Not the Geochemical Reaction You Might Expect
Earlier this semester Dr. Rayburn took a trip to New York City to observe first hand the interesting weathering phenomenon of Cleopatra's Needle in Central Park. We learn in Environmental Geochemical Science how carbon dioxide reacts with water vapor in the atmosphere to make acid rain, and how that acid rain can then react with carbonate minerals in rocks such as limestone or marble. The reaction neutralizes the acid in the rain, but it also dissolves the carbonate rocks. We call this "chemical weathering". The odd thing about Cleopatra's Needle is that it is made of granite, and therefore shouldn't be chemically weathered by acid rain, and yet as you can see the hieroglyphs on two sides are almost completely worn away. It turns out that this is because the needle was half buried in sand in the Egypt's Nile Valley before it was excavated and moved to New York. The side that was buried in the sand was also exposed to salty ground water from time to time. The water would invade the pores of the rock and deposit salt crystals. While the obelisk was in Egypt that was no big deal, but now that it is in New York, where the air is more humid, the salt crystals are hydrating and expanding causing the rock faces on the west and south sides to break apart. This is a form of "physical weathering".
The different faces of Cleopatra’s Needle (the west face is completely weathered)
The different faces of Cleopatra’s Needle (the east face is not weathered)