Download Tectonics III - MSU Billings

Question 13
a. Long columns of hot, less dense rock, rising from deep in the mantle
which are responsible for the volcanism at mid-ocean ridge spreading
zones such as the Mid-Atlantic Ridge
b. Long columns of hot, less dense rock, rising from deep in the mantle
and usually erupting in the middle of oceanic and continental crust, and
occasionally at mid-ocean ridge spreading zones
c. Long columns of hot, less dense rock, rising from deep in the mantle
and resulting in regional uplift at the Earth’s surface
d. Long columns of hot, less dense rock, rising from deep in the mantle
and responsible for about 10% of the Earth’s total heat loss
e. B, C and D are all correct
Convergent Boundaries
• Zones where lithospheric plates collide
– Three major types
• Ocean - Ocean
• Ocean - Continent
• Continent - Continent
– Direction and rate of plate motion influence
final character
Convergent Boundaries
• Convergent boundaries may form
subduction zones
– Occurs in oceanic crust
– Associated with outer swell, trench &
forearc, magmatic arc, and backarc basin
– Associated earthquakes range from shallow
to deep
Convergent Boundaries
• Crustal deformation is common
– Melange produced at subduction zone
– Continental collisions involve strong
horizontal compression
– Extension & compression in backarc basin
and the swell
Convergent Boundaries
• Magma is generated
– Subduction and partial melting of oceanic
crust, sediments and surrounding mantle
• Produces andesitic magma
– Continental convergence produces silicic
magmas from melting of lower portions of
thickened continental crust
Convergent Boundaries
• Metamorphism occurs in broad belts
– Metamorphism is associated with high
pressure from horizontal compression
– High temperature metamorphism may
occur in association with magmas
• Continents grow by addition
Plate Buoyancy
• Processes at convergent margins
influenced by plate density
– Sharp contrast in density of oceanic and
continental crust
– Differences in thickness change density
• Thick oceanic crust forms less dense
lithospheric plate
– Temperature & age also affect density:
• Older oceanic crust is usually colder
1. Ocean-Ocean Convergence
• One plate (oldest and coldest) thrust
under to form subduction zone
– Subducted plate is heated, magma
generated
– Andesitic volcanism forms island arc
– Broad belts of crustal warping occur
Ocean-Ocean convergence
Island Arc Magmatism
• Volcanic islands form arcuate chain
– ~ 100 km from trench
– High heat flow & magma production
– Build large composite volcanoes
• Andesite with some rhyolite
– Volcanoes built on oceanic crust &
metamorphic rocks
– Volcanoes tend to be evenly spaced
The Aleutian Island Chain
Seismic activity in the Aleutian Islands
2. Ocean-Continent Convergence
• Oceanic plate thrust under to form
subduction zone
– Subducted plate is heated, magma
generated
– Andesitic volcanism forms continental
arc
– Broad belts of crustal warping occur
including folded mountain belts
Ocean-Continent convergence
Continental Arc Magmatism
• Volcanic islands form arcuate chain
– ~ 100 - 200 km from trench
– Build large composite volcanoes
• Andesite with some rhyolite
– Plutons of granite & diorite
– Volcanoes built on older igneous &
metamorphic rocks
– Volcanoes tend to be evenly spaced
Fig. 21.19. Magma production at subduction zones
Magma Generation
• Characteristically andesite in composition
– Contain more water and gases than basalt
• Results in more violent volcanism
– Water in slab is released, induces melting of
overlying mantle
• Water lowers mineral melting points
Magma Generation
• Hybrid magma rises & interacts with
crust
– Magma has oceanic crust, sediment mantle
and overlying crust components
– Fractional crystallization enriches the
magma is silica
Magma Generation
• Smaller volumes of granitic magma are
produced at continental collisions
– Melting is induced by deep burial of crust
– Melt forms from partial melting of
metamorphic rocks
– Granites have distinct geochemistry and
include several rare minerals
Mt. Vesuvius
The people of Pompeii; mummified in 5-8 meters of hot ash
in A.D. 79
The smoldering city of Pierre, Martinique
movies
PyroclasticFlow.mov
Redflow.mpg
Thermal Structure of Subduction
• Cold slab
– Cold subducting plate heats very slowly
– Temperature at 150 km
• Cold slab ~ 400oC
• Surrounding mantle ~ 1200oC
– Temperature variation influences slab
behavior
• More brittle & stronger
• Moves downward as coherent slab
Thermal Structure of Subduction
• Hot Arc
– Heat flow is elevated beneath volcanic arc
– Ascending magma carries heat from mantle
– Subducting plate may cause mixing in the
asthenosphere beneath the arc
Thermal structure of subduction zone
Plate Motion
• Direction & rate of plate motion are
important factors in plate dynamics
– Head on collisions form large subduction
zones with intense compression and
igneous activity
– Oblique angle collisions are less energetic
and have smaller subduction zones
Earthquakes - Subduction Zones
• Subducting slab forms inclined seismic
zone
– Angle of plunge between 40-60o
– Reaches depths of > 600 km
– Shallow quakes in broad zone from shearing
of two plates
– Deeper quakes occur within slab
Compression at Subduction Zones
• Unconsolidated sediments form
accretionary wedge
– Sediments scraped off of subducting plate
– Folds of various sizes formed
• Fold axes parallel to trench
– Thrust faulting & metamorphism occur
– Growing mass tends to collapse
Compression at Subduction Zones
• Melange is a complex mixture of rock
types
– Includes metamorphosed sediments and
fragments of seamounts & oceanic crust
• Not all sediment is scraped off
– 20-60% carried down with subducting slab
Compression at Subduction Zones
• Orogenic belts are created at ocean continent margins
– Pronounced folding and thrust faulting
– Granitic plutons develop, add to
deformation
– Rapid uplift creates abundant erosion
Fig. 20.13.
