Extrusive rock

[2] 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.

If the magma contains abundant volatile components which are released as free gas, then it may cool with large or small vesicles (bubble-shaped cavities) such as in pumice, scoria, or vesicular basalt.

[3] Any larger crystals visible to the human eye, called phenocrysts, form earlier while slowly cooling in the magma reservoir.

[4] When igneous rocks contain two distinct grain sizes, the texture is porphyritic, and the finer crystals are called the groundmass.

[3] The extrusive rocks scoria and pumice have a vesicular, bubble-like, texture due to the presence of vapor bubbles trapped in the magma.

IUGS classification of aphanitic extrusive igneous rocks to their relative alkali (Na 2 O + K 2 O) and silica (SiO 2 ) weight contents. Blue area is roughly where alkaline rocks plot; yellow area where subalkaline rocks plot. Original source: * Le Maitre, R.W. ( ed. ); 1989 : A classification of igneous rocks and glossary of terms , Blackwell Science, Oxford.
A volcanic rock from Italy with a relatively large six-sided phenocryst (diameter about 1 mm) surrounded by a fine-grained groundmass , as seen in thin section under a petrographic microscope