July 14th, 2010
Look at this image carefully and you can see a car about halfway down the hole. According to newspaper reports, it’s a Toyota Camry. Sadly, there isn’t a record of what the owner said when they returned to their Toyota. I assume it was something like “Oh, s**t!”
The hole is in Tampa, Florida near the University of South Florida. When it opened up on Sunday, it ‘ate’ this car in the process. The car has now disappeared from view and the growing hole is threatening a car park and a block of flats.
This isn’t the first time Tampa residents have unexpectedly stared into the abyss. In January, a whole rash of holes (or maybe a ‘hole rash of wholes’) suddenly appeared, including a 2.5 m wide pit in someone’s back garden. Another hole near the University of South Florida burst a water pipe.
The underlying reason for Tampa’s problem is rock type combined with water use. Perhaps 1000m thick limestone and dolomite rocks lie under most of Florida. These are covered by sands and clays of various thicknesses.
Limestone and dolomite are mainly made of carbonates – the alkali chemicals used in some indigestion tablets for neutralising stomach acid. As with indigestion tablets, carbonate rocks react with acid water. The products of the chemical reactions are washed away. The water seeps into and widens any rocks that are full of cracks and joints, water seeps into and widens them. Over time, this process forms cavities, underground tunnels and caves.
The carbonate rock under Tampa are riddled with tunnels and cavities. Water has had many opportunities to sculpt the rocks because the sea level around the Florida Peninsula has risen and fallen many times since they were laid down beneath the sea millions of years ago.
Water also gets trapped and stored in the carbonates because they’re holey like Swiss cheese and the surrounding rocks are less permeable. It’s a bit like sandwiching a wet sponge between two glass plates. In fact, the carbonate rocks form an aquifer (a group of rocks that store water) under Florida that holds one-fifth of the water in the US Great Lakes.
Cut forward to the present day and the people of Tampa get their water by drilling into the carbonate rock aquifer and pumping it out. In winter, farmers pump lots of water from the aquifer onto their strawberries to stop them freezing.
The water filling underwater cavities and caves in the carbonates helps support the weight of the cave roof and overlying soil. When the water is pumped out, soil from above moves through cracks in the cavity roof and plugs the gap left by the water.
Sinkholes open up overnight for two main reasons (see a) and d) on the diagram below). First, water flowing downwards to fill the drained cavities eventually washes away the soil plug, causing a sudden slump of soil into the cavity and a hole to open up at the surface (figure d). Alternatively, so much water is removed that it no longer supports the cavity roof and walls. They weaken catastrophically, the cavity roof collapses and a hole opens in the ground (figure a).
One of these explanations is likely to explain the car-’eating’ sinkhole. As an aside, all this doesn’t explain another giant ‘sinkhole’ that consumed a Guatemala clothing factory and road intersection at the end of May. From what I can gather, Guatemala City is underlain by volcanic deposits that were washed away by rain from tropical storm Agatha.
Categories: Short walks