<p>"Basalt" per definition is a fine grained rock (that is, you can't see the individual crystals with the naked eye, aka aphanitic) with a certain chemical composition. The coarse grained form of this rock is called a "gabbro". A diabase (or dolerite) is something in between, but let's ignore it for the meanwhile for the sake of simplicity.</p>

<p>So quoting from your question:</p>

<blockquote>
  <p>I've always understood that basalt is an extrusive rock (formed by
  eruption), and dikes are intrusions (no eruption)</p>
</blockquote>

<p>This is not entirely accurate. Assuming that we are talking about rocks of this specific composition, then every extrusive rock is a basalt but not every intrusive rock is a gabbro (or diabase). Fine grained rocks, and basalts in our case, can also form in hypabyssal settings (underground, for example dykes) if conditions are favourable for very quick nucleation. When a magma of 1100 °C intrudes a 20 °C surface or 100 °C rocks several hundred metres deep, the cooling rate is not too different, causing similar fine grained textures in the rock.</p>

<p>The concept of "basaltic" may differ in context. For example, people commonly use basaltic when the rock is similar to the <em>sensu stricto</em> composition of basalt/gabbro, but not quite. It does not necessarily mean that we are talking about a gabbro with a composition similar to basalt. In a wider context, it may refer to "basaltic" vs "granitic" compositions, whereby their coarse and fine grained forms are called "gabbro" and "rhyolite", respectively. No idea why this terminology is rooted like that.</p>

<p>I'll finish with a photograph of a basalt dyke that I took some years ago:
<img src="https://i.sstatic.net/vvM74.jpg" alt="The Black Heart Dyke"></p>

CC BY-SA 3.0
2014-08-07T18:01:14.307Z
The underlying issue here is the recurrent problem of descriptive vs petrogenetic classification. You have cleared up my confusion by saying that at least in this case basalt is a descriptive classification. Loved the photo; what a magnificent outcrop!
foobarbecue
CC BY-SA 3.0
2014-08-07T18:03:46.867Z
Paul: good point. Are sills thin enough that we get basalt sills as well?
foobarbecue
CC BY-SA 3.0
2014-08-07T18:56:42.707Z
Yes. That photo of the dyke is from an area that is rich in dykes and sills. All are basalt. The interesting thing is that there's also a laccolith in there, that is basically homogenous chemically, but appears as basalt in the margins and as gabbro in the core.
Gimelist
CC BY-SA 3.0
2014-08-07T16:26:18.980Z
2014-12-11T07:49:33.197Z
2014-12-11T07:49:33.197Z
Gimelist
<p>I've always understood that basalt is an extrusive rock (formed by eruption), and dikes are intrusions (no eruption), so I would have thought that a dike could not be composed of basalt rock.</p>

<p>It's come to my attention that many people talk about "basalt dikes" in geology, and many websites, including pages of university geology departments, talk about dikes that are basalt.</p>

<p>The concept of a "basaltic dike" (note the "ic") makes sense to me: a gabbro or diabase dike whose parental magma had a basaltic composition.</p>

<p>So, I know that basaltic dikes exist, but I'd like to have confirmation that there is no such thing as a dike which is basalt rock.</p>

<petrology><igneous>
Is there such a thing as a "basalt dike"?
foobarbecue
# Is there such a thing as a "basalt dike"?

(Asked by: username_0 on 2014-08-07)

I've always understood that basalt is an extrusive rock (formed by eruption), and dikes are intrusions (no eruption), so I would have thought that a dike could not be composed of basalt rock.

It's come to my attention that many people talk about "basalt dikes" in geology, and many websites, including pages of university geology departments, talk about dikes that are basalt.

The concept of a "basaltic dike" (note the "ic") makes sense to me: a gabbro or diabase dike whose parental magma had a basaltic composition.

So, I know that basaltic dikes exist, but I'd like to have confirmation that there is no such thing as a dike which is basalt rock.

**Tags:** petrology, igneous


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## 1 answers

### Answer by username_1 on 2014-08-07. Score: 12 | Accepted answer

"Basalt" per definition is a fine grained rock (that is, you can't see the individual crystals with the naked eye, aka aphanitic) with a certain chemical composition. The coarse grained form of this rock is called a "gabbro". A diabase (or dolerite) is something in between, but let's ignore it for the meanwhile for the sake of simplicity.

So quoting from your question:

> I've always understood that basalt is an extrusive rock (formed by
> eruption), and dikes are intrusions (no eruption)

This is not entirely accurate. Assuming that we are talking about rocks of this specific composition, then every extrusive rock is a basalt but not every intrusive rock is a gabbro (or diabase). Fine grained rocks, and basalts in our case, can also form in hypabyssal settings (underground, for example dykes) if conditions are favourable for very quick nucleation. When a magma of 1100 °C intrudes a 20 °C surface or 100 °C rocks several hundred metres deep, the cooling rate is not too different, causing similar fine grained textures in the rock.

The concept of "basaltic" may differ in context. For example, people commonly use basaltic when the rock is similar to the *sensu stricto* composition of basalt/gabbro, but not quite. It does not necessarily mean that we are talking about a gabbro with a composition similar to basalt. In a wider context, it may refer to "basaltic" vs "granitic" compositions, whereby their coarse and fine grained forms are called "gabbro" and "rhyolite", respectively. No idea why this terminology is rooted like that.

I'll finish with a photograph of a basalt dyke that I took some years ago:
![The Black Heart Dyke](https://i.sstatic.net/vvM74.jpg)

> **Comments:**
> **username_0**: The underlying issue here is the recurrent problem of descriptive vs petrogenetic classification. You have cleared up my confusion by saying that at least in this case basalt is a descriptive classification. Loved the photo; what a magnificent outcrop!  
> **username_0**: Paul: good point. Are sills thin enough that we get basalt sills as well?  
> **username_1**: Yes. That photo of the dyke is from an area that is rich in dykes and sills. All are basalt. The interesting thing is that there's also a laccolith in there, that is basically homogenous chemically, but appears as basalt in the margins and as gabbro in the core.  

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