Transcript Relative Dating - Cloudfront.net

Principles of Relative Dating
What are the rocks telling us about
the history of the earth?
Two ways to tell time with rocks:
• RELATIVE DATING: order or
sequence known, but not the actual
date of occurrence. “Time Line”
(ex) – older or younger
• ABSOLUTE DATING: actual date
determined by radioactive decay
“Clocks in Rocks”
(ex) – 50 million years ago
The Principle of...
• SUPERPOSITION: in a sedimentary
sequence the OLDEST will be on
BOTTOM and YOUNGER will always
be on TOP (if undisturbed – flat and
level).
Superpositionyoungest
to
oldest
GRAND CANYON- LAW OF SUPERPOSITION
YOUNGEST ON TOP
OLDEST ON BOTTOM
• In the 17th C., Nicolas Steno made an
important observation:
"Sediments are usually deposited in
horizontal layers."
He called this
“ORIGINAL HORIZONTALITY”
The Principle of...
• ORIGINAL HORIZONTALITY: Folds and tilts
are younger than rocks themselves
Folds/Tilts
FOLDS - TILTS
The Principle of...
• CROSS-CUTTING: Anything that cuts a
rock must be younger than the rock
a. Igneous intrusion - is younger than rock
it has intruded (cut across). Pre-existing rock
will undergo CONTACT METAMORPHISM
b. Faults – are younger than rock displaced
c. Erosion – layers that get cut are older
than the erosion
Igneous Intrusion - Cross Cutting
Cross Cutting
CROSS-CUTTING
The Principle of...
• INCLUSIONS: pieces of rock found IN
another rock must be OLDER (formed
first) than the rock that they are in.
Included Fragments
Included Fragments
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 Provo
River. 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
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 faunal succession“
• Fossils succeed one another in a definite and
determinable order, and therefore any time period
can be recognized by its fossil content
Eurypterus
Silurian index
fossil
Index Fossils
Index fossils - any animal or plant that
is characteristic of a particular
span of geologic time or
environment.
2 criteria of good index fossils:
•
•
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