Volcano Morphology and Explosive Eruptions in the Red Sea

Abstract:

Latest high resolution bathymetry of the Red Sea Rift reveals (a) cone-shaped, (b) flat-topped, (c) large domed and (d) pancake-dome volcanoes as the main morphological types of volcanoes in the axis. In addition, sheet and lobate flows as well as hummocky volcanic ridges occur – typical for ultra-slow spreading mid ocean ridges. The morphological diversity is the result of numerous (mostly local) factors and their interaction such as e.g., lava viscosity (depending on temperature and magma chemistry), effusion rate, conduit geometry and depth, eruption duration and the morphology of the surrounding seafloor. Some major element variations in basalts – as far as the sparse sampling density allows to say – appear to be systematically related to specific volcano morphotypes. Cone volcanoes reveal the most evolved lavas while sheet lavas appear most primitive. Furthermore, we observe a relative large percentage of flat-topped volcanoes in the central Red Sea compared to the North and South, correlated with significantly decreasing mantle temperatures and volcanic activity in the central Red Sea where the influence if the Afar and Cairo mantle anomalies are less.

Very unique features of the Red Sea Rift are numerous craters with raised rims that are found where submarine salt glaciers (namakiers) blanket the rift valley. These structures are unlikely to result from submarine salt-karst or pockmark formation by degassing or fluid flow as such processes generally do not form rims, and may instead result from phreatic or phreatomagmatic explosions related to magma/salt interaction. Intrusions of >1000 °C, tholeiitic magma raises temperatures far above the melting point of the present hydrous salts (these make up >30% of the salt minerals in the Red Sea evaporites) and fluids that are almost certainly present, migrating through cracks and faults. Due to fast, ductile magma emplacement into the salt cover, sufficient volatile pressure can be generated to blasts away overlaying salt and sediments and create the observed craters. If this hypothesis is correct, craters in salt glaciers mark regions of the oceanic crust where volcanism has taken place after the salt blanketing.