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Forum » SpaceEngine » Science and Astronomy Discussions » Should ice be considered a rock and water a lava? (Discuss within.)
Should ice be considered a rock and water a lava?
Gondor2222Date: Saturday, 10.08.2013, 08:20 | Message # 1
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The sources defining lava that I could find define lava as "the molten, fluid rock that issues from a volcano or volcanic vent.", "The melted rock ejected by a volcano from its crater or fissured sides.", and "refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling"

Therefore there seems to be general agreement that any molten rock expelled by a volcano is considered "lava"

The same sources define volcano as "an opening, or rupture, in a planet's surface or crust, which allows hot magma, volcanic ash and gases to escape from the magma chamber below the surface.", "a vent in the earth's crust through which lava, steam, ashes, etc., are expelled", and "A vent or fissure on the surface of a planet (usually in a mountainous form) with a magma chamber attached to the mantle of a planet or moon, periodically erupting forth lava and volcanic gases onto the surface".

Therefore there seems to be consensus that lava is any molten rock ejected from some sort of fissure in the crust of a planet or planet-mass object.

We have for the definition of rock: "naturally occurring solid aggregate of one or more minerals or mineraloids", "naturally occurring aggregate of solid mineral matter that constitutes a significant part of the earth's crust.", and "mineral matter of variable composition, consolidated or unconsolidated, assembled in masses or considerable quantities in nature"

Therefore we have consensus that magma is any collection of molten minerals expelled from a fissure of some sort in a planet's crust. And i'm pretty sure that most of the water on the planet has at some point been circulated down into the mantle, even if the timescale we need to use is a few billion years.

The International Mineralogical Association has defined "mineral" as such: "A mineral is an element or chemical compound that is normally crystalline and that has been formed as a result of geological processes." The "geological processes" portion serves to exclude minerals created by organic methods (e.g. glucose) from being considered minerals.

However, the term "normally" leaves much to be desired, as on a universal scale water is normally frozen. Clearly, the organization means "normally" to be interpreted as "at STP" but this specification seems to me to make a largely arbitrary line between "non-mineral ordered chemical solid structures" and "mineral ordered chemical solid structures" based purely on their phase at STP.

To make matters more confusing, all "minerals" described before the adoption of the IMA's definition that violated only the "normally" rule were "grandfathered" into the lists to be considered minerals, and hence both Mercury and Ice are considered minerals.

However, this creates a notable inconsistency in the definition. I for one don't believe the boundary line for what is considered a mineral should be determined by the characterization date of the crystalline structure of a material. I find "If it was characterized before 1959, it is a mineral" to be a completely arbitrary rule not worth keeping.

Therefore I feel that this "grandfather" clause should be abolished.

However, we are still left with the before mentioned line of "normally". I don't think that water's lower than STP melting point should mean that ice is not a rock and that water is not a magma. There is also the irritating "geological" portion which means that a diamond formed in the core of a star or graphite crystals formed in a gas giant's atmosphere are not minerals, while identical crystals formed by pressure in a solid environment are. Therefore I propose we do away with the "normally" and "geological processes" altogether and consider a mineral "a crystalline form of an element or chemical compound" Since we have done away with the "geological" portion, organic crystals will now also be considered minerals, and specification of non-biologically-formed crystals will be done by simply calling them such.

This would allow such crystals as glucose, benzene, and butane to be considered minerals and aggregates of them to be considered rocks, and hence any molten form of a crystalline compound that has erupted at some point from the crust of a solid body would be considered lava.

However, even if we DON'T change the definition, ice is still technically a mineral according to the IMA. Therefore aggregates of ice are technically rock, and surface/crustal water, technically being molten rock, is technically lava if it has at any point been in the mantle.

I'd like to see your opinions too guys, so please share.

Also, does anyone have estimates for what proportion of surface/crustal water has at one point cycled through the mantle and back up to the crust? Is it as high (>95%) as I think it is?

Edited by Gondor2222 - Saturday, 10.08.2013, 08:21
HarbingerDawnDate: Saturday, 10.08.2013, 16:12 | Message # 2
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Ice is not rock, and if it shoots out of the ground it's not lava. Regardless of what some organization or other might have as its technical definitions (definitions which all too often have failings), there are specific meanings for the terms "ice" and "rock" in planetary science, and also separate terms to describe what happens when they erupt from the ground in fluid form: a rock volcano is simply called a volcano, and an ice volcano is called a cryovolcano or geyser.

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Edited by HarbingerDawn - Saturday, 10.08.2013, 16:14
WatsisnameDate: Saturday, 10.08.2013, 23:38 | Message # 3
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I concur with HarbingerDawn. Many bodies in the outer solar system have a rock-ice mixture, but pure water-ice is never called rock, nor are eruptions of liquid water called lava, in astrophysics or planetary science. The reason for this is not only a matter of definition, but of physical characteristics, such as the different temperature regimes for the phases of these materials.

Water-ice and other substances that condense at low temperatures are considered volatiles, or ices. Materials that can form at high temperatures, such as silicates and metals, are refractory. Rocks belong to this second category, and magma/lava is associated with molten rock rather than molten ice.

The precise definition of 'mineral' is a matter of some ambiguity and I wouldn't consider labeling water-ice as a mineral to be a strong reason for changing convention.

midtskogenDate: Sunday, 11.08.2013, 06:50 | Message # 4
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I'm not sure physical properties can be used to back up the current convention. For instance, should we call solid mercury "mercury ice"?

WatsisnameDate: Sunday, 11.08.2013, 08:45 | Message # 5
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Where do you find grains of solid mercury in nature?

Edit: Sorry, I should do better than simply answer a question with a new question. tongue
Remember that we're discussing the formation and properties of materials in an astrophysical/geophysical setting. Not the properties of any material in any setting -- that belongs to the subject of chemistry. So unless the formation of solid elemental mercury is found to be an important process in protoplanetary nebulae or celestial bodies, it simply isn't relevant. You could call it 'mercury ice' but it is not the same 'ice' that I'm referring to.

This is somewhat akin to how astronomers refer to all elements beyond helium as 'metals'. The term means something else entirely to a chemist.

Forum » SpaceEngine » Science and Astronomy Discussions » Should ice be considered a rock and water a lava? (Discuss within.)
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