# Determining geological ages

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Determining geological ages
Relative ages – placing rocks and geologic events in their proper sequence, oldest to youngest. Absolute dates – define the actual numerical age of a particular geologic event. For example, large dinosaurs died out 65 mya. The Lavas along Rt 22 and Rt 78 were deposited about 205 mya.

Relative Age Dating assigns a non-specific age to a rock, rock layer or fossil based on its position in the Strata relative to other rocks, rock layers or fossils.

Relative Age Dating is based on a list of principles or rules.

First principle of relative dating
Law of superposition Developed by Nicolaus Steno in 1669 In an undeformed sequence of sedimentary or volcanic rocks the oldest rocks are at the base; the youngest are at the top

-Superposition

Principle of Superposition

Superposition illustrated by strata in the Grand Canyon

2nd principle of relative dating
Principle of original horizontality Layers of sediment are originally deposited horizontally (flat strata have not been disturbed by folding, faulting)

3rd principle of relative dating
Principle of cross-cutting relationships

3rd principle of relative dating
Principle of cross-cutting relationships (example 2)

Cross-Cutting

An Igneous rock is always younger than the rock layer that it has intruded or cut across.

Principle of Cross-Cutting Relationships
The dike is youngest because it cuts across layers 1-4 Layer 1 is the oldest rock layer

Key to Rocks Used in Diagrams

Limestone

Igneous

Metamorphic

Cross-cutting Relationship with multiple overlapping intrusions

Erosional Features and Faults that cut across rock layers are always younger.

Example of Law of Cross-Cutting Relationships
Which came first, the rock layers or the faults?

Cross-cutting Normal Fault

The Law of Embedded Fragments, or Law of Inclusion, states that rocks that are embedded in another rock must be older than the rock in which it is found.

Inclusion

Examples of Law of Inclusions

Inclusion- Conglomerate fragments in overlying Shale

Inclusion-Granite fragments included in overlying Shale

Inclusion- Shale fragments imbedded in Granite intrusion

Another method of examining the Geologic Record involved examining instances where rock layers are missing (Unconformities).

The processes that would bring about the removal of these missing layers require large amounts of time.

Unconformities (loss of rock record)
An unconformity is a break in the rock record produced by erosion and/or nondeposition Types of unconformities Nonconformity – sedimentary rocks deposited above metamorphic or igneous rocks (basement) with time lost Angular unconformity – tilted rocks overlain by flat-lying rocks Disconformity – strata on either side of the unconformity are parallel (but time is lost)

Disconformity Angular unconformity Layered (a) sedimentary rocks
8_9 Nonconformity Igneous intrusive rock Metamorphic rock (b) Younger sedimentary rocks Angular unconformity Older, folded sedimentary rocks (c) Disconformity Trilobite (490 million years old) Brachiopod (290 million years old)

Formation of an angular unconformity

Angular Unconformity Angular Unconformity Erosional Surface

Horizontal younger sediments over tilted older sediments
Cambrian Tapeats sandstone over Precambrian Unkar Group What type of unconformity is this? Grand Canyon in Arizona

Angular Unconformity

Angular Unconformity, Siccar Point, Scotland

Disconformity

Sea level rises, new sediment is deposited
Development of a Nonconformity An intrusion occurs The overburden is eroded away Pennsylvanian sandstone over Precambrian granite is a nonconformity Sea level rises, new sediment is deposited

Nonconformity- Sedimentary Rock layers over older Igneous or Metamorphic

Nonconformity in the Grand Canyon - Sediments deposited over Schist

Cross Cutting Relationships in strata
Zoroaster Granite across Vishnu Schist

Rock Layer Correlation
Correlation is the matching of rock layers from one area to another. Matching rocks in different locations due to their similar characteristics Key Beds Stratigraphic Matching Using Index Fossils (fossils that lived and died in one particular geologic time) to match rock layers

Correlating Rock age using Index Fossils and Stratigraphic Matching

Correlation of rock layers
Matching strata of similar ages in different regions is called correlation

Correlation of strata in southwestern United States
Sections are incomplete Match with fossils and lithology

Matching Rock Layers in Africa and South America

Because of sea-level changes Fossils are more reliable than sequences of sediment facies
However, falling sea level is useful for worldwide correlation. Why? Sauk Sequence WEST EAST Transgression Middle Cambrian Lower Cambrian Note how western BAS is older than eastern BAS

Index Fossil Requirements
Index Fossils must be easy to identify have been very abundant have lived in a wide geographic area have existed for a short geologic time (ie: someone’s picture in a yearbook)

NYS Regents Exam diagram

Absolute Age Dating

Radiometric Dating- Proportion of Parent to Daughter Isotopes
To get amount of parent material for each half-life, know that after one half-life, you have ½ of parent isotope left, then double your denominator for each half-life thereafter.

Half Life The original isotope is called the parent
The new isotope is known as the daughter isotope Produced by radioactive decay All parent isotopes decay to their daughter isotope at a specific and unique rate Based on this decay rate, it takes a certain period of time for one half of the parent isotope to decay to its daughter product Half life – the time it takes for half of the atoms in the isotope to decay

Tree Ring Chronology (Dendrochronology)

Comparison with known tree ring sequences Can go back 10,000+ years Based on living and fossil wood Paleoclimate information Paleohydrology Archeology.

EX: The half life of C-14 is 5,730 years
So it will take 5,730 years for half of the C-14 atoms in an object to change into N-14 atoms So in another years, how many atoms will be turned into N-14? HALF LIFE In another 5,730 years, another half of the remaining atoms will degrade to N-14, and so on. So after 2 half lives, one forth of the original C-14 atoms remain After 3 half lives, one eighth of the original c-14 atoms still remain Keeping cutting in half

Radiocarbon Dating C-14 is useful for dating bones, wood and charcoal up to 75,000 yo Living things take in C from the environment to make their bodies Most is C-12 but some is C-14 The ratio of these two types in the enviro is always the same By studying the ratio in an organism it can be compared to the ratio in the environment presently