Sinkholes By: Cristian Bayona

A sinkhole is a hole in the ground caused by some sort of collapse of the surface layer. Sinkholes come in all different sizes from 1 to 600 m both in diameter and depth, and vary in form from earthy holes to rocky chasms. Sinkholes may form gradually or suddenly, and are found worldwide.
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.
People can create sinkholes when building roads, aquifers, or other types of construction. Altering land in these ways can weaken the underlying rock and make it more susceptible to sinkholes. Sinkholes can open up in the middle of busy streets or in neighbourhoods, especially during heavy rainfall.

Dissolution Sinkholes

The Formation of Solution Sinkholes.

A dissolution sinkhole a hole formed in soluble rock by the action of water, serving to conduct surface water to an underground passage. They can sometimes become ponds if the depression gets lined with debris, trapping water inside. Dissolution sinkholes happen slowly and are generally not dangerous, but ones that become a pond can drain suddenly if water makes it through the protective bottom layer.

Cenotes

This is a cenote. Cenotes and dissolution sinkholes are very similar. The only difference between the two is that a cenote is a deep natural well formed by the collapse of surface usually limestone that exposes ground water underneath.

A cenote in the Yucatan Peninsula.

Meanwhile, a dissolution sinkhole is a hole formed in soluble rock by the action of water, serving to conduct surface water to an underground passage. If dissolution sinkholes get clogged by debris they can hold water like cenotes though. Cenotes are very common on the Yucatan Peninsula of Mexico. There are more than 2,000 cenotes on the Yucatan, and they are a main source of fresh water for people there. Ancient Mayans believed cenotes were passageways to the underworld.

Cenote at Bolochon, Mexico, used as source of water 1842.

Cover - Subsidence Sinkhole

The second type of sinkhole is a cover-subsidence sinkhole. These sinkholes happen in areas where sand covers the bedrock. The sand filters down into openings in the rock, gradually causing the land surface to sink. Continued erosion increases the size of the depression. Like dissolution sinkholes, cover-subsidence sinkholes happen slowly.

Cover subsidence sinkhole in Mount. Gambier Australia.

Cover-subsidence sinkholes tend to develop gradually where the covering sediments are permeable and contain sand. In areas where cover material is thicker or sediments contain more clay, cover-subsidence sinkholes are relatively uncommon, are smaller, and may go undetected for long periods.

Cover - Collapse Sinkhole

The most dangerous type of sinkhole is a cover-collapse sinkhole. Cover-collapse sinkholes may develop abruptly and cause catastrophic damages. In these cases, the bedrock is covered by a layer of clay. Beneath this ground cover, however, water dissolves an underground cavern.

The Formation of a Cover Collapse Sinkhole.

Cover collapse sinkholes form when the bedrock roof of a cave becomes too thin to support the weight of the bedrock and the soil material above it. The cave roof then collapses, forming a bedrock-collapse sinkhole. Bedrock collapse is rare and the least likely way a sinkhole can form, although it is commonly incorrectly assumed to be the way all sinkholes form. Gradually, ground sediments begin to erode into the cavern from the bottom. The ground continues to crumble from beneath until only a thin layer remains between the surface and the underground opening. When that layer collapses, like when someone walks on it, the sinkhole opens up suddenly, swallowing any structures on top.

Where Have Recent Devastating Sinkholes Occurred?

A very devastating sinkhole that happened not to long ago was the 2010 Guatemala City Sinkhole. After being hit by Hurricane Agatha and being hit by a ton of rain, the rain eroded the ancient volcanic ash Guatemala City is built on. This caused the huge 20 metre in diameter and 90 metre in depth sinkhole. It was caused by a corroded sewage pie deep underground. This sinkhole swallowed a whole factory.

Guatemala City Sinkhole 2010.

Other very devastating sinkholes are located in the Sarisarinama field in Venezuela. There are four sinkholes here but the largest is the Sima Humboldt which is 352 metres wide and 314 metres deep. That means it's as deep as the New York Chrysler Building is tall.

Sima Humboldt, Sarisarinama fields in Venezuela.

Blue Holes

Dean's Blue Hole, Bahamas.

Sinkholes can also happen underwater. When this happens they are often called "blue holes" due to the fact that it looks a darker blue in colour. A blue hole is a sinkhole that has developed in a bank or island made of a carbonate bedrock (limestone or coral reef). Blue holes contain freshwater, saltwater or a mixture of the two. They extend below sea level for most of their depth and can allow access to underwater passages. Some well-known examples are located in the South China Sea (Dragon Hole), Belize ( The Great Blue Hole) , the Bahamas (Dean's Blue Hole), Guam, Australia (in the Great Barrier Reef), and Egypt (in the Red Sea).

The Great Blue Hole, Caye, Belize.

Blue holes are different from cenotes because unlike blue hole, cenotes usually contain fresh groundwater rather than seawater as blue holes do.

Cenote at Bolochon Mexico.

Sinkholes are can be very dangerous and destructive swallowing anything ontop of it. They can come in all shapes, sizes and forms. Sinkholes can happen anywhere at anytime and cannot be detected prior to the actual disaster. Some areas are more prone to sinkholes than others and will get hit multiple times by them. Not all sinkholes are catastrophic but sinkholes can continue to deepen after their initial entry to the world. Sinkholes can happen even underwater and can become ponds or lakes if they get lined with debris. To find out if your area is prone to sinkholes

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Created with images by _gee_ - "sinkhole" • sare bear - "cenote"

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