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Reading rock core samples

One important question that the scientists like Dr Phil Shane at The University of Auckland are asking is: “When did the volcanoes in Auckland last erupt?” Answering this question will help them to predict when the volcanoes might erupt again and determine what the consequences could be.

So how do you find out what happened a very, very long time ago? One method involves identifying the age of an eruption by looking at rock core samples. Geologists get rock core samples by pushing or drilling cylindrically shaped tubes down deep into the ground. When the tubes are pulled out, they contain a core sample of the rocks below. Scientists might get hold of these in two different ways – they can use the cylinder of material produced when a pile hole is dug for a new building or to support a road, or they can drill a core sample themselves.

Drilling for data

Geologically, an ideal place to drill volcanic core samples is in a basin. Often, these depressions have filled with water, forming lakes such as the crater lake on Mt Ruapehu or at Lake Pupuke. These lakes protect the ground beneath from erosion. Phil is specifically interested in basins that have been left undisturbed and so contain a record of sediment reaching back over time.

Onepoto Basin on the North Shore is a site in Auckland where Phil is drilling core samples. It was originally a crater lake that was drained in the 1960s and turned into a local park. Sediments that have gathered in this basin over thousands of years now form a record of what has been happening, including volcanic eruptions.

How it’s done

The drilling for core samples doesn’t use a normal drill bit, scaled up from the version that you have in the shed at home. Instead, scientists use 2m long hollow tubes that are driven into the earth by a large weight. As each segment is pushed into the ground, it forms a tube of material that is brought to the surface, a little like a biscuit cutter being pushed into dough. The core that is pulled up is recorded, and the driller hammers the next tube into the ground in the same hole. Each time, the drilling is going 2m deeper. The cores can be analysed, one after the other, to provide a continuous record reaching back in time.

What are they looking for?

Scientists are able to see different layers in the cores. (They call these ‘laminated’, referring to different layers close together.) These layers represent different rock types, for example, ash floating down from a Taupō eruption might look white, lake sediment is a dull brown muddy colour, while basalt rock from local Auckland volcanoes is dark and grainy.

As well as looking at the layers in the cores, scientists take samples of rocks from each of these layers and measure the different types of rocks and minerals in them using an electron microprobe (a specialist piece of equipment that can measure the composition of rocks in very small quantities).

Putting the story together

Phil can read the rock core like pages of a book. He sees each layer as a page that records a period in time:

  • A light-coloured layer of rock at a depth of 60m below the surface corresponds to a massive eruption from the Rotorua area. This is white ash (tephra) that has travelled many miles to settle in the basin. The crater lake has protected this layer of ash and has stopped it being washed away by the rain, as happened elsewhere in the landscape.
  • A layer of sedimentary mud immediately on top of the ash layer shows that there were no eruptions at that time and the lake bottom was naturally forming and settling.
  • Further up the rock core, there is a dark layer, which is basalt rock. Auckland volcanoes erupt this type of rock, so scientists can see how many layers of this darker material there are and how frequently this layering occurs. This gives them the clues about when the volcanoes in Auckland last erupted and when they might do so again.