GEOLOGICAL CONCEPTS
FIVE STEPS TO BECOMING A…
GEOLOGICAL DETECTIVE!
OUR GOAL: TO INTERPRET
GEOLOGICAL STRUCTURES WE SEE,
LIKE THE ROADCUT DEPICTED BELOW
THE 5 CONCEPTS
 UNIFORMITARIANISM
 ORIGINAL
HORIZONTALITY
 LATERAL CONTINUITY
 SUPERPOSITION
 LAW OF CROSS-CUTTING RELATIONSHIPS
CONCEPT:
UNIFORMITARIANISM
USING HISTORY, WE STUDY THE PAST TO UNDERSTAND THE
PRESENT…
BUT USING UNIFORMITARIANISM, WE STUDY THE PRESENT
GEOLOGICAL PROCESSES TO UNDERSTAND THE PAST
GEOLOGICAL PROCESSES (EVENTS THAT HAPPENED
MILLIONS OF YEARS AGO)…
DEFINITION: GEOLOGICAL PROCESSES HAPPENING
TODAY HAPPENED IN THE PAST…
EXAMPLE: TO UNDERSTAND MOUNTAINS THAT FORMED
MILLIONS OF YEARS AGO, WE STUDY HOW MOUNTAINS
ARE FORMING TODAY.
APPLICATION 1 OF UNIFORMITARIANISM:
WHAT ARE THE HORIZONTAL LINES?
CLUE FROM PRESENT PROCESS…
HERE IT IS AGAIN…
USING TIME LAPSE PHOTOGRAPHY
ANCIENT LAKE
DRYING UP
LEAVING TERRACE LINES
APPLICATION 2 OF UNIFORMITARIANISM:
WHAT IS THIS?
CLUE FROM PRESENT PROCESS…
FROM ANCIENT
MUD FLAT
OR SAND DUNE
THEY MUST HAVE SOLIDIFIED INTO ROCK
APPLICATION 3 OF UNIFORMITARIANISM:
EXPLAIN THIS SOLID ROCK…
CLUE FROM PRESENT PROCESS…
GRAVEL
RIVERBED
BEACH
THEY MUST HAVE SOLIDIFIED INTO ROCK
FROM UNIFORMITARIANISM
COMES THE OTHER 4 CONCEPTS
 ORIGINAL HORIZONTALITY
 LATERAL CONTINUITY
 SUPERPOSITION
 LAW
OF CROSS-CUTTING RELATIONSHIPS
CONCEPT:
ORIGINAL HORIZONTALITY
SEDIMENT LAYERS ARE
USUALLY DEPOSITED
HORIZONTALLY
THUS, LOOK FOR LAYERS
NO LONGER HORIZONTAL
TO RECOGNIZE THAT
FORCES ACTED ON THE
LAYERS AFTER FORMATION
APPLICATION OF ORIGINAL HORIZONTALITY:
FINDING TILTED AND FOLDED LAYERS
WHAT HAPPENED TO THE LAYERS
HERE?
BY FORCES
PUSHING
TOGETHER
SUMMARY OF ORIGINAL
HORIZONTALITY
 SEDIMENT
LAYERS ARE USUALLY
DEPOSITED HORIZONTALLY
 SO, LOOK FOR LAYERS THAT ARE NOT
HORIZONTAL, MEANING SOME FORCE
ACTED ON THEM TO FOLD OR TILT THEM
 USE THIS CONCEPT TO FIND FOLDS AND
TILTED ROCK LAYERS
CONCEPT:
LATERAL CONTINUITY
ROCK LAYERS
CAN BE
TRACED
ACROSS WIDE
AREAS
APPLICATION OF LATERAL CONTINUITY:
WHAT HAPPENED TO THE LAYERS
HERE?
BROKEN
THEY WERE
APPLICATION OF LATERAL CONTINUITY:
FINDING FAULTS
FORCES ACTED ON THE ROCK LAYERS TO
BREAK THEM! CREATING A FAULT (BROKEN
ROCK LAYERS).
BROKEN
THEY WERE
2ND APPLICATION OF LATERAL CONTINUITY:
WHAT HAPPENED TO THE LAYER
BETWEEN THESE TWO ARROWS?
