Wednesday, June 13, 2012

Age Dating – and Why it is VERY Important


We have received many queries that implicitly ask a question about how old something is – typically the odd rock that they found in their backyard. A review of age dating, and why it can be critically important, seems appropriate here. 

Q:  
How old is this rock?
- various 

In 1979, a young USGS geologist named Rick H. was mapping flows on the north flank of a beautiful, glacier-covered, symmetric volcano in southwest Washington State. He had some idea of how old the Goat Rock Dome was that he stood on – textures and some sparse historical information suggested that it was very young. He had no idea that in a year the huge outcrop he stood on would be moved many miles to the north – that the spot he then stood on would be more than a thousand feet up in the air. The 1980 eruption of Mount St Helens killed 59 people – that is, that authorities are certain of. The death-toll could have been greater by more than 800 people. By a miracle of governance, those people were being held back at a roadblock until 9am on May 18th. However, 45 minutes before the gate would have been opened - to allow in people who had property around the volcano - the monster blew up catastrophically with a lateral blast. Authorities were certain of 57 people killed, but speculate that there were more caught in the blast that no one knew about. Gray dacite dust fell on cars in Atlanta a day and a half later. 

As the new chief scientist for volcano hazards, I made a point of visiting - and spending time listening to - every single staff member scattered over six different centers. This was not a trivial exercise; it meant lots of airport time and lots of listening. I made a point of spending the same amount of time with the technicians as I did with the senior scientists, and this helped me get a better view for how things were really working within the organization that I had just inherited. In one case, techs clued me in that one of our observatories was no longer functioning, due to a perfect storm of very human conflict starting with a management failure.

The experience was not all grief, however. One of the people I spent time listening to was Andy C., a brilliant young PhD geologist/geochemist who had decided to specialize in age-dating. I also talked with Jim S., a smart, furiously hard-working tech who worked with him. After listening to them, I travelled to the USGS national headquarters in Reston, VA, and tin-cupped around the building. I mean this literally – I carried around a tin cup with me to help break down resistance by disarming people with humor. I was looking for “spare change” in people’s budgets that I could divert to Andy’s laboratory. Spare change in my little 120-person volcano science team usually meant a few thousand dollars from my cost center budget. Two thousand dollars would pay for a young scientist to attend a science meeting, where he would not only learn what was going on in his field, but connect with people he could cooperate with. This meeting attendance had the effect of leveraging our meager funds to accomplish quite a bit more with them by getting others to help us accomplish our objectives.

To put this in perspective, a Stryker combat vehicle costs about $1,500,000. For the Department of Defense, however, I was looking for funding down in their noise-level. For them, our needs were what Sherrie G., a DOD executive, dismissed as “decimal dust.”

But Andy had both energy (he frequently worked 60-hour weeks) and a vision. His vision was to create a center of excellence: a laboratory for high-precision dating. I found and funded him with $250,000 to develop the world’s best state-of-the-art 40Ar/39Ar age-dating laboratory. This has allowed him to steadily refine the ages of very young volcanic rocks, which in turn allows us to put better and better parameters on the eruptive frequency of volcanoes – so we know what we may or may not have to worry about.

In the 10 years that followed, Andy had accumulated a sufficient number of very good age-dates that he was able to start looking in broad brush at the eruptive history of the entire Cascades range - and he can now see episodic pulses of eruptive activity in the past 500,000 years. Importantly, this includes the first hints that we are entering into an unusual period of volcanic activity right now.

In the past 5 years, Andy refined his +/- errors on an age-date from several thousand years to just 400-500 years. He did this by gaining precise control on the atmospheric Argon component in rocks, but also by refining his sample-collecting techniques. He sought the centers of lava flows, parts that were “platy” because they were shearing as they cooled while still flowing. He also avoided porphyries, because the internal crystals formed under different circumstances than the fine-grained that had extruded out onto the Earth’s surface. Other things he avoided included air-vesicles and glass: that is, water-quenched lava that inevitably had an Argon contribution from the water. How did he sort these out? By licking the rock with his tongue. If his tongue stuck slightly to the rock, experience showed that he would get a low-precision date on it. Since he was working long hours to deal with a huge back-log of samples, this kind of “pre-sorting” had gone a long way toward narrowing down his error-bars on dates.

So..... How Does Age-Dating Work?

Wait, you ask, how does age-dating work?  In the pre-radioactive isotope days, crude dating could be done by measuring how fast sediments accumulated in a lake-bottom, measuring the thickness of a stack of those sediments, and noting which sedimentary units lay above (were younger than) another layer. But while you could easily get the relativeages with stratigraphy, you couldn’t get good absolute ages – there were too many variables, like water levels, wind influences, and changing sedimentation rates.

