<p>Obsidian is volcanic glass of <em>felsic</em> composition (i.e. SiO<sub>2</sub>-rich) whereas basalt is a volcanic rock which may or may not be (partially) glassy of <em>mafic</em> composition (i.e. SiO<sub>2</sub>-poor). So when you're saying</p>

<blockquote>
  <p>obsidian as opposed to basalt</p>
</blockquote>

<p>...you're comparing apples and oranges. So instead of discussing obsidian versus basalt, let's discuss <em>phaneritic</em> versus <em>aphanitic</em> versus <em>glassy</em> rocks, in the context of being melted by dragons (because that's what you're asking in your question).</p>

<ul>
<li><p>Phaneritic rocks are those with coarse crystals, easily observable to the naked eye. The best example is granite, like the stuff you see in pavements and kitchen tops. These rocks form by slow underground cooling so this is not something that dragons can make.</p></li>
<li><p>Aphanitic rocks are those with very fine crystals that are only visible with a microscope. Basalt is the most obvious example. To the naked eye they may look like glassy, but they do not have the characteristic conchoidal fractures you see in obsidian.</p></li>
<li><p>Glassy rocks (that I'm pretty sure have a fancy name that starts with <em>hyalo</em>, I just can't remember what it is) have no crystals in them so they're basically glass. Obsidian is the best example.</p></li>
</ul>

<p>Note that there could be combinations of those, just to confuse you even more: for example you can have aphanitic rocks with large crystals of minerals (known as porphyritic textures) or a rock that's mostly glass but has some crystals floating around in it.</p>

<p>When you have dragons melting rocks, the cooling of the rock is done very rapidly in air. This can be minutes to hours, depending on the size of the block being melted. This is definitely rapid enough to form rocks. But, the final form of the rock also depends on the chemical composition. Felsic rocks have very high viscosities and low diffusion rates, and even relatively slow cooling (days?) can form obsidian. That's actually the reason that obsidian is most common in felsic rocks. Mafic rocks, on the other hand, require very rapid cooling to form glass (for example, <a href="https://youtu.be/hmMlspNoZMs" rel="nofollow">erupting into water</a>), and even then they make have some "quench" crystals of olivine or pyroxene.</p>

<p>Felsic rocks also have lower melting points than mafic rocks. This all leads to the conclusion:</p>

<p><strong>Yes, dragons can make obsidian, and it's quite easy</strong></p>

<p>Take a granite (or even sand-bearing mud), melt it with some dragon fire, and let it just cool. The high SiO<sub>2</sub> content will make the melting point rather low (let's say below 1000 °C). The high viscosity will ensure that it stays where it is rather than flow somewhere else (think of <a href="https://youtu.be/_GSp6lPlel4" rel="nofollow">basalt flows of Hawaii</a> versus <a href="https://youtu.be/h6B1myUKAS4" rel="nofollow">Mt St Helens felsic lava domes</a>). Obsidian will surely form.</p>

CC BY-SA 3.0
2016-10-24T16:38:36.740Z
"Take a granite (or even sand-bearing mud), melt it with some dragon fire, and let it just cool." real-world analog: https://en.wikipedia.org/wiki/Fulgurite
wienein
CC BY-SA 3.0
2016-10-24T15:55:25.777Z
2016-10-24T15:55:25.777Z
Gimelist
<p>Specifically, what time period would it take for obsidian as opposed to basalt, to form? (I need to know how much faster obsidian cools than basalt.)</p>

CC BY-SA 3.0
2016-10-20T12:32:09.300Z
[Basalt](http://geology.com/rocks/basalt.shtml) is an igneous rock. Also, basalt comes out of the earth 'as is' in volcanoes, no further chemical process needed. So its hard to say what you mean by basalt 'forming'.
kingledion
CC BY-SA 3.0
2016-10-21T11:55:23.720Z
It depends a lot on the context: 'rapid' could mean anything from a few seconds to thousands of years. There are a lot of different types of igneous rocks and they form on very different timescales.
bon
CC BY-SA 3.0
2016-10-23T03:57:20.000Z
It sounds like you're looking for two ranges of cooling times, one of which would produce obsidian, the other of which would produce basalt. However, the two don't come from the same source melt (felsic versus balaltic lava). So, there may be no good answer for your question.
Daniel Griscom
CC BY-SA 3.0
2016-10-24T00:20:20.773Z
@DanielGriscom that is true, I forgot about that. I'm using this for a fantasy novel in which dragons create giant obsidian statues with their fire and was wondering if this was in any way feasible.
Layla
CC BY-SA 3.0
2016-10-24T00:29:09.633Z
Of course it would be feasible. Or, of course it would be NOT feasible. Depends on how much belief you expect your readers to suspend. In any case, this isn't really an Earth Science question.
Daniel Griscom
CC BY-SA 3.0
2016-10-24T01:12:54.810Z
@DanielGriscom Well even in magic worlds I tend to try and follow hard science as much as possible. Thanks.
Layla
<igneous><world-building><time>
What does "rapid" mean in terms of igneous rock formation?
Layla
# What does "rapid" mean in terms of igneous rock formation?

