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