It’s not hard to get the radioactive decay rates of anything if you have something like a Geiger counter: a certain number of atoms are “popping” every minute, and you could measure how many atoms were in the sample to begin with using some relatively straight-forward chemistry.

Once you have decay rates, age dating - in principle at least - is pretty straightforward.  A mineral solidifies out of a magma mush somewhere with uranium in it. By early in the 20th Century, the decay-process of uranium was well known… there were intermediate “daughter products” with different half-lives, but they all ended up at stable lead, where the decay process ended. All you really needed to do was measure the lead-to-uranium ratio precisely, and with the rate of decay you could get a handle on how long it had been sitting there since the last melt solidified. This wouldn’t work if the rock had been metamorphosed (or “stewed and cooked” as old miners would say it) since the initial solidification. In that case, the age you got was the last re-melt. 

All is not lost, however. Some really smart people eventually worked out how to get something of a handle on even this. It required some good geology, some good chemistry, and some clever statistics… but you could at least get an idea if something had been messed with since original formation.

There were other problems, however. The half-life of uranium-235 is 704 million years. Also, the precision was not that hot when you measure micrograms of uranium and lead in a mass spectrometer - and try to divide that up into 704 million years. In the best of circumstances, you get a rather large plus-or-minus – many thousands, even hundreds of thousands of years. In many situations that didn’t matter all that much. For instance when we were trying to figure which rocks arrived before which in truly ancient southern Venezuela – then 500,000 years or 5,000,000 years one way or the other didn’t matter all that much.
If you have two volcanoes, and one erupts every 20,000 years and the other erupts every 200 years, then this precision doesn’t help you at all. Unless you know the eruption frequency, you have no easy way to know how dangerous the volcano is.

THIS is why age-dating volcanic rocks is so important. Do we pour our meager annual instrumentation budget into Mount St Helens, or into (literally) Crater Lake?

There are other radioactive decay series, of course: rubidium-strontium, carbon-14 (which we use with volcanoes if we can find some fried vegetation under a flow), and the argon series. You can’t always find uranium-hosting minerals, and you usually can’t find rubidium-hosting minerals. If you find something burned under a lava flow, it can’t be older than a few tens of thousands of years, or the 14C is already all gone. But 40Ar has a more useful half-life, and argon is also a significant constituent of the atmosphere. It can be found just about anywhere in volcanic rocks – which came from a subducted ocean floor once exposed to the atmosphere.

Rats. There is yet ANOTHER problem: all that argon in the atmosphere is also going to pollute any measurement you make. Like carbon-14, which is “made” in the upper atmosphere by cosmic rays transmuting nitrogen molecules, Argon-40 comes from atmospheric Argon-39 – which is everywhere, and seeps into just about everything. If you want real numbers – true age dates – you must find a way to get clean and unsullied samples.

Where there’s a smart person, there’s a way. Believe it or not, it comes down to something as low-tech as putting your tongue on a rock. Andy - by trial and error - found that the best rocks to date were the ones in the "platy" middle section of a solidified lava flow. The final test was to see if your tongue sticks to the sample that you hammered out. Does it stick? Chuck it and look for another sample, because it won't give you a reliable age-date.

Bottom line:
It comes down to this: if we are being shot at, it’s important to know how OFTEN we are being shot at. You can plan. You can set up many forms of disaster mitigation to keep a crisis from becoming a catastrophe. Rock-dating information is crucially necessary in order to have even half a chance of predicting the volcano’s future behavior – and roughly calculating the risk it carries. High-risk volcanoes then claim the larger share of our very limited instrumentation budget. Crater Lake (the former Mount Mazama) last erupted catastrophically over 7,000 years ago. Mount St Helens has erupted more frequently than almost all the other Cascades volcanoes combined… the last period of repose was just 24 years. So with good age-dates, we invested the lion’s share of instrumentation on this critical, very-high-risk volcano.

We were relieved that we had done so when the 2004 eruption started with just over a week’s seismic warning. We had seismic and GPS “eyes” already in place and with them we could ”see” what was going on under the volcano. With a huge experience base acquired by studying hundreds of volcanoes in the US, Kamchatka, Japan, Indonesia, and Latin America, the scientists at the Cascades Volcano Observatory could predict (sometimes to within hours) when the next eruptive pulse was coming – and on 1 October 2004 they called for the evacuation of hundreds of people from the Johnston Ridge observatory.

They could not have made that emergency call if the geologists had not already carefully mapped its past eruptive products. And the eruptive history would not have been deciphered without precision age-dating. The 2004 eruption was not nearly as violent as the one in 1980. But even if it had been, the disaster of 1980 would likely not have been repeated. By 2004 we had dates and knew what this volcano was likely to do.
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