unit 6 geologic time
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Unit 6 Geologic TimeChapter 21 Fossils and the Rock Record
21.1 The Rock Record
The geologic time scale- the Earth is 4.6 billion years old. - in order to break up this vast amount of time, scientists divide it into Eons
Eons
The three earliest eons are grouped into the “Precambrian”
Hadean – The formation of the earth
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Had
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Eons
Archaen – Continental plates begin to form and earliest fossils
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Eons
Proterozoic – multicellular life cyanobacteria increases oxygen levels
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Eons
Phanerozoic– “visible life”Most of the fossils with hard parts
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Eons are divided into Eras• Phanerozoic is divided into three Eras
– Paleozoic – ancient life– Mesozoic – middle life– Cenozoic – recent life
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Eras are divided into Periods
• Mesozoic is divided into three Periods– Triassic – Jurassic– Cretaceous
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Periods are divided into Epochs
Eons Eras Periods
Epochs
If 1 Second was 1 year
You were born
15 seconds ago
If 1 Second was 1 year
The Revolutionary War was 4 minutes ago
The First Christmas was
33 minutes ago
The pyramids were built
1.5 hours ago
The Last Ice Age
Was 2 hours and 12 minutes ago
Dinosaurs were wiped out
2 years ago
Pangaea broke apart• 6 years and 4 months ago
Dinosaurs showed up (Triassic)• 8 years ago
Cambrian began
17 years ago
Earth formed• 145 years ago
21.2 Relative-Age Dating
• What happened first? –Oldest and newest?
James Hutton
• Geologist and Physician• Proposed that the earth
was millions of years old based on geologic evidence
Sir Charles Lyell
• Geologist – Proposed that geologic
changes occur slowly over long periods of time.
Uniformitarianism • The idea that the same processes
occurring today have been the same throughout time
• This idea replaced the idea of catastophism which suggested major changes by sudden catastrophic events
Principles for Determining Relative Age• Original horizontality – sedimentary
rocks are deposited in horizontal layers
Superposition
The deeper strata (layers) were deposited first.
Fossils found in lower layers are older than in higher layers.
Cross cutting relationships
Plutons are younger than the rocks they are found in
Principle of Inclusions
Fragments of a rock included in a rock layer are older than the rock layer
Unconformities• Signs of surface erosion that are buried
show a gap in the rock record
Disconformities• Signs of surface erosion that are buried
between sedimentary layers
Nonconformities• Signs of erosion that are found
between a sedimentary layer and metamorphic or igneous layers
Angular Unconformity
Nonhorizontal rock layers indicate uplifting
Correlation• Matching of a unique rock layer or fossil in
one area to similar layers in another area.
Key Bed
A rock layer used as a marker
Fossil Correlation
Using index fossils to compare ages of rock layers
21.3 Absolute-Age Dating
So how old is it?Absolute-age dating gives the
numerical age of rocks.
Radioactive Isostopes
Isotopes are atoms of an element with more or less neutrons
Carbon – has 6 protonsCarbon-12 has 6 protons and 6 neutrons
C-12
Nucleus
Radioactive Isostopes
Isotopes are atoms of an element with more or less neutrons
Carbon – has 6 protonsCarbon-12 has 6 protons and 6 neutronsCarbon-14 has 6 protons and 8 neutrons
C-14
Nucleus
Radioactive Isostopes
Isotopes are atoms of an element with more or less neutrons
Carbon – has 6 protonsCarbon-12 has 6 protons and 6 neutronsCarbon-14 has 6 protons and 8 neutronsCarbon-14 decays into Nitrogen-14
N-14
Nucleus
Radioactive Decay
The decay of radioactive isotopes into the daughter product
Half life
The time required for ½ of the isotopes to decay
Radio Carbon Dating
Isotopes – atoms with a different mass than usual
Half life – the time required for half of the isotopes to decay.
Carbon 14 – half life 5730 yrsPotassium 40 – half life 1.25 billion
years
C-14 isotope levels.
Expect 100 g of C-14 in a fresh sample,
A. Remains have 50 g. ½ of amount = one half-life 5,730
years oldB. Remains have only 25 g ¼ of amount or ½ of ½ = 2 half-
lives 11,460 years old C-14 Limited to about 50,000
years
Dendrochronology • Uses tree rings to determine the
age of an artifact
Ice Cores
Glacial ice sheets are sampled, each years snow is compacted and can be used similar to tree rings
Varves
Alternating bands of light and dark lake sediment.
Dark from summer and light from winter.
21.4 Fossil Remains
Evolution – a change in species over time
Original Preservation
Fossil remains that have been altered very little
Mineral Replacement
Minerals dissolved in rainwater fill in gaps in the fossil. The fossil becomes mineralized
Molds and Casts• The organism is covered with sediment,
the organism decays, but the space left fills in with rock
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