a. principle of original horizontality most sediment settles out of bodies of water = deposited...
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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
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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
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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
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C. Principle of Cross-Cutting 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
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C. Principle of Cross-Cutting 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
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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
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I. Principles of Relative Dating
CWUtrash bin
UWtrash 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.
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I. Principles of Relative Dating
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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
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I. Principles of Relative Dating
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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
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II. Unconformities: missing time in the rock record
I. Principles of Relative Dating
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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
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I. Principles of Relative DatingII. Unconformities:
missing time in the rock record
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B. Angular UnconformityI. Principles of Relative Dating
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B. Angular UnconformityI. Principles of Relative Dating
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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
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I. Principles of Relative Dating
Unconformity animation
II. Unconformities: missing time in the rock record
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I. Principles of Relative Dating
Relative dating animation
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Horizontality, superposition, unconformities, cross-cutting relations, and faunal succession
I. Principles of Relative Dating
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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
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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
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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
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I. Principles of Relative Dating
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Summary1. Principle of Horizontality2. Principle of Superposition
Angular unconformityNonconformityDisconformity
3. Principle of Cross-Cutting RelationsDikes, sills, plutonsFaults
Principle of inclusions4. Faunal Succession
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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
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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
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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
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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
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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
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I. Principles of Relative Dating
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I. Principles of Relative Dating
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Let’s practice
• List events from oldest to youngest (including faulting and erosion
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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
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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
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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)
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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)
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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
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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)
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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.
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