Download The History of the Earth

The History of the Earth
Geologic time
• Planet Earth is approximately
4.5 X 109 years old (4.5 billion
years or 4,500 million years)
–Rocks of the crust provide clues
to Earth’s past
• By analyzing these clues we can
infer events from the past
• Principle of Uniformitarianism
–Major assumption in geology
–Events in the past occurred the same
way that they are occurring today.
Examples Include:
• Weathering/erosion
• Deposition
• Volcanism
• Plate tectonics
James Hutton (1726-1797)
• First to interpret sequence of events in an
unconformity
• Published “Theory of the Earth” in 1785
• He was the one who recognized the uniform
natural laws that govern geologic processes 
“uniformitarianism”
Geologic
Time
• Geologists
have divided
Earth’s
history into
time units
based on the
the fossil
record
Geologic Time
• A study of the fossil record shows
–A great variety of plants, animals,
and simpler life forms have lived on
Earth in the past
–That life forms have evolved
through time
–Most life forms of the geologic past
have become extinct
A Brief History of the Earth
• Eon – largest segment of geologic time
–Era
• Period
–Epoch – smallest segment of geologic time
Let’s compress Earth’s 4.5 billion year
history into 1 year
• January 1 – origin of earth
• Feb. 26 – earliest earth rocks formed
• March 23 – first of simple life forms as simple
bacteria in the sea and photosynthesis
• July 22 – oxygen begins to accumulate in
atmosphere
• Sept. 1 – first green algae and organisms w/ a
nucleus
• November 17 – the proliferation of life; beginning of
• November 21 – the first fish; the first
vertebrate
• November 27 – the first life on land,
plants
• December 2 – amphibians move to land
• December 13 – the first mammals
• December 14 – 26 – age of dinos
December 31st at 11:49 p.m.
• Humans emerge
• Within last minute – all recorded human
history
• 2 seconds before midnight – Declaration of
Independence
• 1 second before midnight – start of Industrial
Revolution
Geologic Time Activity: May 11, 2017
• Get into groups of 2
• Obtain a worksheet from the front of the
classroom and follow instructions on creating
your own geologic timeline
• Markers, colored pencils, scissors, receipt
paper, are all available for you to utilize
RELATIVE TIME
VS.
ABSOLUTE TIME
PLACES EVENTS IN A
IDENTIFIES THE
SEQUENCE BUT DOES
ACTUAL DATES OF
NOT IDENTIFY THEIR
GEOLOGIC EVENTS
ACTUAL DATE OF
OCCURRENCE
EXAMPLE
A LIST IN
THE EXACT TIMES AT
CHRONOLOGICAL
WHICH YOU DID THESE
ORDER OF WHAT YOU
THINGS
HAVE DONE TODAY UP
UNTIL THIS TIME
• Law of Superposition
in undisturbed
sedimentary rocks,
the oldest rock layers
are at the bottom and
the youngest are at
the top
Grand Canyon
Law of Superposition
• Rock layers are older than folds found in
them
– Layers were there before they were folded
Principle of Original Horizontality
• Sediments are deposited in flat layers
(horizontal)
• Most sedimentary rocks occur in the form of
layers called beds or strata
• These layers are the result of sediment
deposition during a natural event such as a
storm or flood
Law of Cross-Cutting Relationships
• An igneous intrusion is younger than the rock it
has intruded into
The fault
is younger
than the
rock it
cuts
Igneous Intrusion – Cross Cutting
Principle of Inclusions
The inclusions are older than the rocks which
contain them
https://www.youtube.com/watch?v=bD5RylDR3
CU
Unconformity
• When a new rock layer is formed atop an
eroded surface
• https://www.youtube.com/watch?v=lMfPSdrrj
ZI
Place the following events in order
starting with the oldest
Principle of Faunal Succession
• Fossils occur in a consistent vertical order in
sedimentary rocks all over the world
• This principle doesn’t depend on any preexisting ideas of evolution
• Geologists interpret fossil succession to be the
result of evolution – the natural
appearance/disappearance of species through
time
Absolute Dating
• Radiometric Dating
• Naturally-occurring radioactive materials
break down into other materials at known
rates. This is known as radioactive decay.
• Radioactive parent elements decay to stable
daughter elements.
• Many radioactive elements can be used as
geologic clocks. Each radioactive element
decays at its own nearly constant rate. Once
this rate is known, geologists can estimate the
length of time over which decay has been
occurring by measuring the amount of
radioactive parent element and the amount of
stable daughter elements.
– How does carbon-14 dating work?
• The half-life is so short (5730 years) that this method
can only be used on materials less than 70,000 years
old. Archaeological dating uses this method.
