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Rough Science 6 Colorado: Mountain Video Diaries: Hermione Cockburn

Updated Wednesday, 16th November 2005

Exclusive video extra in which Hermione Cockburn talks about the challenge for the Mountain programme, from the sixth BBC/OU TV series Rough Science, based in Colorado

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So, just finished the mountain challenge and I have to say this has been my favourite challenge so far, a challenge very close to my heart. What happened? Well on day one Ellen and I were given the task of measuring the mass of Mount Kendall, and Mount Kendall is the mountain that absolutely dominates the skyline where we’re staying. And the first day was a brilliant opportunity to get up the mountain, just walking up it slowly just to kind of get the measure of the mountain, really look at it closely for the first time. And what Ellen and I need to do to get at the mass of the mountain, what we’ve had to do is get at the volume as well as its density. Those are the two parameters that we can use to calculate its mass.

And we divided it up so that Ellen, using her botanical skills, will look at the height of the mountain and get at the volume, whereas I spent day one really looking very closely at the geology of the mountain and what rocks actually comprise it. And I don’t think I have ever mapped such a big area in one day. But fortunately, when we got up to the top the view was incredible and a lot of the lithology up there, a lot of the rock types are exposed, there’s very little vegetation, so I could really visually just map how many different rock types were there. And mercifully there really were only three dominant rock types, and that’s what I decided to focus in on.

So end of day one I collected the three rock types, and they’re all these ash tuffs, they’re volcanic tuff rocks which means they started life as extremely scalding hot volcanic ash being blasted out of very big volcanoes millions of years ago, and that was all consolidated into the rocks that make up Mount Kendall. So I took my samples back to the mill, and that was end of day one. So that was good, but then day two was getting a handle on the density of the rocks. Now for that again we just need two measurements; the mass of those rock samples and their volume. So I just had a sort of day just at the mill building a balance.

Now this was part of the challenge that I though oh yeah, yeah, yeah, just do the density measurements, but actually I needed to be fairly accurate because the density of rocks don’t vary that much but you do actually need to be quite precise. So fortunately Ellen had worked on a balance before so she was able to give me a hand and I just built a very simple device, a ruler suspended on a nail so it could pivot, just a simple pivot balance and with two buckets on each end made out of old water bottles. And to get at the mass we could use a very nice property of water because one gram of water is the same as one millilitre of water or, well very close, certainly close enough for the measurements that I wanted to do. So dry rock samples in one bucket and then I just filled up the other bucket with water until the balance was completely level. I’d soldered on a needle just to make sure that the balance was level. And then that mass of water I just tipped into a measuring cylinder to get the mass of the, to convert the mass of water to millilitres and then back into grams.

So I did that for all the rock samples, and then to get at the volume it’s even more simple using another property of water there, that one millilitre of water is equal to one centimetre cubed. So here was just a simple bucket of water filled to the brim, pop in your rock sample, it displaces a volume of water. It doesn’t matter how irregularly shaped that rock sample is, that’s the benefit of this method. What is spilt out then put that into a measuring cylinder, get it in millilitres and transfer that into centimetres cubed. And then very simply just do the calculations, density is your mass divided by your volume.

So I spent all day on day two doing those density calculations, and what was very interesting is that the three rock types that make up Mount Kendall, they all came out approximating to an average density of 2.5g/cm3. So this was very fortunate because the other side of the problem would have been to work out what rock types comprise what proportion of the mountain, but because the densities were all the same I could just average the whole density of Mount Kendall, the visible part of what we could see, to 2.5g/cm3.

So end of day two all fine, but then at the end of day two I mentioned to Kate that there’s actually more to Mount Kendall than meets the eye. And this was a slightly trickier part of the whole challenge because the visible part of mountains which we can see are supported by very thick crustal roots that project down towards the centre of the earth into the mantle underneath the crust. And so I went to bed at the end of day two, just yesterday, thinking ooh, I’ve got to get my head round these calculations using the density information that I had collected as well as Ellen’s information on the height and volume of the mountain.

So this morning Ellen and I spent a day of brains aching getting our heads around the isostasy model. It’s just a very, that’s essentially the name for this property that the crust is buoyed up by big crustal roots underneath. So working through the isostasy equations we came up with a figure for the mass of Mount Kendall, not just the bit we can see but Mount Kendall in its entirety from its top that we can see to the base of its crustal root. And allowing for a certain amount of error in all our calculations, Ellen and I we came up with a figure of three trillion tonnes of rock. I mean almost a number very difficult to get your head round, but I mean it’s just vast amounts of mass tied up in just a single mountain.

The best thing about the whole challenge was that when Kate opened the envelope from the Professor of Sciences at Boulder University, he too had come up with a figure of three trillion tonnes so a great success, we really could have been way out with our calculations but working through the science and making sensible decisions on where to delimit the mountain and how to approach it, just as this Professor had been able to do with the help of some maps I imagine, we’d come up with the same figure. So a great end to the mountain challenge.





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