Transcript Historical Geology

Historical Geology:
Evolution of the Earth and Life Through Time
6th edition
Reed Wicander and James S. Monroe
Chapter 1
The Dynamic and
Evolving Earth
The Movie of Earth’s History
• What kind of movie would we have
– if it were possible to travel back in time
– and film Earth’s history
– from its beginning 4.6 billion years ago?
• It would certainly be a story of epic proportions
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with incredible special effects
a cast of trillions
a plot with twists and turns
and an ending that is still a mystery!
• Although we cannot travel back in time,
– the Earth’s history is still preserved
– in the geologic record
Subplot: Landscape History
• In this movie we would see
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a planet undergoing remarkable change as
continents moved about its surface
ocean basins opened and closed
mountain ranges formed along continental margins
or where continents collided
• The oceans and atmospheric circulation
patterns would
– shift in response to moving continents
– causing massive ice sheets to form, grow, and then
melt away
• Extensive swamps or vast interior deserts
– would sweep across the landscape
Subplot: Life’s History
• We would also witness
– the first living cells evolving
– from a primordial organic soup
– between 4.6 and 3.6 billion years ago
• Cell nuclei would evolve,
– then multicelled soft-bodied animals
– followed by animals with skeletons and then
backbones
• The barren landscape would come to life as
– plants and animals moved from their watery home.
– Insects, amphibians, reptiles, birds and mammals
– would eventually evolve.
Earth is a Dynamic and
Evolving Planet
• Changes in its surface
• Changes in life
At the End of the Movie
• The movie’s final image is of Earth,
– a shimmering blue-green oasis
– in the black void of space
– and a voice-over says,
• “To be continued.”
The Movie’s Theme
• Every good movie has a theme,
– and The History of Earth is no exception.
• The major theme is that Earth is complex and
dynamic
• Three interrelated themes sub-themes run
throughout this epic:
• The first is that Earth’s outermost part
– is composed of a series of moving plates
• Plate tectonics
– whose interactions have affected its physical and
biological history.
The Movie’s Theme
• The second is that Earth’s biota
– has evolved or changed throughout its history
• Organic evolution
• The third is that physical and biological
changes
– have occurred over long periods of time
• Geologic or Deep Time
• These three interrelated themes
– are central to our understanding and appreciation
– of our planet’s history.
Earth’s Very Early History
• About 4.6 billion years ago, early Earth was
probably
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–
–
–
cool
with uniform composition/density
Composed mostly of silicates, and
iron and magnesium oxides
• The temperature increased because of
– meteorite impacts
– gravitational compression
– radioactive decay
• Iron and nickel melted and Earth’s homogeneous
composition disappeared
Earth’s Differentiation
• Differentiation = segregated into a series of
concentric layers of differing composition
and density
• Molten iron and
nickel sank to form
the core
• Lighter silicates
flowed up to form
mantle and crust
Earth—Dynamic Planet
• Earth is a dynamic planet
– The size, shape, and geographic distribution
– of continents and ocean basins have changed
through time
– The composition of the atmosphere has evolved
– Life-forms existing today differ from those that
lived in the past
Chapter 4
Geologic Time:
Concepts and Principles
Grand Canyon
• When looking down into the Grand Canyon, we are
really looking at the early history of Earth
Grand Canyon
• More than 1 billion years of history are
preserved,
• like pages of a book,
– in the rock layers of the Grand Canyon
• Reading this rock book we learn
– that the area underwent episodes of
– mountain building
– advancing and retreating shallow seas
• We know these things by
– applying the principles of relative dating to the rocks
– and recognizing that present-day processes
– have operated throughout Earth history
What is time?
• We are obsessed with time, and organize our
lives around it.
• Most of us feel we don’t have enough of it.
