Dating Rocks
Ways to tell the age
of a rock
What are relative and
absolute Locations?
2 Ways to Date Rocks:
 Absolute Dating:
 Gives us the true “age” of a fossil or rock
 All you need is a tiny sample of material
(mineral, bone) no larger than a grain of
rice.
 Mainly organic tissue or igneous crystals
 Measures the amount of unstable isotopes
that have “decayed” to figure out age
2 Ways to Date Rocks:
 Relative Dating:
 Places events in geologic history in
the proper order relative to one
another.
 The basis for the geologic time scale
 Does not provide a true “age”
Principles for Relative
Dating:
1. Law of Superposition :
• In any undisturbed
sequence of strata,
• the oldest layer is at the
bottom
• the youngest layer is at
the top.
Newest
Oldest
Principles for Relative
Dating:
Law of Superposition :
• In any undisturbed
sequence of strata,
• the oldest layer is at the
bottom
• the youngest layer is at
the top.
Relative Dating Principles
2. The Cross-cutting Law
• Any feature that cuts across a body of
sediment or rock is younger than the
body of sediment or rock that it cuts
across.
Cross-cutting Igneous Rock
Cross Cutting Relationships - Geologic features that cut
through and across rocks are younger than those rocks.
Relative Dating Principles
1. Law of Superposition
2. Cross-cutting Law
3. Law of Inclusions - Rocks embedded in
other rocks are older than those rocks
they are embedded in.
Law of Inclusions - Rocks embedded in
other rocks are older than those rocks
they are embedded in.
http://www.earth.ox.ac.uk/~oesis/field/medium/xenolith-1365.jpg
Do Class 12: Relative Dating
Exercise 1
Relative Dating Principles
1. Superposition Law
2. Cross-cutting Law
3. Law of Inclusions
4. Law of Original Horizontality (and
Lateral continuity) – layers of sediments
are originally deposited horizontally
Relative Dating Principles
4. Law of Original Horizontality (and
Lateral continuity) …. But stuff can
happen!
• Erosion
• Earthquakes
• Faults
• Deposition
Relative Dating Principles
4. Law of Original Horizontality (and
Lateral continuity) …. But stuff can
happen!
• Erosion
• Earthquakes
• Faults
• Deposition
Relative Dating Principles
4. Law of Original Horizontality (and
Lateral continuity) …. But stuff can
happen!
• Erosion
• Earthquakes
• Faults
• Deposition
Types of Discomformities
•
Angular conformity: younger sediments rest
upon the eroded surface of tilted or folded older
rocks.
•
Disconformity: contact between younger and
older beds is marked by a visible, irregular or
uneven erosional surface.
•
Paraconformity: beds above and below the
unconformity are parallel and no erosional surface
is evident; but can be recognized based on the
gap in the rock record.
•
Nonconformity: develops between sedimentary
rock and older igneous or metamorphic rock that
has been exposed to erosion.
A DISCONFORMITY is a
boundary between two
layers of non-continuous
ages. This boundary is
usually marked
by an erosional
surface and is
often irregular.5.
M&W4 Fig. 17.8; M&W5 Fig. 17.8
An ANGULAR
UNCONFORMITY is a
disconformity between
layers of different angles.
The underlying layers are
first tilted, then erosion
scours away a new,
horizontal surface.
New, horizontal layers form
on top
An NONCONFORMITY is a
disconformity between different
rock types, one of them
sedimentary.
Activity 3: Relative Dating Exercise
A  The Oldest
T  The Youngest
Activity 2: Label the layers from:
A  The Oldest
T  The Youngest
KEY