The San Andreas
transform fault
system
Juan de Fuca Plate
Oregon/Washington
Cascades/Olympics
Idaho
Rockies
Fig. 21.13. Structure of western NA
Montana
3. Continent-Continent Convergence
• One plate thrust over the other
– No subduction zone & associate warping
occurs
– Folded mountain belt forms at suture of
two continental masses
– Orogenic metamorphism occurs with
generation of granitic magmas
Continent-Continent convergence
Compression in Continent Collisions
• Accretionary wedge and magmatic arc
remnants included in orogenic belt
• Continental collision thickens crust
– Tight folds and thrust faulting
– Possible intrusion of granitic plutons
• Substantial uplift associated with erosion
C onv marg.swf
Formation of Himalaya Mountains
Major units in an ophiolite sequence
Extension at Convergent Boundaries
• Extension may be common at
convergent boundaries
– Warping of crust creates extensional
stress
– Extreme extension creates rifting and
formation of new oceanic crust
– Influenced by angle of subduction &
absolute motion of overriding plate
Metamorphism
• Metamorphism driven by changes in
environment
– Tectonic & magmatic processes at
convergent margins create changes in P & T
– Paired metamorphic belts are commonly
associated with subduction zones
Fig. 21.26. Paired metamorphic belts
Metamorphism
• Outer metamorphic belt forms in
accretionary wedge
– Blueschist facies metamorphism
• High P - low T
– Metamorphosed rocks brought back to
surface by faulting
• Include chunks of oceanic crust and
serpentine
Metamorphism
• Inner metamorphic belt forms near
magmatic arc
– High T and varying P conditions
– Contact metamorphism occurs near
magma bodies
– Orogenic metamorphism occurs in
broader area
– Greenschist and amphibolite grade
Regional Metamorphism:
Subduction zones …..
Change in metamorphic grade with depth
Formation of Continental Crust
• Continental crust grows by accretion
– New material introduced by arc magmatism
– Older crust is strongly deformed
– New crust is enriched in silica & is less
dense
– No longer subject to subduction
Accreted Terranes
• Continental margins contain fragments
of other crustal blocks
– Each block is a distinctive terrane with its
own geologic history
• Formation may be unrelated to current
associated continent
– Blocks are separated by faults
• Mostly strike-slip
Accreted terranes
along convergent
margin
Continental Growth Rates
• Basement ages in NA form concentric
rings of outward decreasing age
– Each province represents of series of
mountain building events
– Rate varies over geologic time
• Slow rate during early history - some crust
may have been swept back into mantle
• Rapid growth between 3.5 and 1.5 bya
• Subsequent growth slower
North
American
Craton
Shield
Western North
American Mobile
Belt
Platform
Eastern Nor
American
Mobile Bel
Growth of North American Continent
Volcanism associated with Plumes,
i.e., Hot Spots
Hotspots
• Long, vertical columns of hot magma
– First evidenced in Hawaii
– Shield volcanoes not associated with other
tectonic activity
– Age of islands get progressively older
– Similar trends seen in other linear island
chains
Hotspot Characteristics
• Distribution is linear
– Produces submarine volcanoes
• Some become islands
– Lithosphere moves over mantle plume
• One volcano becomes dormant, a new one
develops
– Magma generation is in the lower mantle
• At least 700 km, maybe at mantle base
Evidence of Mantle Plumes
• Evidence is indirect
– Local zones of high heat flow
– Hotspots do not drift with plate
movement
– Geochemistry of basalt is distinct
• Deep mantle source
– Oceanic islands associated with swells
– Seismic studies
Evolution of Mantle Plumes
• Plumes are a form of convection
– Less dense material at base of mantle
– Less dense material begins to rise
• Diapirs
– Starting plume enlarges
• Large bulbous head grows
• Narrow tail feeds material upward
Fig. 22.6. Plume evolution and geometry
Evolution of Mantle Plumes
• Rising plume swells lithosphere
– Plume rises and spreads beneath
lithosphere
– Reduced pressure allows magma
generation
– Rifting provides conduits for magma
– Most of plume head cools
– Tail may continue to feed new material
Fig. 22.8. A starting plume
Making Magma
• Magma is generated by
decompression melting
– Lower pressure allows material to
partially melt
• Similar process at midocean ridges
• Occurs at ~ 100 km deep
• Less dense magma continues to rise
• Source of material controls geochemistry
