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## IR + CR: rock varnish, iron, manganese, bacteria, limonite

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nicktim Junior | Next Rank: 30 Posts
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#### IR + CR: rock varnish, iron, manganese, bacteria, limonite

Mon Jun 24, 2013 11:32 pm
I am very confused with this CR/inference-embedded IR question. I found the passage confusing and I could not understand the meaning of the passage after reading it a couple of times. Can an expert please help a fella out?

When tackling a "two-part analysis" type of question, should we try to come up with both solutions simultaneously, or should we tackle each question?

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Rock varnish is typically rich in iron and manganese, with the presence of manganese due to bacteria on the surface of the rock. Because the bacteria would not survive on the surface of rocks in the colder, continuously frozen, reaches of Antarctica, scientists were not surprised to discover that rock varnish in the Thiel Mountains area of Antarctica consists only of limonite, a form of oxidized iron. This had penetrated from the surfaces of the rocks into the cracks. However, although moisture is essential to the movement of limonite, snow has not melted in the Thiel Mountains in recent times.

Indicate which statement in the table the given information most strongly suggests is true, and the statement that the given information most strongly suggests is false. Make only two selections, one in each column.

A. Moisture is required for the presence of significant amounts of manganese in the environment.
B. Moisture is not required for the presence of significant amounts of manganese in the environment.
C. When temperatures in a continuously frozen location increase to above freezing, cracks in rocks there begin to take in rock varnish containing significant amounts of manganese.
D. Rock varnish that is especially rich in iron is mostly found in extreme cold.
E. Manganese is unable to penetrate into cracks in significant amounts.
F. Temperatures on rocks in the Thiel Mountains were above freezing at some point in the past.

-----

What must be true? F
What must be false? C

srcc25anu Master | Next Rank: 500 Posts
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Wed Jun 26, 2013 11:44 am
F is true. Since rock varnish in Theil area consists of limonite, it must have penetrated down at some point in the past. Although, the snow did not melt in recent times, this does not exclude the possibility that it never melted. In fact, it must have melted sometime ago (in past) since it did not melt in recent past.

C is False. When temperatures in a continuously frozen location increases to above freezing, cracks in rock begin to take in rock varnish BUT such rock varnish would be rich in iron and not manganese as presence of manganese is due to bacteria on rock surface and bacteria cannot survive on rock surface in such frozen locations. So it is unlikely to discover manganese in rock cracks in Antartica.

kutlee Senior | Next Rank: 100 Posts
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Tue Apr 07, 2015 8:18 pm
Good question.
F should be true because moisture or water is essential to carry limonite into the cracks - So F should be true.
For statement that is not true, i went by elimination

A. Moisture is required for the presence of significant amounts of manganese in the environment.
B. Moisture is not required for the presence of significant amounts of manganese in the environment.
A and B talks about environment. Here we know only about rocks. So we don't know the validity of these statements. So ignore.

C. When temperatures in a continuously frozen location increase to above freezing, cracks in rocks there begin to take in rock varnish containing significant amounts of manganese.
Hold on to C. Cracks are filled with limonite. Not Manganese. This could be the answer

D. Rock varnish that is especially rich in iron is mostly found in extreme cold.
D states something about iron. We have data about iron oxide. Both are chemically different. Confusing. Still better answer is C

E. Manganese is unable to penetrate into cracks in significant amounts.
We don't have data about ability of manganese to penetrate into rocks. So ignore.
F. Temperatures on rocks in the Thiel Mountains were above freezing at some point in the past.

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