Transcript Compared to the desolate surface of the Moon, Earth must

I. Principles of Relative Dating
A. Principle of Original
Horizontality
Most sediment settles out of
bodies of water = deposited
horizontally
Most lava flows are horizontal
or slightly tilted on flanks of
volcanoes (1 -10)
When rocks are not horizontal
they have been tilted
I. Principles of Relative Dating
B. Principle of
Superposition
In a horizontal sequence of
rock layers: the youngest is
on top & oldest at the
bottom.
If rocks are tilted, look for
mudcracks, graded beds,
ripple marks, cross bedding,
or vesicles to determine the
up direction
I. Principles of Relative Dating
B. Principle of Superposition
If rocks are tilted, look for
sedimentary structures such as,
mudcracks, graded beds, ripple
marks, cross bedding, or vesicles to
determine the up direction
I. Principles of Relative Dating
C. Principle of CrossCutting Relationships
Any intrusive formation (dike,
sill, batholith) is younger
than the rock it cuts across
Faults are younger than the
rocks they cut and displace
I. Principles of Relative Dating
C. Principle of CrossCutting Relationships
Any intrusive formation (dike,
sill, batholith) is younger
than the rock it cuts across
Faults are younger than the
rocks they cut and displace
I. Principles of Relative Dating
D. Inclusions: solid
pieces of rock in
another rock
“Inclusions are always
older than the rock
they are in”
Lava flows or intrusions
may pick up pieces of
the surrounding rock
(unmelted) and when
the lava/magma cools
the other rock is an
inclusion
I. Principles of Relative Dating
CWU
trash bin
UW
trash bin
Imagine we are studying two different trash pits recently discovered on the
CWU campus and the UW campus. By carefully digging, we have found
that each trash pit shows a sequence of layers.
Although the types of trash in each pit is quite variable, each layer has a
distinctive kind of trash that distinguishes it from other layers in the pits.
I. Principles of Relative Dating
I. Principles of Relative Dating
E. Principle of Faunal
Successions
Fossils: remains of ancient
organisms
Paleontologists study fossils to
reconstruct the evolution of life
on Earth
Fossils can be preserved in rock
layers
Specific groups of fossils follow,
or succeed, one another in the
rock record in a definite order
Once you know the order, you
can do relative ages and
correlate rocks in different parts
of the world based on the
fossils present
I. Principles of Relative Dating
I. Principles of Relative Dating
II. Unconformities: missing
time in the rock record
Gaps in the geologic record
created by weathering and
erosion or no rocks were
deposited
A. Disconformity
Occurs between parallel
layers of sedimentary rock
or lava flows
Time missing using the fossil
record
Layer below shows erosion irregular surface
I. Principles of Relative Dating
II. Unconformities:
missing time in the
rock record
I. Principles of Relative Dating
II. Unconformities: missing
time in the rock record
Gaps in the geologic record
created by weathering and
erosion or no rocks were
deposited
B. Angular Unconformity
Boundary between
originally horizontal rocks
that have been
deformed/tilted and
eroded and later
horizontal rock deposited
on top
I. Principles of Relative Dating
II. Unconformities:
missing time in the
rock record
I. Principles of Relative Dating
B. Angular Unconformity
I. Principles of Relative Dating
B. Angular Unconformity
I. Principles of Relative Dating
II. Unconformities: missing
time in the rock record
Gaps in the geologic record
created by weathering and
erosion or no rocks were
deposited
C. Nonconformity
Boundary between an
unlayered body of plutonic
igneous or metamorphic
rock and an overlying
layered sequence of
sedimentary rock layers
Underlying rock shows signs
of erosion, e.g., irregular
surface
I. Principles of Relative Dating
II. Unconformities:
missing time in the
rock record
Unconformity animation
I. Principles of Relative Dating
Relative dating
animation
I. Principles of Relative Dating
Horizontality, superposition, unconformities,
cross-cutting relations, and faunal succession
I. Principles of Relative Dating
III. Relative Dating using
Weathering
Weathering is a function of time
In one location, other factors like
climate, organisms, and rock
compositions will be held
constant
A. Weathering Rinds
Rind of chemical weathering
formed as water penetrates
into rock and minerals are
altered to more stable
minerals
Rind gets thicker with time
I. Principles of Relative Dating
III. Relative Dating by
Weathering
Weathering is a function of
time
B. Sharpness of
landscape/topography
Young landscapes are sharp
Old landscapes that have
been well weathered have
rounded features (e.g.,
hills and valleys)
I. Principles of Relative Dating
III. Relative Dating by
Weathering
Weathering is a function of
time
C. Soil Development
More soil development = older
landscape (if other factors
held constant)
Depth that soluble elements
have been dissolved away
Deeper soil = older landscape
(with similar climate)
Development of soil horizons
better developed soil horizon
= older landscape
I. Principles of Relative Dating
Summary
1. Principle of Horizontality
2. Principle of Superposition
Angular unconformity
Nonconformity
Disconformity
3. Principle of Cross-Cutting Relations
Dikes, sills, plutons
Faults
Principle of inclusions
4. Faunal Succession
I. Principles of Relative Dating
Long before radiometric dating
was possible, important
principles of relative ages of
rock units were established.
