Lava Flows - Lava Flows Most lava flows are basaltic in...

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1 Lava Flows Basalt 90% Andesite 8% Dacite/Rhyolite 2% This is because most silicic and intermediate magmas erupt explosively (higher gas content and viscocity) to produce PYROCLASTIC deposits. Only one rhyolite flow has ever been observed – Tuluman Islands (near Papua/New Guinea) (1953-1957). Most lava flows are basaltic in composition There are three main types of lava flow:- ± PAHOEHOE – has a shiny, smooth, glassy surface. It tends to be more fluid (lower viscosity), hence flows more quickly and produces thinner flows (typically 1-3 m). ± AA – a rubbly flow, with a molten core, with higher viscosity (but same composition) which, therefore, tends to move more slowly and produce thicker flows (typically 3- 20 m). ± BLOCKY – similar to Aa, but even thicker (>20 m), with a blocky rather than rubbly surface. Andesites, dacites and rhyolites tend to form blocky flows.
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2 Pahoehoe flows are a silvery gray in color, whereas the Aa flows are a darker gray. This is because the pahoehoe is glassy and the aa is rubbly. Aa and pahoehoe flows on Mauna Loa volcano, Hawaii. Basalts Dacites Thickness 2 - 30 m 20 - 300 m Typically 5-10 m ~ 100 m Hawaii ~ 4 m Aspect Ratio 0.01 – 0.02 0.02 – 0.1 (Height/Area) Length 1 – 90 km 0.5 – 10 km Typically 4 – 5 km ~ 1 km Hawaii 10 - 25 km Volume 0.5 – 1,200 km 3 << 1 km 3 Hawaii < 1 km 3 ( This may need revising) Laki (Iceland) 12 km 3 Roza (CRB) 1,200 km 3
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3 Pahoehoe Flows Pahoehoe flows on Kilauea Volcano, Hawaii Pahoehoe close ups Measuring the temperature of a flow Famous scientist pokes flow with a stick!
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4 Pahoehoe lava takes on a variety of shapes and forms - these are examples of ropy pahoehoe Pahoehoe near the vents is often very gas-rich, inflating to produce shelly pahoehoe. Some bubbles can be quite large! This type of pahoehoe has the splendid name “entrail pahoehoe” - for obvious reasons.
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5 Aa Flows Aa flow on Mauna Loa volcano, 1984. Volcanologist runs away from an aa flow! Why did the aa cross the road? The interrior of an aa flow Measuring the flow temperature
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6 Blocky flows Blocky lava flow completely engulfs church at Paracutin, Mexico. House being consumed by lava flow in Iceland. Note, this flow is transitional between aa and blocky in its characteristics. Blocky lava flows at Newberry Volcano, Oregon (upper left) and Medicine Lake Highland, California.
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7 RHEOLOGY – fluid dynamics of lava flows. Viscosity and Yield Strength are the two most important factors that influence:- ± Surface morphology (flow type) ± Size and shape of the flow
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This note was uploaded on 02/22/2012 for the course GEA 3000 taught by Professor Staff during the Fall '09 term at UMass (Amherst).

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Lava Flows - Lava Flows Most lava flows are basaltic in...

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