Geologic Time


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Geologic TimeScale

Geologists divide the earth´s history into units - in decreasing order of length, the units are eons, eras, periods, and epochs.

Eras are important time units, and are marked by major extinctions. But when you date rocks or fossils, you usually just need to refer to the period, which is a subdivision of the era. Geologic periods are marked by the entrance or exit of certain key species (for example, the Ordovician is the period when fish first appeared.

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Absolute Age Dating

Some rock layers, or events in geologic time, aren´t assigned a specific age in years - rather, they are assigned a "relative" age. But other rocks, or other geologic events, actually can have an age assigned to them. Absolute age dating is the process whereby an age (in years) is assigned to a rock layer or event.

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Undisturbed Layers

If a sequence of rock layers (strata) does not seem to have been disturbed (that is, the units are still horizontal and haven´t been tipped or faulted) you can assume that the oldest layers are on the bottom, and the youngest layers are on the top.

Yeah, it sounds obvious to us now, but in the early days of geology, that was a pretty astute observation known as the Principle of Superposition.

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Cross-Cutting Relations

Here´s another of those relative dating tips that seems obvious to us in our modern sophistication, but which was pretty clever when it was originally developed:

if you have layers of rock which are cut by something - a fault, say, or basalt dike - then the rock layers are obviously older than the fault or dike which cuts them. That´s called the Principle of Cross-Cutting Relations.

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Relative Dating II

In the early days of geology, scientists developed several principles which they used to determine whether a particular rock unit was older or younger than another unit.

For example, the Principle of Original Horizontality stated that sedimentary rock layers (and lava flows) are deposited as relatively horizontal sheets. If they aren´t horizontal now, clearly they were somehow tipped or moved.

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Age of the Earth

Estimates of the Earth´s age have changed over time. ArchBishop Usher´s long-ago estimate of 6006 years has been steadily revised as understanding of geologic processes has improved. It is currently estimated that the Earth is about 4.6 billion years old.

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Angular Unconformities

If a horizontal layer of rock or sediment is underlain by a sequence of tilted rocks, you know two things:

first, the underlying rocks, if they are sedimentary or if they are lava flows, used to be horizontal - so after they were laid down, they were somehow tipped.

second, you know that you are looking at an angular unconformity - that is, there is a gap in the rock record between the tilting of the lower rocks and the deposition of the upper (horizontal) rocks.

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Relative Dating

Relative age dating is how geologists determine when an event occurred (such as deposition of a rock layer, or appearance of a fault) in relation to other events - without necessarily having an actual age in years.

Here´s an example of relative dating: if you have two siblings, you can describe them as older than you or younger than you, without actually giving their ages. That´s relative dating.

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