Critical Thinking about Earth history.
Geologic Time
Chapter 8 Lecture
Natalie Bursztyn
Utah State University
Foundations of Earth Science
Eighth Edition
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Explain the principle of uniformitarianism.
Discuss how it differs from catastrophism.
Focus Questions 8.1
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Mid-1600s
James Ussher stated Earth was only a few thousand years old
Catastrophism
Belief that Earth’s landscapes were formed by great catastrophes
Prevalent during the 1600s and 1700s
Used to fit the rate of Earth’s processes to prevailing ideas of Earth’s age
A Brief History of Geology
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Late 1700s
James Hutton published Theory of the Earth
Uniformitarianism
States that the physical, chemical, and biological laws that operate today have also operated in the geologic past
To understand ancient rocks, we must understand present-day processes
Geologic processes occur over extremely long periods of time
A Brief History of Geology
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Distinguish between numerical and relative dating.
Apply relative dating principles to determine a time sequence of geologic events.
Focus Questions 8.2
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Efforts to determine Earth’s age during the 1800s and 1900s were unreliable
Today radiometric dating allows scientists to accurately determine numerical ages for rocks representing important events in Earth’s past
Relative dates are determined by placing rocks in the proper sequence of formation
Creating a Timescale — Relative Dating Principles
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Principle of superposition
Developed by Nicolas Steno in the mid-1600s
Studied sedimentary rock layers in Italy
In an undeformed sequence of sedimentary rocks, each bed is older than the one above and younger than the one below
Also applies to lava flows and ash beds
Creating a Timescale — Relative Dating Principles
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Creating a Timescale — Relative Dating Principles
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Principle of original horizontality
Layers of sediment are generally deposited in a horizontal position
Rock layers that are flat have not been disturbed
Folded or inclined rocks must have been disrupted after deposition
Creating a Timescale — Relative Dating Principles
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Principle of lateral continuity
Sedimentary beds originate as continuous layers that extend in all directions
Identical strata on two sides of a canyon were continuous before the canyon was carved
Creating a Timescale — Relative Dating Principles
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Principle of cross-cutting relationships
Geologic features that cut across rocks must form after the rocks they cut through
Faults, igneous intrusions
Creating a Timescale — Relative Dating Principles
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Inclusions
Fragments of one rock unit enclosed within another
Rock that contains inclusions is younger than the rock that provided the inclusions
Creating a Timescale — Relative Dating Principles
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Layers of rock that have been deposited without interruption are called conformable
A complete set of conformable strata for all of Earth history does not exist
Interrupting the deposition of sediment creates a break in the rock record called an unconformity
Represents a period when deposition stopped, erosion occurred, and then deposition resumed
Generally, uplift causes deposition to stop and subsidence causes deposition to resume
Unconformities
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Angular unconformity
Consists of tilted or folded sedimentary rocks overlain by younger, more flat lying strata
Deformation occurred during the time that deposition stopped
Unconformities
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Unconformities
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Disconformity
A break in sedimentary rock strata representing a time when erosion occurred
Difficult to identify because layers are parallel
Evidence of erosion (buried stream channel)
Unconformities
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Nonconformity
Younger sedimentary rocks on top of older metamorphic or intrusive igneous rocks
Imply period of uplift of deeply buried rocks
Unconformities
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Unconformities
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Applying Relative Dating Principles
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Define fossil.
Discuss the conditions that favor the preservation of organisms as fossils.
List and describe various fossil types.
Focus Questions 8.3
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Fossils
The remains or traces of prehistoric life
Paleontology
The scientific study of fossils
Fossils: Evidence of Past Life
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Fossils: Evidence of Past Life
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Fossils can be preserved in many ways
Some remains may not be altered at all
Teeth, bones, shells
Entire animals including flesh are not common
Mammoths frozen in Arctic tundra
Mummified slots in a dry cave in Nevada
Types of Fossils
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Permineralization
Mineral-rich groundwater permeates porous tissues
Petrified wood is permineralized with silica
“Petrified” means “turned to stone”
Molds
Form where a structure buried in sediment was dissolved by groundwater
Only the outside shape and surface marking is preserved; no internal structure
If hollow spaces are filled with mineral matter, a cast is formed
Types of Fossils
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Carbonization
Remains are encased in sediment; pressure squeezes out all liquid and gas until only a thin residue of carbon remains
Effectively preserves leaves and delicate animals
Impressions may show considerable detail
Amber
The hardened resin of ancient trees
Seals organisms from atmosphere and water
Preserves delicate organisms like insects
Types of Fossils
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Trace Fossils
Indirect evidence of organisms
Tracks
Burrows
Coprolites
Gastroliths
Types of Fossils
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Types of Fossils
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Only a very small fraction of organisms are preserved as fossils
Rapid burial and hard parts favor preservation
Soft parts are eaten or decomposed
Sediment protects organisms from destruction
Shells, bones, and teeth are much more common in the fossil record
Fossil record is biased
Conditions Favoring Preservation
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What types of organisms are most likely to be missing from, or are very rare, in the fossil record? How might this bias our picture of what life on Earth was like in the past?
Hint: Think about the organisms themselves, but also their ecological context and depositional environment.
Conditions Favoring Preservation
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Explain how rocks of similar age that are in different places can be matched up.
