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Mammoth Caves in Kentucky and Carlsbad Caverns in New Mexico

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Two of the world’s most spectacular caves are Mammoth Caves in central Kentucky and Carlsbad Caverns in southeastern New Mexico . Find out how they formed.

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Underground worlds of mystery and grandeur, caves are labyrinths of passageways and streams, homes to unusual creatures and stunning formations . Two of the world’s most spectacular caves are Mammoth Caves in central Kentucky and Carlsbad Caverns in southeastern New Mexico . Mammoth is known for its long passages while Carlsbad is known for its large rooms and bat colonies . These two caves provide good examples of most of the caves found around the world—limestone caves.

In its pure state, water is acid-free . However, as rainwater falls through the atmosphere and as it passes through decaying organic matter in the soil, it picks up carbon dioxide (CO 2 ), making the water acidic . As this water with carbonic acid makes its way underground through cracks and fractures in the earth’s surface, it reacts with the calcite that makes up most of the limestone, putting it into solution . It is then removed with the help of underground streams . As these areas of limestone are dissolved and removed, large openings under the earth’s surface are left, thus creating caves . In addition to carbonic acid, evidence shows that hydrogen sulfide from oil and gas deposits, and a whole range of microbes, combines with oxygen in the underground water table, producing sulfuric acid . This strong acid causes very aggressive dissolution of passageways at the level of the water table along cracks, fractures, and faults in the limestone.

Caves vary in size, shape and content; but they all include spectacular formations . The two most well-known speleothems are stalactites and stalagmites, but many confuse the two . One way to remember the one on the cave’s ceiling is that they hang “tite” to the ceiling—stalactites . Stalagmites, on the other hand, “mite” rise from the floor of these underground worlds . These formations result when this mixture of water and calcite enters a cave . The solution goes through a reaction where CO 2 bubbles away, and the calcite is deposited in formations called speleothems . Stalagmites result from the calcite collecting on the floor directly below a stalactite, which forms by calcite collecting on the ceiling . If the two join together, they become known as a column . Other speleothems include flowstones, draperies, soda straws, and helectites.

To allow enough time for peer assessments to be performed after learners have submitted their own responses, set the response due date and time at least one week before the peer assessment due date and time.

If the response due time and peer assessment due time are too close together, and a learner submits a response just before responses are due, other learners may not have time to perform peer assessments before peer assessments are due.

Use course discussion posts to provide guidance for peer grading of ORA assignments.

Consider extending due dates to allow the discussion moderation team to monitor course discussions for questions about, or reactions to, peer grading, and to address issues when necessary.

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from peer grading. If there are more widespread issues with peer grading, the course team can reduce the weight of the peer assessment within the final course grade or allow learners to drop the lowest graded assignment from their grades.

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