Text: Historical Geology
Evolution of Earth and Life Through Time
4th edition
by Wicander and Monroe
Chapter 1
The Dynamic and
Evolving Earth
The Movie of Earth’s History
• What kind of movie would we see
– 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
–
–
–
–
with great special effect
and a cast of trillions
twists and turns in its plot
with an unknown ending
• 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
–
–
–
–
–
a planet undergoing remarkable change as
continents moved about
ocean basins opened
mountain ranges grew along continental margins
and where continents collided
–
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form and grow
change circulation patterns
cause massive ice sheets to form and grow
and then melt away
• The oceans and atmosphere would
• 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 says,
• “To be continued.”
The Movie’s Theme
• Every good movie has a theme,
– and “The History of Earth” is no exception
• Three interrelated themes run throughout it
• 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 second is that Earth’s biota
– has evolved or changed throughout its history
• organic evolution
Earth is a System of
Interconnected Subsystems
•
•
•
•
•
Atmosphere (air and gases)
Hydrosphere (water and oceans)
Biosphere (plants and animals)
Lithosphere (Earth’s rocky surface)
Interior (mantle and core)
Interactions in Earth’s
Subsystems
Gases from
respiration
Transport
of seeds and
spores
Interactions in Earth’s
Subsystems
Wind erosion, transport
of water vapor for
precipitation
Mountains
divert air
movements
Interactions in Earth’s
Subsystems
Source of sediment
and dissolved
material
Water
and glacial
erosion, solution
of minerals
This class is about historical geology
What is Geology?
• From the Greek
– geo (Earth) logos (reason)
• Geology is the study of Earth
• Physical geology studies Earth materials,
– such as minerals and rocks
– as well as the processes operating within
– and on Earth’s surface
Historical Geology
• In historical geology we study
– changes in our dynamic planet
– how and why past events happened
– implication for today’s global ecosystems
• Principles of historical geology
– not only aid in interpreting Earth’s history
– but also have practical applications
• William Smith, an English surveyor/engineer
– used study of rock sequences
– to help predict the difficulty of excavation
– in constructing canals
Scientific Method
• The scientific method
– an orderly and logical approach
– Gather and analyze facts or data
• A hypothesis is a tentative explanation
– to explain observed phenomena
• Scientists make predictions using hypotheses
– then they test the predictions
• After repeated tests,
– if one hypothesis continues
– to explain the phenomena,
– scientists propose it as a theory
Formulation of Theories
Theory
• colloquial usage - speculation or conjecture
• scientific usage
– coherent explanation for one or several related
natural phenomena
– supported by a large body of objective evidence
Origin of the Universe
• The Big Bang
– occurred 15 billion years ago
– and is a model for the beginning of the universe
Evidence for the Big Bang
• Universe is expanding
• How do we determine the age?
– measure the rate of expansion
– backtrack to a time when the galaxies
– were all together at a single point
• Pervasive background radiation of 2.7º
above absolute zero
– is the afterglow of the Big Bang
Big Bang Model
• Initial state:
– No time, matter or space existed
• There is no “before the Big Bang”
– Universe consisted of pure energy
• During 1st second:
– Very dense matter came into existence
– The four basic forces separated
• gravity, electromagnetic force, 2 nuclear forces
– Enormous expansion occurred
Big Bang Model (cont.)
• 300,000 years later:
– atoms of hydrogen and helium formed
– light (photons) burst forth for the first time
• During the next 200 million years:
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–
–
–
Continued expansion and cooling
Stars and galaxies began to form
Elements heavier than hydrogen and helium
began to form within stars by nuclear fusion
Features of Our Solar System
•
•
•
•
•
In a spiral arm of the Milky Way Galaxy
Sun
9 planets
101 known moons (satellites)
a tremendous number of asteroids
– most orbit the Sun between the orbits of Mars
and Jupiter
• millions of comets and meteorites
• interplanetary dust and gases
Relative Sizes of the
Sun and Planets
Solar System Configuration
Origin of Our Solar System
Solar nebula theory
• cloud of gases and dust
• formed a rotating
disk
• condensed and
collapsed due to
gravity
• forming solar nebula
– with an embryonic Sun
– surrounded by a rotating cloud
Embryonic Sun and Rotating Cloud
• Planetesimals have formed
– in the inner solar system,
– and large eddies of gas and dust
– remain far from the protosun
The Planets
• Terrestrial Planets
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Mercury
Venus
Earth
Mars
• small, composed of
rock, with metal
cores
• Jovian Planets
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–
–
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Jupiter
Saturn
Uranus
Neptune
• large, composed of
hydrogen, helium,
ammonia, methane,
relatively small
rocky cores
Earth’s Very Early History
• Started out cool about 4.6 billion years ago
– probably with uniform composition/density
• Mostly:
– silicate compounds
– iron and magnesium oxides
• Temperature increased. Heat sources:
– meteorite impacts
– gravitational compression
– radioactive decay
• Heated up enough to melt iron and nickel
Earth’s Differentiation
• Differentiation = segregated into layers of
differing composition and density
• Early Earth was
probably uniform
• Molten iron and
nickel sank to form
the core
• Lighter silicates
flowed up to form
mantle and crust
Forming the Earth-Moon System
• Impact by Mars-sized or larger planetesimal
with young Earth
– 4.6 to 4.4 billion
years ago
– ejected large
quantity of hot
material,
– and formed the
Moon
Forming the Earth-Moon System
• Most of the
lunar material
– came from the
mantle of the
colliding
planetesimal
• The material
cooled
– and crystallized
– into lunar
layers
Forming the Earth-Moon System
• Most of the
lunar material
– came from the
mantle of the
colliding
planetesimal
• The material
cooled
– and crystallized
– into lunar
layers
Moon
• Light-colored
areas are lunar
highlands
– Heavily
cratered
• Provide
striking
evidence
– of massive
meteorite
bombardment
Earth—Dynamic Planet
• Earth was also subjected
– to the same meteorite barrage
– that pock-marked the Moon
• Why isn’t Earth’s surface also densely
cratered?
