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Geological Sciences 107
Our Dynamic Planet
PLANET EARTH IS A DYNAMIC EVOLVING SYSTEM FROM 4.6 BILLION YEARS AGO TO NOW
STRUCTURE & COMPOSITION REFLECT ONGOING EVOLUTION
EVOLUTION DEPENDS ON HOW HEAT- "THE GEOLOGICAL LIFEBLOOD OF
PLANETS" - TRANSFERRED OUT OF COOLING EARTH BY THERMAL
CONVECTION (Hot stuff rises)
THERMAL CONVECTION CAUSES PLATE TECTONICS: PLATES OF EARTH'S
SURFACE MOVE RELATIVE TO EACH OTHER AT A FEW mm/yr
(about speed fingernails grow)
PLATE MOTIONS CAUSE EARTHQUAKES, VOLCANOS, MOUNTAIN BUILDING
AT PLATE BOUNDARIES
PLATE TECTONICS MAKES EARTH WHAT IT IS - DIFFERENT FROM
NEIGHBOR PLANETS (Mars, Venus, & Earth: 3 bears principle)
PLATE MOTIONS ARE CRUCIAL FOR THE ORIGIN OF LIFE, ITS SURVIVAL,
AND OUR CLIMATE
PLATE MOTIONS PROVIDE RESOURCES AS WELL AS HAZARDS TO SOCIETY
"CIVILIZATION EXISTS BY GEOLOGICAL CONSENT"
EARTH- A LAYERED PLANET DUE TO ITS EVOLUTION
THE PLATES
(= LITHOSPHERE)
ARE THE COLD,
STRONG OUTER
BOUNDARY LAYER
("thin scum": 100 /
6371 km)
MOVING OVER
WEAKER
ASTHENOSPHERE
Earth’s surface divided up into about a dozen major lithospheric plates
Three Types of Plate Boundaries (Margins)
NORMAL FAULTING
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Region under extension
Wasatch Fault
Salt Lake City,
Utah
REVERSE
FAULTING
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Region under compression
STRIKE SLIP FAULTING
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Offset fence showing 3.5 m of leftlateral strike-slip motion on San
Andreas fault in 1906 San Francisco
earthquake
Western North America: Three Major Plate Boundaries
CONVERGENT (SUBDUCTION)-
2001 NISQUALLY, WASHINGTON EARTHQUAKE (M 6.8, $2B damage)
Within subducting Juan de Fuca plate
Kirby et al., 1996
Intraslab earthquakes
now recognized hazard
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CASCADE VOLCANOES
INDICATE JUAN DE FUCA
PLATE SUBDUCTION
BENEATH NORTH
AMERICA
Mt. Mt
St.Saint
Helens:
Helens
May1980
18, eruption
1980
USGS
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Mount St. Helens as part of the new dome collapses.
