GLY 150: Earthquakes and Volcanoes Spring 2005: 020105 Lecture

1
GLY 150 Earthquakes and VolcanoesSpring 2005
02/01/05Lecture 6
Puu O o Spatter/Cinder Cone, Kilauea Volcano,
Hawaii
http//volcanoes.usgs.gov/Imgs/Jpg/Tephra/
2
AnnouncementsGLY 150 Earthquakes and Volcanoes

• The next journal assignment is due this Thursday.
  Grading criteria can be found in your syllabus.
  Make sure you meet each of these criteria or
  points WILL be deducted.
• Your latest HW assignment is due this Thursday.
• Instructor office hours have changed to Mon.
  200-300 and Wed. 200-300

3
Figure Sources

• Diagrams of each of the fault types are shown in
  your Natural Disasters test (Chapter 3)
• Figures 3.8 through 3.13 are particularly helpful
• Remember, use your texts to supplement the
  lecture notes. Many of the same figures can be
  found there
• I have placed asterisks on the websites that most
  of the other figures came from. You should
  definitely look at these sites, read the material
  related to the figures, and be sure that you are
  well acquired with the figures.
• The last question on this weeks homework session
  is designed help relate the different faulting
  terminology and provide a study guide for your
  first exam.

4
Events this QuarterSpring 2005
5
Quiz 2 Types of Faults Revisited

• (1 point) What type of faulting is typically
  found at convergent plate boundaries?
• Left-lateral strike slip fault
• Normal fault
• Thrust fault
• Right-lateral strike-slip fault
• Faults rarely occur at convergent boundaries
• (1 point) Which type of fault accommodates only
  horizontal motion?
• Subduction fault
• Strike-slip fault
• Normal fault
• Thrust fault
• Oblique fault
• Based on the classs comments well revisit the
  following
• Hanging wall and footwall definitions
• Types of faults (thrust, normal, right-lateral
  strike-slip, left-lateral strike-slip, oblique)

6
Faulting TerminologyFootwall and Hanging Wall

• The definition of footwall and Hanging wall DOES
  NOT depend on the sense of relative notion across
  the fault
• Definition depends only on fault orientation

Fault Plane
Hanging Wall
Hanging Wall
Footwall
Footwall
Mine Shaft
Hanging Wall
Hanging Wall
Footwall
Footwall
Hanging Wall
Hanging Wall
Footwall
Footwall
7
Divergent Plate Boundaries

• Zone of tension
• Lithospheric extension and thinning
• Crust displaced vertically and horizontally
• Types
• Mid-Ocean Ridges (a.k.a. Spreading Centers)
• Continental Rifts
• Areas of normal faulting

Normal Fault
USGS http//earthquake.usgs.gov/image_glossary/in
dex.html
8
Normal Faults

• Faults generally dip (i.e., they are not
  vertical)
• Hanging wall moves down relative to the footwall
• Earthquakes that rupture surface generate a fault
  scarp

Footwall
Hanging Wall
9
Convergent Plate Boundaries

• Zones of compression
• Lithospheric shortening and thickening
• Crust displaced vertically and horizontally
• Types
• Subduction Zones
• Continent-Continent Collision Zones
• Produce major mountain ranges
• Areas of thrust faulting

Thrust Fault
USGS http//earthquake.usgs.gov/image_glossary/in
dex.html
10
Thrust Reverse Faults

• Faults generally dip (i.e., they are not
  vertical)
• Hanging wall moves up relative to the footwall
• One side of the fault is thrust up and over the
  other side of the fault
• Earthquakes that rupture surface generate a fault
  scarp

Hanging Wall
Fault Scarp
Footwall
11
Thrust FaultingThe Difference

• Low angle thrust faults have a shallow dip (i.e.,
  low dip angle, lt 15)
• High angle thrust faults Reverse faults are
  steeply dipping (i.e., high dip angle , gt 15)
• For the purposes of this class we will not
  differentiate between trust and reverse faults

High Angle Thrust Fault
Low Angle Thrust Fault
Both from http//earthsci.org/struct/struct.html
12
Transform Plate Boundaries

• Zones of shear
• Insignificant changes in lithospheric thickness
• Crust displaced only horizontally
• Types
• Continental Transform Faults (e.g. the San
  Andreas Fault, California)
• Fracture Zones at Mid-Ocean Ridges
• Areas of strike-slip faulting

