1. Historical Geology: Evolution of the Earth and Life Through Time
6th edition
Reed Wicander and James S. Monroe

2. The Dynamic and Evolving Earth
Chapter 1
The Dynamic and Evolving Earth

3. 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
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

4. Subplot: Landscape History
In this movie we would see
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

5. 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.

6. Earth is a Dynamic and Evolving Planet
Changes in its surface
Changes in life
Images from left to right:
Changes in its surface
Artist’s rendition of how Earth is thought to have appeared about 4.6 billion years ago.
Paleogeography of the world for the Late Cambrian Period.
Apollo 17 view of Earth. Africa, Arabian Peninsula, Madagascar, Antarctica, South Atlantic Ocean , Indian Ocean.
Changes in life
Possible precursor of life: bacterium-like proteinoid.
Reconstruction of Middle Devonian reef from the Great Lakes area Shown are corals, ammonoids, trilobites and brachiopods.
Cretaceous Pteranodon.

7. 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.”

8. 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.

9. 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.

10. Earth’s Very Early History
About 4.6 billion years ago, early Earth was probably
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

11. 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

12. 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

13. Geologic Time: Concepts and Principles
Chapter 4
Geologic Time: Concepts and Principles

14. Grand Canyon
When looking down into the Grand Canyon, we are really looking at the early history of Earth

15. 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

16. 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
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

17. 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

18. Relative Geologic Time Scale
The relative geologic time scale has a sequence of
eons
eras
periods
epochs

19. 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

20. Geologic Time Scale The discovery of radioactivity
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

21. 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 ( ) 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.

22. Changes in the Concept of Geologic Time
During the 1700s and 1800s Earth’s age
was estimated scientifically
Georges Louis de Buffon ( )
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

23. 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

24. Relative-Dating Principles
Six fundamental geologic principles are used in relative dating
Principle of superposition
Nicolas Steno ( )
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

25. Relative-Dating Principles
Principle of original horizontality
Nicolas Steno
Sediment is deposited
in essentially horizontal layers
Therefore, a sequence of sedimentary rock layers
that is steeply inclined from horizontal
must have been tilted
after deposition and lithification

26. 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

27. Principle of Original Horizontality
Horizontality: These sediments were originally
deposited horizontally
in a marine environment

28. Relative-Dating Principles
Principle of lateral continuity
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
James Hutton ( )
An igneous intrusion or a fault
must be younger than the rocks
it intrudes or displaces

29. 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.

30. Cross-cutting Relationships
Templin Highway, Castaic, California
A small fault displaces tilted beds.
The fault is younger than the beds.

31. Relative-Dating Principles
Other principles of relative dating
Principle of inclusions
Principle of fossil succession
are discussed later in the text

32. Neptunism Neptunism Werner was an excellent mineralogist,
All rocks, including granite and basalt,
were precipitated in an orderly sequence
from a primeval, worldwide ocean.
proposed in 1787 by Abraham Werner ( )
Werner was an excellent mineralogist,
but is best remembered
for his incorrect interpretation of Earth history

33. 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

34. Catastrophism Catastrophism
concept proposed by Georges Cuvier ( )
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

35. 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

36. Uniformitarianism Principle of uniformitarianism
Present-day processes have operated throughout geologic time.
Developed by James Hutton ( ), advocated by Charles Lyell ( )
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

37. 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

38. Uniformitarianism Hutton viewed Earth history as cyclical
erosion
erosion
Hutton viewed Earth history as cyclical
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”

39. 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
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

40. 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

41. 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

42. 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

43. 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

44. Radioactive Decay Radioactive decay is the process whereby
an unstable atomic nucleus spontaneously transforms
into an atomic nucleus of a different element

45. Half-Lives The half-life of a radioactive isotope
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

46. Half-Lives The length of half-lives for different isotopes
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

47. Uniform Linear Change In this example of uniform linear change,
water is dripping into a glass
at a constant rate

48. Geometric Radioactive Decay
In radioactive decay,
during each equal time unit
half-life
the proportion of parent atoms
decreases by 1/2

49. 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

50. Determining Age Example: how old is the rock?
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 half-lives old.
the rock is 57my x 2 =114 million years old.

51. 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

52. 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

53. Igneous Crystallization
Crystallization of magma separates parent atoms
from previously formed daughters
This resets the radiometric clock to zero.
Then the parents gradually decay.

54. 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

55. Dating Metamorphism 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.
Dating the whole rock yields a date of 700 million years = time of crystallization.

56. 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)
Most of these are useful for dating older rocks
Uranium Lead billion
Uranium Lead million
Thorium Lead billion
Rubidium Strontium billion
Potassium Argon billion

57. 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

58. 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

59. 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

60. 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

61. Geologic Time Scale
Resulted from the work of many 19th century geologists who
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

62. Geologic Time Scale

63. 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

64. 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

65. 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

66. Earth’s Interior Layers
Lithosphere
solid upper mantle and crust
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
Asthenosphere
part of upper mantle
behaves plastically and slowly flows
Core
Solid inner region, liquid outer region
iron and a small amount of nickel

67. 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

68. 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

69. Plate Tectonic Theory
Lithosphere is broken into individual pieces or plates
Plates move over the asthenosphere
as a result of underlying convection cells

70. Modern Plate Map

71. 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

72. Plate Tectonic Theory
Types of plate boundaries

73. 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

74. Plate Tectonics and Earth Systems
Plate tectonics is driven by convection
in the mantle
and in turn drives mountain building
and associated igneous and metamorphic activity
Solid Earth
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
Atmosphere

75. 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
Hydrosphere
Movement of continents creates corridors
or barriers to migration,
the creation of ecological niches,
and transport of habitats into
more or less favorable climates
Biosphere

76. Next time:
Chapter 3: Plate Tectonics