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Are they a natural phenomenon or induced by human activity?Karl Mathiesen investigates
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the wake of a sodden winter, a series of sinkholes have opened up
across Britain. But are they a natural phenomenon or induced by human
activity? With your help, Karl Mathiesen investigates
Join the debate. Post your views in the comments below, email karl.mathiesen.freelance@guardian.co.uk or tweet@karlmathiesen
Human activity has little to do directly with the formation of sinkholes.
But
there are also situations where mines collapse. Llanrhidian Marsh on
the north gower coast had a hole in it one day. Coombe Down in Bath has
suffered collapsing of mines, and there have been one or two cases in
Dorset too.
If Climate change is leading to more intensive
rainfall events, then previous above and below ground drainage systems
will become expanded to cope with that change. This would mean more
sinkholes, spring lines, landslips and the like as land previously
stable is no longer stable. It would also potentially lead to more
collapses of mine adits etc.
Proving a link between one specific
incident and climate change would be tough, but the physics underlying
the geological change is straightforward, as is the link to demonstrate
our actions are making a difference.
Almost inevitably, climate change rears its head. Without getting
too deep down that particular sinkhole I’d like to refer you to last week’s eco audit
in which I looked at the link between the recent storms in the UK and
climate change. We know climate change is making it rain more in
Britain. It follows that we will see more sinkholes as a result.
There is no particular research that I can find which links sinkholes and climate change. But Harley Means, a geologist at the Florida Geological Survey (FGS), told New Scientist that a correlation was not unlikely.
It
is possible, says Means, although there is no direct evidence of this.
Torrential rainstorms and long droughts are both predicted to become
more common in the next century as the planet warms,
particularly in the south-eastern US. “It’s speculation right now,”
says Means. He suggests that research that compares climactic conditions
with periods of high sinkhole activity could provide evidence.
But
Anthony Cooper from the BGS says high incidence rates may only last for
a time. There will be a number of locations close to collapse that
could be pushed over by a spate of abnormal rainfall. This would see the
kind of spike in occurrences we are seeing now. Once these have caved
in, he said the rate decrease.
Underwater sinkholes... The Great Blue Hole, Belize, is a
karst-eroded sinkhole, the result of the repeated collapses of a cave
system formed during lower sea level stands. Photograph: Christian
Fevrier/bluegreenpic
Updated
Surveyors Geoinvestigate said
recent sinkholes in High Wycombe and Hemel Hempstead were likely a
result of natural fissures in the rock or historic chalk mines, which
would not have been recorded. Chalk mines were accessed by a vertical
shaft which creates dangerous conditions for sinkholes.
The
workings date from 1700s – 1900s and have been identified in Nash Mills
and Highbarns areas in the south of the town. Chalk was mined for
building stone and burnt to form lime to spread over clay soils to
improve agricultural land. Typically the workings were small and reached
by a vertical shaft in the chalk which would be infilled at the end of
the shafts life, the miners moving on to dig another shaft nearby.
Because
of water erosion and softening of the ground natural fissures or
openings in the chalk or man made mine opening eventually became so
large that the clay soil which covers and hides these holes can no
longer provide a solid bridge over it. The result is that the ground can
collapse either gradually or as at Oatridge Gardens [Hemel Hempstead]
catastrophically and without warning. Where the ground failure is
progressive and slower it is sometimes possible to see the signs of
problems such as building cracks, building subsidence and the formation
of saucer shaped depressions in the surfaces of driveways, roads, lawns
or agricultural land.
Updated
Humans can cause sinkholes
Anthony Cooper, a geologist at the British Geological Survey (BGS), says humans can certainly play a role in the creation of sinkholes, but they can also be avoided.
“I
don’t think man in this country is aggravating them to a large degree,
although there are exceptions are there are precautions that need to be
taken. If people do things properly, there shouldn’t be an increase.”
He says proper drainage is important to avoid sinkholes, as well as the careful monitoring of pipes and runoff.
The BGS website says:
Several things can trigger sinkholes. The simple process of gradual dissolution can cause a sinkhole to form at the surface.
However, other factors, including humans can induce sinkholes to form, such as:
Heavy
rain or surface flooding can initiate the collapse of normally stable
cavities, especially those developed within superficial deposits.
Leaking
drainage pipes, burst water mains, irrigation or even the act of
emptying a swimming pool are all documented examples of sinkhole
triggers.