• Our common time units are
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seconds
hours
days
weeks
months
years
• Ancient history involves
– hundreds of years
– thousands of years
• But geologic time involves
– millions of years
– even billions of years
Concept of Geologic Time
• Geologists use two different frames of reference
– when discussing geologic time
– Relative dating involves placing geologic events
• in a sequential order as determined
• from their position in the geologic record
– It does not tell us how long ago
• a particular event occurred,
• only that one event preceded another
• For hundreds of years geologists
– have been using relative dating
– to establish a relative geologic time scale
Relative Geologic Time Scale
• The relative geologic
time scale has a
sequence of
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eons
eras
periods
epochs
Concept of Geologic Time
• The second frame of reference for geologic time
is absolute dating
– Absolute dating results in specific dates
• for rock units or events
• expressed in years before the present
– It tells us how long ago a particular event occurred
• giving us numerical information about time
• Radiometric dating is the most common method
– of obtaining absolute ages
– Such dates are calculated
• from the natural rates of decay
• of various natural radioactive elements
• present in trace amounts in some rocks
Geologic Time Scale
• The discovery of radioactivity
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near the end of the 19th century
allowed absolute ages
to be accurately applied
to the relative geologic time
scale
• The geologic time scale is a
dual scale
– a relative scale
– and an absolute scale
Changes in the Concept of
Geologic Time
• The concept and measurement of geologic time
– have changed throughout human history
• Early Christian theologians
– conceived of time as linear rather than circular
• James Ussher (1581-1665) in Ireland
– calculated the age of Earth based
– on Old Testament genealogy
• He announced that Earth was created on October 22,
4004 B.C.
• For nearly a century, it was considered heresy to say
Earth was more than about 6000 years old.
Changes in the Concept of
Geologic Time
• During the 1700s and 1800s Earth’s age
– was estimated scientifically
• Georges Louis de Buffon (1707-1788)
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calculated how long Earth took to cool gradually
from a molten beginning
using melted iron balls of various diameters.
Extrapolating their cooling rate
to an Earth-sized ball,
he estimated Earth was 75,000 years old
Changes in the Concept of
Geologic Time
• Others used different techniques
• Scholars using rates of deposition of various
sediments
– and total thickness of sedimentary rock in the crust
– produced estimates of less than 1 million
– to more than 2 billion years.
• John Joly used the amount of salt carried
– by rivers to the ocean
– and the salinity of seawater
– and obtained a minimum age of 90 million years
Relative-Dating Principles
• Six fundamental geologic principles are used in
relative dating
• Principle of superposition
– Nicolas Steno (1638-1686)
– In an undisturbed succession of sedimentary rock
layers,
– the oldest layer is at the bottom
– and the youngest layer is at the top
• This method is used for determining the
relative age
– of rock layers (strata) and the fossils they contain
Relative-Dating Principles
• Principle of original horizontality
– Nicolas Steno
– Sediment is deposited
• in essentially horizontal layers
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Therefore, a sequence of sedimentary rock layers
that is steeply inclined from horizontal
must have been tilted
after deposition and lithification
Principle of Superposition
• Illustration of the principles of superposition
• Superposition: The youngest
– rocks are at the top
– of the outcrop
– and the oldest rocks are at the bottom
Principle of
Original Horizontality
• Horizontality: These
sediments
were originally
– deposited horizontally
– in a marine environment
Relative-Dating Principles
• Principle of lateral continuity
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Nicolas Steno’s third principle
Sediment extends laterally in all direction
until it thins and pinches out
or terminates against the edges
of the depositional basin
• Principle of cross-cutting relationships
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James Hutton (1726-1797)
An igneous intrusion or a fault
must be younger than the rocks
it intrudes or displaces
Cross-cutting
Relationships
• North shore of Lake
Superior, Ontario
Canada
• A dark-colored dike
has intruded into
older light colored
granite.
• The dike is younger
than the granite.
Cross-cutting Relationships
• Templin
Highway,
Castaic,
California
• A small fault
displaces
tilted beds.
• The fault is
younger than
the beds.
