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In addition
to effusion rate, lava morphology is also related to the specific sequence
of events during an individual eruption. The steps that a lava flow goes
through during its emplacement (advance, ponding, inflation, drainout,
collapse, etc) directly create the lava forms that remain to be observed
when the eruption is over. The following is a step-by-step account of
the interpreted sequence of events that created the morphologies we see
on the 1998 lava flow.
(1) When
lava first emerged from the 1998 eruptive fissure, it initially advanced
as a thin lobate sheet flow, only 20-30 cm thick.
(2) The lava spread out until it became laterally confined by its own
frozen margins. At this point lava flow inflation began (the uplift of
the upper solidified crust by the still-molten interior of the flow).
Basically, once the flow stopped spreading laterally, it had to start
thickening to accommodate the lava that was still being supplied by the
eruptive vent. Lava pillars formed during lava flow inflation (but were
still hidden at this stage, beneath the upper crust within the flow interior).
(3) The
sheet flow continued to inflate upward until it was 3-5 meters thick.
The morphology of the upper crust of the sheet flow at this stage was
lobate everywhere.
(4) Next, the eruption began to wane and the lava supply from the vents
stopped. For a brief period, the lava flow stood at its maximum thickness
before drain-out and collapse began.
(5) Once
the eruption stopped, subsidence of the molten flow interior began due
to lava drain-back. This removed support for the upper solid crust of
the sheet flow causing it to collapse (mostly near the middle of the flow
where it was thickest and where the crust was largely unsupported from
below). After collapse, lava pillars were exposed within the collapse
areas. Pillar tops and the uncollapsed margins of the flow preserve the
initial lobate morphology.
(6) When
the upper crust collapsed, the molten flow interior was suddenly exposed
to seawater, and new crust formed on that subsiding lava surface. This
new crust formed ropy, lineated, or jumbled morphologies within collapse
areas during the drain-back stage.
(7) This
whole sequence of events (spread of thin lava flow, inflation, drainout
and collapse) occurred very rapidly, within about 2 hours. The morphology
of the 1998 lava flow reflects this specific sequence of events: lobate
lava morphology is found on the uncollapsed margins of the flow, and ropy,
lineated, and jumbled morphologies are found in the floors of collapse
areas. Pillow lavas are found only along the outer margins of the flow,
where lobate lava advance was halted by freezing of the lava.
Be sure
to see the animation of this sequence of
events.
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