Volcanology Flashcards
What is half spreading rate?
The rate at which new oceanic lithosphere is added to each tectonic plate on either side of a mid ocean ridge (=half of spreading rate)
What is a volcano?
An opening of the lithosphere from which lava, steam, and tephra erupt onto the Earth’s surface due to underground pressure and the buoyancy of magma
What 3 tectonic settings are volcanoes largely clustered around?
Mid ocean ridges (divergent plate margins at which oceanic crust solidifies)
Convergent plate margins (subduction zones at which a segment of lithosphere is consumed beneath another)
Intra-plate boundaries
What are the defining characteristics of MOR volcanism?
Remarkably uniform magma with limited differentiation and assimilation due to the uniformity of the mantle and crust
Gentle effusion, with pillow lavas the primary eruptive deposit
What are the defining characteristics of convergent plate magmas?
Variation, as magma often ascends through tens of kilometres of compositionally diverse crust, allowing for differentiation and assimilation
What are the 2 types of intra-plate boundary volcanism and their characteristics?
Rifting volcanism occurs when a plate is pulled apart or stretched to create a fault, similarly to MORs
Hot spot volcanism occurs at temporally persistent mantle melting anomalies, where hot upwelling mantle plumes impinge on the lithosphere (either basaltic or silicic eruptions)
Rank volcanic eruption types from least to most explosive
Hawaiian, Strombolian, Surtseyan, Sub-Plinian, Vulcanian, Phreatoplinian, Plinian
Rank volcanic eruption types from least to greatest eruption column height
Hawaiian, Strombolian, Surtseyan, Vulcanian, Sub-Plinian, Phreatoplinian, Plinian
How are lava flows produced?
Un-fragmented magma is erupted or pyroclasts re-weld on deposition in the vent area of the volcano
Which magmas are high viscosity and what sort of lava features do they create?
Rhyolite, dacite, and andesite; lava domes (circular, mound-shaped protrusions) with short runout length, confined to vent region
Which magma is low viscosity and what sort of lava features does it create?
Basalt; it feeds fast moving lava flows that reach up to tens of hundreds of km from the source
What are the 2 types of basaltic lava?
Aa lavas have a rubbly surface of broken fragments of cooled lava crust and interiors of a massive core. Flows are between several to tens of m thick with steep sides. They are mostly formed from high viscosity lavas that have experienced high shear stress
Pahoehoe lava has a near-continuous crust folded into rope-like structures that form flatter, wider flows. They are formed from lower viscosity lavas flowing over gentle slopes
How does the presence of suspended bubbles and crystal phases impact rheology?
Crystals always increase viscosity, with crystal fraction tending to increase as lava cools and degasses during transport
Bubbles tend to increase viscosity in slow shear lavas and decrease viscosity in rapid shear lavas
What are plumes and how are they formed?
Plumes are buoyant mixtures of volcanic gases and tephra typically produced by an explosive eruption. Effusive eruptions can produce gas-only or gas-dominated plumes
What are the 3 parts of a plume?
The jet region: upward motion is driven by momentum from fragmentation due to expansion of resolved gas phases, with surrounding air entrained in turbulent eddies that cause the jet to expand
The convective/buoyant region: exchange of heat from tephra to entrained air gives the mixture buoyancy and causes it to rise, with heat of tephra keeping the mixture buoyant as atmospheric density decreases and entrainment dilutes the plume
The umbrella region: plume reaches neutral buoyancy and expands, with denser tephra falling as the cloud spreads (coarser sooner than finer) with wind susceptibility
