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.
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