How Old is Old? Geologic Time

Relative Age vs. Numerical Age Geologists strive to establish both the sequence of events that produced geologic features AND the date on which each event occurred…. Relative Age: the age of one geologic feature with respect to another Relative dating is placing geologic events in a sequential order as determined from their positions in the geologic record. Relative dating will not tell us how long ago a particular event took place; only that one event preceded another.

Numerical Age (absolute age): the age of a geologic feature given in years Geologists learned how to determine relative age long before they could determine numerical age!

Geologic Principles These principles (1860s!) continue to provide the basic framework within which geologists read the record of Earth history The principle of uniformitarianism: physical processes we observe operating today also operated in the past!

The principle of original horizontality: layers of sediment, when first deposited, are fairly horizontal, because sediments accumulate on surfaces of low relief (gravity!)

The principle of superposition: in a sequence of sedimentary rock layers, each layer must be younger than the one below. The layer at the bottom of a sequence is the oldest, and the layer at the top is the youngest!

The principle of cross-cutting relations: if one geologic feature cuts across another, the feature that has been cut is older.

Unconformities: Gaps in the Record! James Hutton (1726-1797) a Scottish farmer and doctor. One of the fathers of geological dating! He was puzzled by this outcrop at Siccar Point on the coast of Scotland. Do you see any “gaps”?

Devonian “Old Red sandstone” Unconformity Ordovician sandstone and shale The gray sandstone had been deposited, turned into rock, and tilted BEFORE the red sandstone had been deposited!

Hutton deduced that the surface between the gray and red rock sequences represented a time interval during which new strata had not been deposited at Siccar Point and the older strata had been eroded away.

Any surface representing a period of non-deposition and possibly erosion, an unconformity! At a given location, sediments do not accumulate continuously, so unconformities form!

Does the Grand Canyon strata represent all Earth’s history? BUT! By correlating rocks from locality to locality at millions of places around the world, geologists have pieced together….. Because of unconformities, no single location on Earth contains a complete record of Earth history!

Stratigraphic Formations Geologists summarize information about the sequence of sedimentary layers at a location by drawing a stratigraphic column. Then the sequence of layers are divided into stratigraphic formations.

These formations are a sequence of beds of a specific rock type or group of rock that can be traced over a fairly broad region. Typically, a formation has a specific geologic age.

Geologists will use a process called correlation, to find relationships between the layers. They can correlate formations between nearby regions based on similarities in rock type.

And to correlate formations over broad areas, they rely on fossils to define relative ages of the layers.