M. John Roobol

The Madinah eruption, Saudi Arabia: Magma mixing and simultaneous extrusion of three basaltic chemical types

During a 52-day eruption in 1256 A.D., 0.5 km3 of alkali-olivine basalt was extruded from a 2.25-km-long fissure at the north end of the Harrat Rahat lava field, Saudi Arabia. The eruption produced 6 scoria cones and a lava flow 23 km long that approached the ancient and holy city of Madinah to within 8 km. Three chemical types of basalt are defined by data point clusters on variation diagrams, i.e. the low-K, high-K, and hybrid types. All three erupted simultaneously. Their distribution is delineated in both scoria cones and lava flow units from detailed mapping and a petrochemical study of 135 samples. Six flow units, defined by distinct flow fronts, represent extrusive pulses. The high-K type erupted during all six pulses, the low-K type during the first three, and the hybrid type during the first two.Three mineral assemblages occur out of equilibrium in all three chemical types.Assemblage 1 contains resorbed olivine and clinopyroxene megacrysts and ultramafic microxenoliths (Fo90 + Cr spinel + Cr endiopside) which fractionated within the spinel zone of the mantle.Assemblage 2 contains resorbed plagioclase megacrysts (An60) with olivine inclusions (Fo78) which fractionated in the crust.Assemblage 3 contains microphenocrysts of plagioclase and olivine in a groundmass of the same minerals with late-crystallizing titansalite and titanomagnetite; assemblage 3 crystallized at the surface and/or in the upper crust. Each assemblage represents a distinct range in PTX environment, suggesting that their coexistence in each chemical type may be a function of magma mixing. Such a process is confirmed by variable ratios of incompatible element pairs in a range of analyses.All three chemical types are products of mixing. Some of the hybrid types may have developed from surface mixing of the low-K and high-K lavas; however, the occurrence of all three types at the vent system suggests that subsurface mixing was the dominant process. We suggest that the Madinah flow was extruded from a heterogeneous magma chamber containing vertically stacked sections equivalent to the six eruptive pulses. This chamber may have developed contemporaneously with magma mixing when a crustal reservoir containing a magma in equilibrium with assemblage 2 was invaded by a more primitive magma containing cognate microxenoliths and megacrysts of assemblage 1.

Destruction of St. Pierre, Martinique, by ash-cloud surges, May 8 and 20, 1902

St. Pierre, Martinique, was destroyed by ash-cloud surges on May 8 and 20, 1902, that developed from the tops of block and ash flows derived from Mount Pelee. The dome was small during these eruptions, and eyewitness accounts indicate that the eruptive sequence started with vertical eruption columns followed by pyroclastic flows. The flows were topographically controlled and moved down the valley of the Riviere Blanche. Their associated ash clouds expanded to several times the width of the associated flows, and it was these expanding ash clouds that overwhelmed St. Pierre on May 8 and 20. It is our conclusion that the early nuees ardentes of the 1902 eruption of Mount Pelee were formed by column collapse into the crater and not, as commonly stated, by dome collapse or directed blast. The resultant mass poured through a cleft in the southwest side of Pelee, thereby heading the flows in that direction. It was only the later nuees ardentes that appear to have been directly affected by the presence of a dome.

Distal ash hurricane (pyroclastic density current) deposits from a ca. 2000 yr B.P. Plinian-style eruption of Mount Pelée, Martinique: Distribution, grain-size characteristics, and implications for future hazard

Plinian-style activity has not occurred on any volcano in the Lesser Antilles since European settlement; however, pumice-rich deposits from this eruptive style are found throughout the Lesser Antilles, and such eruptions were witnessed by pre-Columbian Native American populations. One of these eruptive events from Mount Pelee on Martinique, occurring between 1800 and 2200 yr B.P., produced pumice-and-ash flow deposits that followed the main drainage channels on the volcano, ash hurricane deposits that both mantle the volcano’s flanks (slopes up to 25°) and extended at least 20 km from the crater. We examined the latter in the context of modern developments in understanding transport and depositional mechanics of pyroclastic density currents. Based on grain-size analysis, they are subdivided into proximal and distal deposits. The distal deposits become progressively more depleted in fines and enriched in crystals with distance from the crater. We conclude these are deposits of dilute, or inertia-dominated pyroclastic overcurrents, which decoupled from their underflows as they surmounted the topographic barrier formed by the older Pitons du Carbet volcanic center. A combination of topography and ingestion of air and water vapor from tropical vegetation perhaps caused the considerable expansion and liftoff, and resulting extreme crystal enrichment. Their areal distribution suggests much of the northern half of Martinique must have been severely affected, and the zone of devastation extended at least to the northern outskirts of the capital Fort-de-France, although their final runout distance may have extended significantly further south. Major coignimbrite plumes dispersing ash and aerosols over thousands of kilometers would today pose hazards to aviation.