Weathering Rates As Exemplified by Cleopatra's Needle in New York City

Abstract

Known weathering rates of granites in different climates are compared with weathering damage on Cleopatra's Needle, an Egyptian obelisk in Central Park, New York City. Damage to this coarse-grained foliated granite has been ascribed by textbook authors to the deleterious effect of the city's atmosphere in 83 years of exposure. About 1½ inches of maximum surface reduction has taken place in New York as shown by the original depth of inscription. Reduction rates of granites in humid climates, were figured with only 0.0076 mm a year, less in deserts. It is concluded that the bulk of damage on the obelisk actually occurred in Egypt through rising moisture loaded with sulfate salts. The hydration pressure of these salts, combined with frost wedging, is responsible for the major damage to the monument, which happened during its first few years in New York.