Sinkhole Science

Sinkhole Science

Two Huge Holes For a few weeks in 2007, people in a neighborhood of Guatemala City felt the ground tremble and shake. No one knew what was causing it. Then on February 23, a huge hole suddenly opened in the street. A dozen houses tumbled into the pit, and several lives were lost. The hole was more than 330 feet (100 m) deep- deep enough to swallow two Statues of Liberty.

In 2010, it happened again, fifteen blocks from the first hole. This time, the hole swallowed a three-story clothing factory and left another gaping chasm in a major intersection. Miraculously, no one was injured. Sinkholes happen all over the world, in the middle of cities and in the middle of nowhere. Though sinkholes take years to form, each time a new sinkhole opens up in the earth, it takes people by surprise. What Causes Sinkholes? The ground we stand on feels solid. Underground, however, a lot is happening. Water trickles and flows, slowly eroding rocks and soil. This creates underground spaces. These result in sinkholes. Sinkholes don't happen just anywhere, though. Most natural sinkholes form where there are certain kinds of soft bedrock. Bedrock is a layer of underground rock. The ground above it and on the surface is called the overburden.

Rain or underground water drips and flows into this bedrock and erodes it. This creates open spaces such as underground caves or caverns. Over time, these spaces get bigger. Eventually, the overburden may slowly drop down to form a sinkhole. Sometimes, though, sinkholes can open suddenly and swallow buildings, cars, and people. Two Types There are two main types of sinkholes. The first is a cover-subsidence sinkhole (subside means to sink or settle). A cover-subsidence sinkhole forms when water wears a space in the underground bedrock. The overburden above the bedrock then shifts downward. Most sinkholes of this kind form slowly and may measure only a few yards across. The overburden may contain a lot of sand that easily slides into the underground space. These sinkholes often create a bowl shape on the surface. They may become ponds or wetlands if water has no way to drain out.

The second type is a cover-collapse sinkhole. This sinkhole forms when a space gets bigger and bigger under the surface. The overburden gets thinner as dirt, sand, and other sediment fall into the space. The ground around this kind of sinkhole may contain a lot of sticky clay. Clay helps hold the overburden together. Without warning, though, the surface may collapse into the underground space.

Areas that get a lot of sinkholes usually have bedrock made of karst. Karst is made of any type of rock that dissolves easily, such as limestone. Karst can also be formed of gypsum, dolomite, salt, or other minerals and rocks. Karst has many holes and cracks that water can flow into. When rainwater soaks into the earth, it picks up a mild acid from leaves and plants. The mild acid dissolves karst. This helps create the underground spaces that cause natural sinkholes.