Transcript Ch. 9
Chapter 09
Geologic Time
Geologic
Time Scale
Relative Dating - Superposition
In
an undeformed sequence of
sedimentary rock, the older rocks
are at the bottom
Rocks and therefore fossils closer
to the surface are more recent
Superposition
Grand Canyon
Relative Dating
Original Horizontality
Layers
of sediment are generally
deposited in a horizontal position
Rock
layers that are flat, have not been
disturbed
Original Horizontality
Relative Dating
Lateral Continuity
Sedimentary
beds originate as continuous
layers that extend in all directions until they
eventually thin out or grade into a different
sediment type
Relative Dating
Lateral Continuity
Relative Dating
Cross-Cutting
Younger features cut across older features
Inclusions
are rock fragments within
another rock
The inclusions are older
Relative Dating
Cross-Cutting & Inclusions
Relative Dating
Unconformities
Layers
of rock that have been
deposited without interruption are
called conformable layers
An unconformity is a break in the
rock record produced by
nondeposition and/or erosion of rock
layers
Unconformities
Angular
Tilted
Unconformity
rocks overlain by flat-lying rocks
Disconformity
Strata
on either side of the unconformity
are parallel
Nonconformity
Found
with metamorphic or igneous rocks
Formation
of an
Angular
Unconformity
Angular Unconformity
Unconformities in the Grand Canyon
Fossils
Traces
or remains of prehistoric life
Sedimentary rock
Rare in metamorphic
Never in igneous
Paleontology - study of fossils
Fossil Preservation
Rapid
Hard
burial
body part
Petrified Wood
Carbon Film of a Bee
Amber
Dinosaur footprint
in limestone
Trilobite Mold & Cast
Fossils and Correlation
Correlation
Matching
rocks of similar ages in
different areas
Relies upon fossils
William Smith (late 1700s)
Fossils and Correlation
Index
Fossil
Short
Fossil
lived & widespread
Succession
Species
evolve or become extinct
Dating Rocks using Fossil Ranges
Figure 9.12
Atomic Review
Nucleus
Protons
+ charge
Neutrons no charge
Atomic
# = Protons
Atomic Mass= Protons + Neutrons
Electrons – charge & no mass
Radioactivity
Isotope
Variant
of same atom
Differs in the number of neutrons
Different mass # than parent atom
Radioactivity
Decay
in the structure of nucleus
Types of Radioactivity
Alpha
emission
Lose 2 protons & 2 neutrons
Mass # reduced by 4
Atomic # lowered by 2
Types of Radioactive Decay
Beta
Emission
Electron (beta particle) is ejected
Mass # stays the same
Atomic number increases by 1
Types of Radioactive Decay
Electron
Capture
Captured electron combines with a
proton to form a neutron
Mass # stays the same
Atomic number decreases by 1
Dating with Radioactivity
Parent
– unstable radioactive isotope
Daughter
Half-life
decay
– isotope formed via decay
– time for ½ parent to
Radioactive
Decay Curve
Problems with Radioactivity
Need
closed system
Need unweathered samples
Dating with Carbon-14
Produced
in the atmosphere
Half-life = 5730 years
Recent Events
End of Chapter 9