A volcano is a crack in the earths crust letting molten magma out as lava. volcanos can happen by subduction (one plate gets pushed under another and the magma gets pushed up), spreading (where the plates spread apart) or hot spots (where there is a hot spot where the magma breaks through). Some examples of volcanos in New Zealand are Ruapehu, Ngaruhoe, Tongariro, Taranaki, Taupo, Auckland, White Island and Rotorua. The ash that comes out of a volcano can contain obsidan, pumice and basalt. Lava has silica in it which makes it sticky.