How do we know how old a rock or fossil is?

Dear Dave,

How do geologists determine the age of an ancient volcanic eruption or a glacial advance? How do we know how many years ago Tyrannosaurus lived?
Bertina H.
Saint Louis, MO

Bertina,

Geologists, paleontologists and archaeologists often want to know the age of a soil deposit, a rock layer (such as a lava flow), or the fossils in the rocks. We speak of two types of 'ages'. 1. Relative age is the relation in time between rock layers (and the fossils they may contain). Relative age can tell us that a particular rock layer is older or younger than another. It CAN'T tell us if a rock or fossil is 3 million or 300 million years old. 2. Absolute age is the time in years before the present time. If the rock can be dated at all, the absolute age can be determined. An example: a lava flow is determined to be 32.4 million years old (almost always with some imprecision, expressed. for example, as " ± 0.2 million years").

This week I'll deal with relative age. I'll tackle absolute age in a future blog post. That will help us understand how long ago Tyrannosaurus lived.

There are some basic laws that state the principles of relative ages of rocks. Geologists apply these to the rock record to determine sequences of time.

The "Law of Superposition" says that any undisturbed sedimentary rock layer that lies above another is the younger of the two. This principal was formalized in the mid-1600s by a Dane, Nicolas Steno, who is among the founders of modern geology.

Steno's Law of Superposition says that the rocks higher in the stack are younger than those below. Sounds like common
sense now, but this was a radical thought in the 1650s, when all sedimentary rocks were held to be the same age: deposited by Noah's Flood over a period of 40 days and 40 nights.

The "Law of Cross-cutting relationships": An intrusion of magma that cuts across other rocks is younger than the rocks it intrudes. Scotsman James Hutton (1726 - 1797)



Diagram illustrating cross-cutting relations in geology. These relations can be used to give structures a relative age. Relative ages are, from oldest to youngest: A - folded rock strata offset by a thrust fault. The folding is younger than the rocks, and the fault is younger than both rock A and the folding; B - large granitic intrusion (cutting through A); C - erosional angular unconformity (cutting off A & B) on which brownish yellow rock strata were deposited; D - volcanic intrusion (a dike), cuts through A, B & C; E - even younger rock strata (overlying C & D); F - normal fault (movement down on the right side) that cuts A, B, C & E. Diagram by Woudloper.

The "Law of Inclusions": Fragments of one rock layer that are enclosed in another rock layer are older than the enclosing layer. This was also recognized by Hutton.

The Law of Inclusions: if Rock B contains fragments of Rock A, then B must be younger than the fragments of rock it contains. The intruding rock (Rock A) must have been there first to provide the fragments.    

The "Law of Faunal Succession": Englishman William Smith (1790) recognized that fossils occur in a definite, invariable sequence in the geologic record.

Law of Faunal succession: fossil remains of living things are present in rock layers at definite intervals, and exist within a discrete period of time. In this instance, using the Law of Superposition, would the age Rock Unit A be older or younger than the age of Rock Unit B?     

If you can see these relationships, you can begin to understand the sequence over time. In the case of Bertina's hypothetical volcanic eruption and glaciation, apply these methods if the deposits are in contact with each other. If they aren't both present in the same rock outcrop, it may be far more difficult to determine relative age. In a future post I'll deal with absolute dating. That involves radiocarbon and other isotopic methods. Stay tuned!
Dave