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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 position in the Strata relative to

other rocks, rock layers or

fossils.

Relative Age Dating is based on a

list of principles or rules.list of principles or rules.

Law of Uniformitarianism.

The physical, chemical, and

biological laws that govern biological laws that govern

processes today have remained

constant throughout time.

• 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 topthe top

Principle of Superposition

Superposition illustrated by strata

in the Grand Canyon

• Principle of original horizontality

• Layers of sediment are originally

deposited horizontally (flat strata have

not been disturbed by folding, faulting)

An Igneous rock is always younger

than the rock layer that it has

.

Principle of cross-cutting relationships

intruded or cuts across.

Principle of cross-cutting relationships

Principle of Cross-Cutting Relationships

The dike is youngest

because it cuts across

layers 1-4

Layer 1 is the

oldest rock layer

Cross-cutting Relationship with

multiple overlapping intrusions

Erosional Features and Faults that

cut across rock layers are always 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 Inclusion, states that

rocks that are embedded in

another rock must be older than 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 layers are missing

(Unconformities).

The processes that would bring

about the removal of these

missing layers require large 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 – 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)

Layeredsedimentaryrocks

Nonconformity

Metamorphicrock

Igneousintrusive rock

Youngersedimentaryrocks

Angularunconformity

(c)

unconformity

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 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

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 characteristicstheir 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 incomplete

Match with fossils and lithology

NYS Regents Exam diagram

Index Fossil Requirements

Index Fossils must

– be easy to identify

– have been very abundant– 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)

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

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 ratespecific 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 woodBased 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 5,730 years, how many atoms will be turned into N-14?

• HALF LIFE • 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 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