unit 6 geologic time
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Unit 6 Geologic Time. Chapter 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. - PowerPoint PPT PresentationTRANSCRIPT
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Unit 6 Geologic TimeChapter 21 Fossils and the Rock Record
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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
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Eons
The three earliest eons are grouped into the “Precambrian”
Hadean – The formation of the earth
542
Had
ean
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Eons
Archaen – Continental plates begin to form and earliest fossils
542
Had
ean
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Eons
Proterozoic – multicellular life cyanobacteria increases oxygen levels
542
Had
ean
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Eons
Phanerozoic– “visible life”Most of the fossils with hard parts
542
Had
ean
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Eons are divided into Eras• Phanerozoic is divided into three Eras
– Paleozoic – ancient life– Mesozoic – middle life– Cenozoic – recent life
542
Had
ean
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Eras are divided into Periods
• Mesozoic is divided into three Periods– Triassic – Jurassic– Cretaceous
542
Had
ean
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Periods are divided into Epochs
Eons Eras Periods
Epochs
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If 1 Second was 1 year
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You were born
15 seconds ago
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If 1 Second was 1 year
The Revolutionary War was 4 minutes ago
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The First Christmas was
33 minutes ago
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The pyramids were built
1.5 hours ago
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The Last Ice Age
Was 2 hours and 12 minutes ago
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Dinosaurs were wiped out
2 years ago
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Pangaea broke apart• 6 years and 4 months ago
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Dinosaurs showed up (Triassic)• 8 years ago
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Cambrian began
17 years ago
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Earth formed• 145 years ago
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21.2 Relative-Age Dating
• What happened first? –Oldest and newest?
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James Hutton
• Geologist and Physician• Proposed that the earth
was millions of years old based on geologic evidence
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Sir Charles Lyell
• Geologist – Proposed that geologic
changes occur slowly over long periods of time.
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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
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Principles for Determining Relative Age• Original horizontality – sedimentary
rocks are deposited in horizontal layers
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Superposition
The deeper strata (layers) were deposited first.
Fossils found in lower layers are older than in higher layers.
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Cross cutting relationships
Plutons are younger than the rocks they are found in
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Principle of Inclusions
Fragments of a rock included in a rock layer are older than the rock layer
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Unconformities• Signs of surface erosion that are buried
show a gap in the rock record
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Disconformities• Signs of surface erosion that are buried
between sedimentary layers
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Nonconformities• Signs of erosion that are found
between a sedimentary layer and metamorphic or igneous layers
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Angular Unconformity
Nonhorizontal rock layers indicate uplifting
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Correlation• Matching of a unique rock layer or fossil in
one area to similar layers in another area.
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Key Bed
A rock layer used as a marker
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Fossil Correlation
Using index fossils to compare ages of rock layers
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21.3 Absolute-Age Dating
So how old is it?Absolute-age dating gives the
numerical age of rocks.
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Radioactive Isostopes
Isotopes are atoms of an element with more or less neutrons
Carbon – has 6 protonsCarbon-12 has 6 protons and 6 neutrons
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C-12
Nucleus
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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
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C-14
Nucleus
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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
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N-14
Nucleus
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Radioactive Decay
The decay of radioactive isotopes into the daughter product
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Half life
The time required for ½ of the isotopes to decay
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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
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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
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Dendrochronology • Uses tree rings to determine the
age of an artifact
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Ice Cores
Glacial ice sheets are sampled, each years snow is compacted and can be used similar to tree rings
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Varves
Alternating bands of light and dark lake sediment.
Dark from summer and light from winter.
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21.4 Fossil Remains
Evolution – a change in species over time
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Original Preservation
Fossil remains that have been altered very little
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Mineral Replacement
Minerals dissolved in rainwater fill in gaps in the fossil. The fossil becomes mineralized
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Molds and Casts• The organism is covered with sediment,
the organism decays, but the space left fills in with rock