|
|
|
Thread Tools | Display Modes |
#1
|
|||
|
|||
Looks of komatiite flows
On 6 nov, 12:03, "John Kepler" wrote:
Fluidity/viscosity has more to do with the chemistry of the material than the temperature! A clarification. This is NOT to imply that komatiite flows weren't hot.....they were hotter than a jalapeno fart, but just that their ultramafic chemistry was responsible for their water-like fluidity. You could heat more felsic dacitic or rhyolitic magma to the same temperature and NOT get the same fluidity as the ultramafic komatiites. BTW, all of this is stretch for an old man living on the Craton digging coal for a living....but I have whacked a couple of komatiite flows up on the Shield....damn laterites! Does it mean that unlike the basalt, let alone rhyolite lavas, which freeze like amorphous substances - get increasingly viscous and pasty on cooling, till they solidify first on both extremities and finally in the interior - the komatiite lavas ought to be fluid at all temperatures down to a sharp freezing point? Then, if we have a freezing komatiite flows, the komatiite crystals ought to sink through the still fluid liquid to the bottom, so the komatiite flow should freeze from bottom up, till the last remaining fluid on the very top freezes - and only then should the surface start cooling below the melting point. How close has anyone got to the active komatiite flows of Io? |
#2
|
|||
|
|||
Looks of komatiite flows
Crown-Horned Snorkack wrote:
Then, if we have a freezing komatiite flows, the komatiite crystals ought to sink through the still fluid liquid to the bottom, so the komatiite flow should freeze from bottom up, till the last remaining fluid on the very top freezes - and only then should the surface start cooling below the melting point. All freezing points are sharp - for a given pressure *and* chemically homogeneity. The problem with your scenario above is that as the komatiite crystallises out, the chemical composition of the remaining melt changes. It's been a long time since I did any igneous petrology... but here goes. If the Mg-rich olivines crystallise first, the remaining melt will become more Si-rich. The crystals, given the v. low viscosity of the magma, will sink, trapping some of the remaining melt in the matrix, but essentially you'll have a liquid sitting on a crystal slush. The Si-rich(er) melt (given the very low initial concentrations of K, Al, Ca and Na) will still be high in Mg - pyroxenes and Mg-feldspars will form. The liquid itself will become increasingly viscous due to not just cooling but to the Si content. I also note that komatiites come out bloody hot - way hotter than flood basalts even. At 1 atm, pure forsterite freezes at 1900oC. So as soon as the temperature drops below this, olivine crystals will start to form. This changes the chemical composition as noted: it also changes the freezing point of the melt, so what remains will stay liquid at lower temperatures. How close has anyone got to the active komatiite flows of Io? rofl Simon Morden -- Visit the *all new* Book of Morden (www.bookofmorden.co.uk) "I haven't had that much fun with a novel for a while." - Bookbag The Lost Art - from David Fickling Books |
#3
|
|||
|
|||
Looks of komatiite flows
On 7 nov, 16:15, Simon Morden
wrote: Crown-Horned Snorkack wrote: Then, if we have a freezing komatiite flows, the komatiite crystals ought to sink through the still fluid liquid to the bottom, so the komatiite flow should freeze from bottom up, till the last remaining fluid on the very top freezes - and only then should the surface start cooling below the melting point. All freezing points are sharp - for a given pressure *and* chemically homogeneity. The problem with your scenario above is that as the komatiite crystallises out, the chemical composition of the remaining melt changes. Some melts freeze into one solid phase with no change of composition in the process. Some melts freeze into two or more solid phases with no composition change at a sharp eutectic melting point. and some melts get increasingly viscous on cooling and freeze into a glass without forming any crystals. It's been a long time since I did any igneous petrology... but here goes. If the Mg-rich olivines crystallise first, the remaining melt will become more Si-rich. The crystals, given the v. low viscosity of the magma, will sink, trapping some of the remaining melt in the matrix, but essentially you'll have a liquid sitting on a crystal slush. The Si-rich(er) melt (given the very low initial concentrations of K, Al, Ca and Na) will still be high in Mg - pyroxenes and Mg-feldspars will form. The liquid itself will become increasingly viscous due to not just cooling but to the Si content. So that once a large amount of olivine has frozen and settled out of the initially fluid komatiite, the remaining melt is more silica-rich and more viscous, more like ordinary lava and ought to form a crust on top like