Dating Rocks
Ways to tell the age
of a rock
2 Ways to Date Rocks:
 Relative Dating:
 Places events in geologic history in
the proper order.
 The basis for the geologic time scale
 Mainly Sedimentary Rocks
 Does not provide a true “age”
2 Ways to Date Rocks:
 Absolute Dating:
 All you need is a tiny sample of material
(mineral, bone) no larger than a grain of
rice.
 Gives us the true “age” of a fossil or rock
 Mainly organic tissue or igneous crystals
 Measure the amount of unstable isotopes
that have “decayed” to figure out age
 Radiometric dating
Principles for Relative
Dating:
• Superposition - The youngest rocks are
on the top, oldest at the bottom.
Relative Dating Principles
• Superposition
• Cross-cutting relationships - Geologic
features that cut through and across
rocks are younger than those rocks.
• Mostly Faults and Igneous intrusions
Cross-cutting Igneous Rock
Cross Cutting Relationships - Geologic features that cut
through and across rocks are younger than those rocks.
Relative Dating Principles
• Superposition
• Cross-cutting relationships
• 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
Relative Dating Principles
•
•
•
•
Superposition
Cross-cutting relationships
Law of Inclusions
Law of Original Horizontality (and
Lateral continuity) – layers of sediments
are originally deposited horizontally
Hikingtripsreport.com
Relative Dating Principles
•
•
•
•
Superposition
Cross-cutting relationships
Law of Inclusions
Law of Original Horizontality (and
Lateral continuity)
• Law of Unconformities
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.
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 2: Label the layers from:
A  The Oldest
T  The Youngest
Activity 2: Label the layers from:
A  The Oldest
T  The Youngest
Name:___________________
What are two ways to date rocks?
1._________________________
2._________________________
Core: 1 2 3 4