Volcanoes (Ch 5) 09:36 Pressure holds gasses in solution Gasses mostly at top of silica-rich melt chamber Decrease in P gasses separate from melt (bubbles) Gas escapes more easily from fluid (mafic) magmas Drop in P as magma ascends Chain-reaction explosions Rhyolitic magmas: Explosive hot ash & gas clouds Can extend 1000s m Bubbles grow as gas separates from melt, decrease in P(pressure)=V(volume) increase P of magma fractures rock (Mount St Helens) Erupted materials Lava, gas, pyroclastics (rock, bombs, ash, dust) (larger pieces fall closer to vent) Lava flows: Aa (“ah ah”): rough jagged, hurts to touch Pahoehoe (“pa hoy hoy”): smooth, ropey, braided, can turn into Aa Lava tubes Long tubular opening in which lava on outside has solified but it has drained out, producing long caves (also found on Mars) Columnar joints 6 sided pillars, produced by slow cooling of basaltic magma Block lavas Short flows
Volcanoes (Ch 5) 09:36 100s m about 2 km Slightly curved, smooth surface, no vesicles on top Pillows Pillow lavas Piles of elliptical, sac-like bodies that form when lava solidifies underwater Indicates that lava either erupted underwater or flowed into water (or ice) Characterized by glassy rind, commonly vesicular (was gas charged) with a crystalline interior Gases Difficult to obtain direct samples (estimate) 1-6% of total weight (mostly H20) Role in clearing pathway from magma chamber to surface Can affect climate, natural atmospheric pollution Pyroclastic Deposit Formed by accumulation of fragments of volcanic rock (pyroclasts) scattered by volcanic ejection into the air When P is released, volatiles escape with explosive force ejecting magma Formed from gas- and Si- rich lavas e.g. rhyolite Types of deposits classified according to size of fragments (commonly mixtures of sizes) Hot ash forms a rock called welded tuff Blocks > 2.5 inches hardened rock
Volcanoes (Ch 5) 09:36 Volcanic bombs ejected as lava & hardened in the air Ash - < 2mm; rock, mineral, glass fragments If particles reach stratosphere, it effects weather Example: Mt. Pinatubo, Philipines Volcanic bombs Detached Volcanic breccias Mixtures of large and smaller angular material Types of volcanic deposits Rhyolitic lavas erupt at 700-900 degrees Celsius, are very viscous, and form steep- sided, bulbous deposits Basaltic lavas erupt at 1000-1200 degrees Celsius, are less viscous. Tend to produce fluid flows that spread out in relatively thin sheets Multiple flows build up to form immense lava plateaus Ex: Snake River Plain Basalts, ID; Columbia River Basalts, OR, WA, ID Styles of eruptions & landforms Shape of volcanic mountains are controlled by types of flows, and possible intermixing of pyroclastic deposits Central Vent eruptions Eruptions from pip-like conduit (central vent)- produces volcanic cones, typically with craters at summit Lava eruptions Lava cone built by successive flows of lavas Shield volcano Broad “shield-shaped” volcano dominated by volcanic lava flows (basalt) e.g. Mauna Loa, Hawaii
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