Determining geological ages Relative ages – placing rocks and geologic events in their proper...

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)

8_9

(a)

(b)

(c)

Layeredsedimentaryrocks

NonconformityMetamorphicrock

Igneousintrusive rock

Youngersedimentaryrocks

Angularunconformity

Older, foldedsedimentaryrocks

Disconformity

Brachiopod(290 million years old)

Trilobite (490 million years old)

Formation of an angular unconformityFormation of an angular unconformity

Angular UnconformityErosional Surface

Angular Unconformity

Horizontal younger sediments over tilted older sedimentsCambrian Tapeats sandstone over Precambrian Unkar Group

What type of unconformity is this?

Grand Canyon in Arizona

Angular Unconformity

Angular Unconformity, Siccar Point, Scotland

Disconformity

Development of a Nonconformity

Pennsylvanian sandstone over Precambrian granite is a nonconformity

An intrusion occurs

The overburden is eroded away

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 strataZoroaster 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 layersCorrelation of rock layers

• Matching strata of similar ages in different regions is

called correlationhttp://www.uwsp.edu/geo/faculty/ozsvath/images/stratigraphy.jpg

Correlation of strata in southwestern United States

Sections are incompleteMatch with fossils and lithology

Matching Rock Layers in Africa and South America

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.

Radioactive Dating- Half Life

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+ yearsBased on living and fossil wood

Paleoclimate informationPaleohydrology

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 5.730 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