Transcript Geologic Time

Crust
• Outermost layer
or shell of the
Earth
• Crust represents
less than 0.1% of
the Earth's total
volume
Mantle
• The zone of the Earth below the
crust and above the core
• Divided into the upper mantle and
the lower mantle, with a transition
zone between
Lithosphere
• The solid portion of the Earth, as compared
with the atmosphere and the hydrosphere
• Includes the crust and part of the upper
mantle and is of the order of 100 km in
thickness
Stress
• Stress is a force that is capable of
greatly deforming rocks, and may
result in folding or faulting of rock,
and even to the building of
mountains
Types of Stress
• There are three types of stress
– Compression
– Tension
– Shearing
Compression
• Opposing forces directed inward along
a single line
• Compression shortens an object along
the axis of compression, and thickens it
in the directions perpendicular to the
stress direction
Before
After
Tension
• Tension is the result of divergence,
pulling an object in opposite directions
along a common axis
• Tension lengthens an object along the
axis of tension, and thins it in the
perpendicular directions
Before
After
Tension Crack Pictures
• Nisqually Earthquake, 2/28/01, in Washington caused
tension cracking.
Shear
• Opposing stress is created by two plates
moving in opposite directions
Deformation
•
•
•
•
Rocks subjected to stress may:
Fracture or crack
Fold
Fault, movement along break
Why do some rocks fold/fault?
Deformation
Folding
Faulting
Very hot
Cooler rock
Ductile
Brittle
Stress applied slowly
Stress applied quickly
Anticline
• If the fold is convex upward, it is
called an anticline
Anticlinal Fold
• Rainbow Gap, Virginia
• Photo: Henry Johnson
Atlas Mountains Anticline
• One of the best
exposures of a
complexly folded
mountain belt
anywhere occurs in
the Atlas Mountain
system of northwest
Africa
Syncline
• If the fold is convex downward, it
is called a syncline
Syncline Photo
• Photo: Duncan Heron
• Synclinal fold exposed by roadcut
Anticline-Syncline Pair
• AnticlineSyncline pair in
Devonian Old
Red Sandstone.
SW Wales, UK
• Note the
different fold
shapes
Domes
• Domes are uplifted
areas
• Caused by magma
pushing up on the
crust
Eroded Dome,
Sinclair, Wyoming
Faults
• A fault is a fracture along which
definite movement has occurred
Fault Terminology
• Foot Wall and
Hanging Wall are
borrowed from mining
terminology
• Ore veins are often
deposited along faults
Strike Slip Fault
Photo: Arthur G. Sylvester.
San Jacinto fault, Anza, Southern California
Right-Lateral Strike Slip
• Block is displaced to
the right, looking
across the fault
Strike Slip Faults
Right Lateral
•Near Coos Bay, Oregon
Left-Lateral Strike Slip
• Block is displaced to
the left, looking across
the fault
Strike Slip Faults - Left Lateral
Near Lillooet, British Columbia
Normal Fault
•Normal faulting results
from tensional forces
•Hanging wall moves
down relative to the
footwall (here, to the
right)
•Places younger rocks
on top of older
Sevier Normal Fault
Death Valley Normal Faults
Reverse Fault
• Reverse faulting results
from compressional forces
• Hanging wall moves up
relative to the footwall
(here, to the left)
• Places older rocks on top
of younger
Reverse Fault
• Reverse faults and associated fold
Thrust Fault
• Thrust faults are low
angle reverse faults
• They sometimes move
large distances (tens of
kilometers)
Lewis Overthrust
Explanation of Lewis Overthrust
• Chief Mountain was moved about forty kilometers and
isolated by erosion
• Chief Mountain is much older (Precambrian) than the rock
upon which it rests (Cretaceous)
Chief Mountain
Older rock above younger, typical of thrust faults
Glacier National Park, Montana
Rift valleys and Fault block
mountains
San Andreas Fault
• Pacific plate, left
• North America, right
San Andreas Offsetting Fence
Mountain building
• Tectonic forces often create mountains, a
process called orogeny
• There are several types of mountains
– Folded
– Faulted
– Upwarped
– Volcanic
Folded mountains
• Plate collisions involving continental plates
can produce high mountains
– Examples:
– Himalayas (India, Tibet, China)
– Alps (Europe)
– Urals (Europe/Asia boundary)
– Appalachians
Himalayan Mountains
Mt. Everest
High peaks in the
