Interpreting Geologic History
 Relative age: The age of one rock or event compared
to others (which is younger or older compared to the
others).
 Absolute age: The actual age of a rock or event
expressed in years or millions of years ago (mya).
 Principle of superposition: Sedimentary rock layers
(and lava flows) at the bottom of an outcrop are older
than those above.
 Original Horizontality: Sedimentary and volcanic
rocks are laid down in horizontal layers extending over
large areas. (Helps with correlation…Matching)!
 Uniformitarianism: The processes that shaped the
landscapes in the past are the same that operate today.
“The present is the key to the past”
 Principle of crosscutting relationships: Anything
that cuts through an existing rock or is found in
another rock must be younger than that rock!!
1. Faults-Breaks in rocks represent a younger event
than the formation of the rock!
2. Inclusions- Any part of another rock that is
inside another rock must be older than the rock it
is inside.
3. Extrusion:-Igneous material that cools and turns
solid on earth’s surface. Lacking contact
metamorphism with any rock that forms above it
(See question #4 on HW & Figure 13-2 RB).
*Contact metamorphism is on bottom ONLY.
4. Intrusions-Igneous material (magma) that has
squeezed or cut into a rock and then COOLS
UNDERGROUND to form an igneous rock.
 Produces a zone of metamorphic rock
adjacent to itself. Contact Metamorphism
all around it!
Contact Metamorphism = Younger Event
AN INTRUSION MUST BE YOUNGER THAN
THE ROCK IT IS WITHIN!
5. Erosion = Unconformity
(INSERT SQUIGGLE LINE AND NON CONFROMITY
HERE).
Unconformity- Buried Erosional Surface
3rd
2nd
1st
Represents a “large gap” in geologic history.
ROCKS = PAGES IN HISTORY BOOK
MISSING ROCKS = MISSING PAGES
REVIEW QUESTIONS:
Correlation: Use of the principles above to show that
certain rocks and geologic events found in different
places have the same or similar relative age (Matching
rocks).
1. Exposed bedrock- Tracing a single layer of rock all
the way to Cortland (see figure 13-4 Pg 281)!
2. Similarities in Rocks-Observing the same layer of
rock across the highway while traveling south on
Route 81 (see figure 13-4 Pg 281)!
3. Use of Index Fossils- special fossils that represent
certain, short periods of geologic time (figure 13-5)!
*Represented by letters on Pg 8&9 of ESRT
a. Lived for a short time.. (Found in few layers)
b. Lived in many places..(More than one outcrop)
BEST CORRELATION METHOD TO USE!
4. Use of time markers-Much like index fossils…Exact
dates are usually known for these deposits.
Volcanic Ash layers- Very thin layer covers a
very large area very quickly!
 Meteorite Deposits- Pieces cover most of
the earth due to huge explosion on impact
leaving a mark or timestamp every place
they are found.

Unconformities- Ancient surfaces of the
earth that have been eroded and then buried
by millions of years of deposition.
(GAPS IN GEOLOGIC TIME)!
*Process of correlation can fill in these gaps!
REVIEW QUESTIONS:/ Relative dating quiz
NEXT…Pgs. 8-9 ESRT Geologic Time Scale with
review book questions!!
Absolute Dating of Rocks Using Radioactive Decay
Half-life- The time it takes for ½ of the radioactive isotope
to decay (change).
Radioactive isotope (PARENT)-The original
UNSTABLE material! (List on pg.1 ESRT).
Decay product (Daughter)-The new STABLE substance
that is formed by decay!
 Radioactive isotopes form within rocks.
 They start to decay SLOWLY atom by atom.
 Compare the # of atoms of “parent remaining” now to
original parent when object formed to determine
Absolute age of object!
(Illustrate decay process…create chart with students)
Carbon-14 dating- Not enough parent material remaining
(due to decay) after 70,000 years to measure.
1. ONLY USEFUL UP TO 70,000 years!
2. ONLY USEFUL FOR ORGANIC REMAINS!