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SYNOPSIS: The solubility of minerals in hot water, especially hot alkaline or acidic water, has been vastly underestimated as a geothermal force. Especially at depths underground in the neighborhood of 1.5 miles, where hydrostatic pressure can be "supercritical" (when steam is denser than water, so boiling essentially does not occur.) This theory postulates that, in the presence of high temperatures, the presence or absence of free water determines when some magmas will remain solid, or begin to flow as a liquid.
BACKGROUND:
Many common minerals that SEEM resistant to solution in water (such as quartz) still have a finite solubility that increases dramatically as temperatures rise, or as the water becomes strongly acidic or basic.
At the surface, water temperatures can rise just so far, and boiling will take place. At great depth, however, water becomes "super-critical", when the pressure is sufficient to prevent boiling taking place, because steam is denser than liquid water.
Many minerals are too refractory to melt directly. Classically, it is held that the melting of rocks involves the first crystals to melt acting as "solvents" for the more refractory minerals.
The presence of water complicates this situation. Silicates (which often make up the most refractory materials in typical rocks) are especially soluble in water as temperatures rise (if pressure is sufficient to prevent boiling). Besides this, the dissolving is not (usually) just simply ordinary solution. Chemical reactions are occurring that are EXOTHERMIC, and produce hydrated silicates, as well as complex silicates (that incorporate aluminum, potassium, sodium, lithium, and other elements).
When the proportion of silicates, particularly quartz, is sufficiently high, the heat given off by dissolving greatly promotes the liquefaction of the magma. The effect of this can be quite striking in a hot porous refractory stone, such as sandstone. Intrusion of liquid water into hot dry sandstone can vitrify the stone in one of two different ways.
If the stone is merely "warm", the water seeps into the pores, and gradually, over time, dissolves and re-deposits the silica, under the influence of gradients of temperature or acidity. This results in a non-porous quartzite or other metamorphic rock. It also tends to seal the pores as it goes, and sometimes limits further incursion of moisture into the stone, until a fracture occurs in the vitrified zone.
If, however, the stone is sufficiently "hot", it is possible for the heat of dissolution of the silica, to combine with the tremendous lowering of melting point (due to the "fluxing" action of the water in forming hydrated silicates), and result in a rapid "melting" of the stone into a silicate rich liquid magma.
Such magma can be extremely mobile (thin and watery), UNTIL it approaches the surface, and vents steam to the atmosphere. This results in dehydration of the magma (boiling) and the formation of the viscous silicate rich magma typical of explosive volcanoes and cinder cones.
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THEORY BASICS:
1. Water, at modest temperatures, tends to vitrify stones by slow solution processes that create geopolymers that "glue" the stone into a non-porous mass over geological spans of time. This almost always requires the water to be strongly alkaline or acidic. Non-neutral waters tend to dissolve minerals (when relatively cool) several orders of magnitude faster than does neutral water. More importantly, they create a "glue" that seals off, and prevents the free flow of water through the stone that might otherwise prevent the minerals from re-depositing in locations resulting in vitrification. Neutral waters usually have little effect on stone until very elevated temperatures are achieved.
2. At higher temperatures (and pressures) there is a sharp point where a "phase change" occurs, such that a silicate dissolved in water occupies less volume than when the water and silicate are separate. Deep inside the earth, this phase change results in the liberation of very considerable energy (in the form of heat). This inherently speeds up the dissolution process.
3. Contraction of the water/silicate mix (on melting) creates a void, a vacuum that must be rapidly filled by either water or stone. It therefore acts as a trigger for one of two mechanisms. EITHER it drives more water into the region of the dissolving stone, by creating more space to fill, OR it results in a geological faulting (earthquake) that permits both in flowing water and out flowing magma through the resulting crack.
4. The available PORE SPACE in stones is, itself, insufficient to allow sufficient water for the liquefaction of most stones. Therefore, this "contraction on dissolution" plays a critical and a "violent" role in the dissolution of silicate based stones that are already approaching the temperature of melting.
5. Hot rocks that begin being quite solid, can have water "intrude "through a new fracture, or through a new" leak into the pore structure of the rock. IF the depth below the surface is greater than 1.5 miles OR if the hydrostatic column is sealed from the surface and the rock column height is sufficient to generate supercritical water (about .75 miles underground), THEN water can have an almost explosive effect on the rocks. Actually, it is an "implosive" effect, caused by a reduction in volume on dissolution.
6. The creation of such "thin watery" hydrous silica magmas create one of the few forces deep underground that is "fast acting" enough to cause rock rupture, rather than just plastic deformation of the rocks.
7. Unlike what intuition tells one, such magmas actually REDUCE subterranean pressures locally, and typically create veins of quartz filling cracks created by the contraction of the dissolving minerals.