• Also useful for dating the Pleistocene Epoch (Ice Ages).
The Standard Geologic Time Scale
• The geologic time scale has been determined
bit-by-bit over the years through relative
dating, correlation, examination of fossils, and
radiometric dating.
• Geochronologic units or time units
• Eon = Largest division of time scale.
In order from oldest to youngest, the
eons are:
– Hadean Eon (early Earth still molten - hot as
hades - no rock record) - beginning about 4.6
billion years ago
– Archean Eon - "ancient or archaic" - oldest rocks
on Earth
– Proterozoic Eon - "beginning life" (2.5 billion to
544 million years ago)
– Phanerozoic Eon - "visible life" (544 million years
ago to present)
• The Archean and Proterozoic are collectively
referred to as the Precambrian (meaning
"before the Cambrian Period").
• Era = A major division of geologic time,
divisible into geologic periods.
The Phanerozoic Eon is divided into
three eras. From oldest to youngest,
the eras are:
– Paleozoic Era- "ancient life" (trilobites)
– Mesozoic Era - "middle life" (dinosaurs)
– Cenozoic Era - "recent life" (diverse mammals)
• Eras are divided into periods. Periods can be
divided into epochs.
- Epochs can be divided into ages.
• Earthquakes are vibrations of Earth produced by
the rapid release of energy from rocks that rupture
because they have been subjected to stresses
beyond their limit.
• This energy, which takes the form of waves, radiates
in all directions from the earthquake's source,
called the focus. The movements that produce most
earthquakes occur along large fractures, called
faults, that are associated with plate boundaries.
• Two main groups of seismic waves are
generated during an earthquake:
• (1) surface waves, which travel along the
outer layer of Earth
• (2) body waves, which travel through Earth's
interior. Body waves are further divided into
primary, or P, waves, which push (compress) and
pull (expand) rocks in the direction the wave is
traveling, and secondary, or S, waves, which shake
the particles in rock at right angles to their direction
of travel. P waves can travel through solids, liquids,
and gases. Fluids (gases and liquids) will not
transmit S waves. In any solid material, P waves
travel about 1.7 times faster than do S waves.
• http://ocean.si.edu/oceanvideos/demonstrating-earthquakes-seismicwaves
• https://www.youtube.com/watch?v=3lie1Myu
XfQ
• The location on Earth’s surface directly above
the focus of an earthquake is the epicenter
• An epicenter is determined using the
difference in velocities of P and S waves
• There is a close correlation between
earthquake epicenters and plate boundaries
• The principal earthquake epicenter zones are
along the outer margin of the Pacific Ocean,
known as the circum-Pacific belt and through
the world’s oceans along the oceanic ridge
system
• Seismologists use 2 fundamentally different
measures to describe the size of an
earthquake – intensity and magnitude
• Intensity – measure of the degree of ground
shaking at a given locale based on the amount
of damage
• The modified mercalli intensity scale uses
damage to buildings in California to estimate
the intensity of ground shaking for a local
earthquake
• Magnitude – calculated from seismic records.
Estimates the amount of energy released at
the source of an earthquake
• Using the Richter Scale, the magnitude of an
earthquake is estimated by measuring the
amplitude of the largest seismic wave
recorded
• Logarithmic scale expresses magnitude (10fold increase in ground shaking corresponds to
an increase of 1 on the richter scale)
4 Major Zones of Earth’s Interior
• 1. Crust – very thin outer layer
• 2. Mantle – rocky layer below crust (2885 km
thick)
• 3. Outer core – characteristics of a mobile
liquid (2270 km thick)
• 4. Inner core – solid metallic sphere with a
radius of 1216 km
• Continental crust – made mostly of granitic rocks
• Oceanic crust – basaltic composition
• Mantle – made up of ultramafic rocks such as
peridotite
• Core – iron and nickel
• Lithosphere – crust and uppermost mantle which
form Earth’s cool rigid outer shell
• Asthenosphere – beneath lithosphere; a soft
relatively weak layer
• Among the most basic geologic structures
associated with rock deformation are folds
– These are flat-lying sedimentary and volcanic
rocks bent into a series of wavelike undulations
– Anticlines – formed by upfolding or arching of rock
layers
– Synclines – downfolds
– Most folds are a result of horizontal compressional
stresses
Anticlines
• Folds in which the
oldest rock lies in the
center or core and
synclines have the
youngest rock in its
center core
Faults
• Faults are fractures in the crust along which
appreciable displacement has occurred
• Dip-slip faults – faults in which the movement
is primarily vertical
– Normal
– Reverse
Normal Fault
• Dip-slip fault
• The block above the fault has moved
