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EARTH’S HISTORY
Unit 12
Review Book: Topic 13
I. Determination of Age
A. Uniformitarianism
• The belief that the present
geological events are the
same as past events.
• This allows us to
understand, and make
inferences about, our
geologic past
B. Principle of Superposition
• relates to the original
horizontality of deposited
sediments
• It is a determination of the
relative age of a rock or
event.
• It states that the youngest
rock layers are found on the
top of a series of rock strata
and that rock age increases
with depth.
• A fold or a fault is younger
than the rock it disturbed.
• An intrusion is younger than
the rock it cuts through
• An extrusion is younger than
the rock it cuts through, but
older than the rock layer that
formed on top of it.
• An inclusion is older than the
rock it is in.
• A joint is younger than the
rock it is in
• The metamorphic rock
formed when an intrusion
cuts through pre-existing
rock is younger than the
pre-existing rock, but older
than the igneous intrusion.
• This is called contact
metamorphism and is
indicated by a hachured
line along the interface
between the pre-existing
rock and the igneous rock
• A vein (formed when
dissolved minerals solidify
in a crack in a rock) is
younger than the rock it is
in.
D. Correlating Rock Layers
• A process by which rock
strata are matched
• A rock outcrop is an
exposed section of bedrock
1. Visual correlation:
matching rock layers
based on similarities in:
rock type
composition
color
thickness
fossils
2. Index Fossils
• Fossils of organisms that
have lived for a relatively
brief amount of time and
have existed over a large
geographical area.
• Index fossils are
• found only in one rock
layer
• and in a variety of
locations
• Index fossils are:
–limited geologically
–widespread geographically
Location A
Location B
Location C
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widespread geographically
Ж widespread geologically
limited geographically
‫ص‬limited geologically
limited geographically
Ш limited geologically
widespread geographically
3. Key Bed
• A volcanic eruption results
in the distribution of a thin
layer of ash over a large
geographical region.
• This makes it effective in
determining relative age
and correlating rock layers
• Rock particles and debris
resulting from the impact
of an asteroid can also
cover a large region in a
very thin layer
• This is equally effective in
correlating rock layers and
determining relative age.
4. Unconformity
• A buried erosional surface
indicates that uplift,
weathering and erosion
has occurred in that region
• This destroys a portion of
the rock record
• which results in a gap in
geologic time!
• An unconformity is
represented by a wavy line
(an irregular surface)
between two rock layers:
C. Absolute Age
• refers to the measured age
of a rock or event in years.
• It is generally determined
by comparing the amount
of radioactive material to
the amount of stable decay
product found in a sample
• Radioactive decay data is
used to determine the
absolute age of rock and
rock layers
• An isotope is a variation of
an element in which the
atomic mass differs
• Radioactive decay occurs
when an isotope is
unstable and releases
particles in order to
become stable.
• A radioactive isotope
decays into a stable decay
product.
1. Half-life
• The half-life of a
radioactive isotope is the
amount of time it takes for
one half of the atoms to
become stable decay
product.
• The half-life of a substance
does not change!
• It is independent of size,
mass, temperature,
pressure and location!
• The half-lives and stable
decay products of common
radioactive isotopes are
listed on the front page of
your ESRT
• When determining the age
of a rock, the older the rock
is presumed to be, the
longer the half-life of the
radioactive isotope must be.
U238 is used to determine the
age of rocks that are
hundreds of millions of years
old.
• C14 is used to determine
the age of relatively recent
materials and all organic
remains.
II. Evolution
• The evolution of conditions
on Earth is recorded in the
rock record.
• The presence of marine
fossils in NYS indicate that
it was once covered by a
shallow sea.
• The presence of warm
climate fossils in NYS
indicate that it once had a
climate similar to that of
low latitude regions.
• Large coal deposits indicate
that conditions in the
region were wet (swampy)
1. Organic evolution shows
how life forms change
through time.
• Climate and environmental
changes result in
variations within a species
making it better able to
survive.
• These variations are
passed on to offspring and
are preserved in the rock
record. The fossil record
offers support to this
theory indicating gradual
changes from an older
species to a newer one.
2. Rapid (punctuated) evolution
• occurs when cataclysmic
events such as volcanic
eruptions, collisions of
comets or asteroids often
result in immense changes
in the environment
• This often spurs rapid
evolutionary changes and
extinctions.
• Evidence exists to support
the belief that the
extinction of the dinosaurs
resulted from the impact of
an asteroid.
III. Earth’s Past
• The earth is estimated to
have formed ~4.6 billion
years ago.
• Heat from impact events,
radioactive decay and
gravity caused the earth to
melt.
• Melting resulted in the
separation of the earth
into density zones: core,
mantle, crust, atmosphere.
• It is estimated to have
solidified with a solid crust
~4.2 billion years ago.
• Gases from the interior
seeped out through the
crust (outgassing) and
created a second
atmosphere of water
vapor, carbon dioxide,
nitrogen…
• The cooling of the earth
resulted in the
precipitation of water to
form the oceans ~4 billion
years ago.
• Ocean salts accumulated
due to the chemical
weathering of the ocean
crust.
• ~3.5 billion years ago,
stromatolites (colonies of
algae and bacteria) formed.
• They used carbon dioxide
and released oxygen
(photosynthesis) changing
the atmosphere to one of
nitrogen and oxygen.
• Oxygen reacted with the
iron in the crust creating
iron oxides…
resulting in an appearance
similar to that of Mars
• ~2.8 billion years ago this
reaction ended…
• allowing more oxygen to
accumulate and the
protective ozone layer to
form.
• Life evolved to sexually
reproducing, hard-bodied
life forms during the
Cambrian period.