wouldn't the different uplifts recorded on different parts of the island back the pore pressurisation theory? Quoting: Anonymous Coward 22580089
if this was that magma, 20 KM down doing this, surely the whole island would uplift by the same amounts roughly, but the facts some are uplifting more than others suggests to me pockets of pressurisations in the volcano?
I want to try and answer this for you. This is only my opinion. It's my theory and so far it has been proven true several times on Hierro, but there may be something else out there that I'm not aware of or considering. Anyway...let me explain it like this.
1. To maintain normal air pressure
2. To put (gas or liquid) under a greater than normal pressure
3. To resist pressure
4. To pressure-cook
5. To subject to excessive stress or strain
It would appear as though you're trying to separate the idea of magma from the idea of pressurisation when in fact it is one in the same; one causes the other. The magma is what is causing the pressure to occur. But it's not not JUST magma. As magma encounters natural gases, water, crude oil, etc., it will either consume that item or it will use it to produce exhaust or upon contact it produces an air gas such as steam from encountering water, etc. Not to mention, heat expands. So...as large groups of magma gets surged under the Island, the natural heat it brings with it will cause expansion as well.
Now...I think maybe what will help you see the big picture is to understand there's not JUST a lake of magma at 20km or shallower under the island. There are huge pools of it no doubt, but there are also crevices, fractures in the rock, underground water ways, oil beds, natural gas pockets, fissures, and other types of cracks and seams in the earth beneath Hierro. So, as one of those fills up, the terrain above that immediate area expands. This is why we don't see the entire island rising all at once, and instead just see unique areas expanding. The ground adjacent and above the actual fissure or crack that is now filled with new incursions of magma will rise when the magma creates stress and pressure. Sometimes what we see when the elevation changes side to side is a large thin wall of magma has moved in somewhere and it is causing part of the island to move sideways. When the elevation drops it's normally because whatever the magma ate is now gone and the ground can settle OR the magma settles and/or moves to another area and the once expanded ground now settles back or even falls a little below zero.
There will be no ground deformation without magma incursions, water escaping through fracking caused by human drilling, or via fault movements. There aren't any known fault lines moving through Hierro besides the miniature ones caused by the previous magma intrusions. So...this is pretty much left to magma to cause these deformations.