ch.6 earth’s history. who’s got the time? relative: order/sequence known, but not the actual...
TRANSCRIPT
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 TimeThe 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
Superposition- youngest
to
oldest
Grand Canyon
IGNEOUS INTRUSION:
• Occurs when magma squeezes into or between layers of pre-existing rock.
• Surrounding pre-existing rocks will undergo contact metamorphism
Intrusions
contact metamorphism on all sidescontact metamorphism on all sides
Extrusions
no contact metamorphism on topno contact metamorphism on top
Is “H ” an intrusion or extrusion?
How can you tell?
Is “H ” an intrusion or extrusion?
How can you tell?
H is an intrusion.Contact
metamorphism on top
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? What is an unconformity? • a buried erosional surface
a part of the rock record
is missing
Rocks above unconformity are younger – rocks below older
a part of the rock record
is missing
Rocks above unconformity are younger – rocks below older
How does it complicate the relative dating of rock layers?
How does it complicate the relative dating of rock layers?
Upper Silurian Carbonates
Tilted Ordovician Shales and Sandstones
Taconic Unconformity
unconformity
What processes could lead to an 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 X2. Letter E3. Letter Q4. 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
- any animal or plant that is characteristic of a particular span of geologic time or environment.
Index Fossils
• lived over a large geographic area (large horizontal distribution)
• lived for a short period of time (small vertical distribution)
• What characteristics must fossils have in order to be
good index fossils?
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?
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
For example, fossils of marine organisms can be found in rocks that are presently high above sea level.
the Earth’s surface has changed.
4. Fossils give clues to
Since coral today live in the warm waters of equatorial regions, between 30° N and 30° S latitude, we can infer that
Earth’s past climate.
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