This week’s sand is from Faga'alu Beach, near Pago Pago, the capital of American Samoa, and is simply a great example of a sand of almost entirely biogenic origins. Bits of coral, sea urchin spines, grains that are quite possibly fragments of bryozoa (although my marine biological knowledge is not extensive) – and our old friends, foraminifera, including the wonderful miniature stars; these latter are the species Baculosypsina, and there are some great stories related to them – but that’s for another day.
The Samoan islands are volcanoes emerging from the depths of the southeast Pacific Ocean, close to the chaos of the great plate boundaries that – as recently in New Zealand – can cause widespread devastation. When the plates are quiet, these Islands have all the characteristics of a Pacific paradise, white sand beaches often fringed by reefs. Faga'alu Beach, the origin of today’s grains, is one such place. As you can see in this Google Earth image, the beach has developed along a strip of rocky shoreline sheltered from the waves by the barrier of the reef. And it’s the reef and its environs, rather than the rocks, that provide the sand.
In this spectacular photo from the air (credit below) reefs can be seen all around the bay of Pago Pago: the town itself is at the head of the bay in the distance, Faga'alu Beach is at the left of the picture.
But when the plates stir, havoc ensues. On September 29, 2009, a major earthquake struck close to Samoa and triggered a tsunami that devastated large areas of the islands. In coastal topography like this, the amplitude of the tsunami would be magnified as it sweeps into the confines of the shallow waters of the bay, and the results catastrophic. In some areas, the tsunami wave reached nearly fifty feet above sea level, and there were, as is often the case, several successive waves, the ocean withdrawing before pounding back onshore again. There are some remarkable videos of the 2009 Samoan tsunami available on YouTube – see, for example, this one, with links to others; a dramatic animated model of the way the waves sweep across the Pacific was generated by NOAA and can be seen here. Incredibly, it was not just one earthquake that caused this, but three. The first, magnitude 8.1, triggered two further 7.8 magnitude events within a couple of minutes. This kind of event – what has been referred to as a “triple whammy”- had not been seen before and was described in detail a year later.
Pacific island sands – paradise beaches waiting to be churned by the paroxysms of plate tectonics.
[The sand sample was kindly provided by Christopher Maslon. The photograph of the bay of Pago Pago is from http://www.pbase.com/aqohana/image/24407276; I attempted by two different routes to obtain explicit permission to use this image, but received no reply. I trust that I have caused no offense by using it.]
Comments