Sinkholes

• Certain types of rocks (like limestone) dissolve easily in water.
• Over geologic time, networks of underground caves and cavities can form as the naturally-flowing groundwater slowly dissolves these rocks.
• Sinkholes are natural features that form when portions of these caves collapse.
• Humans can cause sinkholes to form more rapidly. Rapid pumping of groundwater for drinking and irrigation can draw all of the water out of these underground cavities. The water actually provides support for the roof of the cave, so when water is pumped out the underground caves can collapse. This is one way sinkholes form.
• Circles of fractures often form around the edges of sinkholes when they collapse. The circular shape forms a "bullseye" pattern around the center of the collapse zone as the material above the cavity falls down into it much like sand falling down the hole in an hourglass.

• USGS Overview of Sinkholes USGS Circular 1182 [PDF]
• Sinkholes USGS Fact Sheet [PDF]
• USGS Water Science for Schools - Sinkholes
• What's Up With Sinkholes? (USGS Podcast)

Stockertown Sinkhole Dilemma (Role Playing, grades 6-9)

Misconception: Groundwater flows in vast underground lakes.
Fact: Groundwater usually flows through tiny spaces between individual mineral grains.

Many students have the wrong mental image of groundwater systems -- they frequently picture water flowing in vast underground lakes and rivers. This is not the case for most of the earth! In most rocks on earth, groundwater fills the billions of tiny spaces between individual mineral grains or in narrow fractures within rocks -- a lot like the pore spaces in a sponge. Sinkholes are the rare exception to this sponge-like groundwater system and they actually do form when large cavities develop underground. You can use the existence of sinkholes to help clear up the common misconception by telling students that if groundwater existed in underground lakes everywhere, we would see sinkholes in a lot more places.

Misconception: Groundwater and surface water are separate systems.
Fact: Water from the atmosphere, surface (rivers, lakes, etc.), and groundwater are all connected via the hydrologic cycle and get naturally "recycled" over and over again. Actions people take that impact one of the parts of the system (such as overpumping of ground water) will eventually affect the rest of the system.