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Circular Shaped Scarp
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Location: Near the University of California, Berkeley
About: This road is in need of repairs! Fractures around the edge of this pothole form concentric circles, a lot like rings around a bullseye on a dart board. Near the center of the pothole, there are a lot more fractures and the pattern is harder to see. The pothole formed when a portion of the road compacted or subsided, causes the surface of the road to collapse and fall into the hole. To get a sense of the size of this feature, you can see the painted stripe of a crosswalk in the background, and a small white ruler about 10 cm long near the center-left.
Ring Shaped Scarp
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Location: Near Crooked Lake, Florida
About: Here, circular shaped fractures form the margins of a massive sinkhole that catastrophically caused this house to collapse. Sinkholes are natural features that are relatively common in areas with abundant limestone near the surface. Note how the sinkhole is filled with water, hinting at what caused this catastrophe.

Key Concepts:
  • 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.
Links for further Exploration:
Classroom Activities:

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

Common Misconceptions:

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.

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