Digging archaeology

by Rebecca Priestley / 10 November, 2012
Raking over the past inevitably means looking underground.
Digging archaeology


Question: “Worldwide, archaeologists ‘dig’ to uncover past civilisations’ structures. What puzzles me is why they are underground. We’re not creating more earth; indeed, to the contrary, thousands of tons of silt flow into the sea annually. The earth, occasional earthquakes excepted, is not evidently rising so how is it that all sites of yesteryear are now underground?” – Sir Robert Jones

Answer: Victoria University archaeologist Bruce McFadgen says the reason we find so many archaeological sites underground is that the more exposed sites have been destroyed. Three main processes take place in the landscape, says McFadgen. “There are places where there is erosion, places where there is accumulation and places where neither has happened. Sites in places that are being eroded have disappeared. Sites that are not in places of accumulation tend to remain on the ground surface, where they are vulnerable to damage by animals, people and weathering.

“So the archaeological sites that are best preserved tend to be in places of accumulation. If you look at a river terrace, where there is periodic flooding, silt tends to accumulate. And if there is an archaeological site there, it will tend to become buried. In places like sand dunes, where sand is blowing around, you can also get archaeological sites buried. So it depends entirely on the environment, and it just happens that the
archaeological sites that are preserved are the ones that have been buried.” But other processes are going on too. Through bioperturbation, which is a soil-forming process caused by things like earthworm movement and plant-root growth, things tend to slowly sink into the soil and get buried. “If you drop something on the ground and come back even two years later, chances are it will be under a layer of soil.”

PUMICE MYSTERY SOLVED


In a recent column (“The mystery of the pumice raft”, September 8) I wrote about the floating pumice raft encountered by HMNZS Canterbury on an August voyage to the Kermadec Islands. Chemical analysis, along with satellite photographs, revealed that the pumice came from a July 19 eruption of Havre seamount, an underwater volcano about 700km northeast of White Island.

Niwa marine geologists Richard Wysoczanski and Joshu Mountjoy investigated further, taking the research vessel Tangaroa to map the sea floor around the volcano. Even three months after the eruption, they found floating streams of pumice west of Havre, and with the help of a floating basket that the Tangaroa crew fashioned from a bucket and an onion bag, they were able to haul some pumice samples on board. Once they arrived at Havre, they used the ship’s multibeam sonar to create a 3D picture of the sea floor. By comparing it with a map made in 2002, they discovered that the volcano had grown a new cone, which rises 240m above the previous crater rim. They also dredged up sea floor deposits of waterlogged pumice, shiny black basalt and pure sulphur, with some deposits found up to 100km from the eruption vent.

Wysoczanski says the eruption spewed out several cubic kilometres of magma, mostly in the form of pumice, ash and basalt. “This was a big eruption, a few hundred times the size of the 1995-96 eruption of Ruapehu, and substantially bigger than the Tarawera eruption of 1886. Had it occurred on land, the consequences could have been severe.” Its scale can be appreciated, Mountjoy says, by imagining “a new peak the size of Rangitoto being added to the top of something the size of Ruapehu”.
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