posted on Aug, 21 2003 @ 08:23 AM
With the recent postings concerning the Yellowstone SuperCaldera, I got to ruminating about the properties of lava and what some of the mechanisms of
an eruption (especially in supercalderas) could be. I have never studied volcanism, so my thoughts on this are from a working experience with
thixotropic Non-Newtonian fluids, which pretty much defines magma.
First I’ll define some terms:
A Newtonian fluid is any fluid which has a linear relationship between its shear stress and velocity gradient.
A Non-Newtonian fluid is any fluid which DOESN’T have a linear relationship between its shear stress and velocity gradient. There are different
classifications of Non-Newtonian fluids based on the reaction of the shear stress to increasing shear straing (velocity gradient). There are
Plastics, Dilatants and Thixotropic.
Since magma is thixotropic we will center on this class of fluids.
A thixotropic fluid is one that requires some finite amount of shear prior to flowing (it has a yield point basically). The best example of this is
toothpaste. Toothpaste, if a sufficient shear force is not applied, remains in a “gel” form and can suspend an object (i.e. it can resist a shear
force). Once the required shear force is applied to break the “gel strength” the toothpaste then transitions from a “gel” to a flowable “fluid”.
(Ketchup is another good example.) A flowable fluid cannot support a shear force. As long as the thixotropic fluid is flowing, it will remain in the
“fluid” form. But the minute it comes to a stop, it begins to transition back to the gel form, which means its viscosity (resistance to flow)
dramatically increases and you are back to the “required shear force” to get it to move again. These fluids are also known as “shear-thinning”
Now we must drill down to a smaller subclass of thixotropic fluids in order to get where magma belongs. Not only is it thixotropic, but it can also
transition completely to a solid depending on its temperature and pressure. Materials that fall into this category are cement (this is chemically
dependent instead of just temperature and pressure dependent), molten metal, and magma.
This subclass is very important to what I theorize could happen in a supercaldera. Because it is the gel transition phase to a solid that is the
important thing. During this transition time a problem referred to as gas migration can take place. This problem is battled in any industry that
works with these type fluids. For instance, the casting industry goes to great pains to avoid “worm holes” that gas migration can cause during the
gel-to-solid transition time in metal castings. The oil industry is constantly striving to combat “channeling” that can occur in the cement sheath
when a gas producing zone begins to migrate during the gel-to-solid transition time.
So why is the gel-to-solid transition phase so critical?
Let’s say we have a tall, tall pipe, standing vertical, sealed at the bottom and open at the top. We drill a small hole in the side of the pipe and
start pumping high pressure gas into this hole. The gas will rise up through the pipe and exit out the top. Now, let’s stop pumping the high
pressure gas for a little bit and instead pump a cement slurry in the top of the pipe. Once we have the pipe full, we begin pumping the high pressure
gas back in near the bottom. The column of cement (which right now is still a fluid) creates a hydrostatic head (pressure) at the bottom of the
column dependent on the height of the column. If our column of cement is tall enough (which we are assuming it is) it will have enough hydrostatic
pressure to restrict the gas from flowing upwards.
Now, move forward in time a bit and we get to the gel-to-solid transition time of the cement. We still have our high-pressure gas acting at the
bottom of the cement column. But what is happening now is, the cement slurry is in the process of converting to concrete, and isn’t quite there yet.
It is a gel, with a limited gel strength (like our toothpaste). When the cement hits the gel-transition phase, it no longer acts as a fluid, and it
no longer can produce hydrostatic pressure at the bottom of the column. Hence, our high pressure gas now can “channel” through this gelled cement
because it has enough energy to overcome the gel strength.
Now, in my mind, this channeling is the exact same phenomena that creates volcanoes in the first place. Earth was a big ball of hot stuff in various
forms that coalesced and began to cool…from the outside in. So the outermost layer cooled first but the molten layer below continue to outgas.
Before the outer layer could solidify (during its gel state) the molten layer below “channeled” and then (over large periods of time) continue to move
since it was still fluid…but left its little blow-hole behind in the outer layers.
Now getting to supercalderas, such as that in Yellowstone. We have this massive stream of magma coming up and building pressure through outgassing
and such. If this stream of magma is “mushrooming” over a large space, then the surface area of molten magma contacting the cooler upper crust is
increasing and this will dramatically increase the cooling rate of the top part of the mushroom. If this mushroom top begins a gel-to-solid
transition, it can then no longer hold back the outgassing occurring below, and you’d get a “kick”, or a blow-out.
Am I not correct that part of the concerns here lately are that the heat is rising in an ever-widening area in Yellowstone? Is this a sign that the
magma is mushrooming out? Could we be seeing the signs that the mushroom cap is becoming large enough to start cooling and entering the gel-to-solid
I look forward to any and all inputs concerning my thoughts.