Not Geography Geography Lesson 5
Sneaky Sinkholes
It is that time again - not geography geography time! And this week's super-exciting topic is sinkholes (What? How? Why? Where? So many questions)! They have been in the news quite often in recent years, and are fascinating and kinda horrifying in equal measures - so lets go.
Now That is One Big Ole' Hole!
Sink holes, or 'dolines' in geological terminology, are (predictably) holes in the ground that can open up within minutes, and range in size from a couple of meters to tens of miles across, and many meters deep. Occasionally, people are killed when they're house or car is swallowed up by one of these holes, such as widely-reported Floridian Jeffery Bush in 2013.
So The Earth is Swallowing People... How?
The actual mechanics of sinkholes is really simple. They occur in areas where the underlying rock is easily erodible by water, mostly limestone, topped with unconsolidated or weak rock, like sand or clay. When acidic water from rain, groundwater, aquifers etc. passes through the limestone layer, it erodes the rock, and eventually will do so to such an extent that a significant portion is missing (known as a 'karst') . Now, you have a few thin layers of soil/rock bridging the gap above the empty void, with your house/car/swimming pool/pets/helicopter sitting on top for good measure. At this point, anything which disturbs the balance - heavy rain or building work normally - could cause the top layers to collapse, and everything falls into the hole underneath. You'll be going down fast than a post-sesh kebab. Bad times.
Slightly Worrying - Where?
Because limestone is a very common rock foundation in lots of areas, theoretically anywhere meeting the conditions (limestone + sand/clay + house + trigger) could be prone to sinkholes. But, the last time I checked, large proportions of the Earth weren't doing Swiss cheese impressions. So, there are some other factors to account for - the acidity of the water flowing through the limestone varies greatly geographically (between pH 6.5 - 4), as does the size of the limestone, and the strength and depth of the overlying rock. If your water isn't acidic enough, or your limestone layer is only small, or your top layers are deep and strong, you won't get a sinkhole (boo).
The spread of susceptible areas seems to reasonably even globally, but we only hear about the ones which have affected humans, and they seem to be concentrated in the USA and China. In fact, the USGS have determined that 20% of the US is vulnerable to the ole' sinkhole. Florida particularly is a hotspot, and house insurance is almost impossible to get in some of the worst affected areas.
Is My House Going To Fall Into A Big Hole?
Before anyone starts frantically consulting geological maps and puts their house up for sale, there is good and bad news on this one. Bad news - sink holes are very unpredictable, because lots of places have the right conditions, but we can't tell what is happening under the surface until its already happened. Good news - sinkholes don't often open up quickly, so there is lots of opportunity to evacuate and then rebuild, and they are also pretty rare.
There are some speculations that climate change has and will continue to increase the number of sinkholes, as more extreme rainfall is a trigger, and rain acidification intensifies the erosive capability of that rain. Also, as we build more and more houses in new areas, often unmoderated in poorer countries, we could be unknowingly constructing sinkhole time bombs.
On that cheery note, thank you for reading!
*Rather not shockingly, sinkhole gifs are quite hard to find, so I tried my best okay.
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