UKC

I'm preparing a short presentation for our U3A geology group. The theme is that geological processes have been going on since the Earth was formed 4.5 billion years ago and are going on all the time; some noticeable, others less so.

One of several illustrations I am using is that of the rockfalls in the Grand Couloir of Mont Blanc. Some are really quite spectacular.

Anyway, whilst watching one of the many Youtube videos of the Couloir I was musing as to, with the frequency of such falls, what the scenery up there might look like in however many hundreds or thousands of years.

Only an idle wondering on my part but anyone care to speculate?

In reply to Bojo:

Probably not too different.  As I'm sure you know, hundreds or thousands of years don't mean much in a geological scale.

Perhaps a comparison between a craton area and an area of mountains like the Alps would be good, showing a comparison between human lifetime events like stuff in the Grand Couloir, thousands of years like the last ice age, millions of years like the rise of the Alps of billions of years in a craton area where everything gets worn down but not much else changes, relatively speaking.

I'm sure it'll be good though.  Who doesn't like a good rockfall seen from a safe distance?

T.

Like Tony said, thinking about changes over different time scales is interesting.

Climatic controls will probably produce the biggest in the short term (100s to 1000s). If the highest peaks undergo more freeze thaw weathering, then they will probably have higher erosion rates etc.. Less snow and higher freezing levels might mean more instability and more rockfall higher up.

Currently the Alps are staying about the same size. Uplift due to tectonic forces and erosion are balanced (https://www.google.com/amp/s/phys.org/news/2009-11-alps.amp)

​​​​​​Over tens of millions of years this may change and the Alps might erode away to gentle hills (speculation!), And in 200 million years it could all be at the bottom of the sea again ( enormous speculation!).

I think the relationship between tectonic uplift and erosion is interesting and provides a chance to chat about mountain building and then show some pictures of epic rock falls, and then a bit of climate change chat too.

Good luck.

​

In reply to Tony Buckley & dunnyg:

It is indeed a fascinating subject. Perhaps my timescale of thousands of years was a bit too short.

I read something the other day saying that if the history of the Earth was compressed into 24 hours then humans arrived 78 seconds ago!

The more I walk in the mountains the more I try to take in how the scenery that we know so well came into existence.

Although not the Grand Couloir this one never fails to amze me:

youtube.com/watch?v=5uOiQ__iOmE&

Spectacular big time

In reply to Bojo:

This arm comparison is one I've used before; as it relates to a part of the body, pretty much everyone can relate to it.  The description below, pinched off someone's internet page, is about as exhaustive as you'll ever need.

T.

Hold your arm out with your fingers stretched, which ever you like, your choice. Place the tip of the index finger of your other hand in the middle of your chin, this the formation of the Earth, ~4560 million years ago. The next check point is where your neck and should meet. Here the Earth’s surface first solidifies and is then destroyed when an object the size of Mars collides with the Earth. The debris from this collision becomes our familiar moon. Next, we come to your armpit, it is here, around 4000 million years ago, that the first life appears.

About 3500 million years ago, round about your upper bicep, oxygen is suddenly more abundant in the Earths atmosphere and the chemistry of our planet is changed forever. Currently the blame for this ‘Great Oxidation Event’ is placed on the evolution of photosynthetic cyanobacteria.  After this, nothing much happens until we get to your elbow, around 1600 million years ago. A bacteria swallows another bacteria, and for some reason the larger bacteria does not eat its victim. This is endosymbiosis, the evolution of the complex eukaryotic cell, the smaller bacteria eventually becomes the mitochondria of our own cells. You would think an event like this would revolutionise biology, and it does eventually. But like the bacteria before them, the first eukaryotes do nothing much for a really long time.

This period of ‘nothing much for a really long time’ lasts from your elbow to your wrist, around a billion years. 

From around 800 to 600 million years ago, the base to the top of your wrist, a lot happens. Some eukaryotes do a second endosymbiosis with cyanobacteria and become photosynthetic algae, some also start working together in a single body and become the first sea weeds, who also invent sexual reproduction. Some amoeba start eating other amoeba, who in response either make the first shells, or learn to swim. This triggers a predator-prey arms race that rapidly escalates into a savage war for survival that rages to this day. At 550 million years, the base of your palm, some of the combatants grow large and start leaving visible fossils in the rocks. Some argue these creatures are plants or fungus, but most agree they are simple animals, or at least animal style organisms. At 542 million years, the middle of your thumb pad, life enters its nuclear age. Large shelled organisms appear, things with eyes and mouths and teeth and perhaps the greatest biological innovation of them all; an anus that is separate from the mouth.