Relative-Dating Principles
• Other principles of relative dating
– Principle of inclusions
– Principle of fossil succession
• are discussed later in the text
Neptunism
• Neptunism
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All rocks, including granite and basalt,
were precipitated in an orderly sequence
from a primeval, worldwide ocean.
proposed in 1787 by Abraham Werner (1749-1817)
• Werner was an excellent mineralogist,
– but is best remembered
– for his incorrect interpretation of Earth history
Neptunism
• Werner’s geologic column was widely accepted
– Alluvial rocks
• unconsolidated sediments, youngest
– Secondary rocks
• rocks such as sandstones, limestones, coal, basalt
– Transition rocks
• chemical and detrital rocks, some fossiliferous rocks
– Primitive rocks
• oldest including igneous and metamorphic
Catastrophism
• Catastrophism
– concept proposed by Georges Cuvier (1769-1832)
– dominated European geologic thinking
• The physical and biological history of Earth
– resulted from a series of sudden widespread
catastrophes
– which accounted for significant and rapid changes
in Earth
– and exterminated existing life in the affected area
• Six major catastrophes occurred,
– corresponding to the six days of biblical creation
– The last one was the biblical deluge
Neptunism and Catastrophism
• These hypotheses were abandoned because
– they were not supported by field evidence
• Basalt was shown to be of igneous origin
• Volcanic rocks interbedded with sedimentary
– and primitive rocks showed that igneous activity
– had occurred throughout geologic time
• More than 6 catastrophes were needed
– to explain field observations
• The principle of uniformitarianism
– became the guiding philosophy of geology
Uniformitarianism
• Principle of uniformitarianism
– Present-day processes have operated throughout
geologic time.
– Developed by James Hutton (1726-1797), advocated
by Charles Lyell (1797-1875)
• William Whewell coined the term
“uniformitarianism” in 1832
• Hutton applied the principle of uniformitarianism
– when interpreting rocks at Siccar Point, Scotland
• We now call what Hutton observed an
unconformity,
– but he properly interpreted its formation
Unconformity at Siccar Point
• Hutton explained that
– the tilted, lower rocks
– resulted from severe upheavals that formed
mountains
– these were then worn away
– and covered by younger flat-lying rocks
– the erosional surface
– represents a gap in the rock record
Uniformitarianism
• Hutton viewed Earth
history as cyclical
erosion
deposition
uplift
• He also understood
– that geologic processes
operate over a vast amount of time
• Modern view of uniformitarianism
– Today, geologists assume that the principles or laws
of nature are constant
– but the rates and intensities of change have varied
through time
– Some geologists prefer the term “actualism”
Crisis in Geology
• Lord Kelvin (1824-1907)
– knew about high temperatures inside of deep mines
– and reasoned that Earth
– was losing heat from its interior
• Assuming Earth was once molten, he used
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the melting temperature of rocks
the size of Earth
and the rate of heat loss
to calculate the age of Earth as
between 400 and 20 million years
Crisis in Geology
• This age was too young
– for the geologic processes envisioned
– by other geologists at that time,
– leading to a crisis in geology
• Kelvin did not know about radioactivity
– as a heat source within the Earth
Absolute-Dating Methods
• The discovery of radioactivity
– destroyed Kelvin’s argument for the age of Earth
– and provided a clock to measure Earth’s age
• Radioactivity is the spontaneous decay
– of an element to a more stable isotope
• The heat from radioactivity
– helps explain why the Earth is still warm inside
• Radioactivity provides geologists
– with a powerful tool to measure
– absolute ages of rocks and past geologic events
Atoms: A Review
• Understanding absolute dating requires
– knowledge of atoms and isotopes
• All matter is made up of atoms
• The nucleus of an atom is composed of
– protons – particles with a positive electrical charge
– neutrons – electrically neutral particles
• with electrons – negatively charged particles –
outside the nucleus
• The number of protons (= the atomic number)
– helps determine the atom’s chemical properties
– and the element to which it belongs
Isotopes: A Review
• Atomic mass number
= number of protons + number of neutrons
• The different forms of an element’s atoms
– with varying numbers of neutrons
– are called isotopes
• Different isotopes of the same element
– have different atomic mass numbers
– but behave the same chemically
• Most isotopes are stable,
– but some are unstable
• Geologists use decay rates of unstable isotopes
– to determine absolute ages of rocks
Radioactive Decay
• Radioactive decay is the process whereby
– an unstable atomic nucleus spontaneously
transforms
– into an atomic nucleus of a different element
Half-Lives
• The half-life of a radioactive isotope
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is the time it takes for
one half of the atoms
of the original unstable parent isotope
to decay to atoms
of a new more stable daughter isotope
• The half-life of a specific radioactive isotope
– is constant and can be precisely measured
Half-Lives
• The length of half-lives for different isotopes
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of different elements
can vary from
less than one billionth of a second
to 49 billion years!