The commenter below summarises the way in which human activity can channel water into areas and contribute to sinkholes:
There are a number of contributing factors to consider here:
1.
The increase in population and the spread of the population means that
there is a greater chance of encountering a sinkhole. So the odds go up
simply by being more proximal. 2. The increase in impermeable
surfaces means that rainwater does not infiltrate direction into the
ground and percolate into the water table (again this may be as a result
of urbanisation). This means that where rainwater/surface-water does
finally make it to a place where is can infiltrate into the ground, this
infiltration is concentrated. Combine that with an area where the
erosion of sand and gravel results in a sinkhole and you have an
increased likelihood (though by no means a certainty). 3. Building,
mining, piling etc in prone areas can alter the loading, drainage and
groundwater regime, thus increasing the likelihood of a sinkhole
forming, or possibly speeding the process up.
As someone else
pointed out, the erosion of rock to form a sinkhole is on a geological
timescale (albeit a pretty quick one by those standards) not really a
human one. However the removal (washing out, collapse etc) of infilling
sediment to expose the sinkhole can be abrupt (it's a bit like
Kerplunk). It's also worth remembering that some features attributed to
sinkholes are a result of historic mining and the collapse of addits.
Cornwall (for example) is littered with old shafts and addits, and
historically no-one was very good at keeping track of where they were.
Sinkholes can also be caused by a dramatic reduction in ground
water, says Cooper. “A water filled cavity beneath a bridge of material
will actually support that bridge to a certain degree. The physical
presence of the water helps to support the cavity. So if you the pump a
lot of water out, all of a sudden that material which is water saturated
and heavier suddenly collapses in.”
There have been a number of documented examples in Florida and China (and suspected cases in the UK) in which abstraction for agriculture has caused the earth to subside.
“Plant
City, pop. 32,000, is Florida’s strawberry capital. During a record
cold snap this past winter, farmers pumped millions of gallons of water
onto their strawberries to keep them warm. Cavities in the limestone
aquifer emptied, weakened and gave way,” said the Wall Street Journal in
2010.
Ian Stewart (the geologist, not the mathematician) covered this quite well in Horizon
a few weeks back. He put the blame mostly on urban sprawl, but he also
said that increased agriculture in some areas was causing increasing
volatility in the local water table, which he said could also be a
factor.
According to the BGS,
Lewis Carroll was inspired by the common sinkholes around Ripon, not
far from where he lived as a boy, when he imagined the rabbit hole that
lead Alice to Wonderland.
Alice fell
down- down- down- deep into the earth, following the white rabbit.
Curiouser and curiouser she thought, in this underground wonderland the
walls are made of sparkly gypsum - CaSO4.2H2O (with apologies to Lewis
Carroll). It has been suggested that the author’s vision of Alice
falling down a deep vertical hole into an underground land was inspired
by natural geological events, notably subsidence at Ripon in North
Yorkshire. There is a connection between the author, the city
of Ripon, and dramatic subsidence that occurred at Ure Lodge, where the
alleged model for the published ‘Alice’ illustrations used to live.
Beneath this area, the gypsum has a water-filled cave system within it,
but gypsum dissolves quickly so that the caves enlarge and commonly
collapse. Collapse at Ure Lodge has continued to the present day. It
recently caused the destruction of four modern garages and the
evacuation of several houses, including the Lodge itself. Subsidence in
Ripon, and many other places underlain by gypsum poses a severe
constraint on the development of those areas.
Thanks to Robin Bisson from the Science Media Centre for the tip off.
Updated
Updated
Good question from @moregieves. Can anyone help?
Art critic Jonathan Jones has written for the Guardian
about holes in the earth in art and literature. Rather than fear them,
he says, we should allow them to awe us into delight as they give us an
insight into the momentous forces that create the ever-changing world.
To
build or buy a house, to simply drive to work and expect home to be
there when you get back, is actually to take a massive gamble on a
turbulent hidden planet. Sinkholes are no surprise - just a sudden
revelation of the massive forces that are constantly changing the ground
beneath our feet. Catastrophic change and unimaginable metamorphosis
are the rule, not the exception, in geology. Sinkholes merely make
visible what we choose to ignore.