Himalayas
Owens Valley and the
Sierra Nevada Range
Volcanic Mountains
Isostasy
• The condition of equilibrium, comparable to
floating, of the units of the lithosphere
above the asthenosphere
• Crustal loading, as by ice, water, sediments, or
volcanic flows, leads to isostatic depression or
downwarping
• Crustal unloading, as by erosion, or melting of
ice, to isostatic uplift or upwarping
Isostasy Diagram
Earthquake
• A sudden motion or trembling in
the Earth caused by the abrupt
release of slowly accumulated
strain
• Strain is a change in the shape or
volume of a body as a result of
stress
Focus
• The initial rupture point of an
earthquake, where strain energy is
first converted to elastic wave
energy
• The point within the Earth which is
the center of an earthquake
Epicenter
• The point on the Earth's surface
that is directly above the focus of
an earthquake
Seismograph
• An instrument that detects,
magnifies, and records vibrations
of the Earth, especially earthquakes
• The resulting record is a
seismogram
Principle of the
Modern Seismograph
Example
Seismogram
• Seismogram showing an earthquake - the
three different traces represent vibrations in
different directions
• First peaks are P waves, the second peaks the
S waves
Types of seismic waves
• 1. Primary
• 2. Secondary
• 3. Surface or L waves

P-waves
*Primary, pressure, push-pull
*Travel fastest of the seismic
waves
*Travel through solids and
liquids.
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
S - waves
*Secondary, shaking, shear, side-to-side
*Arrive at a given point after P-waves
*Travel through solids.
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Surface waves
*Often referred to as L-waves or long waves.
*Complex motion. Like waves on the ocean.
* Slowest.
*Causes damage to structures during an
earthquake.
Richter Scale
• Numerical scale of earthquake
magnitude
• Devised in 1935 by the
seismologist C.F. Richter
• The magnitude is based on
multiples of 10, i.e.,101, 102, 103
each number on the scale is ten
times greater than the previous
number. For example magnitude 3
is ten times greater than magnitude
2.
Richter Scale Continued
• Measures the vibrational amplitude
of the earth in response to seismic
waves
• Does NOT measure the energy
release
Mercalli Scale
• Arbitrary scale of earthquake intensity,
ranging from I (detectable only
instrumentally) to XII (causing almost
total destruction)
• Based on human perception of the
earthquake, and damage observed after
the earthquake is over
Number of Earthquakes/Year
Prince William Sound
Earthquake Damage
Earthquake Damage
• Earthquakes can cause damage in a number
of ways
–
–
–
–
–
–
–
Building Collapse
Tsunami waves
Seiche waves
Landslides
Liquefaction
Fire
Disease
Tsunami
• Gravitational sea wave produced by
any large-scale, short-duration
disturbance of the ocean floor
• Disturbances caused principally by a
shallow submarine earthquake, but also
by submarine earth movement,
subsidence, or volcanic eruption
Tsunami Damage
Seward, Alaska. After the “Good Friday” Earthquake in
1964. 9.2 on the Richter scale.
Liquefaction
• Liquefaction is a physical process that takes
place during some earthquakes that may
lead to ground failure
• As a consequence of liquefaction, soft,
young, water-saturated, well sorted, fine
grain sands and silts behave as viscous
fluids rather than solids
Liquefaction Failure
• Overturned building, Niigata Japan Earthquake.
Magnitude 7.5.
Volcano
• A vent in the surface of the Earth through
which magma and associated gases and ash
erupt
• Also, the form or structure, usually conical,
that is produced by the ejected material
• Plural: volcanoes
• Etymology: the Roman deity of fire, Vulcan
Pyroclastic Eruptions
• Magma spews upward with great force
through a central vent
Left: Mt. St.
Helens, 1980
Right:
Kilauea,
Hawaii
Fissure Eruptions
• Volcanic eruptions may occur much more
quietly along long cracks in the ground
Fissure Image
• Eruptive fissure on southeast rim of Kilauea
caldera, Hawaii
Nahuku Lava Tube
• Thurston (Nahuku) lava
tube
• Near summit caldera of
Kilauea Volcano, Hawaii
Volcanoes National Park
Tephra
• General term for fragments of volcanic
rock and lava blasted into the air by an
eruption.
