Kimberlitic Rocks of New York State

Western New York State is noted for its flat-lying, sandstones, limestones, dolostones, siltstones, and shales. However, some very interesting igneous rocks can be found as dikes* intruding these Paleozoic sedimentary rocks in a narrow band that extends from the vicinity of Ithaca, through the Syracuse area, and beyond to Ogdensburg on the St. Lawrence River. These unusual rocks occur as narrow dikes that formed as molten rock from the upper portion of the earth's mantle pushed upwards into joints (vertical cracks) in the surrounding rock, then cooled, and solidified. The dikes range in size from an inch (2.5 cm.) or less, to 195 feet (60 m.) in width.Although igneous rocks characteristically contain radioisotopes that make it possible to determine their ages, several factors make it difficult to date these kimberlitic rocks. For example, different crystals in a sample formed at different times, and sometimes chunks of crystalline rock from the lower crust or upper mantle can become incorporated in the kimberlitic magma as it works its way upward. This results in an incorrect date if portions of it are sampled. Furthermore, post-crystallization alteration and weathering makes it difficult to extract suitable material for dating.