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What causes sinkholes?

A sinkhole is a large depression in the Earth's surface, often shaped like a well or a funnel, that can be up to several miles in diameter. Sinkholes are most common in limestone regions. Underground layers of limestone can be dissolved by groundwater or by seepage from above-ground streams. A crack in underground limestone is often the first step in the formation of a sinkhole. A sinkhole may also be caused by a dramatic event, such as the collapse of a cave roof.

Sources:TheEncyclopedia Americana, International Edition, p. 843; Magill, Frank N. Magill's Survey of Science, vol. 3, pp. 1310-17.

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1. Characteristics
   1. What is the mass of the Earth?
   2. How much of the Earth's surface is land and how much is water?
   3. What are the highest and lowest points on Earth?
   4. What are the highest and lowest elevations in the United States?
   5. Which elements are contained in the Earth's crust?
   6. How does the temperature of the Earth change as one goes deeper underground?
   7. What is the interior of the Earth like?
   8. What are the major eras, periods, and epochs in geologic time?
2. Land
   1. Where are the northernmost and southernmost points of land?
   2. Do the continents move?
   3. What and where is the Continental Divide of North America?
   4. Are there tides in the solid part of the Earth as well as in its waters?
   5. How does a stalactite differ from a stalagmite?
   6. How is "speleothem" defined?
   7. What is the difference between spelunking and speleology?
   8. What causes sinkholes?
   9. Where are the world's largest deserts?
   10. How tall is a sand dune?
   11. Which natural attractions in the United States are the most popular?
   12. How long is the Grand Canyon?
   13. From what type of stone was Mount Rushmore National Monument carved?
   14. What are the LaBrea tar pits?
   15. What is the composition of the Rock of Gibraltar?
3. Water
   1. What is the chemical composition of the ocean?
   2. What causes waves in the ocean?
   3. Where are the world's highest tides?
   4. How deep is the ocean?
   5. What fraction of an iceberg shows above water?
   6. Why is the sea blue?
   7. How salty is seawater?
   8. What is the difference between an ocean and a sea?
   9. Where is the world's deepest Bake?
   10. Where are the five largest lakes in the world located?
   11. What are the longest rivers in the world?
   12. Who invented the ancient Chinese Earthquake detector?
   13. When did the most severe earthquake in American history occur?
   14. Of what magnitude on the Richter scale was the earthquake that hit San Francisco on April 18, 1906?
   15. How many kinds of volcanoes are there?
   16. Where is the "Circle of Fire"?
   17. Which volcanoes have been the most destructive?
   18. When did Mount St. Helens erupt?
4. Ice And Ice Ages
   1. How much of the Earth's surface is covered with ice?
   2. What is a moraine?
   3. How much of the Earth's surface is permanently frozen?
   4. How thick is the ice that covers Antarctica?
   5. Who was the first person on Antarctica?
   6. When was the Ice Age?
   7. If all the ice in the world were melted, how much would the oceans rise?
5. Mapping The Earth
   1. Who is regarded as the founder of U.S. geology?
   2. What is the prime meridian?
   3. What is Mercator's projection for maps?
   4. Which direction does a compass needle point at the north magnetic pole?
   5. What is Foucault's pendulum?
   6. Who was the first person to map the Gulf Stream?