Determining geological ages - Loudoun County Public Time. · • Law of superposition ... cut...

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Transcript of Determining geological ages - Loudoun County Public Time. · • Law of superposition ... cut...

  • 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: someones 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 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