Erratic Boulders

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My education in climatology has roots in meteorology, but lately I’ve been hanging around with the kinds of geoscientists who take the long view. This satisfies me in some ways, as a science fiction reader and a Long View enthusiast. Unfortunately, it leaves me a bit of a layperson for a lot the talks that I get a chance to attend.

Yesterday I went to a “brown bag”, an informal presentation of work in progress. A student presented his work in tracking down the origins of “erratic boulders” on the island of Tonga in the South Pacific. (Nice work if you can get it. Meteorologists never get to go to these exotic places, because the air is pretty much, you know, nearby wherever you go.) He showed us a picture of a rock the size of a large house; thousands of tons.

Erratic boulders are large rock formations which don’t share the formation emchanisms of the immediate surroundings or anything directly uphill from there. Geologists find them interesting because they are indications of the most extreme violence of which nature is capable. Most such rocks are deposited by glaciers, but there have never been glaciers in Tonga.

The rock in question, it appears, was broken off a nearby coral reef, transported about half a mile horizontally and about twenty feet uphill. That’s right, a multi-thousand ton object moved uphill. What possible natural force could account for this?

Water. Apparently this sort of formation is well known in Japan and other tsunami-prone regions. A wall of water a hundred feet high can snap a boulder off a rock formation, and drag it across a half mile, uphill. Fortunately, this is a very rare event. In the case at hand, the event was reliably dated to about 120,000 years ago by standard dating techniques applied to the constituent coral.

This was the part that captured me as an outsider. Most of the talk, though, was about trying to go from the evidence of which way the thing rolled to the geologic origins of the tsunami. This is of interest in understanding the tectonics of the whole Pacific, and eventually the origins and future fate of the earth’s surface. Unfortunately I couldn’t recosntruct the details of the argument for you, though the presenter was in fact quite clear. Although Tonga is not usually in the path of the hugest tsunamis, a specific fault was identified in the right direction that had the capacity of producing such a wave. One more huge and rare event in the history of the earth is thereby diagnosed, and another fact added to the sum of human knowledge.