This is MAUNGAKIEKIE - the largest and most complex volcanic cone fortress in the southern hemisphere.
It formed a complex cone of scoria lapilli when the eruption was fountaining red hot lava.
The hot lava cooled in the air, freezing in the gas that was in the lava forming the vesicles in the scoria.
Some formed into bomb like shapes while in the air and other pieces fell still sufficiently hot to weld together where they fell.
One of the craters formed of scoria lapilli is complete while two others were horseshoe craters, lacking one side where the lava flowed away from the vent.