Download Ch08_Geologic Time

Chapter 8 Lecture Outline
Geologic Time
Focus Question 8.1
• How do catastrophism and uniformitarianism
affect our understanding of Earth history?
A Brief History of Geology
• Mid-1600s
– James Ussher (1581-1656), Archbishop
of Armagh, Primate of All Ireland, ViceChancellor of Trinity College in Dublin
was highly regarded in his day as a
churchman and as a scholar.
– James Ussher stated Earth was only a
few thousand years old
– Created ,October 23, 4004 BC
• 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
A Brief History of Geology
• Late 1700s
– James Hutton published Theory of the Earth, 1788
• 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
– “the present is the key to the past”
– Geologic processes occur over extremely long
periods of time
Creating a Timescale — Relative Dating
Principles
• Efforts to determine Earth’s age during the
1800s and early 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
• 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
Creating a Timescale — Relative Dating
Principles
• 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
• 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
• 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
• Unconformities
• 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
– Can represent a long period of geologic time
Unconformities
• Angular unconformity
– Consists of tilted or folded
sedimentary rocks overlain
by younger, more flat lying
strata
– Deformation occurred
during the time that
deposition stopped
– JamesHutton's
Unconformity at Siccar
Point, eroded outcrop.
• Most famous angular unconformity is the Grand Unconformity in the
Grand Canyon of Arizona. Here tilted sedimentary rocks of Precambrian
age (lower half of photo) are overlain by younger sedimentary rocks of
Phanerozoic age (Cambrian and younger, upper half of photo). The two
packages of strata are clearly separated by an angular unconformity that
is best seen just left of the center of the photo.
Unconformities
• 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)
• Disconformity in Capitol Reef National Park, Utah. The Chinle
Formation (Triassic), the slope forming unit in the central
portion of the picture, has a very sharp contact (black line) with
the overlying Wingate Sandstone (uppermost Triassic, forms
steep cliff).
Unconformities
• Nonconformity
– Younger sedimentary rocks on top of older
metamorphic or intrusive igneous rocks
– Imply period of uplift of deeply buried rocks
• Nonconformity at the base of the Grand Canyon succession.
The dark rocks in the bottom of the gorge are Archean
Vishnu schist, and are overlain by younger Proterozoic and
Phanerozoic sediments.
Angular Unconformities, Nonconformities, and
Disconformities
Unconformities in the Grand Canyon
Applying Relative Dating Principles
Relative Geologic Dating 2
Focus Questions 8.3
• What are the different ways that a fossil can
be preserved?
• Are all organisms that lived in the past
preserved in the fossil record?
Fossils: Evidence of Past Life
• Fossils
– The remains or traces of prehistoric life
• Paleontology
– The scientific study
of fossils
Types of Fossils
• 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 sloths in a dry cave in Nevada
Types of Fossils
• 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 is preserved
– If hollow spaces are filled with mineral matter, a cast is formed
• 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
Types of Fossils
• Amber
– The hardened resin of ancient trees
– Seals organisms from atmosphere and water
– Preserves delicate organisms like insects
• Trace Fossils
– Indirect evidence of organisms
•
•
•
•
Tracks
Burrows
Coprolites
Gastroliths
Types of Fossils
Conditions Favoring Preservation
• 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
Focus Question 8.4
• How can rocks in different areas be
correlated?
Correlation of Rock Layers
• 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
Correlation of Rock Layers
• 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
• 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
Focus Question 8.5
• How can radioactive isotopes be used to
determine numerical ages for geologic
materials?
Reviewing Basic Atomic Structure
• 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
• 99.9% of an atom’s mass is in the nucleus
– Electrons have almost no mass
• # of protons + # of neutrons in an atom = the
mass number
• An isotope has a different number of
neutrons in the nucleus
– Different mass number
Dating with Radioactivity
• 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
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
• Parent Isotope
– Unstable radioactive isotope
• Daughter Product
– Isotope resulting from radioactive decay
Dating with Radioactivity
• 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
• 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.
Radioactive Decay
Dating with Radioactivity
• Five radioactive isotopes are important in
geology:
1.
2.
3.
4.
5.
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
Dating with Radioactivity
• 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
– C-12/C14 ratio constant while alive
– After organism dies C-14 no longer taken in
Dating with Radioactivity
• 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
Focus Question 8.6
• What is the basic structure of the geologic
timescale?
The Geologic Time Scale
• 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 Timescale
• Most detail in the geologic timescale begins
at 542 million years ago
• 4 billion years before the Cambrian is known
as the Precambrian
– Divided into Archean and Proterozoic eons
– Each divided into four eras
– Represents 88% of geologic time
The Geologic Timescale
• Some “unofficial” terms are associated with
the geologic timescale
– Precambrian = eons and eras before the
Phanerozoic
– Hadean = earliest eon of Earth history
(before the oldest known rocks)
Focus Question 8.7
• What are some difficulties associated with
assigning numerical ages to sedimentary
rocks?
Determining Numerical Dates for
Sedimentary Strata
• 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
Relate sedimentary to igneous