8. But earthquakes resulting from shifts of rock masses from large-scale dissolution can create DIFFERENTIAL PRESSURES that encourage the magma to flow from one area to another. In particular, such magma tends to be extruded like toothpaste up the sloping faults typically formed when the mass of rock above settles into the newly created voids.
9. While similar scenarios MAY occur with other magma, silicate based magma creates spectacular volcanoes for two reasons: First, because the contraction upon dissolution in water is more significant. Thus, silicate magmas, as they dissolve in water, generate more strings of earthquakes, that create pores for the transport of even more water, and also a potential crack for the escape of magma towards the surface.
10. Silicate magmas, unlike basaltic lava, suffer from a fatal flaw. The contained water tends to BOIL as the lava approaches the surface, making the lava thicker, resulting in both the venting of steam, and the PLUGGING of the volcanic vent. This typically results in the presence of supercritical water pressure far closer to the surface than it theoretically should ever exist.
11. As long as a volcanic vent is sufficiently plugged, and the liquid lava is restrained from the last stages of boiling, relatively liquid lava containing supercritical water can approach the surface. In doing so, it can exert forces on the surrounding cold stones (as it begins to boil) greatly in excess of their tensile strength.
12. Since boiling lava gets more viscous, the vents ALWAYS plug, and the release of magma is ALWAYS violent.
13. The explosion of water from supercritical fluid magma, in the most extreme cases, create "pumice", a volcanic formed "foam glass" product. This is evidence of the "instantaneous" release of supercritical magma directly to atmospheric pressure. It is a case of a very thin and watery magma (as evidenced by the frothing), boiling off so much water instantaneously that it turns (instantly) back into a solid phase.
14. The existence of pumice is, itself, direct evidence of how extreme of effect that water has on the fluidity of certain magmas. Water MAY have as much, or more, effect on the fluidity of basaltic magma as well. But, in that case, it appears that the chemical reactions with the water (the hydration of the minerals) is more "irreversible". Exposure to atmospheric pressure does not result in explosive boiling, and does not (significantly) result in loss of water or fluidity.
CONCLUSIONS:
1. The presence of water may be the "driving force" behind violent volcanism. Almost as if there is a more direct relation between volcanoes and "geysers" than commonly thought.
2. Injection of water under high pressures into some very hot rocks could trigger explosive volcanism. While it is UNLIKELY that man could trigger explosive volcanism, it is not entirely impossible. It may be very unwise to inject water, or create a path for the natural flow of water, into magmas with the potential for violent dissolution. (It is debatable whether we have the technology to drill into rocks quite THAT HOT. But, if the rocks themselves are somewhat porous, injection of water could be playing with very serious volcanic fire.) Injection of water for geothermal steam production should perhaps be limited to certain conditions. Basaltic or non-explosive volcanic formations should be safer, but not necessarily safe. Injection of water into magma domes known to be already sufficiently below melting point should be safe. But injection of water into any magma where there is the potential for the man injected water to be joined by a vast reservoir of NATURAL water should be scrupulously avoided!
3. If water is injected into hot magma, any resulting fractures MUST NOT be capable of intersecting with a natural reservoir of water. This could (theoretically) result in a natural chain reaction leading to violent volcanism from zones of hot rock of which we are not aware.
4. A "deep geyser" (with water extending to the supercritical zone) more closely related to explosive volcanism than is commonly thought. It provides both a vent (for the outgassing of silicate magma, and its "thickening") and an inlet path (for water for the hydration and dissolution into a liquid of magma.) Only the depth of the hole and the temperature of the magma may differentiate a geyser from a cinder cone being formed. (A geyser or a volcanic vent is a "dry hole" of a volcano, where the water escapes prematurely from liquid magma, resulting in its thickening underground, preventing an explosion from ever happening.
5. The simple act of surface water poring down an exceedingly deep volcanic vent might (in rare cases) trigger the eventual formation of a cinder cone (violent volcano). But, in most cases, the existence of a volcanic vent is already sufficient evidence that sufficient water has escaped that mass of magma to make it too thick to come to the surface.
6. Pumping water into magma that has not yet had an opportunity to vent could be HAZARDOUS, since this may be pouring a FLUX into a potential volcano. IF silica rich rocks are close to melting temperature, AND IF the rocks are relatively poor in water, THEN the simple act of adding water, and fracturing the rock to allow the further (natural and unrestricted) flow of water, could potentially trigger an active volcano.
7. As a general rule, when generating geothermal power: ALWAYS pump water into hot rock that has ALREADY EXPLODED, and never into hot rock that is AWAITING EXPLOSION!
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