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