January 2025
I am told this is a classic papper. Here are some notes / excerpts:
- “Basaltic magma is derived by incongruent partial melting of mantle peridotite, favoured in tectonic settings (e.g. hotspots and rifts) where mantle rock rises adiabatically to relatively shallow levels, or in subduction-zone settings where volatiles decrease the melting temperature of mantle rock.”
- Important magma parameters (pretty uniform for basaltic volcanos)
- Magma density relative to lithosphere density — helps deter- mine the positions of magma chambers and intrusions;
- Viscosity and yield strength determine the geometry and structures of lava flows and intrusions;
- Gas content + viscosity + rheology controls the explosive violence of eruptions by determining the ease with which gases escape from magmas.
- Parameters responsible for diversity: magma-supply rate and involvement of non- magmatic water.
- ”Basaltic systems have a source in the mantle from which magma ascends, mainly because of its positive buoyancy but sometimes aided by tectonic forces, toward the surface. They have one or more conduits by which the magma ascends. Polygenetic volcano systems generally possess a high-level magma chamber, situated at a neutral buoyancy level, which stores magma and modulates its delivery to the volcano and to sub-volcanic intrusions. Deep storage reservoirs may also exist.”
- Types of volcanos
- Shield volcanos
- Stratovolcanos
- Central Volanos.
- Monogenetic volcanoes. These consist of clusters of scattered and mostly small (> 2 km3) volcanoes, each generated by a single eruption. Most commonly a volcano con- sists of a cinder cone associated with outflows of aa lava, but some are lava shields of scutulum- type (e.g. Rangitoto Island, Auckland, and Xitle in M…, and many that occur near the coast or close to lakes are phreatomagmatic tuff-rings or maars.
- Flood basalt fields consist of monogenetic volcanoes erupted from widely scattered vents, but their lava flows cover wider areas, overlap or are superposed to form parallel-stratified successions, and have much greater volumes. Giant flood-basalt fields are distributed through geological time at average intervals of 32 Ma (Rampino & Stothers 1988), and each one formed at the time of inception of a hotspot, on arrival of an ascending mantle plume at the asthenosphere/litho- sphere boundary.
- Volcano Collapse due to instable foundations, layers of pyroclastic or hydrothermally-altered material, intrusive dykes, local updomings in central volcanoes, severe marine erosion.
- Polygenetic vs. monogenetic. “In the polygenetic volcano systems, magma batches ascend sufficiently frequently along the same conduit that the conduit walls are maintained in a hot condition and provide magma with a thermally and mechanically very favour- able pathway toward the surface. In the monogenetic and flood basalt systems magma batches ascend at such long time inter- vals that the pathway taken by one batch has effectivelycooled by the time that the next batch is ready to ascend.”
- Fissures / Rift systems. ‘Most basaltic eruptions occur from fissures, and virtually all basaltic volcano systems have eruptive fissures. Fissures are opened very easily by the hydraulic jacking action of magma, and are the ‘natural’ underground conveyance for low- viscosity magma (Emerman & Marrett 1990). They commonly extend for tens of kilometres and are typically concentrated into rift zones. Magma solidified in fissures forms dykes. Dykes have a high survival potential, and in deeply eroded areas may be virtually all that survives of the volcanic system.
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