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National Park of American Samoa

American Samoa

cover of park brochure

park geology subheading
photo of man sitting on ledge overlooking the coast
National Park of American Samoa, Central South Pacific

The islands that make up America Samoa were created by the same sort of geologic activity that created the islands of Hawaii. Underneath the Samoan Islands, which lie just south of the Equator and just east of the International Date Line, is a hot spot that has been active for millions of years. American Samoa is a living volcano that is on the move - westward towards China at a rate of about 3 inches per year.

The islands that make up the American Samoa are very active geologically. The most recent volcanic eruptions there were in 1905. The lava flows from this eruption destroyed a small village. Not long before that, volcanic eruptions and subsurface earthquakes occured in 1866 which caused smoke and pumice to erupt from the ocean surface into the water for several months.

There are three islands within American Samoa that have units in the National Park: Ta'u (tah-OO), Ofu (OH-foo), and Tutuila (too-too-EE-lä).

map of ta'u
Map of Ta'u - Green Area is Park Unit
Ta'u is the largest of the three units at 190 acres. The highest peak of American Samoa, Lata Mountain (3,170 feet), is located in this unit. Laufuti Stream - the only perennial stream on the island - cuts through the topography of the island and pours down a 1,000 foot cliff at Laufuti Falls.


 

map of ofu
Map of Ofu - Green Area is Park Unit
About nine miles away from Ta'u is the Ofu unit of the park. This island, like others in American Samoa, is surrounded by coral reefs. It is made up of a complex of volcanic cones and is the remnant of an ancient shield volcano. The island has been buried by lava from merging flows and many volcanic features are present here including: lava flows, radiating dikes, collapsed caldera, and pyroclastic beds.


 

map of tutuila
Map of Tutuila - Green Area is Park Unit
The third unit of the park is on the island of Tutuila - the largest island in American Samoa, home to the capital of Pago Pago (PAHNG-go PAHNG-go. This island exhibits substantial weathering which has left most of the area with no visible craters from previous volcanic activity. The exeption to this is the Leone Peninsula, formed about 70,000 years ago. Across the park many closely spaced streams can be found which deeply erode the volcanic slopes made of basalt. Drowned valleys, coastal flats, and volcanic intrusions such as Rainmaker Mountain, can also be found on the island. Rainmaker Mountain is a volcanic feature known as a trachyte plug. This means that it is a volcanic intrusion made of extrusive igneous rocks having alkali feldspar and minor mafic minerals as the main components and a fine-grained, generally porphyritic texture. At Vaisa Point on the northern coast of the park unit, streams drop into the sea from hanging valleys created when the erosion of the crashing waves cuts down faster than the erosion caused by the streams. In the western area of the park, pillow lavas can be found in Amalau Cove. Pillow lavas are formed by extrusion of basaltic lava from the seafloor that solidifies underwater. Because of the intense action of the waves at the beach in this area, the boulders and chunks of coral have become known as the "singing rocks" because they can be heard moving back and forth with the motion of the water.

In addition to the local geological activity on each of the islands, American Samoa is also active on a much larger scale. To understand this concept, one must first understand the movement of plates across the earth's surface. American Samoa is located on the Pacific Plate. This plate also contains the islands of Hawaii and many others. An important feature of the island groups on the Pacific Plate is that the ones located over a hotspot are all oriented in a linear fashion. Since the Pacific Plate is moving in a pretty straight path, all island groups over hotspots were formed in a line that matches the direction of plate movement. This shows that the hotspot isn't moving, but the plate definitely is. The islands on the Pacific Plate grow progressively older as you move from east to west. As it continues its westward trek, the Pacific Plate is making a slow collision with other plates, including the Australian Plate. The collision of these two massive sections of earth causes quite a bit of geologic disruption in the area including earthquakes and volcanoes. Since the two plates cannot occupy the same area on the earth's surface, something must happen when they collide. Recent surveys suggest what may be happening below. The Tongan Trench is an area along the seafloor between Samoa and Australia where the Pacific Plate is subducted under the Australian Plate. Long cracks along the seafloor between Samoa and the Tongan Trench oriented in an east-west direction seem to be formed by the bending of the seafloor. These cracks in the seafloor make it easier for hot magma from underneath the earth's crust to erupt as younger lava on top of older islands. Since we already know that the Pacific Plate is gradually being submerged, it is possible to predict that the Samoan Islands will one day be subducted as well. Slowly, the islands are moving closer and closer to the subduction zone, and at some point they too will be sucked under the Australian Plate. Luckily, American Samoa will be around for millions more years for all to enjoy.



park maps subheading

The General park map handed out at the visitor center is available on the park's map webpage.

For information about topographic maps, geologic maps, and geologic data sets, please see the geologic maps page.

photo album subheading

A geology photo album has not been prepared for this park.

For information on other photo collections featuring National Park geology, please see the Image Sources page.

books, videos, cds subheading

Currently, we do not have a listing for a park-specific geoscience book. The park's geology may be described in regional or state geology texts.

Please visit the Geology Books and Media webpage for additional sources such as text books, theme books, CD ROMs, and technical reports.

Parks and Plates: The Geology of Our National Parks, Monuments & Seashores.
Lillie, Robert J., 2005.
W.W. Norton and Company.
ISBN 0-393-92407-6
9" x 10.75", paperback, 550 pages, full color throughout

The spectacular geology in our national parks provides the answers to many questions about the Earth. The answers can be appreciated through plate tectonics, an exciting way to understand the ongoing natural processes that sculpt our landscape. Parks and Plates is a visual and scientific voyage of discovery!

Ordering from your National Park Cooperative Associations' bookstores helps to support programs in the parks. Please visit the bookstore locator for park books and much more.



geologic research subheading

For information about permits that are required for conducting geologic research activities in National Parks, see the Permits Information page.

The NPS maintains a searchable data base of research needs that have been identified by parks.

A bibliography of geologic references is being prepared for each park through the Geologic Resources Evaluation Program (GRE). Please see the GRE website for more information and contacts.



selected links subheading

NPS Geology and Soils Partners

NRCS logoAssociation of American State Geologists
NRCS logoGeological Society of America
NRCS logoNatural Resource Conservation Service - Soils
USGS logo U.S. Geological Survey

teacher feature subheading

Currently, we do not have a listing for any park-specific geology education programs or activities.

For resources and information on teaching geology using National Park examples, see the Students & Teachers pages.
updated on 01/04/2005  I   http://nature.nps.gov/geology/parks/npsa/index.cfm   I  Email: Webmaster
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