ocn 201: mantle plumes and hawaiian volcanoespattern of mantle convection. •downwelling, however,...
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Eric H. De Carlo, OCN201 F2011
OCN 201:
Mantle Plumes and
Hawaiian Volcanoes
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Seamounts and Guyots
• Seamounts: volcanoes formed at or near MOR or at “hot spots”
• Guyots: Submerged seamounts with flat tops
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Seamounts and Guyots…
• Seamounts that form at MOR become inactive and subside with seafloor as they move away from the ridge axis
• Guyots formed from volcanic islands that are planed off at sea level by erosion, then subside as seafloor travels away from the ridge axis
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…Atolls
• Ring shaped islands or coral reefs centered over
submerged, inactive volcanic seamounts
• Corals can only live within the photic zone in the
tropical regions.
• Coral reefs build upward ~1cm/yr
• If volcanic islands sink sufficiently slowly, coral
growth can keep up, producing an atoll
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Darwin’s Theory of Atoll Formation • Fringing reef grows upward around young island
• Barrier reef develops as corals grow upward but subsiding
island is eroded and lagoon forms
• Atoll develops fully as island subsides further, “motu” form
from accretion/consolidation of storm debris at barrier
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Motu on Barrier Reef of Atolls
Rose atoll
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The Darwin Point
• Darwin Point is where atolls “drown” because coral growth can no longer keep up with subsidence
• When temp. becomes too low for coral to grow efficiently…
• Rate of volcanic edifice subsidence becomes greater than (upward) coral growth rate…
• In Hawaii this occurs ~ 29oN (i.e., just N. of Kure Atoll)
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Mantle Plumes or “Hot Spots”
• First
hypothesized
by J. Tuzo
Wilson
(1963) to
explain linear
island chains
in the Pacific
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Mantle Plume or “Hot Spot” Theory
• Proposes that “hot spots” are point sources of
magma that have apparently remained (relatively)
fixed in one spot of the Earth’s mantle for long
periods of time.
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The Hawaiian hot spot currently lies
beneath the Big Island of Hawaii.
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Age of the Hawaiian Islands increases
with distance from Kilauea
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Linear Island Chains: II
• Bend in Hawaiian-Emperor chain reflects change in
direction of motion of Pacific Plate…
• End of chain is ~90 Ma, bend is ~ 40 Ma.
• Change may have resulted from collision of India with
Asia, shutting down 1200 km of subduction zone.
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Eric H. De Carlo, OCN201Sp2010
• Hot spots represent major zones of upwelling in overall pattern of mantle convection.
• Downwelling, however, is not so localized as upwelling that occurs at hot spots…
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Mantle Plumes: II
• Mantle plumes may be derived from near the core-mantle boundary, as demonstrated in this computer simulation from the Minnesota supercomputing lab.
• Note the bulbous plume heads, and the narrow plume tails.
• Plume heads flatten as they impinge on the outer sphere (the base of the lithosphere)
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• Hot spots commonly occur on or near the MOR
(Easter Island, Iceland, St. Helena, Tristan da Cunha).
• This led Morgan (1972) to suggest hot spots play an
important role in driving plate motion…
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• Hot spots near MOR create aseismic ridges that extend
outward from spreading axis (e.g., Tristan da Cunha,
which produced the Walvis Ridge and Rio Grande Rise
in the S. Atlantic).
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• Hot spots also
occur in middle
of oceanic plates
(e.g., Hawaii,
Reunion) and
under continental
crust (e.g.,
Yellowstone).
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Southern Idaho
Trace of the
Yellowstone hot spot:
the Snake River Plain,
Oregon to Wyoming
The Snake River Plain
from space
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The Yellowstone hot spot: a beautiful
interaction between a deep reservoir
and a surface reservoir…
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But massive
eruptions are
dangerous!
(Mt. Pinatubo, Philippines,
1991: 3-5 km3)
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Hot Spots of the World
• Because they remain more or less fixed in mantle,
hot spots lead to linear track of volcanism that can
be used to trace absolute plate motion.
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Large Igneous Provinces
• Initiation of of new mantle plume is thought
to cause formation of a LIP, because head
of the plume is large.
• Similar features are
found on the Moon,
Venus, and Mars.
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Growing Mantle Plume
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Eric H. De Carlo, OCN201Sp2010
Large Igneous Provinces
• On land, LIPs are called “flood basalts” (e.g.,
Columbia River, Deccan Traps in India)
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Eric H. De Carlo, OCN201Sp2010
Large Igneous Provinces
• On the seafloor, LIPs are called “oceanic
plateaus”
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Eric H. De Carlo, OCN201Sp2010
LIP Sizes…
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Eric H. De Carlo, OCN201Sp2010
Flood Basalts: I
Deccan Traps in India (67Ma)
• Several 100 km across
• A few km thick
• Erupted in <1My 2- 8 km3/yr
• Produced Chagos-Laccadive Ridge
and Mascarene Plateau
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Deccan Traps • India migrated to current position over past 67
Ma from East of Madagascar
• Present position of hot spot is Reunion Island
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Eric H. De Carlo, OCN201Sp2010
Flood Basalts: II Siberian Flood Basalts (248Ma) largest on land
• Accompanied by most severe biotic extinction ever
for multicellular life, at the end of Paleozoic
• 95% of marine life vanished
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Columbia River Plateau (17 Ma)
• Original area larger than NY State
• Erupted within 1.5 My
Flood Basalts: III
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CRB Volcanism in Foothills
of (current) Rocky Mountains • Eruptions during Miocene
• Not all gentle lava flows…
• Large ash eruptions also occurred and
buried local area, e.g., Clarkia Lake bed…
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Oceanic Plateaus: I Ontong-Java (122 Ma)
• Largest in the world (36 Mkm3)
• Formed in <3 My 12 km3/yr
• 25 X larger than Deccan
• 2/3 size of Australia
• Plume head (at 5-30% melt): 600-1400 km (up to ½ thickness of mantle)
• Eruption would have raised sea level by ~10 m and raised mean atmospheric T ~7-13oC
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Oceanic Plateaus: II
Kerguelen Plateau
(122Ma)
• Southern Indian Ocean
• 2nd largest in ocean
• Formed within 4.5 My
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Global Effects of LIP
• Raise sea level when erupt on sea floor
• Raise seawater temperature
• Raise atmospheric temperature
• Potentially caused mass biotic extinctions
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Hot Spots: Mantle Plumes?
• Important because they represent:
– Third type of volcanism on Earth (MOR: basalt,
Volcanic Arc: andesite, Hot spot: basalt)
– The major mode of mantle upwelling (focused
point sources)
– A measure of absolute plate motion (motion with
respect to fixed point in mantle rather than
relative to other plates)
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Eric H. De Carlo, OCN201Sp2010
Hot Spots of the World
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HURL, PISCES V
Eric H. De Carlo, OCN201Sp2010
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Lo`ihi
Hydrothermal
Vents (2004)
from PISCES V
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Eric H. De Carlo, OCN201 F2011
Aloha, see you next time
Lo`ihi Volcano