Transcript geologic time scale

ES 16
Historical
Geology
Spring 2009
Spheres of the Earth
• When we view the Earth from space what
Earth systems are observable?
• What is most obvious?
• Are these systems independent or do they
interact with one another?
Spheres of the Earth
• Lithosphere: Earth’s solid rocky mass
• Hydrosphere: All of earth’s water
• Atmosphere: The thin gaseous layer
above Earth’s surface
• Biosphere: All of earth’s life forms
Gases from respiration
Transport
of seeds and spores
Wind erosion, transport
of water vapor for
precipitation
Mountains
divert air
movements
Slide 2
Fig. 1-1, p. 3
Source of sediment
and dissolved
material
Water
and glacial
erosion, solution
of minerals
Historical Geology:
A study of the dynamic and evolving Earth
• Changes in its surface
• Changes in life
Three Themes dominate the
story of the evolving Earth
• Solid Earth is composed of plates that move over
Earth’s surface over time. This is explained by the
Theory of Plate Tectonics
• Earth’s biota – all of living things – has evolved or
changed through history and is explained by the
Theory of Organic Evolution
• All of the geologic processes take place within an
extensive geologic time scale spanning 4.6 billion
years of Earth’s history
Historical Geology
applies geologic principles to
help explain processes on Earth
from past to present
• William Smith was an
English surveyor who
realized that rock types
and fossils occur in
repeated patterns. He
was able to predict rock
sequences that would be
encountered in
constructing canals
• Smith mapped the
geology of much of
England. (1815)
This map took many years to
complete and helped establish the
geologic time scale.
Slide 12
Hypothesis or Theory?
• The scientific method brings an orderly and
logical approach to decoding geologic evidence.
• A hypothesis is a tentative explanation for
observations
• Scientists make predictions using hypotheses –
then they are tested
• After repeated testing, a theory may be proposed
• Some phenomena cannot be tested or explained
• Some are discovered by sheer chance!
A theory is formed
• A theory is more than an “educated guess”
• A theory explains natural phenomena and
may relate several observations
• A theory is well-tested, well-supported and
widely accepted.
• Examples include the Plate Tectonics
Theory and the Theory of Organic
Evolution
Where do scientists look for
evidence for the following?
• The origin and age of the universe
• The origin and age of the solar system
• The origin and age of the Earth and Moon
• The origin of life on Earth
• Evidence of plate movement on Earth
• Explanation for large scale extinctions on Earth
How old is the universe?
• When? Scientists believe the universe was
formed about 15 billion years ago
• How? The Big Bang is a model for the
“beginning” of the universe
• “Show me”! What is the evidence?
Edwin Hubble and Albert Einstein:
contemporaries in time and space
•
Birth of a modern hypothesis
• Hubble, an Oxford Rhodes scholar, a former lawyer,
and boxer, with a PhD in astronomy, was an
infantry soldier in WW I.
• After the war he went to work at Mt. Wilson
observatory using the 100” Hooker Telescope.
• Here he observed the existence of galaxies outside of
the Milky Way
• Hubble used light, which travels over finite time from
one part of the universe to another, to measure distance
to stars outside of the Milky Way.
• By studying variable stars with predictable cycles, he discovered
that objects are moving uniformly away from our Galaxy
• He used the red shift of stellar spectra to measure distances and
velocities of deep space objects
• Hubble concluded that the universe is expanding and that distant
stars and galaxies are moving away from the Milky Way.
•
• A few years earlier, Einstein had predicted the
universe would be found to be expanding.
• He proposed his theory of relativity and
proposed that the objects in the universe are
not moving apart; rather space is expanding
and the objects are carried along
•
The effects of gravity on light
Hubble’s Constant quantifies the movement
Using predictable variable stars, along with redshift observations,
Hubble predicted that a galaxy twice as far away from us is receding
twice as fast.
• This was predicted by Einstein’s theory of
relativity.
Hubble’s constant was tested and continues to
be refined today as the ability to observe the
outer reaches of the universe improves.
So……….
• The universe is expanding
• Think of a deflated balloon with
markings on it,
• Being blown up
• Causing the markings to move apart.
Other evidence of the Big Bang
• Pervasive background radiation of 2.7o
above absolute zero is observed in space
--Afterglow of the Big Bang
To Find the Age of the Universe:
Determine rate of expansion
Backmodel to a time when the
galaxies would be together in space
Big Bang hypothesis
• Initial state: NO time, NO matter, NO space
• Universe was pure ENERGY
• During the FIRST second of time:
--very dense matter came into existence
--The four basic forces separated:
gravity, electromagnetic force, strong
and weak nuclear forces
--Enormous expansion occurred
Big Bang Model
• 300,000 years later:
– Atoms of hydrogen and helium formed
– Light (photons) burst forth for the first time
• Next 200 million years:
– Continued expansion
– Stars and galaxies began to form
– Elements heavier than hydrogen and helium began
to form with stars by nuclear fusion
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
• Jovian
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•
•
•
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Mercury
Venus
Earth
Mars
Small in size.
Composed of rock.
Metallic cores.
Asteroid Belt
Jupiter
Saturn
Uranus
Neptune
• Large in size.
• Composed of hydrogen,
helium, ammonia,
methane
• Small rocky cores
• Pluto ?????????
Relative Sizes of the
Sun and Planets
The sun is
believed
To be about 5
billion years old
– about half way
through its life
cycle.
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 Marssized planetesimal
with early Earth
• 4.6 to 4.4 billion
years ago
• Ejected large
quantity of hot
material
• Formed the moon
Most of the
lunar material
Came from
the mantle of
the colliding
planetesimal
Slide 8
Light colored surface areas are lunar
Highlands – heavily cratered.
Evidence of massive meteorite
Bombardment
Mare are areas of lava flows
Fig 1-5, p. 9
The material
cooled and
Crystallized
into lunar
layers
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
Plate Tectonic Theory:
A Revolutionary Concept!
• 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
Modern Plate Map
Plate Tectonic Theory
• A major Breakthrough in the 1960s:
• comparable to Darwin’s theory of evolution in
biology
• Explains and provides a working framework for
– interpreting many processes on Earth on a global
scale
– relating many seemingly unrelated phenomena
– interpreting Earth history
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 long 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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–
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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 later dated in years
– using radiometric dating techniques
Geologic
Time Scale
Principle of Uniformitarianism:
The Present is the key to the past
• 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
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–
–
–
–
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