Relative Dating: Which Came
First?
(No we aren’t talking about dating
your cousin even though we live in
Alabama.  )
What is relative dating?
• It is determining the age of a rock or
fossils based upon the rocks or fossils
around the fossil or rock in question.
– Example:
• If you go visit a preschool, even at 11 or 12 years
of age the kids at the preschool will think you are
old. In this case your age, relative to your
surroundings is old.
• If you go visit a nursing home, the people there
generally being elderly, will think you are just
“babies” because you are so much younger than
they are. In this case your age, relative to your
surroundings, is young.
Relative dating of rock layers
The Principle of Superposition
• Rock layers are put down one on top of
the other over millions of years.
• The oldest layers are at the bottom, and
the youngest layers are at the top if the
rock layer is undisturbed.
Notice the
different
color
bands
that
indicate
different
layers of
rock.
Which layer is the oldest according to
superposition? How can you tell?
Disturbing Forces
• Not all rock sequences are youngest to
oldest.
• Natural forces can move these layers.
– Things such as faulting, folding, tilting, and
intrusions. These things disrupt the rock layer
the same way tearing pages from a book
disrupts the book.
• Geologists, like crime scene investigators,
must use clues trapped in the rock layers
to put the rocks back in sequence and get
a true look at Earth’s history.
Disturbing Forces
• Fault: break in the Earth’s crust where a
hanging wall is pushed up or down.
• Folding: rock layers are bent into synclines or
anticlines.
• Intrusion: molten rock from Earth’s interior
squeezes into existing rock layer, cools, and
appears to have cut across it.
• Tilting: occurs when forces inside the Earth
slants the rock layers.
Disturbing Forces
Geologic Column
• To solve the mystery trapped in disturbed
rock layers, scientists combine data
from undisturbed rock layers around
the world to form the ideal rock
sequence that contains all known
fossils and rock formations on Earth
arranged from oldest to youngest.
• This sequence is the geologic column and
does NOT exist naturally on Earth.
An Example of a Geologic Column
Disturbed Rock Layers
• Another tool aiding geologists in age rocks
is cross cutting features, such as an
intrusion.
• The intrusion or cross-cutting feature is the
youngest thing present since the other
layers had to be present before it could cut
across them.
Cross-Cutting Feature: Look at
Letter R
Gaps in the Record: Unconfomities
• Dating rock layers with folds, faults, etc. is
difficult, but imagine dating rocks that have
missing layers.
• The missing layers are caused by
erosion or nondeposition—they weren’t
put there to start with—and they are called
unconformities.
Unconformities
Three types of Unconformities
• Disconformities: part of a sequence of parallel
rock layers is missing.
– This is common in the Grand Canyon.
• Nonconformities: sedimentary rock layers lie
on top of an eroded surface of non-layered
igneous or metamorphic rock.
• Angular unconformity: found b/w horizontal
layers of rock that have been tilted or folded.
Disconformity
Disconformities
Nonconformities
Nonconformity
Angular Unconformity
Angular Unconformity