A sinkhole is a subsidence feature resulting from the downward movement of surficial material into a pre-existing subsurface void.
Sinkholes come in many shapes and sizes, and although caused by naturally occurring processes, their formation can be accelerated by human activities.
Sinkholes in Pennsylvania
The booklet Sinkholes in Pennsylvania (PDF) is a good primer about sinkholes. It includes information on the following topics:
How the underlying geology contributes to sinkhole development
How sinkholes form and their characteristics
How human activity that contributes to sinkhole development
Recognizing a subsidence problem
Repairing a sinkhole
Improving Sinkhole Awareness
Know the history of your property -- Many subsidence issues can be related to relic structures or materials left underground that can convey water and contribute to creating subsurface voids
Become informed about the geology of your area --Realtors should be able to identify if a property lies within a limestone area; libraries, colleges, county planning commissions, and state and federal agencies are other sources of information
Know the location of any nearby underground mines
Ensure that municipalities maintain and regularly inspect water-bearing utility lines and that requirements are in place for development in subsidence-prone areas
Maintain sinkhole insurance
Geologic Conditions That Contribute to Sinkholes
Sinkholes, along with caves, are a definitive part of Pennsylvania’s landscape known as karst. The chemical and physical processes that helped to form this unique landscape have taken place over hundreds of millions of years.
Karst is common to areas underlain by carbonate bedrock (limestone and dolostone). These rocks are more easily dissolved than other rocks by a weak, natural acid formed by the mixture of water and carbon dioxide. The dissolving process is enhanced along the many fractures found within the bedrock, and over time this has created a unique subsurface plumbing network.
Just as we have drains and pipes in our homes that help move water from one place to another, the karst system uses the widened fractures in the carbonate bedrock to help convey water to the water table. Typically, the drains in the karst network are clogged with soil, but at times, water can act as a de-clogging agent and flush the karstic drains open, creating a sinkhole.
The water-driven nature of karst systems lends them to be more sensitive to changes in land use. Rapid and widespread groundwater contamination or the sudden “unclogging” of a karst drain is a public safety as well as an economic concern.
When we consider how to manage storm-water runoff, infrastructure layout and design, and utilize groundwater as a resource, it becomes important to understand the relationships between activities at the surface and their potential impact beneath.
Pennsylvania Sinkhole Data on an Interactive Map
Sinkhole and surface depression locations are shown on this interactive map of Pennsylvania. Additional information on the local density of the karst features can be seen with a click of a data point.