Friday, February 1, 2013

WHAT’S UNDER THE WAVES – OF HAWAII?


The surf may always be “up” in Hawaii, but what’s under the waves is one of the earth’s great geologic “observations” – a tectonic plate in motion!

This is a topographic image of Hawaii minus Pacific Ocean. In addition to being drop-dead gorgeous, it was made as part of a study to find the behavior of potential Tsunami waves along the rugged structure of the islands. At last count, there are eight major islands, numerous small islets, several atolls, and dozens of submerged seamounts in Hawaii. That sort of “star shaped” form of the Big Island is controlled in part by volcanic intrusions at its points; northern islands seem buried around their margins by sediments, carbonates building up around them. Since the southern part of the islands are still active, in all probability there will be newer islands added in the foreseeable geologic future – here’s why:

The Hawaiian Islands are located at the southern-most extreme of the Emperor – Hawaiian Volcanic Chain. A simple visit to the “Pacific Hemisphere” of the world as viewed on Google Earth shows this mostly submerged mountain range starting between the Kamchatka Peninsula and Aleutian Islands in the north, continuing nearly due south until as it nears the Midway Islands, then turns an abrupt corner and continues east-south-east onward until it finally terminates, 6,200 km from its start, with the “Big Island” of Hawaii.

Just about all of that “big blue” part of the globe that hosts Hawaii is the Pacific Tectonic Plate, the largest on Earth. It is encircled by what’s been called “The Ring of Fire,” that is, the belts of active volcanism and earthquakes that mark its plate boundaries. Hawaii, though certainly volcanically active, is not part of this ring – it is a prominent example of mid-plate volcanism. Comparing the ages of the volcanoes in the Emperor-Hawaiian chain, the direct motion of the Pacific tectonic plate, and the speed at which it is moving, are readily observable.

The oldest volcano is the furthest north, nearly at Kamchatka; the submerged seamount of Mejii is dated to be about 80 million years in age. Going south, the volcanoes of the Emperor-Hawaii Chain decrease in age, each one to the south younger than its neighbor to the north. Ages decrease by Midway Island to 25 million years old; Kauai in Hawaii just ~5 million years, and the Big Island is half a million years old to, well, it’s still forming (and erupting lava) at present. The reason given for this age range is that beneath the Pacific plate is a relatively immobile plume of heat and heated rock upwelling from a depth associated with the mantle/core boundary: at the position where this plume penetrates the earth’s crust, magma is intruded, eruptions occur, volcanoes grow. While all this activity is going on, the Pacific plate is moving relative to the position of this hot spot – okay, you can do the math along with me: 6,200 kilometers is the distance traveled by the plume in 80 million years, thus its general rate of speed is… I got 7.75 cm a year assuming I didn't drop a zero somewhere. Did you?

This is a fairly rapid speed as plate tectonic movement goes. And it’s still continuing. To the south of Hawaii, visible in the topographic image, a new seamount is forming which has already been given a name – Lo’ihi. Presently, Lo’ihi is about 975 m below sea level and is expected to become an island in 10,000 to 100,000 years from now.

Whether it will have prime surfing beaches remains to be seen...


Thanks, Bill, for the picture that inspired this.

Image: I downloaded the image from Drew Kapp (who also has maps of the other islands at his links below).The map is the handiwork of the USGS: (http://www.usgs.gov/)
http://www2.hawaii.edu/~dkapp/hawaiian_islands_map.jpg
http://www2.hawaii.edu/~dkapp/122_maps.html
The USGS provides 637 public domain photos of Hawaii:
http://libraryphoto.cr.usgs.gov/cgi-bin/search.cgi?search_mode=noPunct&free_form
Other good sources:
http://www.hawaii.edu/environment/ainakumuwai/html/ainakumuwaiislandformation.htm
http://www.geolsoc.org.uk/en/Plate-Tectonics/Chap3-Plate-Margins/Mid-plate/Hawaiian%20islands



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