23 December 2013
A Christmas recipe for perfect landslides
Posted by Dave Petley
Context for the recipe
As Christmas approaches the newspapers in the UK are full of recipes for the perfect mince pie / christmas pudding / turkey. That set me thinking about the recipe for the perfect landslide. This is what I came up with:
Raw ingredients
You will need a pile of weak rocks. If these aren’t available, and shops do tend to sell out of useful things this close to Christmas, a pile of strong rocks with many joints will be a satisfactory alternative, and indeed may give some more spectacular large failures. Avoid very strong rocks with very few joints – this will give you a Scandinavian landscape, which is not completely without landslides but will tend to disappoint in terms of the frequency of events:
http://photorator.com/photo/7668/-pulpit-rock-preikestolen-norway-sergey-l
..
Volcanic soils are a perfect introduction to landslide generation for those looking for a quick win.
Initial preparation
Subject your rocks to a few million years of rapid uplift (I find that something over 5 mm per year will be ideal). This should create an over-steepened landscape, but to try to avoid making your landscape too steep – in this case you’ll tend to get rockfalls rather than landslides. Your landscape does need to have a decent slope gradient though. To create the perfect conditions you will need a landscape that has steep slopes right down to the rivers, but with occasional flatter areas (perhaps old river terraces) – you will see why below.
You will now need to marinade your landscape in a warm humid climate to generate really deep weathering – ideally to 20 metres depth or more. Daily temperatures of over 30 C and a millimetres of rainfall every day will be ideal. If you can get your soil to look like this then you will be doing well:
http://www.latinamericanminerals.com/gallery/photographs/
..
Shortly before you want to cook your landslides, I suggest that you completely deforest your landscape, ideally with aggressive clear-cutting to leave completely exposed soil, like this:
http://webeyedea.info/studentwork-11-12/3-greenpeace/daniel/pages/deforestation.html
..
Creating the correct oven environment
To get your landslides going properly you will need two key aspects to your oven environment. First, you will need very heavy rainfall. Ideally this should be occasional (every few years), very large rainfall events, either as individual storms (a strong tropical cyclone is a good example) or more prolonged rainfall with occasional cloud burst events (such as the SW monsoon or the heavy rainfall in some parts of the world associated with El Nino events). Second, you should subject your landscape to occasional strong earthquakes. Generally speaking, rare very large earthquakes will give better landslides than frequent smaller events. Very satisfactory results can be achieved by a combination of a strong earthquake followed by a large rainstorm.
Advanced landslide cooking
For those looking for a truly exceptional set of landslides, you can pep up the recipe with the actions of people. Ideally, your population should be poor, living on the isolated flatter areas you created in the landscape above, and rapidly developing. In doing so, they should throw away their traditional agricultural techniques to grow inappropriate cash crops on steep land that is being cleared. To get these crops to market the local authorities should build new roads without any assessment of the stability of the route corridor and with no engineering measures to provide drainage or to stabilise the soil. The international aid agencies will probably provide funds to support this type of approach. This example from Nepal will provide a good template:
..
The impact of the landslides will be increased significantly if you encourage the local people to move from their safe terraces to live beside the road.
Serving suggestion
A light sprinkling of global warming, and in particular increased storminess and higher rainfall intensities, should keep your landslides going well into the future.
Dave Petley is the Vice-Chancellor of the University of Hull in the United Kingdom. His blog provides commentary and analysis of landslide events occurring worldwide, including the landslides themselves, latest research, and conferences and meetings.
Great post
, but for the gratuitous global warming sprinkling. The globe’s been warming for 18 thousand years, give or take a couple.[Thanks, but you are wrong about global warming – D]
Wrong that the globe’s been warming for the past 18,000 years. You really deny that? Speak of global warming denial![Absolute rubbish. The temperature rose rapidly in the early part of this period as the earth emerged from the Devensian glaciation. Since then temperature has varied somewhat, but without a long term warming trend. This changed of course about a century ago when humans started to drive up terrestrial heat content. Skeptical Science debunk this myth thoroughly, based on sound science – D]
D –
So, suddenly you’re way back to the Devensian. That helps the conversation. Speaking more recently, there’s not been global warming since the end of the last ice age, over the past 18,000 years? There wasn’t a glacier, example, in what today is Lakes Cuomo or Superior?18,000 years isn’t long enough for you, but the past 100 years is? Speak of absolute rubbish.[Sigh – I am unsure as to what you are trying to gain from this exchange, other than to demonstrate that you do not understand the climate of the last 18,000 years. A simple lesson:
1. The Devensian is a technical word for the last ice age. Note – it is not the same as the Devonian in case anyone is confused). The Devensian extended from about 110,000 to 11,700 years BP, so the period that
you mentioned in your first comment includes the end of the Devensian, and then the Holocene.
2. As per my first reply to you, the first few thousand years of the period you mentioned, the end of the Devensian, was characterised by rapid warming as the earth emerged from the ice age. This ceased quite early in the Holocene.
3. Most of the Holocene, starting 11,700 years BP, has not been characterised by a long term warming trend. The early part of the Holocene saw the tail end of the end of the warming from the last glaciation; thereafter temperatures have varied but there was no net warming over a long time-scale until the start of the modern era.
4. As numerous papers have demonstrate, the last century has been characterised by rapid warming, the effect of human emissions of, primarily, carbon dioxide.
5. Lake Superior was glaciated during the Devensian. As the ice retreated around at the end of the Devensian, a large pro-glacial lake formed, known as Lake Duluth. There is a good diagram showing the post-glacial history of this area here. You will see that the response of the ice is to the late Devensian / early Holocene warming.
6. By Lake Cuomo I assume you mean Lake Como in Italy. The glaciers in the Como area also retreated at te end of the last glaciation. This study shows that it was ice free at 12,000 years BP.
I feel that I have spent a great deal of time, during my vacation, explaining very basic palaeoclimate information to you. I will not post further comments from you on this issue unless they are based on sound science – D].
Dave – don’t forget that the recipe should be shaken, not stirred!