ordinary lava does? I also note that komatiites come out bloody hot - way hotter than flood basalts even. At 1 atm, pure forsterite freezes at 1900oC. So as soon as the temperature drops below this, olivine crystals will start to form. This changes the chemical composition as noted: it also changes the freezing point of the melt, so what remains will stay liquid at lower temperatures. But what about iron and fayalite? Are komatiites rich in iron? And does iron freeze out along with magnesium or remain in melt while forsterite freezes out? How close has anyone got to the active komatiite flows of Io? rofl You are right - no one has been much further than Moon, and therefore everyone has been exactly as close as Earth is to Io. However, a fair number of probes have passed Jupiter and satellites, carrying instruments and sending back photos. Pioneer, Voyager, Galileo... How thoroughly is Io known, between those probes? Also, komatiite is supposed to be very hot and fluid. If so, it would be expected to erupt quietly, spread in smooth floods filling holes and valleys, like the gentle basalt shields of Hawaii and yet gentler flood basalts. Io seems to have violent volcanic explosions. Even more violent than on Earth. How do those happen, and what is the petrology of those explosion products? |
#4
|
|||
|
|||
Looks of komatiite flows
Crown-Horned Snorkack wrote:
On 7 nov, 16:15, Simon Morden wrote: Crown-Horned Snorkack wrote: Then, if we have a freezing komatiite flows, the komatiite crystals ought to sink through the still fluid liquid to the bottom, so the komatiite flow should freeze from bottom up, till the last remaining fluid on the very top freezes - and only then should the surface start cooling below the melting point. All freezing points are sharp - for a given pressure *and* chemically homogeneity. The problem with your scenario above is that as the komatiite crystallises out, the chemical composition of the remaining melt changes. Some melts freeze into one solid phase with no change of composition in the process. Some melts freeze into two or more solid phases with no composition change at a sharp eutectic melting point. and some melts get increasingly viscous on cooling and freeze into a glass without forming any crystals. It's a rule of thumb that the faster the cooling, the smaller the crystal size - obsidian cools too quickly for nucleation points (though there might be some post-freezing ie snowflake obsidian). A change in bulk composition will occur if, frex, a biphase melt AB freezes out crystals of A which then get removed from the remaining melt B - either by gravity or by mechanical means. It's been a long time since I did any igneous petrology... but here goes. If the Mg-rich olivines crystallise first, the remaining melt will become more Si-rich. The crystals, given the v. low viscosity of the magma, will sink, trapping some of the remaining melt in the matrix, but essentially you'll have a liquid sitting on a crystal slush. The Si-rich(er) melt (given the very low initial concentrations of K, Al, Ca and Na) will still be high in Mg - pyroxenes and Mg-feldspars will form. The liquid itself will become increasingly viscous due to not just cooling but to the Si content. So that once a large amount of olivine has frozen and settled out of the initially fluid komatiite, the remaining melt is more silica-rich and more viscous, more like ordinary lava and ought to form a crust on top like ordinary lava does? For some values of ordinary, yes. From the New Zealand simulation, they suggest that the lava tubes will stretch a long way. I imagine that for a large, turbulant flow, the crust will be continually reabsorbed until the eruption has all but ceased. Komatiite is *so* rich in Mg, so poor in other metals, that left to its own devices, it'll be almost all olivine. Wiki tells me that the lavas often have a super-cooled appearance - densely packed small crystals, whilst magma chamber deposits have massive aggregations of olives and pyroxenes. It does ultimately depend on the cooling rate. And probably the pressure, too - I don't have a phase diagram of P-T variations for olivines to hand... The nearest analogue I can think of is the cement product called a levelling compound: it's a cement slurry that flows like water (finding its own level). Then it sets solid to produce an exactly flat surface. I also note that komatiites come out bloody hot - way hotter than flood basalts even. At 1 atm, pure forsterite freezes at 1900oC. So as soon as the temperature drops below this, olivine crystals will start to form. This changes the chemical composition as noted: it also changes the freezing point of the melt, so what remains will stay liquid at lower temperatures. But what about iron and fayalite? Are komatiites rich in iron? And does iron freeze out along with magnesium or remain in melt while forsterite freezes out? I don't know about the Fe content. But with