SINKHOLE COLLAPSE

PART 1:  Sinkholes Near Orlando, Florida

The Orlando metropolitan area is situated in a region where sinkholes can develop unexpectedly as the land surface collapses into underground cavities (void spaces that range in size from small openings to large caverns).  These cavities occur in the limestone bedrock, which is covered by 90 to 150 feet of sandy clay and an additional 30 to 60 feet of sand (see geologic cross section).   The limestone bedrock also serves as an aquifer (called the Floridan Aquifer) for the residents of this area.

At least 140 new collapse sinkholes formed in the Orlando area (Orange and Seminole Counties) between 1961 and 1986, with an average of 11 new sinkholes appearing each year (Wilson and Beck, 1992).  Data from Wilson and Beck (1992) show that the appearance of new sinkholes is not equally distributed throughout the year (see Figure 1). 

             Figure 1:  Number of new sinkholes in Orange and Seminole Counties, Florida by 
                             month for the years 1961 through 1986.  Note that the month of the year
                             is represented with a number, starting with January as 1 and ending with
                             December as 12.

Questions

1.  During which months of the year are sinkholes most likely to form in the Orlando area?

2.  What hypothesis can you suggest to explain this pattern of sinkhole development?

3.  What data would you need to test your hypothesis?

References

Davies, W.E., and I. M. Morgan, The Geology of Caves; U.S. Geol. Survey and National Park
       Service; http://wrgis.wr.usgs.gov/docs/parks/cave/cave.html

Wilson, W.L., and B.F. Beck, 1992, Hydrogeologic Factors in Affecting New Sinkhole
       Development in the Orlando Area, Florida; Ground Water, Vol. 30, No. 6, p. 918-930.

PART 2:  Mining Company Sued for Causing Sinkhole

In the fall of 1996, a man driving his minivan near Westminster, Maryland plunged to his death in a sinkhole that suddenly opened up in the road.  His widow sued a local mining company for $13.5 million, claiming that it was responsible for the development of this sinkhole.  As is common with newspaper articles, this one does not provide all of the information we might like to have to evaluate the validity of the widow's claim.  However, using the geologic information available for this region and your understanding of how sinkholes form, it is possible to at least have an "informed opinion" about what happened and what additional information would be needed to further evaluate this case.

Question

Read "Mining Firm Sued For Millions Over Sinkhole", taken from a local Maryland newspaper.

Westminster is situated near the center of Carroll County,  about 7.5 miles to the northeast of
New Windsor along State Highway 31.   The Maryland Geological Survey has published a map 
of the bedrock in Carroll County, including a close up view of the New Windsor area (to find out
which rock type is represented by a given map pattern, click on that pattern's tile in the legend).
The Maryland Geological Survey has also published a map of caves in Maryland.  A news story
from July 2003 reveals that Westminster has also had trouble with sinkholes.

Given the information in the article, the Maryland Geological Survey maps, and your knowledge
of how collapse sinkholes can form, explain how you (as a scientist!) would determine if Robert
McKnight's widow's case against the mining company is valid.  Cite any additional information
that you would need to collect.