USDA Forest Service photograph by taken moments
after a 3.2 magnitude earthquake triggered the event
on July 18, 2005
www.fs.fed.us/gpnf/volcanocams/msh
TRANSFORM- San Andreas Fault System:
Motion Between North American & Pacific Plates
San Andreas LakeSan Francisco Bay
Area
1906 SAN FRANCISCO
EARTHQUAKE (Magnitude
7.8)
~ 4 m of slip on 450 km of
San Andreas Fault
~2500 deaths, ~28,000
buildings destroyed (most
by fire)
San Andreas Fault- Carrizo Plane, California
San Andreas Fault- Palmdale, California
DIVERGENT: Seafloor Spreading in the Gulf of California
Plate Boundaries in Western North America Evolve Over Time
Western North America Tectonics- The Movie
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ICELAND - Part of the Mid-Atlantic Ridge
- Made of Recently Erupted Basalt
- Formed in past 15 million years
North
American
plate
20 mm/yr
Thingvellir
Eurasian
plate
1973 Eruption on the island of
Heimaey, Iceland
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East Africa Rift - New Spreading Center Forms by
Rifting Continental Crust
Africa is splitting up into Nubia (West Africa) and
Somalia (East Africa)
2001 Eruption near Goma, Congo
Along the East African Rift
DECEMBER 2004 INDIAN OCEAN EARTHQUAKE
AND TSUNAMI
INDIAN PLATE MOVES NORTH COLLIDING WITH EURASIA
COMPLEX PLATE
BOUNDARY ZONE
IN SOUTHEAST
ASIA
Northward motion of India
deforms large region
Many small plates
(microplates) and blocks
Molnar & Tapponier, 1977
India subducts
beneath Burma
microplate
Earthquakes occur
at plate interface
along the Sumatra
arc (Sunda trench)
Largest & most
destructive
earthquakes results
from many years of
accumulated motion
BETWEEN
EARTHQUAKES:
SUMATRA TRENCH
BURMA
INDIA
India subducts beneath
Burma at about 20 mm/yr
Fault interface is locked
Tsunami generated
EARTHQUAKE:
Fault interface slips,
overriding plate
rebounds, releasing
accumulated motion and
generating tsunami
Stein & Wysession, 2003
HOW OFTEN:
Fault slipped ~ 10 m -->
10000 mm / 20 mm/yr = 500 yr
Faults aren’t exactly periodic - intervals vary
Crucial for assessing hazards
Earthquakes rupture a patch
along fault's surface.
Generally speaking, the
larger the rupture patch, the
larger the earthquake
magnitude.
Initial estimates from the
aftershock distribution show
the magnitude 9.0 SumatraAndaman Islands Earthquake
ruptured a patch of fault
roughly the size of California,
and modeling seismic waves
show that most of the slip
occurred in the southern 400
km of the patch.
For comparison, a magnitude
5 earthquake would rupture
a patch roughly the size of
New York City's Central Park.
MAGNITUDE 9 EARTHQUAKES ARE INFREQUENT
Stein & Wysession after IRIS
TSUNAMI - water wave generated by earthquakes,
volcanic eruptions & landslides
NY Times
TSUNAMI GENERATED ALONG FAULT, WHERE SEA
FLOOR DISPLACED, AND SPREADS OUTWARD
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Hyndeman and Wang, 1993
Red - up motion, blue down
http://staff.aist.go.jp/kenji.satake/animation.gif
TSUNAMI SPEED IN
DEEP WATER of
depth d
c = (gd)1/2
g = 9.8 m/s2 d = 4000 m
c = 200 m/s = 720 km/hr
= 450 m/hr
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Tsunami generated
along fault, where sea
floor displaced, and
spreads outward
Reached Sri Lanka in 2
hrs, India in 2-3
http://staff.aist.go.jp/kenji.satake/animation.gif
TSUNAMI WARNING
Deep ocean buoys can measure
wave heights, verify tsunami and
reduce false alarms
Because seismic waves travel much
faster (km/s) than tsunamis, rapid
analysis of seismograms can identify
earthquakes likely to cause major
tsunamis and predict when waves will
arrive
HOWEVER, HARD TO PREDICT EARTHQUAKES
recurrence is highly variable
Sieh et al., 1989
Extend earthquake history
with geologic records paleoseismology
M>7 mean 132 yr s 105 yr
Estimated probability in 30 yrs 7-51%
PLATE TECTONICS IS
DESTRUCTIVE TO HUMAN
SOCIETY BUT CRUCIAL FOR
HUMAN LIFE
Plate boundary volcanism produces
atmospheric gases (carbon dioxide
CO2 ; water H2O) needed to support life
and keep planet warm enough for life
("greenhouse" )
May explain how life evolved on earth
(at midocean ridge hot springs)
Plate tectonics raises continents
above sea level
Plate tectonics produces mineral
resources including fossil fuels
Press & Siever
“CIVILIZATION EXISTS BY
GEOLOGICAL CONSENT
- SUBJECT TO CHANGE
WITHOUT NOTICE”
The same geologic processes
that make our planet
habitable also make it
dangerous