Measuring Relative Time

Geological time is the same 24 hour day, one week, year, decade, and century as customary time that measures the age of the Earth. So, how is time measured for these past events? Scientists have developed numerous ways to estimate time. There are ‘relative’ dates and ‘numerical’ (‘absolute’) dates. Relative dates give approximate times and can be used to correlate events from one place to another including human activities as reflected in the archaeological record. Correlation, or deducing events that occurred at the same time in different places, allows the reconstruction of Earth history.

One hint of the duration of time is the measuring of sedimentary deposition rates. That is, how fast (or slow) different types of sediments (limestone, shale, sandstone, coal, etc.) accumulate under different conditions. The duration of time, then, can be estimated based on the sediment type and the thickness of the sedimentary unit. Sedimentary varves, for example, are a seasonal record just as tree rings. These methods do not take into account the time in the interval when deposition did not occur or when sediment was removed by erosion. The use of such approaches is useful, but limited.

Another qualitative estimate of time can be made by the erosion rate on the denudation of the land surface, usually by wind or water; the rates normally are higher in young (high-relief) terrains and lower in old (low-relief) ones. Thus, in mature areas such as the US Midcontinent, they are steady but with accelerated rates in times of crisis such as floods or wind storms (e. g., tornadoes). Normal rates generally prevail between the catastrophic events. The rates of denudation can be estimated by measuring the amount of material carried by streams and rivers in a unit of time. This may be reported as so many inches (or centimeters) per decade or millennium. Occasionally, some unique technique allows another type of measurement.