Focus Question 8.4
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Correlation is matching up rocks of similar age in different regions
Reveals a more comprehensive picture of the sedimentary rock record
Correlation by walking along outcropping edges is possible within limited areas
Rock layers made of distinctive material can be identified in other places
Widely separated areas require the use of fossils
Correlation of Rock Layers
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Correlation of Rock Layers
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William Smith
1700s to 1800s
Noted that rock formations in canals contained fossils unlike the fossils in the beds above and below
Distinctive fossils can be used to identify and correlate widely separated sedimentary strata
Principle of fossil succession
Fossil organisms succeed one another in a definite and determinable order, therefore any time period can be recognized by its fossil content
Fossils document the evolution of life through time
Correlation of Rock Layers
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Index fossils
Geographically widespread and limited to a short span of geologic time
Important for correlation
Fossil assemblage
Can be used when there aren’t index fossils
Fossils are useful environmental indicators
Correlation of Rock Layers
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Correlation of Rock Layers
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Discuss three types of radioactive decay.
Explain how radioactive isotopes are used to determine numerical dates.
Focus Questions 8.5
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Each atom is made up of protons, neutrons, and electrons
Protons have a positive charge
Electrons have a negative charge
Neutrons are neutral
Elements are identified by atomic number
Number of protons in the nucleus
Reviewing Basic Atomic Structure
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99.9% of an atom’s mass is in the nucleus
Electrons have almost no mass
Number of protons + number of neutrons in an atom = the mass number
An isotope has a different number of neutrons in the nucleus
Different mass number
Reviewing Basic Atomic Structure
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Some isotopes have unstable nuclei with bonds that are not strong enough to hold the protons and neutrons together
These nuclei will break apart (decay) in a process called radioactivity
Dating with Radioactivity
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Three common types of radioactive decay:
Alpha particle = 2 protons and 2 neutrons
Mass number reduced by 4 and atomic number decreased by 2
Beta particle = electron from the neutron
Neutron is actually a proton and electron combined
Mass number remains the same, but atomic number increases by 1
Electron capture
Captured by the nucleus and combined with a proton to form a neutron
Mass number remains the same, but atomic number decreases by 1
Dating with Radioactivity
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Dating with Radioactivity
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Parent Isotope
Unstable radioactive isotope
Daughter Product
Isotope resulting from radioactive decay
Dating with Radioactivity
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Radiometric dating
Reliable method of calculating ages of rocks
Rate of decay for many isotopes does not vary
Rate of decay has been precisely measured
Daughter product has been accumulating at a known rate since rocks were formed
Dating with Radioactivity
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Half-life
Time required for one-half of the nuclei in a sample to decay
One half-life has transpired when quantities of parent and daughter are equal (1:1 ratio)
If half-life of an isotope is known and parentdaughter ratio can be measured, then age can be calculated.
Dating with Radioactivity
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Dating with Radioactivity
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Five radioactive isotopes are important in geology:
Rubidium-87
Uranium-238
Uranium-235
Thorium-232
Potassium-40
Only useful if the mineral remained in a closed system
No addition of loss of parent or daughter isotopes
Dating with Radioactivity
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Dating with Radioactivity
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Radiometric dating methods have been used to determine the age of the oldest rocks on Earth
3.5-billion-year-old rocks found on all continents
Oldest rocks: 4.28 billion years old (Quebec, Canada)
3.7 to 3.8 billion years old in western Greenland
3.5 to 3.7 billion years old in the Minnesota River Valley and northern Michigan
3.4 to 3.5 billion years old in southern Africa
3.4 to 3.6 billion years in western Australia
Dating with Radioactivity
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Radiocarbon dating
Using the carbon-14 isotope to date very recent events
Half-life of carbon-14 is only 5,730 years
Only useful for dating events from historic past and very recent geologic history
Carbon-14 is present in small amounts in all organisms
Dating with Radioactivity
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Distinguish among the four basic time units that make up the geologic time scale.
Explain why the time scale is considered to be a dynamic tool.
Focus Questions 8.6
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Geologic history divided into units of variable magnitude
Developed during the nineteenth century
Based on relative dating
Eons represent the greatest span of time
Phanerozoic Eon began about 542 million years ago
Eons divided into eras
Phanerozoic includes Paleozoic, Mesozoic, and Cenozoic
Bounded by profound worldwide changes in life-forms
Eras divided into periods
Periods divided into epochs
The Geologic Time Scale
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The Geologic Time Scale
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Most detail in the geologic time scale begins at 542 million years ago
4 billion years before the Cambrian is known as the Precambrian
Divided into Archean and Proterozoic eons
Together are divided into seven eras
Represents 88% of geologic time
The Geologic Time Scale
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Some “unofficial” terms are associated with the geologic time scale
Precambrian = eons and eras before the Phanerozoic
Hadean = earliest eon of Earth history (before the oldest known rocks)
The Geologic Time Scale
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Geologic time scale must be updated periodically to include changes in unit names and boundary age estimates
A few years ago, Cenozoic divided into Tertiary and Quaternary periods
Today, former Tertiary is divided into Paleogene and Neogene periods
The Geologic Time Scale
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Explain how reliable numerical dates are determined for layers of sedimentary rock.
Focus Question 8.7
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Rocks can only be radiometrically dated if all minerals formed at the same time
Works for igneous and metamorphic rocks
Sedimentary rocks contain particles of many ages
Must be related to datable igneous masses
Determining Numerical Dates for Sedimentary Strata
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Determining Numerical Dates for Sedimentary Strata
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