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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.

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"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

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  • $\begingroup$ 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! $\endgroup$
    – foobarbecue
    Commented Aug 7, 2014 at 18:01
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    $\begingroup$ It might also be worth noting that dykes are relatively thin meaning a small ratio of magma volume to surface area in contact with cooler rock, leading to faster cooling. $\endgroup$
    – user26
    Commented Aug 7, 2014 at 18:01
  • $\begingroup$ Paul: good point. Are sills thin enough that we get basalt sills as well? $\endgroup$
    – foobarbecue
    Commented Aug 7, 2014 at 18:03
  • $\begingroup$ 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. $\endgroup$
    – Gimelist
    Commented Aug 7, 2014 at 18:56

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