A view of a large crack on a house in
Magadalen's Close, Ripon after a huge 25ft wide sinkhole opened up in
the street. Photograph: Anna Gowthorpe/PA
Updated
Another view of Britain’s soluble rocks, this time with the
different types of rock. The timescale of erosion of limestone is
thousands of years, according to Anthony Cooper of the BGS. Whereas
gypsum can disappear in tens of years.
What causes sinkholes?
Water. Water. Water. Anthony Cooper from the British Geological Survey
(BGS) says the single most important element in any sinkhole event is
water, either in absence or abundance. Too much water can cause soluble
rocks such as gypsum and chalk to dissolve and erode, creating
underground shafts. Conversely, if ground water is removed, through
abstraction or prolonged drought, underground rocks can crumble under
the pressure from above.
Sinkholes can occur slowly or
dramatically. This is dependent on the material that coats the surface.
Sand will subside along with the material beneath. Meaning a gradual
sinking. But a more robust material like clay can hold together for much
longer, leaving a chasm beneath. Sinkholes 101 with the BBC’s David Shukman.A fish tank demonstration of sinkholes from the British Geological Society.
The Guardian’s John Henley investigated what causes sinkholes in 2013:
Natural
sinkholes – as opposed to manmade tunnel or cave collapses – occur when
acidic rainwater seeps down through surface soil and sediment,
eventually reaching a soluble bedrock such as sandstone, chalk, salt or
gypsum, or (most commonly) a carbonate rock such as limestone beneath.
In a process that can last hundreds, sometimes thousands of years, the
water gradually dissolves small parts of the rock, enlarging its natural
fissures and joints and creating cavities beneath.
As the process
continues, the loose, unconsolidated soil and sand above is gradually
washed into these cracks and voids. Depending on how thick and strong
that top layer is (sand will not last long; clay can hold out for
millennia), and how close to the surface the void beneath is, the land
may be able to sustain its own weight – and that of whatever we build on
top of it. But as the holes grow, there will come a day when the
surface layer will simply give way.
“Once those caves start to
collapse, the materials above will simply funnel in,” says Dr Anthony
Cooper, a principal geologist at the British Geological Survey,
which maps the country for rock types susceptible to sinkholes and
carries out surveys for developers, builders and individuals worried
about the prospects of the land caving in beneath them. “It’s just like
an eggtimer, really.”
Updated
When does a pothole become a sinkhole?
Updated
Pockmarked Britain
Britain’s skin has broken out in a rash of sinkholes this winter.
Normally, Britain averages one reported sinkhole each month. But in
February five significant holes have opened. A woman is in hospital in Swansea after her car fell into a hole in her garden yesterday. Another in High Wycombe swallowed a Volkswagen Lupo. A sinkhole which swallowed a car on a driveway in High Wycombe, Buckinghamshire. Photograph: Steve Parsons/PA
Scientists from the British Geological Survey (BGS) say this
winter’s incessant rain is the major factor in the number of sinkholes.
But the other important factor is the type of rock below the surface.
Soluble rocks like gypsum and chalk, which abound in parts of England
and Wales, can erode over time, leaving a chasm beneath our feet.
In
Britain, scientists differentiate between sinkholes caused by erosion
of rocks and the collapse of old mine shafts. Mineral extraction has
left the country riddled with unmarked holes, which were not gazetted
until the 1870s. This means Britain is prone to both sinkholes and mine
collapses. Sinkholes in Britain in February and the occurrence of soluble rocks. Photograph: Guardian
Updated
Welcome to the eco audit
Sinkholes speak to the same fear as earthquakes and volcanos, they
are the moment when our benign home reveals its true and violent nature.
We see Earth as an immovable object. “Solid ground” is our habitat, our
safe, intransigent anchor in a dangerous universe. It’s also a myth.
Earthquakes
and volcanos are created by forces that transcend humanity, they are
beyond our control. But this is not the case with sinkholes. Some are
caused by humans. A yawning pit that swallowed a factory in Guatemala
City in 2010 was caused by a leaking sewage pipe. A sinkhole at a street intersection in Guatemala City Photograph: Luis Echeverria/AP
Britain has seen a spike in the occurrence of sinkholes in the past
month. Geologists say water is the primary factor in any sinkhole event,
and Britain has certainly had its fair share of that. But what other
factors are at play? Are sinkholes becoming more common? What role do
humans have in their creation? And can we control them?
Please join in the discussion by contributing in the comments below,tweet me or email me. If you are quoting figures or studies, please provide a link to the original source. Later I will return with my own verdict.
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