alt ▁has ▁a ▁fine - gra ined ▁min eral ▁texture ▁due ▁to ▁the ▁mol ten ▁rock ▁cool ing ▁too ▁quickly ▁for ▁large ▁min eral ▁cry st als ▁to ▁grow ; ▁it ▁is ▁often ▁por phy rit ic , ▁containing ▁larger ▁cry st als ▁( phen oc ry st s ) ▁formed ▁prior ▁to ▁the ▁extr usion ▁that ▁brought ▁the ▁mag ma ▁to ▁the ▁surface , ▁embedded ▁in ▁a ▁fin er - gra ined ▁matrix . ▁ ▁These ▁phen oc ry st s ▁usually ▁are ▁of ▁ol iv ine ▁or ▁a ▁calci um - rich ▁pl agi oc l ase , ▁which ▁have ▁the ▁highest ▁mel ting ▁temper atures ▁of ▁the ▁typical ▁min er als ▁that ▁can ▁cry st all ize ▁from ▁the ▁m elt . ▁ ▁Bas alt ▁with ▁a ▁ves icular ▁texture ▁is ▁called ▁ves icular ▁bas alt , ▁when ▁the ▁bulk ▁of ▁the ▁rock ▁is ▁mostly ▁solid ; ▁when ▁the ▁ves icles ▁are ▁over ▁half ▁the ▁volume ▁of ▁a ▁spec imen , ▁it ▁is ▁called ▁sc oria . ▁ ▁This ▁texture ▁forms ▁when ▁dissol ved ▁g ases ▁come ▁out ▁of ▁solution ▁and ▁form ▁b ub bles ▁as ▁the ▁mag ma ▁decom press es ▁as ▁it ▁reaches ▁the ▁surface , ▁yet ▁are ▁tra pped ▁as ▁the ▁er upt ed ▁la va ▁hard ens ▁before ▁the ▁g ases ▁can ▁escape . ▁ ▁The ▁term ▁bas alt ▁is ▁at ▁times ▁applied ▁to ▁shall ow ▁intr us ive ▁rocks ▁with ▁a ▁composition ▁typical ▁of ▁bas alt , ▁but ▁rocks ▁of ▁this ▁composition ▁with ▁a ▁ph