Strike-Slip Fault
USGS http//earthquake.usgs.gov/image_glossary/in
dex.html
13
Strike-Slip Faults

• Motion is purely horizontal There is no vertical
  motion
• Faults are usually close to vertical
• No fault scarps produced unless there is
  topography
• Get mole tracks instead

14
Strike-Slip FaultsRight and Left-Lateral

• When looking across the fault, the opposite side
  moves to the
• Right if right-lateral
• Left if left-lateral

15
Oblique Faults

• Slip direction is a combination of strike-slip
  (i.e. horizontal) and either normal or thrust
  (i.e., vertical) motion
• Results from geometrically complexity
• Curved faults, combination plate boundaries, etc.

I.
II.
III.
IV.
16
Convergent Plate Boundaries
Plate 1
Plate 2
faulting

• Convergence, compression, mountain building,
  thickening of the lithosphere, thrust faulting
• Types
• Ocean-ocean (subduction island arc volcanoes)
• Ocean-continent (subduction long, linear
  mountain chain volcanoes)
• Continent-continent (continental collision zone
  wide, high mountainous region no volcanism)

17
Convergent Plate Boundaries

• Zones of compression
• Lithospheric shortening and thickening
• Crust displaced vertically and horizontally
• Types
• Subduction Zones
• Continent-Continent Collision Zones
• Produce major mountain ranges
• Areas of thrust faulting

Thrust Fault
USGS http//earthquake.usgs.gov/image_glossary/in
dex.html
18
Thrust Reverse Faults

• Faults generally dip (i.e., they are not
  vertical)
• Hanging wall moves up relative to the footwall
• One side of the fault is thrust up and over the
  other side of the fault
• Earthquakes that rupture surface generate a fault
  scarp

Hanging Wall
Fault Scarp
Footwall
19
NormalFaulting
12-inch ruler
1959 M 7.7 Hebgen Lake, Montana earthquake
1983 M 7.3 Borah Peak, Idaho earthquake
http//www.ngdc.noaa.gov/cgi-bin/seg/m2h?seg/haz_v
olume1.menFaults,I13
20
Thrust Faulting
1964 Alaska Earthquake
Fault scarps created during earthquake
1999 Chi-Chi, Taiwan Earthquake
21
Fault Creep

• Continuous, extremely slow movement along the
  fault that does not generate detectable seismic
  waves, i.e. does not produce earthquakes
• Slowly displaces curbs, houses, streets, fences
  anything lying directly atop the fault trace
• Displacement rates a few mm/yr

Calaveras Fault, California
http//www.ngdc.noaa.gov/cgi-bin/seg/m2h?seg
22
Quiz
23
Quiz 3 Earthquake Causes?

• (1 point) Earthquakes are a result of behavior on
  the fault surface.
• plastic
• stick-slip
• slippery
• stable sliding
• creep
• (1 point) In thrust faulting the footwall moves
  ______ relative to the hanging wall.
• up
• down
• strike-slip
• right
• left
• (2 points) Write 2-3 complete sentences
  discussing concepts you are having difficulty
  with, topics you think are particularly
  interesting, specific questions you might have,
  or topics you want to hear more about, etc.

24
Subduction ZonesBenioff Zones Aleutian Arc

• Note volcanism along island arc

25
Subduction ZonesBenioff Zones Aleutian Arc

• Note intense shallow seismicity
• Deep earthquakes as subducted slab breaks apart.
• Note single and double Benioff Zones

26
Mainshock Aftershocks

• Mainshock Largest earthquake in a series of
  events that are concentrated in a restricted
  crustal volume
• Aftershocks Smaller events that follow the
  largest earthquake in a series of events that are
  concentrated in a restricted crustal volume

http//earthquake.usgs.gov/image_glossary/mainshoc
k.html
27
Foreshocks

• Smaller events preceding the largest earthquake
  in a series of events that are concentrated in a
  restricted crustal volume
• Unfortunately for prediction purposes, we dont
  know if a foreshock is really a foreshock unless
  a larger mainshock occurs

http//earthquake.usgs.gov/image_glossary/foreshoc
ks.html
28
Aftershocks
Cross Sections

