Boulder dash

One of the questions I tried to answer in my PhD was whether the huge blockfields of northeast Tasmania were formed during the last Ice Age. The largest block deposits are 1.5 km long and extend around the slopes of the mountains stretch for miles. Together with the blockstreams of the Falkland Islands, these are the biggest block deposits on Earth. In one of the most empirical pieces of work I’ve seen, geomorphologist Nel Caine carefully studied these blockfields in the 1960s by spending months walking over them measuring thousands of individual boulder’s dimensions and orientations. Nel made educated guesses as to the age of the blockfields by estimating rates of weathering and geomorphological processes.

I wanted to use the new technology of exposure dating using cosmogenic nuclides to put more quantitative estimates on block age. I decided to use the Carrvilla blockslope at Ben Lomond (named in honour of the Scottish one), so I travelled there with my supervisor John Stone in 1997. We sampled a number of blocks down a transect, it was clear that the boulders at the bottom were older because they were more weathered. Brainstorming with John, we figured this experiment would only work if the boulders stayed upright during their journey through the ages down the slope of the mountain. This looked unlikely given the jumble of blocks with huge steps and pits. John had the idea to sample for exposure dating under the same boulders we had sampled on the top. This was far from easy and we scrambled to get access even under these giant blocks up to 8 m long.

Exposure dating sounds like it comes right out of Star Trek. It works because of nuclear reactions in the rock caused by cosmic rays bombarding the Earth from outer space. However, cosmic rays aren’t rays and aren’t all cosmic either. Most cosmic rays begin their life as the nucleus of an atom ejected out of an exploding star, a super nova. These nuclei travel through interstellar space and can acquire ridiculous amounts of energy through interactions with shock waves to become the most energetic particles in the galaxy. Some even approach the speed of light. Most cosmic rays striking the Earth aren’t that powerful and virtually all interact at the top of the atmosphere. This collision smashes atoms to bits creating an enormous shower or cascade of particles that rains upon the Earth. Because they have no charge to interact, it is usually the neutrons that survive to interact with rocks (and everything else) at the surface. Given their high energy, these secondary cosmic rays can still smash atoms in the rock, creating exotic nuclei and almost any element on the periodic table. The cosmic rays can travel through tens of metres of rock but most are stopped in the top 3 m. The longer a rock is exposed at the surface, the more exotic nuclei or cosmogenic nuclei are made, minus any that decay away. As long as we know the production rate of a particular nuclide from a site of known age we can convert the abundance of the nuclide into an exposure age.

We figured that if a sample from the underside of the block had a significant concentration of cosmogenic nuclides, the block had spent some time upside down at the surface. By accounting for this, we’d have a better estimate of travel time. However, cosmic rays can leak through the sides of blocks, so we tried to make a 3D model of the block to account for this. Given the best tools we had, our 3D block looked like the wireframe space station (or the aptly named Boulder spaceship) from the Commodore 64 game Elite. The calculations proved challenging and John had little time, so we held off publishing the model, but it was clear that there might be some extra exposure at the base1. Greg Balco and colleagues published something similar to what we came up with in 2011 when studying precariously balanced rocks and were faced with the same challenge of modelling block shape.

During a student field trip to Iceland a few years ago I was confined to the field area closest to the hotel because of the flu, so I decided to see if I could make a 3D model of a boulder using structure from motion. I found a recently deposited glacially striated boulder on the moraine of Svínafellsjökull, where the ice planet in Interstellar was filmed. The model (above) worked better than I imagined it would. It is a simple matter to calibrate size measurements into this model so the dimensions can be calculated and the shielding of cosmic rays accounted for. It might be time to dig that old data out.

1. Barrows, T. T., Stone, J. O., and Fifield, L. K. (2004). Exposure ages for Pleistocene periglacial deposits in Australia. Quaternary Science Reviews, 23: 697-708.


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