11-13 Blocky lava-rings: Collapsed tumuli in volcanic flows

Session: Using Volcanic Deposits to Help Us Understand Volcanic and Magmatic Processes

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Abstract:

Blocky lava-rings in pahoehoe flows, often mistaken for volcanic explosion pits, are produced by deflation of lava-rise tumuli.  Tumuli margins vary from blunt, jointed lava crust to steep, blocky slopes depending on the extent of stretching as the flow inflates.  If inflation stretches the flow margins significantly, the lava crust may fracture enough to allow interior lava to escape along the flow margins. The tumuli are surrounded by radiating lava channels. Leakage of fluid, interior lava causes the lava-rise plateau to deflate, and the surface crust collapses to pre-inflation levels. The interior lava crust adjacent to the flow front is stretched during the deflation and develops a blocky nature similar to the outward facing flow margin. A raised blocky lava-ring is formed.  The rings average 100-200 m in diameter and rise 3-10 m in height. They are intermediate in size between rootless shields and maar craters. The rings may be circular, oval, or irregular in plan-view.  The ring material consists of 0.3-1 m blocks of fractured lava flow crust.  The interior floor is at the same level as the exterior lava flows and is identical to those outside of the ring. The interior lava floor may also be stretched enough to form floor-transecting clefts. The resulting landform is a raised ring of blocky material encompassing lowered lavas that formerly capped the tumuli.  Blocks of the raised ring overlie the lava flows both inside and outside the rings. The Aden flow (182-15.7 ka) in Dona Ana County New Mexico is proposed as a type locality exhibiting a progression of landforms—rootless shields, blocky lava-rings, partially collapsed lava tumuli, and fully developed lava plateaus--that chronicle the stages of blocky ring formation.  Collapsed tumuli are common in Holocene monogenetic volcanic fields that contain lava-rise plateaus and tumuli.  Verification of a collapse tumulus requires determination that the ring is composed of jointed surface lava and not pyroclastic material.  Recognition of collapse tumuli as opposed to explosion pits is important in interpretation of structural control of vent location. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6019

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