2ND APPLICATION OF LATERAL CONTINUITY:
FINDING EROSION
EROSION REMOVED THE MATERIAL AS THE
RIVER CUT DOWNWARD
2ND APPLICATION OF LATERAL CONTINUITY:
FINDING EROSION
EROSION REMOVED PART OF THE LATERALLY
CONTINUOUS LAYERS AFTER THE LAYERS WERE FOLDED
UPWARD
WHAT HAPPENED HERE?
SUMMARY OF LATERAL
CONTINUITY
 ROCK
LAYERS ARE DEPOSITED (SO CAN
BE TRACED) ACROSS WIDE AREAS
 SO, LOOK FOR BREAKS IN THE
CONTINUITY
 AND USE THIS CONCEPT TO FIND FAULTS
AND AREAS THAT HAVE EXPERIENCED
EROSION
CONCEPT:
SUPERPOSITION
YOUNGER ROCK
LAYERS
YOUNGER
USUALLY OVERLIE
OLDER
OLDER ROCK
LAYERS
AS NEW SEDIMENT DEPOSITS ON TOP OF EXISTING SEDIMENT
APPLICATION OF SUPERPOSITION:
WHICH ROCK LAYER IS YOUNGER?
A?
OR
B?
APPLICATION OF SUPERPOSITION:
DETERMINING RELATIVE AGES OF LAYERS
ROCK LAYER A IS YOUNGER
A
NOT
<<<<< YOUNGER
OLDER >>>>>
B
2ND APPLICATION OF SUPERPOSITION:
WHICH ROCK LAYER IS YOUNGER?
OR
A?
B?
2ND APPLICATION OF SUPERPOSITION:
DETERMINING RELATIVE AGES OF LAYERS
ROCK LAYER A IS YOUNGER
OR
A?
B?
SUMMARY OF SUPERPOSITION
 YOUNGER
ROCK LAYERS USUALLY
OVERLIE OLDER ROCK LAYERS
 SO, LOOK FOR ROCK LAYERS ONE ON TOP
OF THE OTHER
 USE THIS CONCEPT TO FOLLOW THE
HISTORICAL SEQUENCE OF EVENTS
HAPPENING AT A GIVEN SPOT
CONCEPT:
LAW OF CROSS-CUTTING
RELATIONSHIPS
MAGMA INTRUSION:
CALLED A DIKE
MOLTEN ROCK:
CALLED MAGMA
INTRUDING
ROCK IS
YOUNGER THAN
THE ROCK INTO
WHICH IT
INTRUDES
APPLICATION OF CROSS-CUTTING
RELATIONSHIPS:
WHICH ROCK IS YOUNGER?
A?
OR
B?
APPLICATION OF CROSS-CUTTING
RELATIONSHIPS:
DETERMINING RELATIVE AGE OF ROCKS
“B” IS YOUNGER BECAUSE “B” INTRUDED
THROUGH LAYER A
INTRUSION
A?
OR
B?
SUMMARY OF CROSS-CUTTING
RELATIONSHIPS
 INTRUDING
ROCK IS YOUNGER THAN THE
ROCK INTO WHICH IT INTRUDES
 SO LOOK FOR INTRUSIONS, ROCK THAT IS
DIFFERENT FROM AND CUTS THROUGH
OTHER TYPES OF ROCK
 USE THIS CONCEPT WITH SUPERPOSITION
TO FOLLOW THE HISTORICAL SEQUENCE
OF EVENTS HAPPENING AT A GIVEN SPOT
REVIEW 1
1
2
3
1. HOW CAN YOU APPLY ORIGINAL
HORIZONTALITY?
A. LOOK FOR LAYERS NOT
HORIZONTAL TO FIND FOLDS AND
TILTED ROCK
2. HOW CAN YOU APPLY LATERAL
CONTINUITY?
A. LOOK FOR BREAK IN CONTINUITY
TO FIND FAULTS AND EROSION
3. HOW CAN YOU APPLY
SUPERPOSITION?
A. TO DETERMINE THE HISTORICAL
SEQUENCE OF EVENTS HERE
REVIEW 2
WHICH IS YOUNGER…
1. LAYER 1 OR 2?
A. LAYER 2
SUPERPOSITION
2. LAYER 2 OR DIKE B?
A. DIKE B
CROSS-CUTTING REL.
3. VOLCANO A OR B?
A. VOLCANO A
LAYER 2
LAYER 1
4. FAULT A OR B?
A. FAULT A