I don't think that's the case. From the Wikipedia article for "Obsidian":
Obsidian has low water content when fresh, typically less than 1% water by weight,[4] but becomes progressively hydrated when exposed to groundwater, forming perlite.
Umm... looking at that article,
Because obsidian is metastable at the Earth's surface (over time the glass becomes fine-grained mineral crystals), no obsidian has been found that is older than Cretaceous age. This breakdown of obsidian is accelerated by the presence of water. Obsidian has low water content when fresh, typically less than 1% water by weight,[4] but becomes progressively hydrated when exposed to groundwater, forming perlite.
That refers to obsidian that has already cooled being chemically altered by water. That would be similar to (already cooled) igneous rocks being turned into metamorphic rocks by weathering. That would be entirely different from forming a different substance based on the way in which it was cooled.
I'd just like to point out, because I'm a nitpicker like this, that, while space is exceedingly cold, vacuums are very good insulators. As there isn't a lot of matter for heat to transfer to, things in space cool very slowly, which is why realistic stealth devices would never exist in real life, because the spaceship would be hot from it's engines firing and life support and would be incapable of bleeding that heat into it's surroundings.
It also has zero pressure, and pressure can radically alter how or if crystaline structures form and rapid pressure changes can lead to corresponding temperature changes. The wikipedia article I was reading specifically refering to how they form on comets and outer planets.
Not a bad go at the felsic/intermediate/felsic system at all 
Generally you will find only one of those basic pairs at a single volcano - the magma chamber and derived dykes and sills will dontain the intrusive version, the eruptive products will comprise the extrusive (i.e. faster cooled, therefore finer grained) versions.
However, ALL magmas are originally mafic in composition. It is the process of cooling as the magma rises through the crust which causes variation. Minerals with high melting points will crystallis out first, leading to a process known as fractional crystallisation. Because the early-crystallising minerals are generally rich in iron and magnesium, and low in silica, as they are removed the remaining melt is relatively enriched in silica and depleted in iron and megnesium.
Now imagine a magma chamber under a volcano: It is periodically recharged with pulses of mafic material from the mantle, but a constant process of fractional crystallisation is going on. If an eruption taps that magma chamber at intervals, then the type of magma erupted will depend on where in the fractionation cycle the magma chamber is at that point, leading to a variety of magma compositions being possible at any given vent.
This can be further complicated by volcanoes which have multiple active magma chambers, each of which may be tapped individually or in combination, often with their own volumes, cooling rates, and recharge periods. It can get very complex very quickly. However, broadly speaking volcanoes sit within one of the three broad magma groups stated above (in actual fact there are numerous subdivisions and classifications within those three broad groups).
The simplest way to think about volcanoes is those which have a mafic, mantle derived source produce runny lava and form broad low-angle slopes, such as those on Hawaii, while the opposite end of the spectrum has the highly evolved, silica rich magmas. These are very viscous and unable to move effectively as lava, so tend to form large explosive eruptions such as Mt St Helens and Pinatubo. These volcanoes are steep sloped (about 30 degrees). 
On the subject of obsidian formation - you tend to see this produced mainly in rhyolite (i.e. felsic) eruptions - usually localised as spires or domes of material are extruded from the vent over a period of months or years. Mixing magma and water should only really produce a very deadly pyroclastic cloud, full of steam, ash and death. 
Hmm... as cool as it would be to have a way of measuring how much a magma flow changes as it starts to cool and evolve as a magma flow, that's pretty much just beyond DF's current capabilities, where all magma is the same temperature.
Since magma in the game now is basically either from the magma sea, or from a "volcano" that is really just a cyllindrical hole in the planet filled with magma straight down to the magma sea, I guess we should be thinking of it as mafic, and capable of freely mixing with the mafic magma at the parts of the center of the planet that are not Hell?
Hmm... apparently, while Wikipedia isn't helping much, google searching is giving me some hits on just plain "basatlic glass" that appear to be a little different from the named, pumice-like glasses such as tachylite or
Scoria, and seem to be a little more obsidian-like in nature, but they all generally note that they weather very quickly into other stones.
http://www.geo.uni-bremen.de/Ozeankruste/Research/Research_Weathering.htm (Talks about biological weathering of basaltic glass in deep sea vents - the stone he's holding looks somewhat obsidian-like, compared to the more pumice-like Scoria.)
http://www.flickr.com/photos/42398031@N02/4037065202/ (The stuff in the center is a sand with basaltic glass fragments)
Ah... This one is actually more helpful -
http://www1.newark.ohio-state.edu/Professional/OSU/Faculty/jstjohn/Volcano%20stuff/Kupaianaha.htm - apparently, the black, glassy (non-vescular, or filled with holes like pumice is) surface is tachylite, so maybe I was getting things a little wrong, since this page is saying tachylite is essentially the mafic version of obsidian.
Apparently, tachylite can refer to both the mafic version of obsidian, but also to certain types of pumice when basaltic volcanos erupt pyroclastically, although wikipedia seemed to imply more of the latter. This also says that mafic obsidian displays many of the same properties of regular obsidian, except that its iron impurities will make it turn rust red as it oxidizes.
Armed with that knowledge, I guess I would say that perhaps we could have the mafic tachylite as an alternative to the obsidian we normally get, if we simply give it similar properties to the obsidian we already get (although wikipedia gave me the impression tachylite was somewhat more brittle than even obsidian).
Given the last paragraph of what Xenoc said, however, I think maybe just creating obsidian is wrong. If we are adding water, and making "pyroclastic clouds of death", perhaps we
should be having pumice, along with a little of the obsidian (mafic or felsic). Theoretically, rather than instantly creating an obsidian wall out of any tile where magma and water contact, first some pumice stones may be created, which are flung at high velocity out of the tile, potentially causing a little mayhem as they rain back down to earth (if this was not in an inside controlled chamber), after which, obsidian may form as normal (or have a small chance of forming, based on how deep the magma was, if we want to be more exact, as a small puddle of magma might entirely be turned into that exploding pumice rain.)
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Topic: Magma Shouldnt Always Cool Into Obsidian (Read 8305 times)