The geology of the Samoan islands is surprisingly interesting. First, we are living on a volcano, which is resting quietly at the moment. Second, our volcano is on the move -- it's traveling towards China with us on it. And, finally and most unfortunately, our volcano is doomed and it will eventually sink back into the dark ocean depths.

It is not really an exaggeration to call the Samoan islands 'active volcanos'. These islands were indeed formed by volcanism, and the volcanos are still active, in a geologic timeframe of course, and due to some unusual circumstances as described below.

About 1.5 million years ago, our volcano spewed forth enough lava to rise up out of the ocean and become “Tutuila Island”. Actually, just the tip of the volcano is visible to us -- most of the mountain is underwater. While the tallest mountain peak on Tutuila is about one half mile high, the mountain extends another 2 miles below the sea surface.

The most recent volcanic eruptions were a lot more recent than many people realize. In western Samoa, major eruptions occurred in 1905 when lava flows destroyed a village. In the Manu'a islands, subsurface volcanic eruptions and earthquakes occurred in 1866, causing dense clouds of smoke and pumice to erupt from the ocean surface for several months. One hundred years ago is just a blink of the eye to a volcano, which measures time in the millions of years. We humans tend to forget how briefly people have lived upon these shores. Human habitation on Tutuila, even considering the whole 3000-year period that Samoans have dwelt here, represents a mere 0.2% of the time since the sun first shone on this new land.

To explain our volcano's march towards China, we first need to review the nature of the earth's surface or crust. The earth's outer layer, the one we live on, is several miles thick, but that is a thin skin compared to the total size of the earth. This outer layer is made up of many separate sections that seemingly float on top of the earth's molten core and move about in very slow motion. Geologists call these outer sections “plates”. You may recall, for example, that the continents of Africa and South America were once joined together when the earth first formed, but the two continents slowly drifted apart to where they are today. The same process applies to the plates under the Pacific Ocean. The plate we're on is called the Pacific Plate and it is moving westward (towards China) at a leisurely speed of about 3 inches per year. At this rate, in one million years we will be 50 miles closer to China.

Used with permission from S. Hart, Woods Hole Oceanog. Inst.

It is not accidental that the islands of American Samoa and western Samoa lie roughly in a straight line. Directly underneath us is what geologists call a “hot spot” of thermal activity in the earth's core. It's a volcano just waiting to happen. When the pressure builds up at the hot spot, molten magma bursts up through the Pacific Plate and forms a volcanic island. Then the hot spot calms down for awhile, perhaps a million years or so. During this peaceful interval, the Pacific Plate keeps marching onward, so when the hot spot acts up again, it forms a new volcanic island rather than build upon the previous one. In other words, the hot spot stays in one place but the plate above it keeps moving.

Used with permission from S. Hart, Woods Hole Oceanog. Inst.

The islands formed therefore generally lie in a straight line that is oriented in the direction the plate is moving. The new islands form on the eastern end of the chain, so the islands become progressively older as you move from east to west. For that reason, the islands in western Samoa are about 1 million years older than the islands in American Samoa. (Early geologists got this direction backwards.) The newest volcanic eruption in our island chain is forming about 30 miles east of Ta'u Island, but it will probably be another 100,000 years before this sub-surface volcano, named Vailulu'u, breaks the sea surface.

But something else really exciting also happens in our area. As Tutuila Island glides westward, a part of our plate collides with another plate to the west of us (the Australian Plate), and our plate actually rips in two at this point (see diagram). One piece of our plate continues moving towards China, but the other piece slides down into the 6-mile deep Tongan Trench and under the Australian Plate, never to be seen again.

The collision of these two colossal pieces of the earth's surface causes the seafloor to bend and rip, which in turn probably causes some earthquakes and renewed volcanic activity. Recent surveys suggest how this might be happening. Long cracks in the seafloor have been discovered between the Samoan islands and the Tongan Trench. The cracks are oriented in an east-west direction and seem to be formed as the seafloor bends southward down into the top of the Tongan Trench. These cracks may make it easier for the hot magma beneath the crust to spew upward and emerge as young lava on top of our old islands.

And all this is happening a mere 100 miles south of Tutuila Island. We live in a very unique area.

Finally, all oceanic volcanos must come to an end. As time passes, two things happen. Our volcano erodes continuously as ocean waves attack its shorelines and rivers gouge into its terrain. In addition, the weight of a newly formed volcano is so heavy that it causes the volcano to sink slowly back down into the sea. Rose Atoll and Swains Island are good examples of sunken volcanos. In the distant past, Rose and Swains may have been magnificent mountainous islands with beautiful rainforests and coral reefs. But that's ancient history now, because those islands, over a period of several million years, eventually sank out of sight. All that remains are tiny amounts of coral that grew up from the peaks of the mountains as they slipped below the sea surface.

Not to worry. Tutuila Island should be around for a few more million years.

P. Craig, NPS