I. Principle of original
horizontality:
Because sedimentary particles
settle under the influence of
gravity, sedimentary layers of
rock are deposited horizontally.
Sedimentary rock layers that are
not horizontal have been folded
or tilted by a tectonic event.
Deposition of the sedimentary
rocks predates the tectonic
event.
I. Principles of Relative Dating
2. Principle of superposition:
In any sequence of undisturbed
layers of sedimentary rocks, the
oldest layer is on the bottom
and successively higher layers
are successively younger.
Layers later can be tilted and
deformed, even turned upside
down by later tectonic events.
The original top and bottom of a
sedimentary unit often can be
determined from sedimentary
structures, such as mud cracks,
cross beds, and ripple marks.
I. Principles of Relative Dating
3. Principle of inclusions:
Fragments of rock that are
enclosed within another
rock are older than the
enclosing rock.
Example: unconformities,
fragments of the older,
underlying rocks are eroded
and incorporated into the
overlying, younger
sedimentary rock.
I. Principles of Relative Dating
4. Crosscutting
relationships: If an igneous
intrusion or a fault cuts a
rock unit, then the rock unit
is older than the intrusion or
fault.
Evidence for intrusion can
include baking of the
intruded rocks.
If you date the igneous rock,
you have a limit on the
youngest absolute possible
age of the rocks (minimum
age, i.e., they cannot be
younger than the intrusion).
I. Principles of Relative Dating
5. Superposition of volcanic
rocks:
If sedimentary rocks are
overlain by a lava flow, they
must be older than the
flow.
If you date the igneous rock,
you know the youngest
possible absolute age for
the sedimentary rocks.
I. Principles of Relative Dating
I. Principles of Relative Dating
Let’s practice
• List events from oldest to youngest (including faulting and
erosion
Let’s practice
• List events from oldest to youngest (including faulting and
erosion)
• Deposition of Abo Formation
• Yeso Formation,
• Moenkopi Formation, and
• Agua Zarco Formation
Let’s practice
• List events from oldest to youngest (including faulting and erosion)
• Deposition of Abo Formation, Yeso Formation, Moenkopi Formation, Agua
Zarco Formation
• Fault (covered) offsets the four sedimentary units
Let’s practice
• List events from oldest to youngest (including faulting and erosion)
• Deposition of Abo Formation, Yeso Formation, Moenkopi Formation, Agua
Zarco Formation
• Fault (covered) offsets the four sedimentary units
• Erosion (especially of Moenkopi)
Let’s practice
• List events from oldest to youngest (including faulting and erosion)
• Deposition of Abo Formation, Yeso Formation, Moenkopi Formation, Agua
Zarco Formation
• Fault (covered) offsets the four sedimentary units
• Erosion (especially of Moenkopi)
• Emplacement of Bandelier Rhyolite (as hot ash flow)
Let’s practice
• List events from oldest to youngest (including faulting and erosion)
• Deposition of Abo Formation, Yeso Formation, Moenkopi Formation, Agua
Zarco Formation
• Fault (covered) offsets the four sedimentary units
• Erosion (especially of Moenkopi)
• Emplacement of Bandelier Rhyolite (as hot ash flow)
• Erosion
Length of Geologic Time
Age of the Earth
A. How Old is the Earth
i. Weathering of rocks and Sediment Production
ii. Settling of clay particles
(1) Clay particle .5 microns in diameter
89 days/m = 243 years/1 Km
1701 years/7 km
iii. Rates
(1) Mississippi Delta Advance
25 m/year = 100 km/4000 years =
5 advances/20,000 yrs.
(2) Uplift of mountains 0.5 to 1.5 m /100 years
1.5 m -- 100 years
15 m -- 1000 years
150 m -- 10,000
1500 m -- 100,000
3000 m (9800 ft) -- 200,000
Everest -- 29028 ft. 600,000 years
(assuming no erosion is occurring)
Length of Geologic Time
Age of the Earth
A. How Old is the Earth
iii. Rates
(3) Clay Deposition--Deep Ocean
(a) 1 cm/1000 years = 100 m/10 million years
(4) Cooling of the Earth from an initial melt
(1) 20-30 million years (Lord Kelvin --1846)
(5) Opening of the Atlantic Ocean
(a) Rate = cm/year ----- 180 Million Years.