• Seismologists use the distribution of aftershocks
  to map out the fault plane that ruptured during
  the mainshock
• Aftershocks clean up stress concentrations and
  other heterogeneities left after the mainshock

Map View
29
Aftershocks

• The number of aftershocks decays exponentially
  with time
• Aftershocks are larger and more frequent
  immediately after the mainshock
• How might this affect rescue efforts after a
  large earthquake?

http//earthquake.usgs.gov/image_glossary/aftersho
cks.html
30
Volcanoes
31
Plate Tectonics
32
Plate Tectonics(Putting the Pieces Together)
Convergent
Convergent
Convergent
Divergent
Divergent
Transform
33
Where Volcanoes Occur
Intraplate Earthquake

• Volcanism

No volcanism at continent-continent collision
zones or transform boundaries
Modified from Dynamic Earth http//pubs.usgs.gov/
publications/text/Vigil.html
34
Mid-Ocean Ridge Systems

• Volcanoes occur only along the rifts
• There are no volcanoes along the fracture zones

Older
Volcanoes
Younger
35
Worldwide Distribution of Volcanoes
Fig. 5.1 Pipkin Trent, 2001 similar to figure
in preface, Decker Decker, 1998
36
The Mid-Ocean Ridge SystemDivergent Boundary
Oceanic-Oceanic
Dynamic Earth http//pubs.usgs.gov/publications/t
ext/baseball.html
37
Where is Magma Erupted?

• a. Mid-Ocean Ridges
• Greatest volume of erupted magma, basaltic
• b. Subduction Zones
• Slab sinks and water migrates into overlying
  mantle
• Lowers melting temperature of rocks so that they
  melt
• Ocean-ocean subduction zones -gt island arcs
• Oceanic-continental subduction zones -gt long,
  narrow mountain ranges
• c. Within a Tectonic Plate
• Hot mantle plumes (i.e. hot spots), perhaps
  rising from core-mantle boundary
• e.g., Hawaii, Iceland

38
Divergent Plate Boundaries
Mid-Ocean Ridges (a.k.a. Spreading Centers) new
oceanic crust is created
Continental Rifts
May also contain central rift valley and fault
blocks
39
Convergent Plate BoundariesSubduction Zones
Oceanic vs. Oceanic

• Narrow mountain belts with volcanoes.
• Since they are detached from any continent, these
  mountain belts are called island arcs (e.g. a
  narrow linear chain of volcanic islands).
• The oldest, coldest, densest slab subducts

Figure in This Dynamic Earth (http//pubs.usgs.gov
/publications/text/historical.html)
40
Convergent Plate BoundariesSubduction Zones
Oceanic vs. Continental
narrow mountain belts w/ volcanoes
Lithosphere
trench
Lithosphere

• oceanic lithosphere is denser so it subducts
  (i.e., is recycled into the Earths mantle)

This Dynamic Earth (http//pubs.usgs.gov/publicati
ons/text/historical.html)
41
Hot SpotsLocations of Intraplate Volcanism

• Formed by rising plumes of hot mantle material
• As the tectonic plate moves over these stationary
  plumes, a line of volcanoes is formed
• Only the volcanoes immediately above the plume
  are active
• Volcanoes that are farther away from the plume
  are older
• Seamount undersea volcanic edifice

Lithosphere
Asthenosphere and Mantle
Core-Mantle Boundary
42
Hot SpotsFormation of Hot Spot Tracks

• Plumes are stationary for millions of years
• Provide a continuous source of magma
• As the tectonic plate moves over stationary
  mantle plumes, a line of volcanoes is formed
• Only the volcanoes immediately above the plume
  are active
• As volcanoes move away from the plume they become
  extinct and the ocean starts to erode them

43
Hot SpotsWorldwide Locations

• Total number of hot spots still debated

http//pubs.usgs.gov/publications/text/hotspots.ht
ml
44
What is an Explosion?

• Explosion a large-scale, rapid expansion or
  bursting out or forth of gas and other solid
  objects propelled by the explosive blast
• Blast Rapid change in air pressure that
  propagates away from the region of an explosion.
• A sharp jump in pressure is known as a shock wave
  and a slow rise is known as a compression wave.
• A blast wave is produced by an explosion because
  the explosive event displaces the surrounding air
  rapidly.

