Magma comes from the mantle, about 100km deep - the
hot spot. But they can also be formed by subduction
sometimes.
The magma has very little water or dissolved gas,
so eruptions are very gentle.
Eruptions are also gentle because the magma is
runny and very hot, therefore it travels a long way before cooling.
Therefore the volcanoes build up with very gentle slopes.
Caldera and Dome Volcanoes
Formed from viscous rhyolitic magma which is very
viscous because it has a high amount of silica.
Gas, especially water vapour and carbon dioxide
from melted carbonates, gets trapped in the sticky lava which results in
very explosive eruptions forming calderas.
Sometimes the gas escapes so that the magma is
still very sticky without trapped gas. This forms dome volcanoes.
Steep cone volcanoes like Mt Taranake and Mt
Ruapehu are formed when alternating layers of slightly sticky andesite
lava and ash build up a cone around the vent.