• Radioactive decay
– is geometric, NOT linear,
– and produces a curved graph
Uniform Linear Change
• In this example
– of uniform
linear change,
– water is
dripping into a
glass
– at a constant
rate
Geometric Radioactive Decay
– In radioactive
decay,
– during each
equal time unit
• half-life
– the proportion
of parent atoms
– decreases by 1/2
Determining Age
• By measuring the parent/daughter ratio
– and knowing the half-life of the parent
• which has been determined in the laboratory
– geologists can calculate the age of a sample
– containing the radioactive element
• The parent/daughter ratio
– is usually determined by a mass spectrometer
• an instrument that measures the proportions
• of atoms with different masses
Determining Age
• Example:
– If a rock has a parent/daughter ratio of 1:3
– or a ratio of (parent)/(parent + daughter) =
1:4 or 25%,
– and the half-live is 57 million years,
• how old is the rock?
– 25% means it is 2 halflives old.
– the rock is 57my x 2 =114
million years old.
What Materials Can Be Dated?
• Most radiometric dates are obtained
– from igneous rocks
• As magma cools and crystallizes,
– radioactive parent atoms separate
– from previously formed daughter atoms
• Because they are the right size
– some radioactive parents
– are included in the crystal structure of cooling
minerals
What Materials Can Be Dated?
• The daughter atoms are different elements
– with different sizes
– and, therefore, do not generally fit
– into the same minerals as the parents
• Geologists can use the crystals containing
– the parent atoms
– to date the time of crystallization
Igneous Crystallization
• Crystallization of magma separates parent atoms
– from previously formed daughters
• This resets the radiometric clock to zero.
• Then the parents gradually decay.
Sedimentary Rocks
• Generally, sedimentary rocks can NOT be
radiometrically dated
– The date obtained would correspond to the time of
crystallization of the mineral,
– when it formed in an igneous or metamorphic rock,
– and NOT the time that it was deposited as a
sedimentary particle
• Exception: The mineral glauconite can be dated
– because it forms in certain marine environments as
a reaction with clay minerals
– during the formation of the sedimentary rock
Dating Metamorphism
Dating the whole rock
yields a date of 700
million years = time of
crystallization.
a. A mineral has just
crystallized from magma.
b. As time passes, parent
atoms decay to daughters.
c. Metamorphism drives
the daughters out of the
mineral as it
recrystallizes.
d. Dating the mineral today
yields a date of 350
million years = time of
metamorphism, provided
the system remains closed
during that time.
Long-Lived Radioactive
Isotope Pairs Used in Dating
• The isotopes used in radiometric dating
– need to be sufficiently long-lived
– so the amount of parent material left is measurable
• Such isotopes include:
Parents
Daughters
Half-Life (years)
Uranium 238
Uranium 234
Thorium 232
Rubidium 87
Potassium 40
4.5 billion
704 million
14 billion
48.8 billion
1.3 billion
Lead 206
Lead 207
Lead 208
Strontium 87
Argon 40
Most of these
are useful for
dating older
rocks
Theory of Organic Evolution
• Provides a framework
– for understanding the history of life
• Charles Darwin’s
– On the Origin of Species by Means of Natural
Selection, published in 1859,
– revolutionized biology
Central Thesis of Evolution
• All present-day organisms
– are related
– and descended from organisms
– that lived during the past
• Natural selection is the mechanism
– that accounts for evolution
• Natural selection results in the survival
– to reproductive age of those organisms
– best adapted to their environment
History of Life
• The fossil record compelling evidence
– in favor of evolution
• Fossils are the remains or traces
– of once-living organisms
• Fossils demonstrate that Earth
– has a history of life
Geologic Time
• From the human perspective, time units are
– seconds, hours, days, years
• Ancient human history
– hundreds or thousands of years ago
• Geologic history
– millions, hundreds of millions, billions of years
Geologic Time Scale
• Resulted from the work of many 19th century
geologists who
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gathered information
from numerous rock exposures, and
constructed a sequential chronology
based on changes in Earth’s biota through time
• Ages subsequently were assigned to the time
scale
– using radiometric dating techniques
Geologic
Time Scale
How Does the Study of Historical
Geology Benefit Us?
