Transcript Lecture 2b: Hot spots

Lecture 2b: Hot spots
• Questions
– Why are there volcanoes in the middle of plates?
– How do such volcanoes grow and evolve?
– What is the connection between hotspots and flood basalts?
• Tools
– Plate tectonics, geochronology, igneous petrology, isotope
geochemistry, etc.
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Hot spot chains
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Hot spots, flood basalts, LIPs
• Chains of volcanoes in the middle of plates, if long-lived and with
an age progression along the chain, are called hot spots.
– There are continual arguments over how many hot spots there are – some are
obvious, some are marginal (usually because it is hard to establish the age
progression). The most common catalogue has ~40, others go over 100.
– Hawaii and Iceland are biggest,
by buoyancy flux and by volume
of volcanism.
Hot spots: correlated with geoid?
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Hot spots: correlated with geoid?
Burke et al, 2008 (10.1016/j.epsl.2007.09.042)
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Hot spots: correlated with geoid?
Burke et al, 2008 (10.1016/j.epsl.2007.09.042)
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Mantle Plumes
The initiation of a new plume is thought
to involve a very large blob of hot
material arriving at the base of the
lithosphere and hence a large episode of
excess volcanism.
This is supported by the association
between many active hotspots and older
continental flood basalts or oceanic
plateaux (collectively, large igneous
provinces or LIPs).
An experimental starting plume (in glucose syrup)
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Flood Basalts and hotspot tracks
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Flood Basalts and hotspot tracks
Today
120 Ma ago
Flood Basalts and hotspot tracks
Burke et al, 2008 (10.1016/j.epsl.2007.09.042)
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Flood Basalts
• Flood basalts are big and erupt very quickly:
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–
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Siberian traps, 2 x 106 km3 within ~1 Ma at 250 Ma (Permian-Triassic)
Deccan traps (India), 106 km3 within ~1 Ma at 65 Ma (K-T)
Columbia River Basalts, 2 x 105 km3 within ~1 Ma at 16 Ma.
It may or not be coincidence that big flood basalt eruptions coincide with
major extinction events in the fossil record!
• Flood basalt petrology and chemistry: there is a general
progression through…
– small volume of early alkali basalt and olivine tholeiite with mantle
isotope signatures and high 3He/4He
– massive volume of quartz tholeiite with isotopic signature of
subcontinental lithosphere
– small late eruptions with wide range of compositions and evidence of
crustal components
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Flood Basalts
• We can think of this sequence as the result of emplacement of a
big thermal anomaly at the base of the continental lithosphere.
– The first products are small degree, high-pressure melts of the plume
itself, that escape quickly.
– Then heat flow, a slow process, raises the temperature of the cold nonconvecting part of the mantle attached to the base of the continent until it
melts over a wide area, in a process that is characterized by positive
feedback between melting and heat flow, giving high magma flux for a
short time.
– Finally, heat from the plume head reaches the crust itself, mostly by
advection of magmas, and some crustal melts occur.
• Oceanic plateaux are basically similar to flood basalts, except
they presumably occur when a plume head comes up under
oceanic lithosphere.
– The biggest on earth is the Ontong-Java plateau, which is really two
oceanic plateaux on top of each other, one 122 Ma and one 90 Ma. At
that time the Pacific plate was hardly moving, and two plume-head like
blobs came up the same conduit and hit the same area of lithosphere.
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Ocean Island Volcanoes
• Mid-plate volcanoes in age-progressive chains are presumably
the product of long-lived plume “tails”.
• They show a sequence driven by motion of new lithosphere over the plume
(rather than arrival of new plume under lithosphere).
• At least at Hawaii, the lifecycle of one volcano is typically divided into four
stages:
• Preshield – low flux of alkali basalt,
erupted submarine, very pure plume
component (high 3He/4He, etc.)
• Shield-building stage – very large
flux and large volume of tholeiite,
progressing towards an upper
mantle/oceanic lithosphere affinity.
• Alkalic capping stage – small flux
of alkali basalts, no steady shallow
magma chamber.
• Posterosional stage – very small
volume of extremely alkalic lavas
that erupt ~2 Ma after end of
capping stage.
Ocean Island Volcanoes
• In addition to characteristic chemistry,
these stages generate characteristic
morphology and structures:
• The pre-shield stage, erupted underwater, is
mostly a big mound of pillow basalts, relatively
steep sided. At present, this stage is only known
from Loihi seamount; its role in other volcanoes
is inferred.
• The main shield stage creates an edifice that
emerges above sea-level.
• Subaerial tholeiite
flows have low
viscosity and long
cooling times and can
travel far down low
slopes, allowing the
volcano to assume the
characteristic shield
shape, perhaps 50 km
in diameter for each 1
km above sea level at
the summit.
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Ocean Islands: Main Shield Stage
 A large summit caldera develops when the roof collapses into a
shallow (<1 km below summit) magma chamber. Most lavas
ascend to this summit magma chamber and degas and
differentiate there, even if they erupt down on the…
 Rift zones that develop when gravitational stresses and push from
intruding dikes break the edifice into three (or two if buttressed
by an older volcano on one flank) sectors. The ongoing Pu`u-O`o
eruption of Kilauea is on Kilauea’s southeast rift zone.
• Occasionally, large sectors of a Hawaiian volcano will fail
catastrophically and produce an enormous landslide, with the
potential to drive km-high tsunamis. There are frequent
earthquakes on shield volcano islands, sometimes on nearly
horizontal faults.
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Ocean Islands: Main Shield Stage
By way of advertisement, when you get to end of our Ph.D.
program, you will have the opportunity to participate in project
Pahoehoe and see a shield volcano for yourself. Here is Caltech
undergrad Laura Elliott lava-dipping in 2004.
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Ocean Islands: Hawaii
• Topography and bathymetry show the shield shapes, summit calderas, rift
zones, Loihi seamount, and sector-collapse landslide deposits.
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Ocean Islands: Post-shield stage
• The alkalic lavas of the post-shield capping stage are smaller in volume and
more viscous. They build a steeper mound on top of the shield (see Mauna
Kea at present), with many near-summit cones, but no major caldera. These
lavas frequently carry xenoliths from the oceanic crust and the cumulate pile
inside the volcano, implying that they pond at the base of the crust and do not
pause at any shallow magma chamber.
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Ocean Islands: Post-erosional stage
• The post-erosional lavas are easily recognized as a series of cinder cones and
explosive craters on top of a major unconformity and soil horizon from 1-2
Ma of erosion and weathering. See Diamond Head on Oahu. These flows may
carry mantle xenoliths, including garnet peridotites, indicating rapid ascent
from mantle depths with no permanent magma chamber at any level.
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Oceanic basalts, conclusion
• In addition to the obvious morphological differences between mid-ocean
ridges and ocean island volcanoes, there are important petrological differences
relating to degree of melting, volatile content, and extent of melting.
• Moreover, there are essential, first-order differences in trace-element ratios
and radiogenic isotope ratios.
– Broadly, MORB is from a depleted and degassed source, presumably the
upper mantle
– OIB sources tend to be less depleted, nearly primitive, or even enriched
relative to bulk earth and show evidence for a primordial noble gas
component, hence they are thought to sample the lower mantle in some
way.
– The existence of distinct isotopic reservoirs in the mantle constitutes the
essential geochemical commentary on issues of whole-mantle vs. layered
mantle convection, a subject on which geophysicists also have opinions.
– We will return to the global geochemical dynamics of the mantle as
expressed through oceanic basalts at the very end of the course.
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