Earth Science
Chapter 9
Section 2
A. Relative and Absolute Ages:
The relative age of a rock is its age compared to the age of other
rocks. Example: What is your age relative to your brother or
sister?
Younger or older? This is relative age.
The absolute age of a rock is the number of years since the rock
formed. This is not exact, the absolute age can be determined with
a certain number of years.
B. The position of the Rock:
The method used to find the relative age of a layer of rock is
known as the Law of Superposition.
According to the Law of Superposition, in horizontal sedimentary
rock layers the oldest layer is at the bottom. Each higher layer is
younger than the layers below it.
C. Other Clues to Relative Age:
1. Clues From Igneous Rock:
Lava that hardens on the surface is called an extrusion. The
rocks below an extrusion are always older than the extrusion.
Magma may push into a mass of rock and harden. This
formation is known as an intrusion. The intrusion is always
younger than the rock around and below the intrusion.
2. Clues From Faults:
A fault is a break in the Earth’s crust. A fault is always
younger than the rock it cuts through. To find the relative
age of a fault, a geologist finds the relative age of the most
recent rock layer through which the fault slices.
D. Gaps in the Geologic Record:
The geologic record of sedimentary rock layers is not always
complete.
The surface where new rock layers meet a much older rock surface
beneath them is called an unconformity.
An unconformity is a gap in the geologic record.
An unconformity shows where some rock layers have been lost to
erosion.
E. Using Fossils to Date Rock:
Fossils can be used to date rock in different locations.
Fossils called index fossils are used to give a relative date to rock
layers according to the distribution of fossils of organisms that
briefly existed during certain periods of time.