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For an effusive eruption to occur, magma must be permeable enough to allow the expulsion of gas bubbles contained within it. If the magma is not above a certain permeability threshold, it cannot degas and will erupt explosively. Additionally, at a certain threshold, fragmentation within the magma can cause an explosive eruption.
Magma consists of liquid rock that usually contains suspended solid crystals. [14] As magma approaches the surface and the overburden pressure drops, dissolved gases bubble out of the liquid, so that magma near the surface consists of materials in solid, liquid, and gas phases.
The lava flows are more viscous, and therefore shorter and thicker, than the corresponding Hawaiian eruptions; it may or may not be accompanied by production of pyroclastic rock. Instead the gas coalesces into bubbles, called gas slugs , that grow large enough to rise through the magma column, bursting near the top due to the decrease in ...
Because it contains so few gas bubbles, the molten lava is denser than the bubble-rich cinders. [8] Thus, it often burrows out along the bottom of the cinder cone, lifting the less dense cinders like corks on water, and advances outward, creating a lava flow around the cone's base. [ 8 ]
Magmatism along strike-slip faults is the process of rock melting, magma ascent and emplacement, associated with the tectonics and geometry of various strike-slip settings, most commonly occurring along transform boundaries at mid-ocean ridge spreading centres [1] and at strike-slip systems parallel to oblique subduction zones. [2]
Composite or stratovolcanoes often have andesitic magma and typically form the extrusive rock andesite. Andesitic magma is composed of many gases and melted mantle rocks. [2] Cinder or scoria cones violently expel lava with high gas content, [2] and due to the vapor bubbles in this mafic lava, the extrusive basalt scoria is formed. [6]
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Fragmentation occurs when bubbles occupy ~70–80 vol% of the erupting mixture. [5] When fragmentation occurs, violently expanding bubbles tear the magma apart into fragments which are ejected into the atmosphere where they solidify into ash particles. Fragmentation is a very efficient process of ash formation and is capable of generating very ...