• Dust - smallest ,like flour
• Ash - grain of rice
• Cinders - golf ball size
• Bombs - largest, as big as a car
Volcanic Ash Fall
• Mount Pinatubo (Philippines - 1991)
Effect on Climate
• Large volcanic eruptions can block a great
deal of the sun’s energy from reaching the
earth’s surface
• This cools the climate until the tephra
particles sink to the surface
Stratovolcano
• A volcano that is constructed of alternating
layers of lava and pyroclastic deposits,
along with abundant dikes and sills
• Synonym: composite volcano; composite
cone
Mt. Fuji, Japan
Vent and Steam Explosion
• Mt. St. Helens
Crater Lake,
Oregon
Crater Lake National Park, with Wizard Island Cinder Cone
• Crater Lake, despite the name, is a caldera,
formed after the eruption of ancient Mt.
Mazama about 6600 y.b.p.
Cinder
Cone
Crater Lake National Park, with Wizard Island Cinder Cone
• Wizard Island, within Crater Lake, is a cinder
cone, and one of the tallest in the world
Shield Volcano
• A volcano in the shape of a
flattened dome, broad and low,
built by flows of very fluid basaltic
lava or by rhyolitic ash flows
• Shield volcanoes are the largest
volcanoes on Earth that actually
look like volcanoes (i.e. not
counting flood basalt flows)
Mauna Loa
Shield Volcano - note the low slopes
Kilauea Eruption
•(Upper left) Explosive
eruptions
•(Center left) Gas bubbles
splash lava over the edges
(with sound)
•(Lower left) Lava flowing
after initial surge
•(Right) Upwelling lava –
note rapid change in color –
yellow is hottest, then orange,
and red is the coolest
Pahoehoe Flow
• Toes of a pahoehoe flow
advance across a road in
Kalapana on the east rift
zone of Kilauea Volcano,
Hawaii
• Photograph by J.D.
Griggs on 16 July 1990
Alfred Wegener,
1880-1930
Wrote The Origin of
Continents and
Oceans in 1915
Continental Drift
• 550 MYBP
136 MYBP
220 MYBP
65 MYBP
Fossil Plant Evidence
Glossopteris
• Extinct group of seed plants that arose
during the Permian on the great southern
continent of Gondwana
How Can a Continent Move?
• The biggest objection was the lack of a
mechanism for moving continents
• Wegener spent the rest of his life
looking for evidence to support his
ideas
• He died in Greenland in 1930, while
seeking more evidence
Sea-floor Spreading
• Concept came from oceanographic
investigations
• Uses convection cells, an idea
Wegener would have been familiar
with
Hess-Dietz Hypothesis
The Oceans
• Mid-ocean ridges
• Rift valleys
• Heat flow
• Age of ridge
Magnetic
Stripes
• As magma rises, it hardens and its magnetic field
matches the present field of the earth - after a
polarity reversal, it will be aligned against the
earth’s field
Magnetic
Stripe
Creation
• Picture from USGS animation of magnetic stripe
creation
• Dark gray area are have magnetic orientation
pointing north, light gray to the south
Age of Ocean Fossils
• Continental fossils are at least 3.5
billion years old
• Oldest marine fossils, found on the
ocean floor, are about 180 million
years
• Since life is though to originate in the
oceans, why aren’t ocean fossils older?
Plate Movement
• Plates move slowly (up to 15
cm/yr)
• Plates may collide, move apart, or
slide past each other
• Friction during plate movement
often generates earthquakes
Plate Motions
• Two plates move relative to each other
– Convergent - Plates move toward each
other, often a head-on collision
– Divergent - Plates move away from each
other
– Sideways - Plates move past each other
along transform faults
Converging Plates
• When two plates collide, the
denser plate will sink (subside)
beneath the less dense plate
• Density differences as small as 1%
are enough to cause subduction
Subduction Zones
The key to subduction is the density of
the rock types involved
Density = mass/unit volume
Continental rock has a density of about 3
g/cc
Oceanic rock is about 3.28
Plate Types
• At any given point, a plate is either
oceanic or continental
• Interactions between plates are thus:
– Ocean-ocean (O-O)
– Ocean-continent (O-C)
– Continent-continent (C-C)
Plate Interactions
Conv.