By the top of your palm, large life forms are everywhere. They fill the oceans, they cover the land, they have infiltrated every single niche available, and they have made as many new niches. In the middle segment of your index finger, big lizards appear. A bit later they vanish when a rock the size of small mountain, and travelling at 250 thousand km per hour, tries to visit Mexico. Very quickly the lizards are replaced by furry warm-blooded, live birthing organisms called mammals. Around the middle of the last segment of your index finger the Earths vast tropical forests are replaced by grass lands. As grass is shorter and harder to climb than trees, some of the arboreal mammals called apes decide to live on the ground. To avoid being eaten they start walking upright to see over the grass. By the last quarter of the last segment of your index finger, some of those apes have started using simple stone tools to help them catch and prepare food. Shortly after, they also figure out how to make fire and how to attach a sharp stone to a long stick so they can catch food from a distance. Suddenly the Earth gets very cold and the upright apes decide to leave their warm homeland and travel to some of the coldest parts of the Earth.

This brings us to the very end of your index finger, the final fraction of a millimetre of your fingernail. This thinnest sliver of your body, and of Earth History, represents all of recorded human history and civilisation, it represents every ‘modern’ human who has ever lived or ever will live. Around the last 300 thousand years or so. The entirety of human history, both written, archaeological and biological, is just the thinnest wisp of breath in the lifetime of our planet

In reply to Tony Buckley: While geologic processes work slowly on average in the short term they can be quite rapid, and large features can form very quickly (glacial floods can do a lot of work very fast as an example ) - in this case I'd say that while the rockfall on the grand couloir will erode in current conditions roughly as we see now, it wasn't many thousands of years ago we were in an ice age and things were very different.  That's before we start going back to before the alpine orogeny etc.

In reply to Tony Buckley:

Au contre - lots of UK uplands had alps levels of glaciation not 18,000 years ago. I’d expect an evolution towards a “eryrification” (if you will) of the Euro alps in 20k years if climate change continues as it is.

In reply to wbo2:

Another fun thing which really bucks geological time is the refilling of the med after the Messinian Salt crisis - which is estimated to have taken about 10(ten) years

In reply to Tony Buckley:

Actually, there was a very important period of crustal growth and tectonism in the Late Archaean.

In reply to Tony Buckley:

Interesting to note that had the humble lawn mower been invented several tens of millions of years earlier we might all still be walking on all fours.

In reply to Tony Buckley:

I'm a geography teacher and I use a 100m timeline to do the same. Can't remember the figures exactly off the top of my head but recorded human history is something like 0.2mm at that scale.

In reply to John Stainforth:

Wll yes, and the Late Heavy bombardment and a few other things, but then there's a fairly long period where nothing much happens (!) except a prolonged period of craton accretion.

In reply to Bojo:

There is a good read on SummitPost about the Les Drus rock falls. As I remember there is a stunning video on the internet of the one in 2011 from across the valley.

https://www.summitpost.org/the-fall-of-the-alps/794108

Edit to add: Maison de la Montagne in Chamonix has an exhibition on climate change impacting the Alps, including the Les Drus rock falls. 

In reply to wbo2:

A lot of what happens isn't preserved in the rock record.

In reply to Stefan Jacobsen:

I've yet to see a copy but this recent French book from Editions Guérin might be of interest

'Coup de chaud sur les montagnes' by Bernard Francou et Marie-Antoinette Mélières (see https://www.editionspaulsen.com/les-livres/les-livres-guerin/les-livres-gue... )

In reply to John Stainforth:

Tell me about it!

In reply to wbo2:

Yes, it was a deliberate gross understatement.

Last night I came across a report by the education  department in Victoria, Australia. It stated that many students(and, I suspect, many adults) have a poor grasp of geological time scales. Many apparently believe that the Earth has existed in it present form as long as it has existed. They believe that the Earth never changes and that, for instance, mountains were formed solely by volcanic activity a few million years ago and have remained in their current state since. Even worse, of course, is the biblical myth that the Earth is only 4000 years old and was "created" in six days. Then, of course, there are those folk who, when told that Yr Wyddfa is comprised of volcanic rock run away with the idea that Llyn Glas below the summit occupies an  old volcanic crater.

In reply to Bojo:

People haven't really needed to understand things over those temporal and spatial scales, so it doesn't come naturally. I've got a background in geology and it regularly boggles me still!