Geologic Hazards

A geologic hazard is an extreme natural event in the crust of the earth that poses a threat to life and property. Sinkholes and landslides are examples of geologic hazards.

 A sinkhole is a hole in the ground that forms when water dissolves surface rock. Sinkholes are common where the rock below the land surface is limestone, carbonate rock, salt beds, or rocks that can naturally be dissolved by groundwater circulating through them. Typically, sinkholes form so slowly that little change is noticeable, but they can form suddenly when a collapse occurs. 

A landslide is the movement of rock, earth, or debris down a sloped section of land. They tend to occur after heavy rainfall, and tend to happen in areas where they have occurred before. Landslides are often also called mudslides, debris flows, mudflows, or debris avalanches.

Sinkholes and landslides are the main types of geologic hazards.

What causes geologic hazards?

A sinkhole is a hole in the ground that forms when water dissolves surface rock. Often, this surface rock is limestone, which is easily eroded, or worn away, by the movement of water. In a landscape where limestone sits underneath the soil, water from rainfall collects in cracks in the stone. These cracks are called joints. Slowly, as the limestone dissolves and is carried away, the joints widen until the ground above them becomes unstable and collapses. The collapse often happens suddenly and without warning.

Sinkholes also form when the roofs of caves collapse. Sinkholes are often funnel-shaped with the wide end open at the surface and the narrow end at the bottom of the pool. They vary from shallow holes about 3 feet deep to pits more than feet deep. Sinkholes can occur naturally, especially where there is abundant rainfall. 

Landslides are caused by disturbances in the natural stability of a slope. They can accompany heavy rains or follow droughts, earthquakes, or volcanic eruptions. Mudslides develop when water rapidly accumulates in the ground and results in a surge of water-saturated rock, earth, and debris. Mudslides usually start on steep slopes and can be activated by natural disasters. Areas where wildfires or human modification of the land have destroyed vegetation on slopes are particularly vulnerable to landslides during and after heavy rains.

Sinkholes and landslides can occur in every state and can cause significant damage. 


There are things you can do to prepare in advance for landslides or sinkholes:

  1. Build an emergency kit. Keep one in your vehicle in case of hazards on the road, and one at home in case of a landslide evacuation.
  2. Have a backup communications plan in place. Know how you will contact friends and family and reconnect if separated.
  3. If you know you live in an area with landslides, have an evacuation route planned.
  4. Sign up for your community’s alert and warning systems.
  5. Know the warning signs of geologic hazards and what is common in your area. Become familiar with the land around you.

Stay Alert

During a Landslide

If you suspect a landslide is about to occur:

  1. Evacuate immediately if you are in a landslide path. Always follow the warnings and evacuation notices from local authority. They provide the latest recommendations based on the threat in your community.
  2. Listen for sounds of moving debris, cracking trees, or moving rocks. If you do get stuck in the path of a landslide move uphill as quickly as possible.
  3. If you are near a stream or channel and the water turns muddy or changes in flow speed, it may indicate debris flow upstream. These can be signs that a landslide is coming. Evacuate the area.

Using alert systems is crucial to being informed before and during hazardous events. Our alert systems page has resources for a number of alert options.

Local Geologic Hazards Risks

History of geologic hazards in Williamson County

Tennessee Landforms has information on current sinkholes in the state of Tennessee, including a map of sinkholes across the state and a map of the biggest sinkholes in the state. 

A lines of cars pile up behind a flooded road during the May 2010 flood.
A damaged road is blocked off during the May 2010 flood.
Sinkholes, bumps and cracks appeared in a road from flood damage in May 2010.

Ranking Local Vulnerability

Williamson County uses simple system called a vulnerability calculator to determine each jurisdiction’s vulnerability to hazardous events, as shown in the charts below.

Geologic vulnerability calculator for Williamson County.

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