The islands formed 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 of movement 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 few hundred years before this sub-surface volcano, named Vailulu'u, breaks the sea surface (in 2005 it was 1800 feet below the surface but growing rapidly).
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.
About 1.3 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.
It is not really an exaggeration to call the Samoan islands 'active volcanoes'. These islands were indeed formed by volcanism, and the volcanoes are still active, in a geologic timeframe of course, and due to some unusual circumstances as described below.
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 slow-motion 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.
NATURAL HISTORY GUIDE