"Ma"- or "How do geologists cope with the span of geologic time?"

Mark Schmitz, Geosciences Department, Boise State University

As a geochronologist I use a variety of mass spectrometric methods to measure the abundance of long-lived radioactive and radiogenic isotopes in minerals like zircon, and infer their numerical dates from those isotope ratios. These numerical dates are then interpreted within the geological and petrological context of the sample as the age of a geologic phenomenon or process.

This last—the focus on phenomenon and process—is an important aspect of a geologist's working conceptualization of time, and reveals several strategies by which scientists cope with the expanse of geologic time. These strategies in turn are potentially useful for teaching about geologic time.

The first strategy is the construction of a relative age framework for a given problem prior to investing in the acquisition and interpretation of absolute ages. Whether making a geologic map, or correlating sedimentary sequences, or hypothesizing on a fossil succession, nearly every historical problem in geology is first cast in terms of a relative sequence. Absolute ages test this sequence of neighboring, contrasting or correlative relative ages.

The second strategy involves a common narrowing of the focus of investigation on relatively limited temporal intervals. Paleontologists and biostratigraphers construct biozones and stages, which generally encompass only hundreds of thousands to a few million years of time. Petrologists consider the lifespans of magma chambers, volcanoes, or plutons, which similarly span tens of thousands to millions of years. Admittedly, tectonicists perhaps most commonly stretch this period to the timescales of orogens or plate boundaries, which stretch into the tens or even hundreds of millions of years.

The third strategy is the translation of processes operating over a large span of time into rates, which are in turn expressed in terms of more tangible timescales. Global plate motions over millions of years are expressed in terms of centimeters per year. The deposition of thousands of meters of sediment over millions of years is translated into millimeters per year. Vast outpourings of basalt forming oceanic crust at the mid-ocean ridges are translated to cubic kilometers per year.

Finally, a fourth strategy takes advantage of specifying appropriate time units that reduce the number of numerical values to a tractable range. The most common time unit in geology is probably the abbreviation for millions of years: "Ma". Why? One likely reason is that it reduces the framework of Phanerozoic time into just 542 units—a number which is easy to assimilate. Similarly, major events in the Earth system—for example major extinction events, or the formation and fragmentation of continents—are separated by tens or hundreds of Ma, again very tractable numerical quantities. By contrast the Precambrian, or its constituent Proterozoic or Archean Eons, span sufficiently longer intervals that the natural unit for the Precambrian becomes billions of years, or "Ga". Use of these alternative time units effectively short-circuits the cognitive task of assimilating the vastness of geologic time, while emphasizing the arguably more important relative differences between the ages of geologic events or phenomena.