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Unit #5
Geologic Time
21.1 Organizing Time
O Fossils are preserved remains or traces of ancient
organisms.
O Fossils are located within the layers of rock on Earth
O Studying these rock layers and the fossils in them enable
scientists to reconstruct Earth’s history and past climates
21.1 Organizing Time
O Using the information
from fossils and rock
layers geologists have
created the geologic time
scale
O Geologic time scalerecord of Earth’s history
from its origin 4.6 billion
years ago to the present
21.1 Geologic Time Scale
O The geologic time scale
O
O
O
O
begins with Earth’s
formation 4.6 bya.
Geologists organize
Earth’s history according
to groupings called eons.
Each eon contains eras
Each era contains
periods.
Each period contains
epochs.
These divisions are
defined by different life
forms found within the
rock
O Eons
21.1 Geologic Time Scale
O The earliest eon is the
Precambrian
O Makes up 90% of
Earth’s history
O Began when the Earth
first formed until it
became hospitable to
simple life
O After the Precambrian
there the Phanerozoic
eon
21.1 Geologic Time Scale
O Eras
O Usually 10-100’s of
millions of years in
length
O The three era found in
the Phanerozoic eon
O Paleozoic-Ancient Life
O Mesozoic- Middle Life
O Cenozoic- Recent Life
O Periods
O Generally 10’s of
million of years in
length
21.1 Geologic Time Scale
O Epochs
O Generally 100’s of thousands to millions of
years
O The most current epoch is the Holocene
which started 12,000 years ago to current
time
21.1 Succession of Life Forms
O During the Phanerozoic Eon multicellular life diversified
becoming more complex
O During the Paleozoic era small segmented animals like
trilobites became abundant
O The end of the Paleozoic is marked by largest mass
extinction in Earth’s history
O About 97% of marine organism became extinct
O The large loss of species in a relatively short period of time
is a mass extinction.
21.1 Succession of Life Forms
O The era after the Paleozoic is called the
Mesozoic
O Know for the emergence of dinosaurs
O The end of the Mesozoic is marked by another
mass extinction even which includes the
extinction of the dinosaurs
O The era after the Mesozoic is the Cenozoic
O Mammals began to dominate in numbers and
diversity
O Humans arise
21.2 Interpreting Geology
O In the past it was thought that the Earth was only
a few thousand year old (time spans that the
human could relate)
O James Hutton (1700’s) was the first scientist to
think that the Earth was very old
O Hutton attempted to explain the process that
occur on Earth (erosion, weathering ect.) if the
Earth was only a few thousand years old and he
couldn’t
O He realized that it would take a much longer
amount of time for those processes occur
21.2 Uniformitarianism
O Hutton’s work led to the development of
uniformitarianism
O Uniformitarianism states that geologic processes
occurring today have been occurring since Earth
formed
21.2 Principles for Determining
Relative Age
O Relative-age dating is the process that is used to
put geologic events in order
O There are several method used to determine
relative ages
O Original Horizontality
O Superposition
O Cross Cutting Relationships
O Inclusions
21.2 Principles for Determining
Relative Age
O Original Horizontally
O Sedimentary rocks
are deposited in
horizontal or
nearly horizontal
21.2 Principles for Determining
Relative Age
O Superposition
O The oldest rocks are found at the bottom of a
sequence of rocks
O Each layer above get progressively younger
21.2 Principles for Determining
Relative Age
O Cross-Cutting Relationships
O States that an intrusion is younger that the
rock it cuts across
O It also applies to faults that cut across rock
layers
21.2 Principles for Determining
Relative Age
O Inclusions
O Rocks can contain pieces of other rocks called inclusions
O The principle of inclusions states that inclusions
(fragments from other rocks) must be older than the rock
that contains them
21.2 Principles for Determining
Relative Age
O Unconformities
O Because Earth’s surface is constantly changing it can be
difficult to relatively date some sequences of rocks
O Many time rocks have been weathered away, removed, or
changed in other ways
O Unconformity occurs where a layer(s) of rock have been
removed then sediments are deposited on top of the missing
layers
21.2 Principles for Determining
Relative Age
O There are three types of unconformity
O Disconformity- When sedimentary rock is
eroded away and new sedimentary rock is
deposited
Sedimentary Rock
Area where erosion
occurred
Sedimentary Rock
21.2 Principles for Determining
Relative Age
O Nonconformity
O A layer of sedimentary rock that is deposited
over a layer of metamorphic rock
Erosional
Surface
21.2 Principles for Determining
Relative Age
O Angular
Unconformity
O When horizontally
deposited layers of
rock are deformed