• Survival of the human species
– depends on understanding
– how Earth’s various subsystems
– work and interact
• By studying what has happened in the past
– on a global scale,
– and try to determine how our actions
– might affect the balance of subsystems in the future
We “Live” Geology
• Our standard of living depends directly on
– our consumption of natural resources . . .
– resources that formed millions and billions of
years ago
• How we consume natural resources
– and interact with the environment
– determines our ability to pass on this standard of
living
– to the next generation
Earth’s Interior Layers
• Crust
– Continental (20-90
km thick)
– Oceanic (5-10 km
thick)
• Mantle
– 83% volume
– composed largely of
peridotite
– dark, dense igneous
rock, rich in iron and
magnesium
• Core
– Solid inner region,
liquid outer region
– iron and a small
amount of nickel
Earth’s Interior Layers
• Crust
– Continental (20-90 km
thick)
– Oceanic (5-10 km thick)
• Lithosphere
– solid upper mantle
and crust
• Mantle
– 83% volume
– composed largely of
peridotite
– dark, dense igneous
rock, rich in iron and
magnesium
• Core
– Solid inner region,
liquid outer region
– iron and a small
amount of nickel
• Asthenosphere
– part of upper
mantle
– behaves plastically
and slowly flows
Earth’s Interior Layers
• Lithosphere
– solid upper mantle
and crust
– broken into plates
that move over the
asthenosphere
• Asthenosphere
– part of upper
mantle
– behaves plastically
and slowly flows
Earth’s Crust
• outermost layer
• continental (20-90 km thick)
– density 2.7 g/cm3
– contains Si, Al
• oceanic (5-10 km thick)
– density 3.0 g/cm3
– composed of basalt and
gabbro
Plate Tectonic Theory
• Lithosphere is broken into individual pieces or
plates
• Plates move over the asthenosphere
– as a result of underlying convection cells
Modern Plate Map
Plate Tectonic Theory
• Plate boundaries are marked by
– Volcanic activity
– Earthquake activity
• At plate boundaries
– plates diverge,
– plates converge,
– plates slide sideways past each other
Plate Tectonic Theory
• Types of plate boundaries
Plate Tectonic Theory
Influence on geological sciences:
• Revolutionary concept
– major milestone, comparable to Darwin’s theory of
evolution in biology
• Provides a framework for
– interpreting many aspects of Earth on a global scale
– relating many seemingly unrelated phenomena
– interpreting Earth history
Solid Earth
Plate tectonics is driven by convection
in the mantle
and in turn drives mountain building
and associated igneous and metamorphic activity
Atmosphere
Plate Tectonics and
Earth Systems
Arrangement of continents affects
solar heating and cooling,
and thus winds and weather systems.
Rapid plate spreading and hot-spot activity
may release volcanic carbon dioxide
and affect global climate
Biosphere Hydrosphere
Plate Tectonics and
Earth Systems
Continental arrangement affects ocean currents
Rate of spreading affects volume
of mid-oceanic ridges and hence sea level
Placement of continents may contribute
to the onset of ice ages
Movement of continents creates corridors
or barriers to migration,
the creation of ecological niches,
and transport of habitats into
more or less favorable climates
Next time:
Chapter 3: Plate Tectonics