GEOS 211
Rates and Dates: Calibrating the Rock Record Fall 2024
Division III W Writing Skills

Late in the eighteenth century, the Scottish naturalist, James Hutton, argued that Earth had “no vestige of a beginning, no prospect of an end” challenging the widely held biblical view that Earth was a mere 6,000 years old. Yet it was not until the discovery of radioactive decay that geologists were able to accurately date rocks and assign absolute ages to the geologic time scale, which had been developed using fossils and relative dating of rocks. Before radiometric dating, there were numerous attempts to estimate the age of Earth using the rates of natural phenomena, but these early approaches were plagued by faulty assumptions about geologic processes. We still endeavor to estimate the rate of a wide variety of geologic processes, and many are critical to society, such as climate change, sea-level rise, plate motions, and mass extinctions. In this tutorial, we explore the methods of radiometric dating that allow us to determine the age of igneous rocks that cooled from a magma, estimate when deeply buried metamorphic rocks cooled below certain temperatures, and determine the age of organic materials from their radiocarbon signatures. We then examine methods used to estimate the rates of geologic processes with particular emphasis on diffusion — the movement of matter or energy in response to a gradient in concentration, temperature, or potential energy — and the explicit and implicit assumptions that are critical to rate calculations. Topics include the basic isotope systematics of geochronology (U-Pb, K-Ar, and 14C ages) and thermochronology (U-Th/He or 40Ar/39Ar), as well as the rates of processes such as plate motion, sea-level rise or fall, glacial advance or retreat, magma storage and ascent, and/or mineral growth rates. There will be weekly tutorial meetings with pairs of students, and students will alternate writing papers on assigned topics. This course is in the Solid Earth group for the Geosciences major.