(Asked by: username_0 on 2016-10-20)

Specifically, what time period would it take for obsidian as opposed to basalt, to form? (I need to know how much faster obsidian cools than basalt.)

**Tags:** igneous, world-building, time

> **Comments:**
> **username_1**: [Basalt](http://geology.com/rocks/basalt.shtml) is an igneous rock. Also, basalt comes out of the earth 'as is' in volcanoes, no further chemical process needed. So its hard to say what you mean by basalt 'forming'.  
> **username_2**: It depends a lot on the context: 'rapid' could mean anything from a few seconds to thousands of years. There are a lot of different types of igneous rocks and they form on very different timescales.  
> **username_3**: It sounds like you're looking for two ranges of cooling times, one of which would produce obsidian, the other of which would produce basalt. However, the two don't come from the same source melt (felsic versus balaltic lava). So, there may be no good answer for your question.  
> **username_0**: @username_3 that is true, I forgot about that. I'm using this for a fantasy novel in which dragons create giant obsidian statues with their fire and was wondering if this was in any way feasible.  
> **username_3**: Of course it would be feasible. Or, of course it would be NOT feasible. Depends on how much belief you expect your readers to suspend. In any case, this isn't really an Earth Science question.  
> **username_0**: @username_3 Well even in magic worlds I tend to try and follow hard science as much as possible. Thanks.  

---

## 1 answers

### Answer by username_4 on 2016-10-24. Score: 4

Obsidian is volcanic glass of *felsic* composition (i.e. SiO2-rich) whereas basalt is a volcanic rock which may or may not be (partially) glassy of *mafic* composition (i.e. SiO2-poor). So when you're saying

> obsidian as opposed to basalt

...you're comparing apples and oranges. So instead of discussing obsidian versus basalt, let's discuss *phaneritic* versus *aphanitic* versus *glassy* rocks, in the context of being melted by dragons (because that's what you're asking in your question).

* Phaneritic rocks are those with coarse crystals, easily observable to the naked eye. The best example is granite, like the stuff you see in pavements and kitchen tops. These rocks form by slow underground cooling so this is not something that dragons can make.
* Aphanitic rocks are those with very fine crystals that are only visible with a microscope. Basalt is the most obvious example. To the naked eye they may look like glassy, but they do not have the characteristic conchoidal fractures you see in obsidian.
* Glassy rocks (that I'm pretty sure have a fancy name that starts with *hyalo*, I just can't remember what it is) have no crystals in them so they're basically glass. Obsidian is the best example.

Note that there could be combinations of those, just to confuse you even more: for example you can have aphanitic rocks with large crystals of minerals (known as porphyritic textures) or a rock that's mostly glass but has some crystals floating around in it.

When you have dragons melting rocks, the cooling of the rock is done very rapidly in air. This can be minutes to hours, depending on the size of the block being melted. This is definitely rapid enough to form rocks. But, the final form of the rock also depends on the chemical composition. Felsic rocks have very high viscosities and low diffusion rates, and even relatively slow cooling (days?) can form obsidian. That's actually the reason that obsidian is most common in felsic rocks. Mafic rocks, on the other hand, require very rapid cooling to form glass (for example, [erupting into water](https://youtu.be/hmMlspNoZMs)), and even then they make have some "quench" crystals of olivine or pyroxene.

Felsic rocks also have lower melting points than mafic rocks. This all leads to the conclusion:

**Yes, dragons can make obsidian, and it's quite easy**

Take a granite (or even sand-bearing mud), melt it with some dragon fire, and let it just cool. The high SiO2 content will make the melting point rather low (let's say below 1000 °C). The high viscosity will ensure that it stays where it is rather than flow somewhere else (think of [basalt flows of Hawaii](https://youtu.be/_GSp6lPlel4) versus [Mt St Helens felsic lava domes](https://youtu.be/h6B1myUKAS4)). Obsidian will surely form.

> **Comments:**
> **username_5**: "Take a granite (or even sand-bearing mud), melt it with some dragon fire, and let it just cool." real-world analog: https://en.wikipedia.org/wiki/Fulgurite  

---

