Earth History- Table of Contents
• RELATIVE vs. ABSOLUTE
• LAWS
• UNCONFORMITIES
• GEOLOGIC SECTIONS
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”
First Things First…or…
“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).
• CROSS-CUTTING: an igneous intrusion is
younger than rock it has intruded (cut across).
• INCLUDED FRAGMENTS: pieces of rock
found IN another rock must be OLDER (formed
first).
• FOLDS/TILTS: younger than rocks
themselves
Superpositionyoungest
to
oldest
Grand Canyon
IGNEOUS INTRUSION:
• Occurs when magma squeezes
into or between layers of preexisting rock.
• Surrounding pre-existing rocks will
undergo contact metamorphism
Cross Cutting
Igneous Intrusion - Cross Cutting
Included Fragments
Included Fragments
Folds/Tilts
Folds/Tilted
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
Upper Silurian
Carbonates
Tilted
Ordovician
Shales and
Sandstones
unconformity
Taconic Unconformity
Mr. Orgonik
pointing out
the Taconic
Unconformity
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
Practice: what happened here?
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
Index fossils
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
Eurypterus
NY State Fossil
Silurian index
fossil
OTHER METHODS OF
CORRELATION
• Layers of bedrock exposed (outcrops) on
either sides of river valleys/excavations
“walking the outcrop”
• Rock similarities
• Volcanic ash – large eruption – widely
distributed – represents a small time interval