Wednesday, 13 May 2015

Sinkholes: Nature’s most awful trick

It is easy to forget that the planet we live on is in many ways itself a living thing. Part of what makes the Earth so amazing is that it is one of the few places in the universe we know about that is geologically active. Internal forces churn magma, tectonic plates shift and crash into each other, volcanoes spew rock and ash sometimes with explosive force. The thing that these forces have in common is that they generally work slowly but surely beneath our feet before revealing themselves in some cataclysmic event – an earthquake, an eruption, etc. We can live with this because science (and a little common sense) lets us know where the danger zones are so we can avoid them or at least know about the risk. However, every once in a while the Earth quite literally pulls the rug out from beneath us.

Anyone who regularly watches the news has at some point seen footage of the geologic event known as a sinkhole and it probably boggled their mind. The picture is usually the same: a busy street in the middle of an urban area punctuated by a massive, seemingly bottomless hole in the ground, the black maw swallowing any light foolish enough to fall on it. Sinkholes that form suddenly to suck up a building or car are scary things. They often occur without much warning and if you are the unlucky sap driving over top when one decides to open up, there really isn’t much you can do about it.

Sinkholes form because of the layered nature of the Earth’s crust. Ask any kid with a shovel and they will explain that as you dig down into our planet, you will encounter various layers. To keep things relatively simple let’s focus on two key ones: the top bit we all know and love comprised of sand, soil, clay and the odd rock (occasionally and unceremoniously called “overburden”) and the layer of more solid rock beneath it called bedrock.

Bedrock can be made from any number of different types of stone, some extremely hard and tough and others that are more porous and relatively weak. Weaker forms of bedrock generally include things like salt, gypsum, limestone or dolomite. While you certainly wouldn’t want to bash your head against a piece of limestone, it is pretty wimpy in the world of rocks. Limestone and its kin are easily eroded by water that is even a little acidic. As water works its way through the ground is often absorbs chemicals like carbon dioxide that increase its acidity, allowing it to eat away at bedrock.

If you’ve ever explored a cave or been to the grand canyon you’ve seen what water can do to rock given enough time. Problems occur when the overburden on top of a layer of eroded bedrock has a little rigidity to it. Sometimes. seemingly solid ground we are walking, driving or building on is really just a soft cap covering a gaping chasm of nothingness. When enough weight is place on top of such a cap, or enough water flows through it to weaken it, the whole thing can come tumbling down.

But the really unfair thing about sinkholes is that they can sometimes form in places that we know are solid. One particularly awesome example of such an event is what happened in Louisiana at a place called Lake Peigneur on November 20, 1980. The thing about Lake Peigneur is that it sits on top of a massive salt deposit that is covered by a thin layer of soil. It also has some modest oil reserves. With that in mind, in November 1980 there were two industrial operations going on beneath the water: a salt mine and an exploratory oil drilling project.

Unfortunately, the team drilling for oil made a miscalculation about where one of the channels for the salt mine was. They drilled through the soil beneath the lake and right into the chamber where people were working. The men in the mine we barely able to escape before water started rushing in through the 14 inch hole the oil rig had created. Essentially they pulled the cork on the bathtub that was Lake Peigneur.

The thing about water, as we have learned, is that is can dissolve salt. As the water filled the salt mine it ate away at the walls, expanding the cavern. Over the course of a few hours the entire lake drained into the ground, pulling the surrounding Earth down with it. The canal that ran out of the lake into the Gulf of Mexico even changed direction, following inland and creating  a 150 foot waterfall (the biggest ever in Louisiana). The pressure and displaced air even created a 400 geyser. It was a scene out of the end of the world.

The vortex of draining water sucked up barges and fishing boats until enough water had been pulled in from the Gulf to equalize things, at which point everything that was sucked down popped back to the surface. Nowadays if you go to Lake Peigneur you can see the chimneys of houses that used to sit next to the lake poking out the salty water.

Keep that in mind next time to feel like you have your feet planted on solid ground. Sometimes it’s hard to know what lies beneath.

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