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Samoan islands heading for China
« on: January 19, 2010, 07:03:41 AM »
Samoa  lives on a volcano
The geology of the Samoan islands is surprisingly interesting: We are living on a volcano, which is resting at the moment; our volcano is on the move— it’s traveling towards China; and finally, and most unfortunately, our volcano will eventually sink back into the dark ocean depths.

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 2-3 miles below the sea surface.

It is not much of an exaggeration to call the Samoan islands ‘active volcanoes’. These islands were 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 not that long ago. In 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.

100 years ago is just a blink of the eye to a volcano, which measures time in the millions of years. We 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.

It is this movement of the earth’s plates that caused the 8.3 magnitude earthquake on September 29, 2009. That massive movement resulted in the tsunami that struck the Samoan Islands.

It is not accidental that the islands of American Samoa and western Samoa lie in a rather 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 building upon the previous one.

In other words, the hot spot stays in one place but the plate above it keeps moving. The islands 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 westward. For that reason, the islands in western Samoa are about 1 million years older than the islands in American Samoa. 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 and growing).

But something else interesting 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.

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 sea floor to bend and rip, which in turn causes some earthquakes.

Recent surveys suggest how this might be happening.

Long cracks in the seafloor have been discovered between our 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. Perhaps these cracks 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 unique area.

Eventually most oceanic volcanoes disappear. As time passes, two things happen:

•    The volcano erodes continuously as ocean waves attack its shorelines and rivers gouge into its terrain.

•    Additionally, 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 volcanoes. 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.

There is an additional factor to consider— a major change in sea level occurred during the last ice age over 10,000 years ago.

At that time the world’s sea level was about 390 feet lower than it is today because vast quantities of water were locked up in glaciers and in the polar ice caps. We can see evidence of this locally.

New benthic mapping by NOAA indicates that Tutuila was somewhat larger when the sea level was lower, and it was surrounded by a ring of barrier islands that were separated from Tutuila itself by lagoons.

Taema Bank across from Pago Pago Harbor was one of these barrier islands. It must have been gorgeous. But later, as the glacial ice melted, the growth of these coral reefs was unable to keep pace with the rising water so the reefs “drowned”. They are now a necklace of submerged seamounts around Tutuila.

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


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