the Mg content so unspeakably high, there's not much room for Fe. If they are, as suspected, formed by partial melting and differentiation elsewhere before accumulating, the Fe has probably been excluded in the first stage melt. How close has anyone got to the active komatiite flows of Io? rofl You are right - no one has been much further than Moon, and therefore everyone has been exactly as close as Earth is to Io. However, a fair number of probes have passed Jupiter and satellites, carrying instruments and sending back photos. Pioneer, Voyager, Galileo... How thoroughly is Io known, between those probes? Also, komatiite is supposed to be very hot and fluid. If so, it would be expected to erupt quietly, spread in smooth floods filling holes and valleys, like the gentle basalt shields of Hawaii and yet gentler flood basalts. Io seems to have violent volcanic explosions. Even more violent than on Earth. How do those happen, and what is the petrology of those explosion products? Violence in volcanic eruptions is almost always due to the presence of volatiles. Gas-rich (including steam) magmas which get trapped beneath ground or behind a plug of cool lava are spectacularly bad news. What volatiles Io has entrained in its magmas is a matter for speculation - I would guess at compounds of sulphur. Simon Morden -- Visit the *all new* Book of Morden (www.bookofmorden.co.uk) "I haven't had that much fun with a novel for a while." - Bookbag The Lost Art - from David Fickling Books |
#5
|
|||
|
|||
Looks of komatiite flows
obsidian cools too quickly for nucleation points (though
there might be some post-freezing ie snowflake obsidian). Actually it isn't even a solid, but a super-cooled liquid, and it never stops flowing! John |
#6
|
|||
|
|||
Looks of komatiite flows
John Kepler wrote:
obsidian cools too quickly for nucleation points (though there might be some post-freezing ie snowflake obsidian). Actually it isn't even a solid, but a super-cooled liquid, and it never stops flowing! John Also true - but it does shatter into a whirling cloud of razor-sharp shards when struck with sufficient force, frex with a geology hammer...* Simon Morden *not me, fortunately. -- Visit the *all new* Book of Morden (www.bookofmorden.co.uk) "I haven't had that much fun with a novel for a while." - Bookbag The Lost Art - from David Fickling Books |
#7
|
|||
|
|||
Looks of komatiite flows
Also true - but it does shatter into a whirling cloud of razor-sharp
shards when struck with sufficient force, frex with a geology hammer...* Been there, done that, got the scars......a flow north of Las Vegas! John |
#8
|
|||
|
|||
Looks of komatiite flows
John Kepler skreiv:
obsidian cools too quickly for nucleation points (though there might be some post-freezing ie snowflake obsidian). Actually it isn't even a solid, but a super-cooled liquid, and it never stops flowing! Sorta like glass ? /ducks and covers/ Eivind |
#9
|
|||
|
|||
Looks of komatiite flows
Sorta like glass ? No, EXACTLY like glass, which is what it is....so is pyroclastic ash! John |
#10
|
|||
|
|||
Looks of komatiite flows
"John Kepler" wrote in message ... obsidian cools too quickly for nucleation points (though there might be some post-freezing ie snowflake obsidian). Actually it isn't even a solid, but a super-cooled liquid, and it never stops flowing! John Can you point to a cite describing the actual observation of obsidian flow at condtions near STP? Recall of course that even crystalline materials (solid by any definition) can be found to flow (deform) in geologic formations since they are subjected to high (but sub-melting) temperatures and pressures over long periods of time. Silica glasses near room temperature are amorphous solids and not liquids. They are rigid and do not flow at all, their atoms locked into place even though they are not in crystals. There is a glass transition temperature below which a fundamental thermodynamic process begins to take place as it cools further, which is reflected in the change of the specific heat and density of the glass just like the true phase change of solidification that occurs in crystalline solids. It is true that the glass transition to a solid is not abrupt like crystallization solidification, and that there is a zone below the glass transition temperature where viscosity increases to very high levels and the physical state is ambiguous. But the glass does not remain in this state of ambiguity regardless of how low the temperature goes, there is a point where the transition is complete. |
|
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Twin Solar Space Flows Confirmed In Outer Space | nightbat[_1_] | Misc | 8 | February 24th 07 12:12 AM |
Scientists zero in on why time flows in one direction (Forwarded) | Andrew Yee | News | 0 | November 1st 04 07:34 PM |
Scientists zero in on why time flows in one direction (Forwarded) | Andrew Yee | Astronomy Misc | 0 | November 1st 04 07:32 PM |