Mt. St. Helens, May 18, 1980
http//vulcan.wr.usgs.gov/Volcanoes/MSH/Images/may
18_images.html
45
Volcanic BehaviorViscosity

• Viscosity Ability to flow
• The lower the viscosity the more fluid the
  behavior
• Water (low viscosity) flows faster then honey
  (high viscosity)
• Low viscosity magma flow like ice-cream on a hot
  day
• High viscosity magma hardly flows at all
• Higher Temperatures lowers viscosity
• Silica and oxygen content increase the viscosity
• Increased content of minerals (i.e. crystallized
  minerals) increases the viscosity

46
Factors Affecting Magma ExplosivityVolatile
Content

• Volatile Content how much gas is contained in
  the magma
• Pepsi has a higher volatile content than water
• Volatiles include water/steam, carbon dioxide,
  etc.
• Gas content can range from lt 1 (Kilauea) to gt 5
  (Mt. St. Helens) by weight
• The higher the volatile content, the more
  explosive the magma

Mageik Volcano, Alaska
http//volcanoes.usgs.gov/About/What/Monitor/Gas/s
ample.html
47
Volcanic BehaviorVolatile Content

• Volatile
• Dissolved gas contained in the magma
• Solubility in magma increases as pressure
  increases and temperature decreases
• Analogues to a soda pop under pressure by the
  bottle cap.
• When the cap is removed, reducing the pressure
  volatiles (CO2) gas escapes
• As the uncapped bottle warms, more volatiles are
  released (i.e. the soda goes flat)
• In low viscosity magmas gas easily escapes so
  pressure in the magma does not build up leading
  to non-explosive eruptions
• In high viscosity magmas gas becomes trapped in
  the magma causing pressures to increase.
• When the pressure is reduced, the dissolved
  gasses, the gasses expand in volume.
• Because they cannot escape the high viscosity
  magma an explosion results

48
Gas/Fluid MixturesPressure Dependence

• At high pressure, gas molecules are floating
  around in the magma separately from each other.
  The gas is "dissolved" in the magma.
• As the pressure begins to drop (the magma begins
  to rise), the gas molecules start to glom
  together ("nucleate") into clusters.
• When the concentration of molecules in each
  cluster becomes high enough, it becomes an actual
  bubble of gas, surrounded by the magma.
• The bubbles continue to grow in size, making up
  an ever-increasing percentage of the total
  volume.
• When the bubbles comprise 75 of the total
  volume, the magma disrupts (a.k.a. fragments)
  into a spray of magma blobs (which, when they are
  erupted become pyroclasts) surrounded by a stream
  of gas.

Magma
Magma
Gas
Gas
Magma
Decreasing Pressure Decreasing Depth
Dissolved Gas Molecules
49
How a Volcano Erupts

• Eruptions expel internal heat
• Magma generated by melting of existing rock
• Lowering the pressure
• Raising the temperature
• Increasing the water content
• As solid, mobile hot rock rises upward,
  experiencing decreasing pressures
• As pressure decreases, rock spontaneously melts
  with no increase in temperature (called
  decompression melting)
• Ready-to-melt rocks exist in asthenosphere
• Melted rock increases in volume as it moves
  upward, creating fractures in the overlying rock
• Eventually rising rock liquefies into magma which
  in turn causes more rock to melt
• As the magma rises towards the surface, pressure
  continually decreases and volatiles come out of
  solution to form gas bubbles that expand with
  decreasing pressure

From Natural Disasters, pg. 157-158
50
Formation of Explosive EruptionsBubble Formation
(a.k.a. vesiculation)

• As the pressure decreases as magma rises to the
  surface, bubbles form when volatiles initially
  dissolved in the magma come out of solution to
  form gas bubbles. This process is called
  vesiculation.
• As the magma rises further, the gas bubbles
  become more numerous and, and if the magma is
  fluid enough, existing bubbles also grow larger
  and escape as the magma as it rises

51
Formation of Explosive EruptionsBubble
Fragmentation

• In viscous magmas, the pressure inside the
  bubbles becomes so great that they burst (a.k.a.
  fragment) allowing the gas inside the bubbles to
  suddenly and catastrophically expand
• This sudden expansion of gas is what propels
  explosive eruptions (gas thrust region)
• Once volatiles are expended in main explosive
  eruption, thick viscous lava flows may follow
• Results in alternating ash and lava layers
  observed at stratovolcanoes

