Emplacement of the 1998 lava flow
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.
lava first emerged from the 1998 eruptive fissure, it initially advanced
as a thin lobate sheet flow, only 20-30 cm thick.
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
(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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