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).

Superposition- youngest to oldest Click anywhere in picture to skip back to LAWS.

GRAND CANYON- LAW OF SUPERPOSITION YOUNGEST ON TOP OLDEST ON BOTTOM

“ORIGINAL HORIZONTALITY” 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 Click to skip back to LAWS.

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

(1) A fault exists between outcrops A and B. 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 Uplift – area of crust uplifted above sea level (deposition – under water) Erosion – some time after Submergence (subsidence) below sea level Deposition – new sediments deposited on top of the buried eroded surface

Layers formed according to superposition Something happens to uplift area Erosion wears away the uppermost layers Area submerges and deposition begins again AT THE INTERFACE BETWEEN STEPS 3 AND 4 THERE IS A BURIED EROSIONAL SURFACE AKA AN UNCONFORMITY

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Sediments deposited, sea-level lowered, layers intruded, layers tilted, erosion and deposition under sea, sea-level lowered again. Click arrow to continue.

Practice: what happened here? Click to see arrows shoot in one at a time. #1= included fragments #2=intrusion, cross-cutting #3= angular unconformity #4=erosion. This is the outcome of the previous slide. Unconformities are usually shown in block diagrams with a wavy line to represent the buried eroded surface interface between layers

Taconic Unconformity 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

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1. Which layers are the same? 2. Which layer is older E or F? 3. What is correct sequence of rock layers from oldest to youngest? 4. An unconformity is represented by the interface between which 2 layers?

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

Which letter would make a good index fossil? Location A Location B Location C Rock layer 1 W W Z Rock layer 2 W Z Y Z Rock layer 3 W X X 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