downward relative to the block below
• Occurs in response to extension – usually in
the Western U.S. Basin and Range Province
and along oceanic ridge systems
Thrust Faults
• Low-angle reverse faults
• Upper block above the fault plane moves up
and over the lower block
• Common in areas of compression such as
regions where one plate is subducted under
another (Japan)
Strike-Slip Faults
• Exhibit mainly horizontal displacement parallel
to the fault surface
Plate Tectonics
• Continental Drift Theory
– In the early 1900s Alfred Wegener set forth his
continental drift hypothesis
• A supercontinent called Pangaea began breaking apart into
smaller continents about 200 mya and then the smaller
continental fragments “drifted” to their present positions
• To support the claim that the now-separate continents were
once joined, Wegener and others used the fit of S. America
and Africa, the distribution of ancient climates, fossil evidence,
and rock structures
• One of the main objections to the continental
drift theory was the inability of its supporters
to provide an acceptable mechanism for the
movement of continents
Plate Tectonics
• Far more encompassing than continental drift
• Says that Earth’s rigid outer shell (lithosphere)
consists of 7 large and numerous smaller
segments called plates that are in motion
relative to each other
• Most of Earth’s seismic activity, volcanism, and
mountain building occur along the dynamic
margins of these plates
• A major departure of the plate tectonics
theory from the continental drift hypothesis is
that large plates contain both continental and
oceanic crust and the entire plate moves
– In contrast, in continental drift, Wegener
proposed that the sturdier continents “drifted” by
breaking through the oceanic crust, much like ice
breakers cut through ice
Boundaries
• Divergent plate boundaries occur where
plates move apart, resulting in upwelling of
material from the mantle to create a new
seafloor
– Most occur along the axis of the oceanic ridge
system and are associated with seafloor
spreading, which occurs as rates between about 2
and 15 cm per year
https://pubs.usgs.gov/gip/dynamic/topomap.html
• New divergent boundaries may form within a
continent (for example the East African rift
valleys) where they may fragment a landmass
and develop a new ocean basin
• http://pubs.usgs.gov/gip/dynamic/East_Africa
.html
Convergent Plate Boundaries
• Occur where plates move together, resulting
in the subduction of oceanic lithosphere into
the mantle along a deep oceanic trench
– Convergence between an oceanic and continental
block results in subduction of the oceanic slab and
the formation of a continental volcanic arc such as
the Andes of South America
http://www.geographicguide.net/earth/pictures/an
des.jpg
Oceanic-Oceanic Convergence
• Results in an arc-shaped chain of volcanic
islands called a volcanic island arc
• http://www.johnpratt.com/items/docs/Ids/m
eridian/2005/images/aleutian.jpg
• http://www.earthweek.com/2008/ew081128/
ew081128xLARGE.jpg
• When 2 plates carrying continental crust
converge, both plates are too buoyant to be
subducted. The result is a “collision” resulting
in the formation of a mountain belt such as
the Himalayas
• http://www.eduspace.esa.int/casestudy/imag
es/subhim_meris_t2.jpg
Transform Plate Boundaries
• Occur where plates grind past each other
without the production or destruction of
lithosphere
– Most transform faults join 2 segments of an
oceanic ridge. Others connect spreading centers
to subduction zones and thus facilitate the
transport of oceanic crust created at a ridge crest
to its site of destruction, at a deep ocean trench.
Others, like the San Andreas Fault, cut through
continental crust
– http://www.geo.cornell.edu/geology/classes/geol
326/images/sanandreas.gif
Plate Tectonics Support:
• 1. Paleomagnetism – direction/intensity of
Earth’s magnetism in the geologic past
• 2. Global distribution of earthquakes and their
close association with plate boundaries
• 3. Ages of sediments from the floors of deep
ocean basins
• 4. Existence of island groups that formed over
hot spots and provide a frame of reference for
tracing the direction of plate motion
• http://www.worldofstock.com/slides/SES1016
.jpg
Mantle Convection
• Slab-pull – cold, dense oceanic lithosphere is
subducted and pulls the trailing lithosphere
along
• Ridge-push – gravity sets the elevated slabs
astride oceanic ridges in motion. Hot buoyant
plumes are considered the upward flowing
arms of mantle convection
Ridge-Push
• One model suggests that mantle convection
occurs in 2 layers separated at a depth of
660km
• Another model proposes whole-mantle
convection that stirs the entire 2900-km thick
rocky mantle
• Another suggests that the bottom third of the
mantle gradually bulges upward in some areas
and sinks in others w/o appreciable mixing
Volcanoes