Friday, October 19, 2007

Pocket Money

'If your parents told you one day that they were going to give you and your brother pocket money, but they were going to give your brother more pocket money than you, what questions would you ask them (after you'd trashed your brother's bedroom, raided his CD collection and let down his bike tyres?) During the subsequent hours of solitary confinement in your room, you might start wondering the following:

1. HOW MUCH money is HE going to get?
2. HOW MUCH money am I going to get?
4. Is the difference such a big deal?
5. WHATS' MY BROTHER DONE to deserve more pocket money than me?
6. How much does the pocket money we are being offered shape up against the pocket money my friends get from their parents?

If you ask these questions, then you are a good sediment core geologist. That's because they are asking pretty much the same things:

1. EXACTLY HOW OLD are the layers of rock in the core?
2. Is "this" sediment layer older or younger than "that" sediment layer?
3. What's the EXACT DIFFERENCE in age between them?
4. In the scheme of things are the age differences big or small?
5. WHAT HAS HAPPENED IN THE PAST TO MAKE THE SEDIMENTS DIFFERENT in age, in thickness and in character?
6. Where does the sediment sequence we are looking at fit with other sediment sequences we already know about?

Understanding the RELATIVE AGE of rocks need not be too hard, especially perhaps with rocks that have formed from sediments that have built up on the sea floor. Unless there has been a major upheaval of the earth's crust since the sediments were laid down (and if there had been there would be ample evidence) it is fair to assume that the sediments at the BOTTOM are OLDEST, and the sediments at the TOP are YOUNGEST. That is exactly what is assumed for our core of sediment from under the Ross Ice Shelf.

However, knowing the EXACT age of rocks is the "Rosetta Stone" of geology, and an accurate method to do it has only been possible within the last 100 years with the discovery of the fact that some elements in the rocks of the earth are unstable and change into other elements at a fixed rate through time. It is the principles of this technique, used on the core along with many others, that I want to introduce to you here with a story:

Let's go to a place we can see. It is a village in a mountain range. For the sake of argument let's say this village is in Switzerland and is called Sion, in the canton of Valais. The village completely encircles a hill, on top of which are boulders. Great table-sized boulders. These boulders are just in balance, but they are highly unstable. Just the slightest push sets them off. They would rather be at the bottom of the hill than at the top.

Luckily for the boulders the boys from the village are well aware of their predicament and they take great delight in scrambling up the hill and giving some of them the gentlest of nudges to help them on their way. But these boys aren't idiots; they want their fun to last. So they have a rule. By the very end of the year they will have only pushed down HALF THE TOTAL NUMBER of hilltop boulders that were there at the very beginning of the same year.

Ken, the village scoundrel in his youth who by some strange twist of fate is now the mayor at the tender age of 24, is under pressure from the villagers. After many years of the game being played there aren't many boulders left on the hilltop, but one boulder in particular is perched directly above the town hall. The villagers are all keenly aware of the rules the boys abide by. Nevertheless, despite the unpredictable nature of the game they still want to know what the chances are that this particular boulder will be set off within the year.

The date is January 1st. Ken invites a geologist to Valais to investigate the matter. The geologist spends many days in the village. He talks to the boys and becomes educated as to the boulder-rolling rules, he examines the offending boulder, he counts all the boulders at the top of the hill and all the boulders in amongst the village houses. Eventually it his time to make report back:

"I'm sorry Ken, I can't help you," he replies, "the boulder above your town hall is totally insecure, just like all the other boulders at the top of the hill. The boys could push it down at any moment. On the other hand it could be up to four years before they push it off. However, the one thing I have learned is this: don't blame the boys. They are just continuing a tradition that began in this village before they were old enough to know better. In fact if anyone is to blame, it's the generation of boys that started this ridiculous game in the first place. You wouldn't happen to know who that was, would you Ken?"

Ken paid the geologist twice his usual fee, told him not to announce his findings to a single soul in the village, and ushered him on his way.

So here are some questions:

1. The geologist counted 8 boulders on the hilltop and 8184 within the village. How many boulders used to be on top of the hill before the game began?

2. Exactly one year after the game began how many boulders would have been pushed down into the village?

3. How many boulders will have been at the top of the hill at the BEGINNING of the FIFTH year of the game starting?

4. If there were 7168 boulders at the bottom of the hill at the end of the year, for how many years had the game been played?

5. How many years ago did the game begin and why do you think that Ken paid the geologist twice his fee and instructed him not to speak to the villagers?

6. Show how the geologist knew that the boys would have pushed the boulder down within four years.

Answers and more on radiometric dating for real in the next blogs. Don't forget to keep those entries coming!
There has been a lot of maths in this blog page, so that's why I put a photo of a cool snowplough I saw at lunch today at the top. The drivers leave their engines running all lunchtime so the diesel doesn't freeze.

Everyone asks whether I've seen any penguins ... the answer is I've not seen any yet, although as the sea ice breaks up we may get to see some. Up until then, if you are staying in McMurdo, this is the closest you can get!

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