GEOLOGICALLY SPEAKING
Welcome to The Deep science and technology column where we cover topics from the deep sea to deep space and beyond. Visit our website at www.thedeepradioshow.com
As many of you know, in addition to writing The Deep column I’m also the Star Lady. And in my capacity as Planetarium Director, I’d like to invite you to the UOG Planetarium this Thursday, Friday or Saturday for the Planetarium show “Winter Skies” which tells you all about the things you can find in Guam’s lovely dry-season skies. The doors open at 6:00 p.m. and the show starts at 6:30 p.m.
After “Winter Skies” we’ll have “Quality Time with the Star Lady” where I answer your space-related questions until it gets dark enough that we can go outside and try to find some the sky objects in the real sky (I call it “The BIG Planetarium!). So join us this weekend for “Winter Skies” and remember the show is FREE!
As we all struggle to get back into the daily grind after the holidays, I thought I’d share some news items about Mother Earth: specifically about her geology.
FLIPPING POLES
One of the things you learn when you study space and the Earth is that while we humans live our lives in minutes, days and years, the average human life span is not even a blink of the eye in recorded time (see my article Time Is (As) Money.) When I talk about a ‘recent event’, I’m probably talking about last year or perhaps something that happened in the 1990’s. When a geologist talks about something that happened recently, the event could have occurred a hundred thousand year ago.
Last week, I talked about a research team that was examining the Mid-Atlantic Ridge. The first cores drilled there pointed the way to one of the most dramatic discoveries of the 20th century. The Mid-Atlantic Ridge is a long chain of volcanoes that are constantly active. The molten rock flows down the mountainside and eventually hardens. This astounding flow is pushing the Americas away from Europe and Africa and eventually, the Atlantic will be wider than the Pacific.
Earth has a large magnetic field, which is generated by a big core of molten iron at the Earth’s center. When the scientists examined their cores from the Mid-Atlantic Ridge, they discovered something really interesting. The rocks contained iron, just as they expected, but some of the magnetized rock didn’t point north; it pointed south. When they mapped the pole reversals with their distance from the spreading center (the active volcanoes) they constructed a map of pole reversals frozen in the rock layers.
Earth’s poles have ‘flipped’ at least 400 times over the last 180 million years. The time intervals between the ‘flips’ vary wildly from 50,000 years to 40 million years. The ‘average’ interval is about 500,000 years.
The most recent reversal occurred about 780,000 years ago. This is longer than ‘average’ (but nowhere near 40 million years) and scientists have found some clues that we may indeed be at the beginning of another pole reversal.
Scientists have recently discovered a shallow core field in the Earth’s magnetism. They believe the shallow core field may play a role in determining whether the main field polarity flips while weakened or whether it recovers its strength without reversing. Current evidence suggests we are now approaching one of these transitional states because the main magnetic field is relatively weak and rapidly decreasing, he says. While the last polarity reversal occurred several hundred thousand years ago, the next might come within only a few thousand years.
Right now, historic records show that the strength of the magnetic field is declining very rapidly. Anomalous magnetic ‘poles’ have formed in widely separated spots from Brazil to Siberia. We all know that a compass doesn’t point to due north, it points to ‘magnetic north’. And the pole wanders. In the 1700’s, it was in southern Canada. It’s now in northern Canada and is drifting towards Siberia at an accelerating rate. The pole was moving about 6 miles per year at the beginning of the 20th century, but the drift rate was 25 miles a year in 2003. It is also unknown if the drift will continue to accelerate.
The researchers say that if current trends continue, in 1,500 years Earth’s magnetic field will be as weak as it’s ever been and the poles will probably reverse. We certainly won’t see reversal, and neither will our grandchildren. Fifteen hundred years is a very long time for humans, but the blink of an eye for geologic history.
