Fast-rising magma may warn of major eruptions
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“Geologist [I'm Climate Change]”
Since: Mar 07
#1 Aug 5, 2013
An excellent article.
Notes on crustal relaxation earthquakes indicate that these events Rebound/long duration mercalli X (r/lx) rebound the crust of the overriding plate and forearc back up the trench generating a bloody great big tsunami (such as Chile 1960, Alaska 1964, South Andaman 2004, Chile 2010, and Tohoku 2011); have notable effects on the stratovolcanoes sitting in the arc.
The initial surface effect was a spread & a sink into the arc graben. The other effect is not so easy to see... This is the opening of subvertical fractures in dike orientation in the base of the crust & upper mantle over the magma discs that accumulated at the base of the crust derived from magma extracted from the descending plate. This is the origin of the arc magma that eventually erupts through the stratovolcanoes.
The eruptions of the stratovolcanoes occur in the recompression stage as the fractures perpendicular to the runout vector are compressed by the converging recompressing arc. The magma initially goes sideways along the pressure gradient in the fracture (50 to 120Km down depending on the age of the descending plate & thickness of the overriding crust [thinnest in backarc basins]). being insulated by hot solid rock there is little crystallisation during this stage but some crystals do form & magmatic gas concentrates in the residual magma with time.
The fun bit is when the fracture set hits a "wall" (usually a segment boundary transform fault), & can go no further sideways; @ that point further arc compression drives the magma in the only direction left (up). This results in rapid ascent of magma which then reaches the surface relatively quickly. The usual result is emergence through an established stratovolcano because previous magma ascent leaves the base of the stratovolcano warm and relatively uncompressed (and eventually with one or more magma chambers on the way up).
The result is that following r/lx and with an interval of time measures in years to decades there is a rapid onset eruption @ a stratovolcano. This may not be the same stratovolcano as was active prior to r/lx as the r/lx runout vector varies and as a result the stratovolcanoes may be on in one cycle and off in the next.
After the rapid onset eruption when the fracture set magma breaches the base of the volcano & hits the surface, there is often a chain of medium to low level eruptions caused by the magma remaining in the fracture set being squeezed in pods along the fracture and into the volcano resulting in eruption series and the growth of a photogenic cone in the initial onset crater (such as Arenal for example). This results in the classic cone shape of young stratovolcanoes.
More than one fracture may form in the sector leading to one particular volcano so rapid onset eruptions may occur both early & late in the r/lx cycle (Irazu was a late one). The usual mechanism is for a sub parallel fracture in the late stage is to break into a pre existing magma filled fracture squeezed in the early stage to a thin hot vein. The result is a large late eruption at a volcano previously active earlier in the r/lx cycle. Late event eruptions may be very large (Tambora 1815, Krakatau 1886) being examples from the Java arc. The intensity of the rapid onset late stage events depends strongly on the amount of magma in the late event fracture set. In the case of Tambora, most of the magma hit the surface at once, and in the case of Krakatau a tail of magma was left in the fracture set which is now forming a series creating Anak Krakatau.
The earthquake set is not magmatic but a response to sudden dilation of the deep arc by the magma travelling along the fracture set The result is that the top of the arc graben tends to fall in over the dilating fracture.
Post eruptions when all of the magma filled fracture set has been squeezed flat, a series of thrust quakes may invert the arc graben prior to r/lx. A late dome may also appear @ the volcano.
Have a nice day: Ag
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