52
Factors Affecting Magma ExplosivitySuddenness of
Pressure Release

• Suddenness of the pressure surrounding a magma
  body is changed
• If magma rises to the surface slowly over months
  to years, gasses can escape slowly through cracks
  in the rock
• If magma suddenly uncorked over minutes to
  hours the shallow magma and superheated
  groundwater will expand explosively
• Explosion driven by rapid expansion of volatile
  gasses

53
Its All in the Bubbles

• The key to avoiding an explosive eruption is
• Allowing the volatiles to escape
• Allowing gas bubbles to expand as the pressure
  deceases as the magma approaches the surface
• High viscosity magmas with high volatile contents
  are more explosive because volatile gasses
  contained in the magma
• As magma rises, dissolved gasses come out of
  solution to form bubbles these bubbles cannot
  expand as the magma rises (pressure decreases) to
  the surface
• Because the fluid cannot flow to allow the gas
  bubbles to expand, the gas bubbles remain small
  as the magma approaches the surface
• Cannot rise to the surface of the magma body and
  escape
• Analogy As compared to water, an air bubble
  rises to the surface much slower in a bottle of
  honey
• When the magma does reach the surface, the
  bubbles are no longer confined. The pressure
  inside the bubble drops instantaneously and it
  explodes/fragments
• Analogy uncorking a shaken a champagne bottle

54
Volcanic BehaviorSummary I
Determine eruptive style
The 3 Vs can help you define eruptive styles and
landforms
Makes magma more viscous
Makes magma less viscous
.i.e. Volatile Content
Determines eruptive style
From Natural Disasters, Table 6.5
55
Types of Volcanic EruptionsEffusive
(non-explosive)Oceanic
http//www.pmel.noaa.gov/vents/coax/coax.html

• Spreading centers ideal locations for volcanism
• Above high-temperature asthenosphere
• Magma is high-temperature, low-volatile content,
  low viscosity, and low SiO2
• Oceanic plates pull away from one another such
  the magma rises at low pressure

CoAxial Segment, Juan de Fuca Ridge
Mid-ocean ridge segment off Pacific northwest
coast (colors denote elevation)
56
Mid-Ocean Ridge VolcanismUnderwater Eruptions

• Pillow Basalts
• Because of the pressure of overlying water,
  undersea volcanoes often do not erupt explosively
• Instead lava squeezes out like toothpaste through
  the thin skin of the flow
• The surface of the exposed lava quickly
  solidifies, forming a pillow like structure

57
Types of Volcanic EruptionsEffusive
(non-explosive)Oceanic

• Form pillow lava as molten rock flows into the
  ocean and rapidly cools

Photograph by Gordon Tribble and courtesy of U.S.
Geological Survey.
http//volcano.und.nodak.edu/vwdocs/vwlessons/lava
/part1.html
Pillow lava. Photograph courtesy of U.S.
Geological Survey
58
Types of Volcanic EruptionsEffusive
(non-explosive)Continental

• May initiate with short-lived episode of lava
  fountaining (10-500 meters high)
• Dominated by gentle outpourings of lava onto the
  ground, usually from a central vent or conduit
• Eruptions often long-lived (days, weeks, years)

Lava lake at Puu O o Cinder Cone, Kilauea
Volcano, Hawaii
http//hvo.wr.usgs.gov/gallery/kilauea/erupt/1986t
o1991.html
59
Types of LavaPahoehoe
Both from http//volcanoes.usgs.gov/Products/Pglos
sary/
Ropy Pahoehoe

• Smooth, hummocky, or ropy texture
• Typically advances as a series of small lobes and
  toes that continually break out from cooled crust
• Very fluid (i.e., not very viscous)

Pahoehoe toe
Kilauea Volcano, Hawaii
60
Types of LavaAa

• Rough, rubbly surface composed of broken lava
  blocks called clinkers
• Aa flows composed of
• Spiny, irregular surface
• Massive, dense inner core
• Viscous
• Difficult and slow to traverse once solidified

Kilauea Volcano, Hawaii
http//volcanoes.usgs.gov/Products/Pglossary/