A. Principle of Original Horizontality Most sediment settles out of bodies of water = deposited...

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


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


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


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



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.

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


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




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 DatingII. Unconformities:

missing time in the rock record

B. Angular UnconformityI. Principles of Relative Dating



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



Unconformity animation



Relative dating animation

Horizontality, superposition, unconformities, cross-cutting relations, and faunal succession


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


III. Relative Dating by Weathering


B. Sharpness of landscape/topography

Young landscapes are sharp

Old landscapes that have been well weathered have rounded features (e.g., hills and valleys)


III. Relative Dating by Weathering


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


Summary1. Principle of Horizontality2. Principle of Superposition

Angular unconformityNonconformityDisconformity

3. Principle of Cross-Cutting RelationsDikes, sills, plutonsFaults

Principle of inclusions4. Faunal Succession

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.


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.


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.


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).


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.


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


Zarco Formation• Fault (covered) offsets the four sedimentary units• Erosion (especially of Moenkopi)

Let’s practice


Zarco Formation• Fault (covered) offsets the four sedimentary units• Erosion (especially of Moenkopi)• Emplacement of Bandelier Rhyolite (as hot ash flow)

Let’s practice


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.

A. Principle of Original Horizontality Most sediment settles out of bodies of water = deposited...

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