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extrusive
This term is a specialized geological descriptor used to categorize volcanic rocks that solidify on the surface. It carries a connotation of suddenness and external exposure, contrasting sharply with intrusive rocks that cool slowly beneath the crust. The term evokes an image of molten material being pushed outward and rapidly quenched by the atmosphere or ocean.
In technical discourse, it is used almost exclusively within the context of petrology and volcanology. It describes the physical process of extrusion, where the rapid cooling rate typically results in fine-grained or glassy textures, such as those found in basalt or obsidian.
Meanings
Examples
The geologist pointed to the obsidian and explained that it is an extrusive rock.
Extrusive magma cools much faster than intrusive magma because it is exposed to the air.
Look at these fine grains; this must be an extrusive formation.
The island was formed by successive layers of extrusive volcanic material.
The island was formed by successive layers of extrusive volcanic material.
Is basalt considered an extrusive or intrusive rock?
The landscape is dominated by extrusive flows from the ancient caldera.
I wonder if this sample is truly extrusive or if it cooled just below the surface.
Rhyolite is a common example of an extrusive igneous rock.