Divg.
Trans.
O-O
SID Quakes
Oceanic Arc
Volcanism
Japan
SI Quakes
Fissure
Volcanism
MOR
S Quakes
No volcanism
Oceanic
transform faults
O-C
SID Quakes
Stratovolcanic
Chains
Cascades, Andes
Not known
Not known
C-C
SI Quakes
No Volcanism
Himalayas
SI Quakes
Alkaline
volcanism
East Africa
S Quakes
No volcanism
San Andreas
Fault
Hot Spot
Diagram
• Diagram showing
creation of several
Hawaiian Islands
• Age of islands
should be
progressively older
as they move away,
and this is observed
Map of Major Tectonic Plates
Mid-ocean Ridge Map
Geologic Time
4,600,000,000 Years - Estimated
Age of the Earth
Relative Dating
• Relative Age is the answers to a
question like, “Which is younger?”
• Relative ages allow us to compare
different geologic formations, and
determine which is the oldest, next
oldest, etc.
Principle of Superposition
• Layers on the bottom were
deposited first, and are the
oldest (A older than B, B
older than C, etc.) - think of
paint layers on a wall
• In any unaltered sequence of
rocks, the oldest is at the
bottom, the youngest at the
top
Index Fossils - Short existence on earth helps to
pinpoint the age of the rock in which they are found
• Organisms with
specific
characteristics:
– Short lived
(geologically)
– Widespread
occurrence
– Readily recognized
Unconformity
• Gaps in the rock record,
unconformities mark boundaries
between rocks of different ages
• Can be produced by:
• A. erosion & sedimentation,
• B. faulting
• C. intrusions & extrusions
Unconfomity
– intrusions
(black basalt
& white
quartz)
• A feature, such as a dike or fault, that
cuts formations is younger than the
formations it cuts
Unconformity: cross-cutting by faults &
intrusions/extrusions
• The uncomformity is younger than the rock it offsets
Unconformity in Volcanic Ash
• Outcrop photo of volcanic
ash layers in Japan
• There is an erosional
discontinuity
(unconformity) that
separates earlier folding in
the lower half from folding
(above) after later ash
flows were deposited.
Absolute Age
• Determination of the absolute age is
usually done using radiometric dating
• Absolute ages are expressed in years, or
millions or billions of years, before
present
Radiometric Dating
• A parent isotope that is
radioactive decays to yield a
daughter isotope at a known rate
– Example:
– 14C  14N
– Radioactive decay follows an
exponential decay law
Illustration of Radiometric Decay
Half-life, t½
• The time necessary for half of the
original atoms of the parent isotope
to decay to the daughter isotope is the
half life
Parent and Daughter Isotopes
• In the previous example,
– 14C  14N
• 14C is the parent, and 14N is the daughter
• The half-life, t½, is 5730 years
Isotope Systems
Parent
Daughter
Half-life
Dating Range
Materials
Dated
Rb87
Sr87
47 billion
10 million to 4.6
billion
IgneousMetamorphic
U238
Pb206
4.5 billion
10 million to 4.6
billion
IgneousMetamorphic
U235
Pb208
713 million
10 million to 4.6
billion
IgneousMetamorphic
K40
Ar40
1.3 billion
100,000 to 4.6
billion
IgneousMetamorphic
C14
N14
5730
100 to 100,000
C- bearing
material
Geologic Time Scale
• Eons - Largest divisions of time, beginning
with the Hadean (4.6 to 3.8 billion years ago)
– Eras - Largest subdivision, defined by
dominant life forms
• Periods - Divisions of eras, based on smaller
scale changes
» Epochs - Divisions of periods in the
Cenozoic era, based on detailed, smaller
scale changes
Geologic Time
Recent History of the Earth
Earth’s recent history is divided into
eras:
Paleozoic – 570 to 245 million years
before present (MYBP)
Mesozoic – 245 to 65 MYBP
Cenozoic – 65 MYBP to present
Paleozoic Era
• Trilobite fossil,
early Paleozoic
era
Mesozoic Era
• Dinosaurs were the dominant life forms
Cenozoic Era
• Kangaroos are
marsupials, a
type of mammal
Geologic Time Line