I. Principles of Relative Dating

Name: I. Principles of Relative Dating
Uploaded: 2017-06-29T15:45:37+00:00
Duration: PTM14S53
Channel: Rachael Worth
Description: I. Principles of Relative Dating

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

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

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 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

II. Unconformities: missing time in the rock record

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

II. Unconformities: missing time in the rock record

B. Angular Unconformity

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

II. Unconformities: missing time in the rock record 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 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)

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

Grand Canyon: Metamorphic rocks: oldest because if rock nearby had been nearby it would be metamorphosed too Igneous intrusion Nonconformity: between metamorphic and igneous rock and layered sedimentary rocks Unkar Group: Proterozoic and Archean Eons Angular Unconformity: between Proterozoic and Phanerozoic Eons Tonto Group: Cambrian Disconformity: Ordovician and Silurian missing Redwall Limestone: Devonian and Mississipian Disconformity:

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

IV. Problem set, due Monday Page 2: C: two feeder dikes, where they cross is blank, must decide relative age D: pieces of B in C Page 4 and 5 go through the layers Key on page 4 for both Come look at the originals if unsure! Come get help from me if you need it! I will walk through it with you! Can work with a partner Please do not copy, it won’t help you in the long run!! NEAT—do in pencil!!!

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.

Question of the week! Draw and label an angular unconformity

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

Practice makes perfect?
Now for a little more complicated example

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 m/year = 100 km/4000 years = advances/20,000 yrs. (2) Uplift of mountains 0.5 to 1.5 m /100 years m years m years 150 m -- 10,000 m ,000 m (9800 ft) ,000 Everest 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) million years (Lord Kelvin ) (5) Opening of the Atlantic Ocean (a) Rate = cm/year Million Years.

I. Principles of Relative Dating

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