(tilted) then are
weathered
O Then sedimentary
rock layers are
deposited on top of
the tilted layers
21.2 Principles for Determining
Relative
Age
O Correlation
O The matching of rock layers of fossils in one region to similar
rocks in another location
21.2 Principles for Determining
Relative Age
O Fossil Correlation
O The use of fossils
in sedimentary
rock layers to
compare rocks
from different
regions
21.2 Principles for Determining
Relative Age
O Key Beds
O Distinctive rock layers that are deposited over
a large geographic area
O They are used as a marker to help date the
rock layers around it
21.3 Radioactive Isotopes
O Absolute-age dating- helps scientists determine the numerical
ages of rocks
O Researchers use radioactive isotope to find the age of rocks
O Radioactive Decay is the emission of radioactive particles and
the resulting change into other isotopes
O Radioactive isotopes emit nuclear particles at a constant rate
no matter the conditions
21.3 Radioactive Isotopes
O The original radioactive isotope is called the
parent
O The isotope that results from radioactive
decay is called the daughter
21.3 Radiometric Dating
O As the number of parent atoms
decreases the number of
daughter atoms increases
O The ratio of parent to daughter
product in a mineral shows the
amount of time that has passed
since it formed
O The time it takes for ½ of the
original parent isotope to decay
into daughter product is called
half-life
O After 1 half life 50% of the
parent has turned into daughter
O After 2 half lives 75% of the
parent material has turned into
daughter product
21.3 Radiometric Dating
21.3 Dating Rocks
O Radiometric dating is useful for dating
igneous and metamorphic rocks
O Sedimentary rocks cannot be dated using
radioactive isotopes because they are
formed form other rocks
O To get the age of sedimentary rocks
geologists can use surrounding igneous and
metamorphic rocks to get a more accurate
age
21.3 Radiocarbon Dating
O Uses carbon-14 (radioactive isotope of carbon)
O Has a much shorter half-life than other radioactive
isotopes
O Used to determine the age of organic materials
O Bone, charcoal, amber
O Used for dating geologic events that occurred with in
the last 60,000 years
21.3 Other ways to Determine
Absolute Age
O Tree rings
O growth rings in a tree
that can show how
old a tree is
O Can also show certain
events in the past
O Wet or dry periods
O Forest fires
O Can date up to
10,000 years in the
past
O Dating using tree
rings is called
dendrochronology
21.3 Other ways to Determine
Absolute Age
O Ice Cores
O Used a lot like tree rings
O Contain a record of past environmental conditions like
temperature and atmospheric composition
21.3 Other ways to Determine
Absolute Age
O Varves
O Alternating light and dark
O
O
O
O
sediments of sand, clay
and silt
Show seasonal deposition
of sediments, usually in
lakes
Summer sediments are
generally sand sized
particles with traces or
organic matter (lighter in
color)
Winter darker fine grained
sediments
Can date up to 120,000
years
21.4 Fossil Record
O Fossils provide evidence
of the past existence if
life forms
O Also provide evidence
for how species have
evolved over time
O When geologists find a
fossil in a rock they
know that the rock is
about the same age as
the fossil
21.4 Fossil Record
O Original Preservation
O Remains of plants and
animals that have been
changed very little since the
organisms death
O Requires freezing, arid or
oxygen free environments
21.4 Fossil Record
O Altered Hard-Parts
O Usually the soft parts of an organism decay rapidly
O The hard parts can become fossils with altered parts
O Shells, bones. Cell walls, exoskeletons
O Altered part fossils are the most common type of
fossil
O They form from two processes
O Mineral Replacement
O Recrystallization
21.4 Fossil Record
O Mineral Replacement
O the pore spaces of on organisms hard parts
are filled in with minerals from ground water
O This gradually replaced the organisms
original hard parts all together with minerals
21.4 Fossil Record
O Recrystallization
O Occurs when buried
hard parts are
subjected to changes
in temperature and
pressure
O The original mineral in
the organism is
transformed into a
new mineral
21.4 Fossil Record
O Molds and Casts
O Fossils that do not contain
any of the original or altered
material of an organism
O Mold- Sediments cover the
original hard part of and
organism and the hard part
in later removed by erosion or
weathering leaving a
hollowed out impression
O A cast forms when the mold
become filled in with
materials
21.4 Fossil Record
O Trace Fossils
O indirect evidence that an organism existed
O Foot prints, tunneling burrows
21.4 Index Fossils
O Fossils that are
easily recognized,
abundant, widely
distributed
geographically and
existed for a short
period of geologic
time
O These types of
fossils can help
scientists date
sedimentary rock
layers