Relative Dating
 Relative dating is the process of placing events in the sequence
in which they occurred
 Relative dating does not identify the actual dates on which the
events occurred
 Scientists use relative dating to help them determine the
relative ages of Earth’s rocks
 The technique is based on certain principles for telling relative
time
1. The Principle of Superposition
- sedimentary rock forms after particles settle out of a fluid
and, over time, are compressed into layers (strata)
- the principle of superposition states that in an undisturbed
sequence of sedimentary strata, the oldest rock layer will be
at the bottom and the youngest will be at the top
- most layers of sediment are deposited in a horizontal position
- this is the basis for all relative dating
2. The Principle of Cross-Cutting relationships
- magma can intrude the strata, cutting across the horizontal
layers
- the principle of cross-cutting states that an igneous intrusion
is always younger than the rock it has intruded or cut across
3. Embedded Fragments
- rocks that are embedded in another rock must be older than
the rock in which they are contained
- i.e. the embedded rock must have existed before the
surrounding rock
a) another way of reading the rock record involves the
examination of unconformities
b) a unconformity indicates where layers of rock are
missing in the strata sequence
c) the processes that cause unconformities require huge
expanses of time
d) strata may be missing because they were never
deposited
e) more likely, the rock layers may have been deposited and
later removed
4. Angular Unconformity
- original horizontal sediments are sometimes tilted during
uplift
- over time the exposed surface is worn down
- an angular unconformity results when younger, flat strata are
deposited on top of older strata
5. Index Fossils
- these fossils are the remains of animals that lived and died
within a particular time segment of Earth’s history
- most of the animals then became extinct
- therefore each layer contains fossils unlike those in the layer
above or below it
- an index fossil has four characteristics:
a) fossils are easily recognizable (unique/easy to tell similar
fossils apart)
b) index fossils are abundant
c) the fossils are widespread (found over a wide geographical
area)
d) index fossils are the remains of organisms that existed for
only a brief period
- index fossils found in rock layers in different parts of the
world indicate that the layers were formed at the same time