Earth History- Table of Contents
• RELATIVE vs. ABSOLUTE
• LAWS
• UNCONFORMITIES
• GEOLOGIC SECTIONS
Which graph shows the relative duration of
geologic time for the Precambrian, Paleozoic,
Mesozoic, and Cenozoic time intervals?
Who’s got the TIME?
• RELATIVE: order/sequence
known, but not the actual date of
occurrence. “Time Line”
• ABSOLUTE: actual date
determined by radioactive decay
“Clocks in Rocks”
“How’d that get there?”
• In the 17th C., Nicolas Steno made an
important observation:
"Sediments are usually deposited in
horizontal layers."
He called this
“ORIGINAL HORIZONTALITY”
Finding Relative Time
The LAW of...
• SUPERPOSITION: a sedimentary sequence will be
OLDEST on BOTTOM (if undisturbed – flat and level).
• CROSS-CUTTING:
a. Igneous intrusion is younger than rock it has
intruded (cut across). Pre-existing rock will undergo
CONTACT METAMORPHISM
b. Faults – younger than rock displaced
• FOLDS/TILTS: younger than rocks themselves
• INCLUDED FRAGMENTS: pieces of rock found
IN another rock must be OLDER (formed first).
Superpositionyoungest
to
oldest
GRAND CANYON- LAW OF SUPERPOSITION
YOUNGEST ON TOP
OLDEST ON BOTTOM
What is the geologic age of layer B?
(1) Cambrian
(3) Devonian
(2) Ordovician
(4) Permian
Cross Cutting
CROSS-CUTTING
Igneous Intrusion - Cross Cutting
Folds/Tilts
FOLDS - TILTS
Included Fragments
Included Fragments
What is the correct order of development from oldest to youngest?
Unconformities – Buried Eroded
Surfaces
• Sometimes layers of rock are missing
• There is a break or gap of geologic time not represented
by the layers in an area. The gap represents an unknown
length of time
• No way of knowing exactly what happened but we do
know UPLIFT exposed rocks to weathering and erosion.
• Rocks above unconformity are younger – rocks below
older
Bedrock outcrops A and B are located at two different locations along the Genesee
River in western New York State. Rock layers 1, 2, and 4 are the same in both
outcrops.
Which statement best explains why rock layer 3 is missing from
outcrop B?
(1) A fault exists between outcrops A and B.
(2) Erosion created an unconformity between rock layers 2 and 4 in
outcrop B.
(3) A volcanic eruption destroyed rock layer 3 in outcrop B.
(4) Metamorphism of outcrop A created rock layer 3.
4 steps produce an unconformity
1. Uplift – area of crust uplifted above sea
level (deposition – under water)
2. Erosion – some time after
3. Submergence (subsidence) below sea
level
4. Deposition – new sediments deposited on
top of the buried eroded surface
1. Layers formed according to superposition
2. Something happens to uplift area
3. Erosion wears away the uppermost layers
4. Area submerges and deposition begins again
AT THE INTERFACE BETWEEN STEPS 3 AND 4 THERE IS
A BURIED EROSIONAL SURFACE AKA AN
UNCONFORMITY
Practice: what happened here?
Unconformities are usually shown in block diagrams with a
wavy line to represent the buried eroded surface interface
between layers
Upper Silurian
Carbonates
Tilted
Ordovician
Shales and
Sandstones
unconformity
Taconic Unconformity
Mr. Orgonik
pointing out
the Taconic
Unconformity
Applying Principles of Relative
Dating to Determine Geologic
History of an Area
• The process of matching rocks or geologic
events occurring at different locations of the
same age is called
CORRELATION
Correlation of rock layers often
relies upon fossils
• William Smith (late l700’s) noted that rock layers
in widely separated areas could be identified and
correlated by their distinctive fossil content
• This led to the "principle of fossil succession“
• Fossils succeed one another in a definite and
determinable order, and therefore any time period
can be recognized by its fossil content
Which fossil might be found in Devonian rock
layers?
Eurypterus
NY State Fossil
Silurian index
fossil
Index fossils – Page 8-9 ESRT
Index fossils - any animal or plant that
is characteristic of a particular
span of geologic time or
environment.
2 criteria must be met
•
•
Life form lived over a wide
geographic area – horizontal
distribution
Life form existed for a short period
of time – short vertical distribution
Location A
Location B
Location C
Rock layer 1
W
W
W
Rock layer 2
W
Rock layer 3
W
Z
X
Z
Y
X
Z
X
Z
Which letter would make a good index fossil?
OTHER METHODS OF
CORRELATION
• Layers of bedrock exposed (outcrops) on
either sides of river valleys/excavations
“walking the outcrop”
• Volcanic ash – large eruption – widely
distributed – represents a small time interval
LIKE AN INDEX FOSSIL
VOLCANIC ASH LAYER AT THE SAME TIME AS
THE DINOSAURS BECAME EXTINCT LEAD
SCIENTISTS TO LOOK FOR METEORITE CRATER
Chicxulub Crater 65 mya 170 km. Wide