Subaerial Mafic Lava Flows
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 fissures-Flood Basalts
• Fissures associated with shield volcanoes• Fissures associated with composite
volcanoes-Andesites
Length and Morphology
• 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 forms-Pahoehoe
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
Smooth, rolling upper surfaceRopy or corded surfaceShelly surface- highly vesicularand fragile-like walking on eggShellsSheet flood pahoehoe-highly mobile flows that advance as a coherent unitwith a smooth, rolling motion-plateau lavas
thin flows (<3m)Advance slower than aa so Surface able to resist constanttearing and fragmentationPahoehoe can turn into AaDown slopeLess viscous than aa and advancas tongues or toes-interiors easilyrestrained by surface crust
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 throughthe cracked crust and dribbles down side of the dome.
Agglutinate :Accumulation of flattened,Plastic fragments-from gasCharged, spattering lava
Hornitos (driblet spires):Conelets of agglutinate
Blocky Lavas
• 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
Spiricle into vesiclecylinder
Vesicle CylinderPipe Vesicle
Jointing:
In flows due to coolingAnd shrinkage of the lava.
This cuases it to fissure.
Jts form at right angles toThe cooling surface
Thin flows- right angles to each other
Thick flows jts tend to develop in3 directions at 60 to eachother
Basalt Flows
• Erupted from large extensive fissures to form flood basalts (Columbia River, mid-ocean 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