Fig. 22.9. Decompression melting
Making Magma
• Source of magma appears to be mantle
material contaminated with ancient
oceanic crust and sediments
– Cold oceanic crust is metamorphosed
during subduction
– Resulting material is very dense
– Dense material sinks to base of mantle
Mantle
Composition of different
basalts
Mantle Plumes - Oceanic
• Tail plumes create island chains
– Large shield volcanoes produced over
plume tails
• Quiet flows of basaltic lava
• Collapse caldera forms at summit
• Vertical tectonic processes from high heat flow
and weight of volcano
Mantle Plumes - Oceanic
• Starting plumes generate flood basalts
– Broad oceanic plateaus
– Extremely large volcanic event
• Oceanic crust increased in thickness by up
to 5x
• No large shield volcanoes
• Magnetic stripes hidden
• Eruption rate similar to all of ridge system
Hawaiian Plume
• Best example of still-rising tail plume
– Hawaii is active portion of chain of islands
• Remaining islands are extinct volcanoes
• Most are now below sea level
• Hawaii has 2 active volcanoes
–Mauna Loa & Kilauea
Fig. 22.5. Formation of island chain
Fig. 22.1. Hawaiian Island chain
http://www.ngdc.noaa.gov/mgg/image/2minrelief.html
Hawaiian Volcanism
• Volcanism dominated by basalt
– Partial melting of mantle material
– Low water and volatiles content compared
to subduction zone basalts
– Few andesites or rhyolites
• No continental crust component
– Eruptions commonly form along fissures
Fig. 22.16. Evolution of a volcanic island
http://www.youtube.com/watch?v=488BkTUsMa4&feature=related
Size comparison of various volcanic features
Hawaiian Earthquakes
• Earthquakes are relatively small and
infrequent
– Most are shallow associated with magma
movement or slumping
• Usually magnitude 4.5 or less
– Some quakes form in the mantle
• May be larger, up to 6.2 magnitude
Mantle Plumes - Midocean Ridges
• Plumes may form coincident with
midocean ridge system
– Iceland formed at intersection of ridge
and hotspot tail plume
• Combination produced island
– Basalt geochemistry shows mixing
• Rhyolite forms as basalt partially melts
– Plume may have assisted in initial rifting
Mantle Plumes - Continents
• Plumes beneath continents create
regional uplift and bimodal volcanism
– Lithosphere gently warps from rising plume
– Flood basalts erupted
– Rhyolite forms from melting of crust
– May initiate continental rifting
Yellowstone Plume
• Yellowstone plume has evolved from
head to tail stage
– Starting plume produced Columbia River
flood basalts
– Uplift created rifting in Nevada
– NA has moved SW over plume
– Tail plume forms Yellowstone volcanics
and geysers
Fig. 22.21. Cenozoic features of NW U.S.
Fig. 22.23b. Cross section of Yellowstone plume
Continental Rifting
• Rifting may be initiated by mantle
plumes
– Rising starting plume spreads out beneath
continental lithosphere
– Buoyant plume domes lithosphere
– Extension may lead to rift development
– Etendeka and Parana basalt provinces
Continental Rifting
• Plumes do not always cause rifting
– Major mantle plumes produce
continental flood basalts
– Rifting occurs in an intraplate
environment on a plate already in
motion
• Siberian flood basalts - latest Paleozoic
• Lake Superior - Precambrian
• Yellowstone
Flood basalts with several thick and
thin layers. Each layer represents a separate eruption.
Plumes, Climate & Extinction
• Mantle plumes may affect Earth’s climate
and magnetic field
– Starting plumes create enormous amount
of volcanism over short time period
• May change composition and circulation in
ocean and atmosphere
• Large volumes of volcanic gases produced,
including CO2
Plumes, Climate & Extinction
• Flood basalts may be correlated with
climate change and extinction events
– Ontong-Java Plateau - Cretaceous
warming
– Deccan Plateau - Cretaceous Tertiary
boundary
– Siberian flood basalts - late Paleozoic
extinctions
Plumes, Climate & Extinction
• Plume events may correlate with polarity
reversals
– Large number of plume events correlates
with decreased polarity changes during
Cretaceous
– Plume events may remove heat from outer
core area, slowing convection
Evolution of Seamounts & Islands
• Grow by extrusion of lava at various points
of their surface - mostly subaqueous
– Intrusive features also form
• Base subsides as volcano grows upward
• Submarine mass movements, mainly along
faults
• Islands subject to subaerial erosion
• Subsidence occurs away from hotspot