Collapsing ice cap above Katla (Iceland Review – English, and Icelandic ) (Remember that the black cover is because of the ash from Eyjafjallajokull last year)As the fractures from the earthquakes continue to fracture to the surface, so the paths that they create allow water to migrate down to the underlying hotter rock. This then converts to steam, and flows back up helping to further melt the underside of the ice sheet, with some water possibly escaping down the mountain, under the ice. These floods can be sudden, as was the one that took out the ring road around Iceland. That has now been repaired, a week after the flood took it out.
The narrowness of the fracture paths, at least initially, will slow magma migration and this will likely allow more additional activity at the surface, although Katla does not give a lot of warning before it erupts, apparently.
One of the misfortunes of having retired and given away many of my books is that I no longer have the references that relate quake size to rock damage. A lot of this work was done in the South African gold mines, which go down over 3 km (2 miles) and are thus at the depths of many of the current quakes around Katla. As the mines extended the workings so the weight of the surrounding rock shifted, and the speed of this (usually following a mine blast with explosives that broke out some rock) would cause rock fracture in the area.
Quakes could be created that were up to a magnitude 4, similar to some of those around Katla recently, and resulted in significant rock movement and fracture. Underground this leads to problems with keeping the tunnels open and safe for the workers (since the rock bursts can be violent and hurl rock fragments a long way, as well as creating air blasts that can also be dangerous). There have also been larger ones.
But I was trying to get some sense of displacement and damage as a function of seismic strength, since when the rock fractures there is often some crushing of the rock along the interface of the fracture, and this would then be removable by the upward pressure of magma, and the fractures released the overlying confinement. The initial crushed zone probably measures just a few inches, but will be eroded out by water/steam passage and the later passage of magma, since fractured rock is easily removed. This then gives the open passages for magma to move. If the fractures intersect then the intervening rock will likely also be removed since the magma has a higher density and thus more power than mere water or steam flows. There have now been several dozen quakes within the region of the Katla caldera, and knowing the damage zone from each would allow a better estimate of how damaged the rock is. Bear in mind that the re-healed fractures (healed by cooling magma) from earlier eruptions are likely the weakest links that are now failing and opening, and in the process, therefore rebuilding the network of passages that are needed for flow. The levels of permeability generated are orders of magnitude greater than that from a typical oil well.
Where the rock is attached to the overlying ice, then cracks in the rock can also occur in the overlying ice. And you may note that in the above picture apart from the circular rings of fractures, there are three well defined line cracks that cross the circular fractures. If you also look at the web cam you can see (in daylight) some of the lines where magma has flowed up to the surface in earlier eruptions (the picture shown in the last post).
So we now watch as the process continues to unfold.
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