extrusive rock

Crystals in aphantic rocks are small in size due to their rapid formation during eruption.
Igneous rocks can be either intrusive (plutonic and hypabyssal) or extrusive (volcanic).
The main effect of extrusion is that the magma can cool much more quickly in the open air or under seawater, and there is little time for the growth of crystals.
The texture of extrusive rocks is characterized by fine-grained crystals indistinguishable to the human eye, described as aphantic.

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Wikipedia

Extrusive rock (Wikipedia) Extrusive rock refers to the mode of igneous volcanic rock formation in which hot magma from inside the Earth flows out (extrudes) onto the surface as lava or explodes violently into the atmosphere to fall back as pyroclastics or tuff. In contrast, intrusive rock refers to rocks formed by magma which cools below the surface.