologic Resources Division 

Natural Resource Program Center 

P.O. Box 25287 

Denver, CO 80225 

http://www.nature.nps.gov/geology/inventory/ 

(303) 969-2090 

EXPERIENCE YOUR AMERICATM 

National Park Service 

U.S. Department of the Interior