▁as ▁ves icles . ▁These ▁formed ▁when ▁g ases ▁such ▁as ▁steam ▁and ▁carbon ▁dio x ide ▁escaped ▁as ▁the ▁compress ive ▁pressure ▁got ▁less ▁in ▁the ▁mol ten ▁rock ▁as ▁the ▁la va ▁reached ▁the ▁surface . ▁The ▁largest ▁numbers ▁of ▁b ub bles ▁occur ▁when ▁the ▁hard ened ▁skin ▁of ▁the ▁la va ▁surface ▁prevent ed ▁the ▁b ub bles ▁from ▁rising ▁to ▁the ▁surface ▁and ▁burst ing . ▁Oliv ine ▁cry stal ▁faces ▁are ▁visible , ▁a ▁min eral ▁rich ▁in ▁sil ica , ▁iron ▁and ▁mag nes ium . ▁The ▁size ▁of ▁the ▁faces ▁shows ▁that ▁the ▁la va ▁co o led ▁relatively ▁slowly , ▁allowing ▁the ▁cry st als ▁time ▁to ▁' g row '. ▁ ▁Sp ier ' s ▁is ▁close ▁to ▁the ▁lim estone ▁quar ries ▁of ▁Broad stone , ▁Hill head , ▁Middle ton , ▁D ock ra ▁and ▁Tre ar ne . ▁Wh inst one ▁has ▁been ▁quar ried ▁along ▁a ▁di ke ▁at ▁Bar rm ill ▁and ▁is ▁still ▁quar ried ▁( 2 0 1 1 ) ▁at ▁Lo an head . ▁ ▁DM ▁Be ith ▁ ▁A ▁large ▁Def ence ▁Mun itions ▁( DM ) ▁centre ▁is ▁located ▁at ▁Be ith . ▁The ▁site ▁was ▁originally ▁developed ▁in ▁ 1 9 4 3 ▁as ▁a ▁conventional ▁Royal ▁Naval ▁Arm aments ▁Dep ot , ▁m un itions ▁store , ▁for ▁the ▁Royal ▁Navy . ▁It ▁now ▁processes ▁and ▁stores ▁Spe ar fish , ▁Storm ▁Sh adow , ▁Tom ah awk ▁and ▁Br im