Thursday, January 05, 2006

Most of the geological record is missing!

A thought that came to me as I was looking at photos of cores of rocks deposited millions of years ago. The record as we see it shows what is preserved, it lacks what wasn't. An obvious statement perhaps, but consider this: what we see tends to stay in the mind, what we can't see tends to be ignored. So it is with geological cores. An example:

A sequence known geologically as a turbidite consists of sediments that were deposited in deepwater having been rapidly dumped from a shallow water environment. For example, a mass of unstable sediment sitting near the edge of the continental shelf, can be triggered by an earthquake into a huge underwater flow down the continental slope onto the abyssal plain. Such processes happen all the time (see this explanation).

In the geological record we may see hundreds of discrete turbidite beds stacked one of top of the other. They are quite easy to detect, having unique characteristics defined as Bouma sequences. What is interesting is that an entire Bouma sequence from coarse sand at the base to fine silt at the top might have only taken days to form. In truth, most of the bulk of sediment arrived almost instantaneously. Eventually conditions reverted to "normal" and a slow, fine rain of mud created the final part of the sequence until another underwater avalanche rushed down the slope. Such turbidity currents might occur over widely spaced intervals. The result is that the sedimentary evidence is biased toward what we see and that is a lot of material deposited in a day or two compared with not much fine material that might have taken hundreds of years to accumulate.

A similar situation exists onshore in a desert. We see sand dunes marching across a desert (well, they move quite slowly, but they do move!) and then see a frozen dune in a Triassic outcrop and can identify its origin by the internal texture and structure that is preserved. But we are not seeing a moving dune, rather the last position of that dune system before it was buried as geological history.

The science of geology allows us to reconstruct what probably happened in either case (for example we can predict current and wind directions) but what the geological record can't tell us is what might have been happening during the much larger gaps in the record.

This phenomenon mirrors the major misunderstanding that under-pins most modern scientific thinking on global warming - we spend far too much time emphasizing what we can see and measure, not enough time on thinking about what we can't see and measure. If we could we wouldn't make so many sweeping statements that translate into fear and panic in the non-scientific layity (and that includes the politicians!)

3 Comments:

Anonymous Grant said...

An interesting point and the sort of thing that the general public probably don't think much about in any aspect of human knowledge. ANd I count myself in that grouping though thoughts similar to the point you make came to me 3 or 4 years ago.

Even our own historical knowledge is related to an incredibly short period of time and even then much of the even the last millenium is guesswork.

I wonder what could be found under the sands of the Sahara if only we knew where to look. Or along the crumbled coastlines of the world. That ever more ancient things seem to be found ever more frequently (well, at least things that can survive well like flints) might suggest that much may have come and gone, perhaps related to climate changes, without leaving much to trace.

Of course that could be a completely stupid view but somehow I find the apparent gain and loss of civilisations like the Greek and Roman eras suggest the mankind has a propensity to learn and destroy but retaining some previous knowledge. Without that rapid change in science and technology as we have experienced in the last 100 years seems unlikely. I mean, why only now?

In geoligical terms our 100 years would be hard to spot and pinpoint for researchers in the future should some major catastrophe destroy much of what we have.

In fact whet of the 20th century would survive? Is there anything constructed, other than some concrete foundations for major engineering works perhaps, that would survive an extended period of planetary turbulence?

I doubt it. Especially not computers.

Which then led me to wonder, based on some of the strange things that seem to have eixsted many thousands of years ago, whether mankind has in fact reached something akin to our current stage of development before. Perhaps several times, only to be thwarted by nature in one way or another.

The idea may be ludicrous - but not more so than the concept that we can control the climate of the planet by controlling CO2 levels.

Thanks for the blog. It's good to hear the questioning voices more and more frequently.


Grant

1:04 AM  
Anonymous Paul said...

The Nazca Lines might be a good representative of a pre-historic event that is barely recognizable today and very poorly understood. It is interesting how the most popular "theory" is that they are the work of aliens.

8:26 AM  
Anonymous Anonymous said...

not the most popular theory among scientists, infact, the idea that the nascan tribe was under the influence of narcotics at the time is now more widely accepted, and much more amusing too

10:47 PM  

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