▁form ▁explos ive ▁la va ▁er u ptions ▁but ▁occasionally ▁this ▁will ▁happen ▁by ▁tra pping ▁of ▁the ▁la va ▁within ▁the ▁vol can ic ▁thro at ▁and ▁bu il dup ▁of ▁vol can ic ▁g ases . ▁Hawai ʻ i ' s ▁Ma una ▁Lo a ▁vol cano ▁er upt ed ▁in ▁this ▁way ▁in ▁the ▁ 1 9 th ▁century , ▁as ▁did ▁Mount ▁T ara wer a , ▁New ▁Zealand ▁in ▁its ▁violent ▁ 1 8 8 6 ▁er u ption . ▁Ma ar ▁vol cano es ▁are ▁typical ▁of ▁small ▁bas alt ▁t uff s , ▁formed ▁by ▁explos ive ▁er u ption ▁of ▁bas alt ▁through ▁the ▁cr ust , ▁forming ▁an ▁apr on ▁of ▁mixed ▁bas alt ▁and ▁wall ▁rock ▁bre cc ia ▁and ▁a ▁fan ▁of ▁bas alt ▁t uff ▁further ▁out ▁from ▁the ▁vol cano . ▁ ▁Amy g dal oid al ▁structure ▁is ▁common ▁in ▁re lict ▁ves icles ▁and ▁beautiful ly ▁cry st all ized ▁species ▁of ▁ze ol ites , ▁quart z ▁or ▁cal cite ▁are ▁frequently ▁found . ▁ ▁Column ar ▁bas alt ▁▁ ▁During ▁the ▁cool ing ▁of ▁a ▁thick ▁la va ▁flow , ▁contra ction al ▁joint s ▁or ▁f ract ures ▁form . ▁If ▁a ▁flow ▁co ols ▁relatively ▁rapidly , ▁significant ▁contra ction ▁forces ▁build ▁up . ▁While ▁a ▁flow ▁can ▁shr ink ▁in ▁the ▁vertical ▁dimension ▁without ▁f ract uring , ▁it ▁cannot ▁easily ▁accommod ate ▁shr inking ▁in ▁the ▁horizontal ▁direction ▁unless ▁cra cks ▁form