FINDING THE HOT SPOTS
Of course, the Mid-Atlantic Ridge isn’t the only spot for undersea volcanoes. There are other ‘hot spots’ where a plume erupts from the mantle and causes a chain of volcanoes as the sea floor drifts above it. The most famous example of this is the Hawaiian Islands. The island chain extends over thousands of miles including eroded seamounts that are below the surface of the water. The Hawaiian chain even records a right angle turn in the spreading direction millions of years ago. But Hawaii isn’t the only island chain created by a hot spot. New research shows that the Samoan Islands were created in the same way.
A new study that determines Samoa is indeed on the path of a geologic “hotspot” trail is adding fuel to a vigorous scientific debate over the origins of volcano chains – especially in the Pacific Ocean. Some scientist think they are created by hot spots, others advocate the idea that the world’s 50,000 seamounts – most of which have never been explored – may have been formed due to plate tectonic forces that locally have put the Pacific plate under stress and extension. Oregon State University marine geologist Anthony Koppers says there’s room for both theories.
“There are areas, including Samoa, where a clear linear age progression of the islands or seamounts indicates a hotspot trail, and there are areas where alternative theories make more sense,” Koppers said. “The whole debate underscores the need for more research, because these are fundamental questions about the processes that formed, and continue to form, the Earth’s crust.”
Samoa has been an enigma among scientists because the ages determined from sampled lava don’t seem to line up in a linear fashion with the movement of plates. Some scientists took that to indicate that the creation of the islands was related to magma seeping up through cracks in the seafloor in stressed fracture zones.
In their study, funded by the National Science Foundation, Koppers’ team found that previous age determination studies of the Samoan islands focused on land that had risen above sea level and was much “younger” geologically than older rocks at the base of the seamounts.
Samoa is close to the Tonga Trench, Koppers said, where the Pacific plate submerges beneath another tectonic plate and bends sharply. “The resulting stress on the plate complicates the volcanic processes,” he said.
“As we get better data sets, we’ll gain a clearer understanding of the processes that are shaping the planet,” Koppers said. “Until those models are refined, we’ll have to collect data and evaluate each seamount trail on a case-by-case basis.”
A narrow strait (just left of center) separates the two triangle-shaped islands of Ofu (left) and Olosega (right) in eastern Samoa. The islands are formed by two eroded, coalescing basaltic shield volcanoes. (Credit: Peter Craig, 1995 / U.S. National Park Service)
T
HE BIG ONE
One thing that can happen when undersea volcanoes erupt is a tsunami. We all remember the incredible devastation all along the Indian Ocean coastline only four years ago. A line of massive boulders on the western shore of the island of Tonga may be evidence of a volcano-triggered tsunami that makes the Boxing Day tsunami look puny. These seven coral boulders are up to 30 feet high and weigh up to 3.5 million pounds and they’re located 300 to 1,300 feet from the coast. The house-sized boulders were likely flung ashore by a wave rivaling the tsunami generated by the 1883 eruption of Krakatoa in Indonesia, which is estimated to have been 115 feet high.
These giant coral rocks did not form at their present location on Tongatapu, Tonga’s main island. Because the island is flat, the boulders could not have rolled downhill from elsewhere. The boulders are made of the same reef material found just offshore, which is quite distinct from the island’s volcanic soil. In fact, satellite photos show a clear break in the reef opposite one of the biggest boulders. And some of the boulders’ coral animals are oriented upside down or sideways instead of toward the sun, as they are on the reef.
The boulders are such an unusual part of the Tongan landscape that tales of their origins appear in local folklore. According to one legend, the god Maui hurled the boulders ashore in an attempt to kill a giant man-eating fowl.
Of course, we’re luck here on Guam. A tsunami is an ocean-deep wave that must enter shallow water to have the run-up necessary to cause massive on-shore destruction. If you’ve ever looked at a topographic map of the water around Guam, you’ll realize that there is NO shallow water to act as a run-up for a tsunami. We have lots of things to worry about here, but tsunamis aren’t one of them.
Cruise on over to the Deep Website at www.thedeepradioshow.com to learn more about tsunamis on Guam and many other topics. Enjoy!
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