Transcript Geologic History - Valhalla High School

Ch.6 Earth’s History
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
Intrusions
contact metamorphism on all sides
Extrusions
no contact metamorphism on top
Is “H” an intrusion or extrusion?
How can you tell?
H is an intrusion.
Contact metamorphism
on top
Cross Cutting
Igneous Intrusion - Cross Cutting
Which is older:
F or S
How do you know?
Included Fragments
Included Fragments
Folds/Tilts
Folds/Tilts
What is an unconformity?
• a buried erosional surface
How does it complicate the
relative dating of rock layers?
a part of the rock record
is missing
Rocks above unconformity are
younger – rocks below older
Upper Silurian
Carbonates
Tilted
Ordovician
Shales and
Sandstones
unconformity
Taconic Unconformity
What processes could lead to
an unconformity?
• weathering , erosion &
deposition
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?
Using the diagram to
the left, identify where
the unconformity is
located by drawing an
arrow and writing the
word “unconformity”
next to it.
The unconformities shown in
the cross section represent
1. buried erosional surfaces
2. locations of index fossils
3. volcanic ash deposits
4. boundaries between oceanic
and continental crust
The unconformities shown in the
cross section represent
1. Letter X
2. Letter E
3. Letter Q
4. Letter Y
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
Index Fossils
- any animal or plant that is
characteristic of a particular span of
geologic time or environment.
• What characteristics must
fossils have in order to be
good index fossils?
• lived over a large geographic area (large
horizontal distribution)
• lived for a short period of time (small
vertical distribution)
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
In the diagram below, a geologist has matched up
rock layers based on index fossils.
Where is there an
Unconformity?
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
D. Interpreting Fossils
1. Fossils indicate that
many different kinds of life forms existed at different
times in Earth’s history.
2. When fossils are arranged according to age, they show that
Certain living things have changed or evolved over time.
3. Fossils indicate how the Earth’s surface has changed.
For example, fossils of marine organisms can be found in
rocks that are presently high above sea level.
4. Fossils give clues to Earth’s past climate.
Since coral today live in the warm waters of equatorial regions,
between 30° N and 30° S latitude, we can infer that
N.Y.S. had a warmer climate in the past.
5. Fossils tell about the
appearance and activities of past life.
For example, fossil teeth give clues
about the kind of food the animal
ate.
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
How to Use the Geologic History
of NYS in the ESRT
• ESRT p.8
• ESRT p.9