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My textbook for a historical geography class mentions a couple of geological features it calls "basalt sills" (the sill of Rosh Pinah and one other).

I'm a total newbie to geology, but the term basalt sill is confusing me. As best I can tell... 1. basalt is an extrusive rock but 2. a sill is an intrusive formation. Is basalt sill a contradiction in terms?

Thanks for your help.

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2 Answers 2

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I think I know where the confusion comes from. You need at least two criteria to define a magmatic rock: a composition, and a texture. A magma of the same composition can yield two different rocks depending on the texture. For instance, a mafic melt can yield a gabbro if the texture is coarse-grained, or a basalt if it is fine-grained. Same with a felsic melt and granite/rhyolite.

Now, usually, intrusive rocks tend to be coarse-grained, because they cool slowly, hence crystals have time to grow. And usually, extrusive rocks tend to be fine-grained because they cool quickly, so crystals don't have enough time to grow. That's why one could think that basalt is restricted to extrusive rocks. But if you take a mafic melt, and cool it quickly in an intrusive setting, then you end up with a mafic, fine-grained rock, i.e., a basalt. This can happen with sills, which can be quite thin (< 1 m in some cases) thus can cool quickly due to the contact with the cold, host rock.

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  • $\begingroup$ That being said, the first link posted by @Fred (which starts by calling basalt "extrusive") states: "The term basalt is at times applied to shallow intrusive rocks with a composition typical of basalt, but rocks of this composition with a phaneritic (coarser) groundmass are more properly referred to as diabase (also called dolerite)". So it would be intersting to know if the sills mentioned in your textbook are truly basalt, or if it is one such case of improper use. $\endgroup$ Oct 23, 2022 at 9:30
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Basalt is an igneous rock - derived from a molten rock. When molten basalt is injected, or simply enters an underground void, or when it enters between two easily separable geological layers, it intrudes into the existing geology. Basalt can also be deposited on the surface of the Earth via volcanic eruptions.

In geology, a sill is an intrusion that is horizontal, or subhorizontal, whereas a dike is an intrusion that is vertical or subvertical.

So, a basalt sill is not a contradiction, but tells you two things: firstly the type of rock, which is basalt and secondly, the way it was formed: as a subhorizontal intrusion into existing geological formations.

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