– Because Earth is a dynamic and evolving planet
– Craters have long since been worn away
Earth’s Interior Layers
• Crust - 5-90 km
thick
– continental and
oceanic
• Mantle
– composed largely
of peridotite
– dark, dense
igneous rock
– rich in iron and
magnesium
• Core
– iron and a small
amount of nickel
Earth’s Interior Layers
• Crust - 5-90 km
thick
– continental and
oceanic
• Lithosphere
– solid upper mantle
and crust
• Mantle
– composed largely
of peridotite
– dark, dense
igneous rock
– rich in iron and
magnesium
• Core
– 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
Plate Tectonic Theory
• Lithosphere is broken into individual pieces
called
plates
• Plates move over the asthenosphere
– as a result of underlying convection cells
Modern Plate Map
Plate Tectonic Theory
• At plate boundaries
– Volcanic activity occurs
– Earthquakes occur
• Movement at plate boundaries
– plates diverge
– plates converge
– plates slide sideways past each other
Plate Tectonic Theory
• Types of plate boundaries
Divergent
Mid-oceanic plate
ridge
boundary
Transform
plate
boundary
Continentalcontinental
convergent
plate
boundary
Continentaloceanic
convergent
plate
boundary
Divergent
plate
boundary
Trench
Oceanicoceanic
convergent
plate
boundary
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
Theory of Organic Evolution
• Provides a framework
– for understanding the history of life
• 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 provides perhaps
– the most 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 in
– seconds, hours, days, years
• Ancient human history
– hundreds or even thousands of years
• 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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pieced together information
from numerous rock exposures,
constructed a sequential chronology
based on changes in Earth’s biota through time
• The time scale was subsequently dated in years
– using radiometric dating techniques
Geologic
Time Scale
Uniformitarianism
• Uniformitarianism is a cornerstone of geology
– is based on the premise that present-day processes
– have operated throughout geologic time
• The physical and chemical laws of nature
– have remained the same through time
• To interpret geologic events
– from evidence preserved in rocks
– we must first understand present-day processes
– and their results
• Rates and intensities of geologic processes
– may have changed with time
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
• Study what has happened in the past,
– on a global scale,
– to try and 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
Summary
• Earth is a system
– of interconnected subsystems
• Geology is the study of Earth
• Historical geology is the study
– of the origin and evolution of Earth
• Scientific method is
–
–
–
–
an orderly, logical approach
to explain phenomena,
using data,
formulating and testing hypotheses and theories
• Universe began with
– a big bang 15 billion years ago
Summary
• Solar system formed 4.6 billion years ago
– by condensation and gravitational collapse
– of a rotating interstellar cloud
• Earth formed 4.6 billion years ago
– as a swirling eddy in the solar system nebula
• Moon may have formed
– when a planetesimal collided with Earth
– 4.6 to 4.4 billion years ago
• Earth probably started solid
– then differentiated into layers
– as it heated and melted
Summary
• Earth’s layers mostly solidified
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into the core, mantle and crust,
with the upper mantle and crust
making up the soft asthenosphere
and the solid lithosphere
• Lithosphere is broken into plates
– that diverge, converge and
– slide sideways past each other
• Plate tectonics is a unifying theory
– that helps explain features and events
– including volcanic eruptions,
– earthquakes and mountain forming
Summary
• Central thesis of organic evolution is
– that all living organisms evolved
– from organisms that existed in the past
• An appreciation
– of the immensity of geologic time
– is central to understanding Earth’s evolution
• Uniformitarianism holds that the laws
– of nature have been constant through time
• Geology is part of our lives
– and our standard of living depends
– on our use of natural resources
– that formed over billions of years