determining geological ages - loudoun county public … time.… · • law of superposition ......
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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.
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 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
The dike is youngest
because it cuts across
layers 1-4
Layer 1 is the
oldest rock layer
Erosional Features and Faults that
cut across rock layers are always cut across rock layers are always
younger.
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.
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)
Horizontal younger sediments over tilted older sediments
Cambrian Tapeats sandstone over Precambrian Unkar Group
What type of unconformity is this?
Grand Canyon in Arizona
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
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
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
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)
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
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
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