Blue holes are drowned collapsed sinkholes probably formed during the Ice Ages.
SOMETIMES we may use the expression, “I’m feeling down in the dumps!” Depression is not only related to psychological matters for there are numerous forms of depression worldwide that are related to the earth’s surface.
Most of the latter are natural landforms on the land surface or on the seabed.
However, man’s activities have created several other forms through mining, quarrying and warfare.
NATURAL LAND DEPRESSIONS
Sinkholes are frequently found in limestone, chalk, gypsum, salt and sandstone rocks.
All these sedimentary rocks were once deposited in long lost seas.
Tectonic plate movements heaved and thrust these sediments from the ocean floor upwards to create the land surface.
In this gradual process these sedimentary rocks gradually dried out and in so doing vertical shrinkage cracks occurred, referred to as joints.
Horizontally to the near vertical joints, most sedimentary rocks possess bedding planes, marking sequences in the deposition of calcium carbonite (limestones) or sand grains (sandstones) as these sediments accumulated on the sea floor.
These joints and bedding planes allow water to naturally percolate downwards into the rock.
Gradually, the acidity of the water enlarges these joints and bedding planes through the process of solution until the water reaches an impermeable rock below.
High inputs of rainwater charged by atmospheric carbon dioxide or from the soil layer above the limestone creates an aggressive form of limestone solution in the form of carbonic acid.
The natural rate of limestone solution is five times faster in tropical climates than in temperate ones merely because the higher temperatures speed up the chemical reaction.
Caving in In time, the land surface collapses into the cave system below as the roof of the cave can no longer bear the weight of the overlying rock and a big depression thus occurs.
Rivers then descend into the caves below, often in the form of waterfalls.
These collapsed surface features are commonly known as potholes, sinkholes, swallow holes, swallets, dolines, or just ‘sinks’.
My travels as a geomorphologist have brought me to some of the largest sinkholes in Europe and Southeast Asia.
My first sinkhole encounter was in the carboniferous limestone area of the Pennine mountain range – the so-called northern backbone of England – at Gaping Ghyll, where a stream (Fell Beck) cascades 98 metres downwards into the cave system below.
In the Julian Alps in Slovenia, I swam in a large crater-like lake created by the collapse of a limestone sinkhole onto the impermeable rock layer below.
A few years later, when exploring Niah Caves in Sarawak, through the darkness, a huge of shaft of sunlight suddenly appeared with a waterfall.
There, too, the cave roof had collapsed with the surface water today plunging downwards onto the cave floor.
Sinkholes occurring under the foundations of a building is a likelihood in areas of limestone scenery.
Several instances of houses collapsing into sinkholes have been recorded in the US state of Florida, which is famous for its limestone scenery.
Florida is pockmarked with thousands of sinkholes and small circular lakes, so much so that insurers are obliged by law to cover the cost of the possibility of house subsidence into a sinkhole.
DISAPPEARING RIVERS
Normally a river increases its volume and velocity downstream but there are examples in limestone type rocks where this does not happen.
On geomorphological field trips I used to take my students to test this normal theory in the River Mole in Surrey, England.
Little did they appreciate that the underlying bedrock beneath the alluvium of the river’s deposits was chalk.
They found that this river, in the section they surveyed, did not increase its volume or velocity downstream.
When asked to explain the reasons for these phenomena, they scratched their heads until I revealed the secret.
There were seven sinkholes in that section of the river bed, with eddies occurring around each where the river had excavated holes into the chalk below and some of the river’s volume was disappearing underground! No one won the prize for the correct answer.
BLUE HOLES
Undersea sinkholes, often known as blue holes, have been located by divers in reef locations in Australia, the West Indies, the Bahamas and Belize.
The colour of the surface seawater gives them their name.
These features are no more than drowned collapsed sinkholes, which were initially formed when the sea level was much lower, probably during the Ice Ages.
On a visit to Malta and its island of Gozo, in the Mediterranean Sea, I swam in the crystal clear waters of the Blue Lagoon created in limestone rock.
Recently one such blue hole, now declared to be the largest in the world, was discovered by Chinese researchers under a coral reef of the Xisha islands in the South China Sea.
With a diameter of 130 metres and a depth of 301 metres, this circular depression is indeed a wonder of the natural world and is appropriately named as Dragon Hole.
Many other smaller blue holes have been found in China’s waters and in translation these are referred to as ‘sky holes’ or ‘heavenly holes’.
Prior to this year’s recent discovery, the deepest undersea sinkhole was located in the Bahamas at Dean’s Blue Hole with a mere depth of 202 metres.
SINKHOLES BY MAN
In the UK, there are often reports of houses and roads collapsing into sinkholes.
So far there has been relatively little loss of life.
Ninety per cent of these housing disasters have been occasioned by the collapse of former underground horizontal mining shafts, many of which are hundreds of years old.
Such incidents have occurred in the former coal mining areas of South Wales and Staffordshire.
In West Cornwall, a once prosperous deep tin mining area in the 18th to early 20th century (until more easily accessible alluvial tin was produced by dredging methods in the Ipoh area of Kinta Valley in Perak), the gradual collapse of former underground workings has occurred with the loss of some houses.
In Cheshire, sometimes houses sink into cavernous former salt mines.
The 1980s saw the collapse or subsidence of buildings into former chalk mines in North Kent near the Liquid Petroleum Gas (LPG) terminals.
These mines were used by an importing LPG company to store the gas by freezing the chalk and houses were subsequently built on the ground surface.
With a reduction in the need for LPG the underground storage caves were closed down and the once refrigerated chalk thawed out allowing rainwater again to run into the permeable chalk, resulting in cavern roof collapses.
Lessons have been learned by housing developers and constructors, but still prospective homeowners buy properties without paying a bit extra for a structural survey.
This should include a geology report on the underground rock structure.
Rest assured, in Kuching, deep piling into solid rock beneath the surface alluvium provides a firm foundation on which to build.
In other areas of Malaysia where limestone or any rock of a permeable nature is found, caution should be taken before agreeing to sign a purchasing document.
Today’s insurance companies carry out risk assessments of the area in which a property is located before issuing policies and the cost of covering building subsidence is an added extra.