Subaerial Mafic Lava Flows
Mafic Lava Flows
• Most common volcanic feature on earth
• Outpourings of molten rock
• Most silicate compositions erupt between
800-1200 degrees centigrade
• Rare exceptions- Komatiites, sulfur flows,
carbonate flows
Morphology and Extent
• Erupted from large, extensive fissuresFlood Basalts
• Fissures associated with shield volcanoes
• Fissures associated with composite
volcanoes-Andesites
Length and Morphology
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Effusion Rate-Most Important for Lithology
Volume-Important for length
Slope
Physical Properties-temperature, volatile
content, composition
Duration typically days to months-Kilaueu- years?
Most can be out walked, exceptions- Hawaii and Nyiragongo
Lava Flows
• Most lavas are
crystallizing on eruption
• Heat loss to ground and
air
• As a result flows form
either channels or tubes
• This concentrates motion
along a small number of
paths
• Flow initially forms a tube or a channel
according to whether or not the top can
develop a continuous crust
• Air chills top of flow to strong, solid crust
within minutes of eruption
• Same time new crust pulled forward by
more mobile lava below
• If forward pull of
mobile lava is large
enough the crust will
continuously break
into fragments- open
channel-Aa, Blocky
• If forward pull is small
continuous crust can
develop across whole
flow-tube formsPahoehoe
Flow Fronts
• When disruption
dominates (channels)
front moves forward
as a single unit
• When crust
dominates front
moves forward by
oozing small tongues
or toes of lava
through fractures in
the crust
Kinds of Mafic Lava
• Classification based on appearance of
flow crust
• Three types
• Aa- SiO2 < 55%
• Pahoehoe- SiO2 < 55%
• Blocky- SiO2 > 55%
Aa Lava
• Surface covered by a jumble of angular fragments
• Rare for one part of front to move far ahead of neighboring sections
• Crumble at front to form a “snout” of angular debris
• Flow creeps forward by collapse of flow front
• Fronts 20m or less high
• Lengths in 10’s of kms
• Tend to form on slopes
• Vesicles contorted and deformed
• can’t walk on
thin flows (<3m)
Advance slower than aa so
Surface able to resist constant
tearing and fragmentation
Pahoehoe can turn into Aa
Down slope
Less viscous than aa and advanc
as tongues or toes-interiors easily
restrained by surface crust
Smooth, rolling upper surface
Ropy or corded surface
Shelly surface- highly vesicular
and fragile-like walking on egg
Shells
Sheet flood pahoehoe-highly mobile
flows that advance as a coherent unit
with a smooth, rolling motionplateau lavas
Tumuli: small, dome-shaped structures on Pahoehoe or Aa lava flows.
Oval in ground plan, 150 feet in length and 25-30 feet high. Form where crust of
the lava has been heaved up into domal structures. Lava then rises up through
the cracked crust and dribbles down side of the dome.
Hornitos (driblet spires):
Conelets of agglutinate
Agglutinate :
Accumulation of flattened,
Plastic fragments-from gas
Charged, spattering lava
Blocky Lavas
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Similar to AA but:
More Viscous than Pahoehoe or AA
Shorter flows
Covered by angular fragments
More siliceous-Andesite
Pahoehoe-Round vesicles or amygdules
Aa and Blocky- angular and contorted
Up to 50% of flow- increase in % up through
flow unless very thin. Can make top look like a pincushion
Thicker flows- massive center, vesicular top and bottom
Spiricle into vesicle
cylinder
Pipe Vesicle
Vesicle Cylinder
Spiricle
Jointing:
In flows due to cooling
And shrinkage of the lava.
This cuases it to fissure.
Jts form at right angles to
The cooling surface
Thin flows- right angles to each other
Thick flows jts tend to develop in
3 directions at 60 to each
other
Basalt Flows
• Erupted from large extensive fissures to
form flood basalts (Columbia River, midocean ridges
• Or from fissures associated with central
volcanic complexes such as shield
volcanoes
• Flood lavas- extensive and voluminous
• Roza Member->300 km long, volume >
than 1500 km3- 3 times larger than most of
the worlds largest stratavolcanoes!
• Recent work has shown that some of the
Deccan Trap lavas are over 700 miles
long! Volume > than 5000 km3
Fissure-fed from shield volcanoes-much less voluminous,
cover smaller areas. Flank eruptions at Mauna Loa rarely
exceed 0.5 km3