Subaerial Felsic Lava Flows and Domes
Occurrence Alone or in linear and arcuate chains up to 20 km long Margins of calderas or volcanic depressions. Feeder occupies synvolcanic fault (ring fracture). Extrusion of lava preceded by explosive eruptions- thus associated with pyroclastic rocks. Large caldera- central portions- product of magma resurgence after cataclysmic eruptions. Crater of Stratovolcano- Mt. St. Helens
Felsic (Dacite Rhyolite) flows grade laterally into domes. Magma so sticky doesn t flow far (10,000 to 100,000 times more viscous than basalt) [important exception] Thus relatively short flows
Flows: 10 s of meters to more than 500 meters thick Average 100 meters (much thicker than mafic lava flows) < ½ km to 6-8 km length (Hawaiian lavas- 20-25 km)
Two Main Shapes: A)Coulees- B)Asymetric about a central vent- short, steep flows talus covered margins
Flow Morphology: 1) Obsidian (devitrified =)- Glassy, chilled carapace around flow 6-10 m thick at top, thinner at bottom 2) Interior- Lithic (cryptocrystalline rhyolite-dacite. Some flows all obsidian 3) Top- Flow Bx- obsidian to vesicular-pumiceous lava 4) Bottom- flow Bx, similar to top but thinner.
Surface- blocky and rough. Flow fronts are steep and normally terminated by a talus apron of bx. Breccia results from flow advance and mass wasting Basal Breccia- flow overrides frags that avalanche down front Surface Features
Internal Features: Flow Banding- foliated layers Of obsidian and vesicular-massive Lithic rhyolite-dacite Due to friction along conduit walls During ascent then deformation in Response to velocity changes as lava Advances. Flow lamina mm s to several meter s thick. Most < 2cm.,
Ideally flow banding is Parallel at base of flow (Flow Contact) and vertical-contorted In core Individual Lamina-bands recognized by: ) Color differences ) Textural differences: Spherulitic vs non spherulitic Crystalline vs glass
Perlitic Rhyolite: Hydrated Obsidian Gives onion skin appearance due to expansion of glass during hydration
Lithophysae (stone lilies): Gas Phase- gas cavities filled by Vapor phase minerals- Sanidine, Tridimite, Fayalite
Domes Steep sided, mound shaped masses of viscous lava that pile up over and around vent or fissure Domes usually form after explosive eruptions Domes are volatile poor
Domes have an exterior composed mostly of rubble with banks of talus covering sides and top. May make dome unrecognizable
In plan view domes. Like flows, are either circular to oval in shapeand related to a central vent Or- elongate and related to a fissure
Single Domes- occupy explosion Craters at tops and sides of Stratovolcanos or Center of tuff rings and cones Clusters of domes- above And along fissures- Margins of calderas Rift Valleys More rarely in stratovolcano
Surface Features-3 Texturally Distinct Parts Outer and upper zone of breccia- talus Intermediate zone of fractured rock- coherent Inner zone- massive to flow banded Gradational Bx Fct Massive
Talus Breccia: Ash- block-size fragments Form due to expansion and Cracking of outer surface as Dome rises, expands, grows Magma-semisolid shell Along with explosions. Top and sides
Continued growth leads to constant formation of breccia and a continuous increase in the thickness of the breccia pile on top and sides. Also get fragments from collapsed spines. Fragments cemented by secondary minerals.
Spines- Monoliths of solid magma pushed Up through dome rubble- Dykes Slickensides
Explosions Rain or sea-water entering fractures at top and sides of the dome- hot rocks-cold water Vesiculation and violent escape of gas Dome covers vent
Hazard Talus rubble plus rapid dome growth-expansion Gravitationally unstable- collapses Avalanche down slope of talus and hot magma- OR Explosion at base of growing dome- avalanche of material down slope Block and ash flows Can collapse to form debris flows 100 s of years after activity-chaos Crags-5km to form Chaos Jumbles
Domes Characterized by: High glass content-> 90 % obsidian Cores may be cryptocrystalline to crystalline and massive or vesicular margins often more vesicular Phenocryst content- 0-40% crystals typically quartz, feldspar, biotite and/or horneblende. crystals can parallel flow banding or define it. Domes associated with calderas have Low crystal contents- blown out
Typs of Domes Can be classified by: Morphology Surface texture Eruptive style
Types of Domes Domes have a great passion for selfdestruction- throughout formation constant fragmentation- explosions, spines, talus, avalanches, earthquakes End up with a pile of rubble Can happen quickly then weathering and mass wasting finish dome off
Types of Domes Plug Spiny to Lobate to Axisymmetric -Vary based on extrusion rate and cooling rate Crypto Either case- domes grow rapidly- 0ne to five years
Plug- Lassen: extruded lava solidifies as a plug (steep-sided) and is not much wider than the central vent. Types of Domes
Types of Domes Spiney-lobate-axisymetric- Steep sided and circular to irregular in plan view. smooth to block covered upper upper surface punctuated by spines and talus aprons, Lots of avalanches and block and ash flows Explosions may occur Spines up to 700 feet tall
Types of Domes Cryptodomes-High level intrusions May not breach surface Subaqueous- important