The tuff rings comes in explosive eruptions hydromagmatic (or phreatomagmatic) when there is magma rising contact with surface water or groundwater levels.
The explosive nature of the associated explosion is due to the ratio water/small magma and the presence of pyroclastic products of surge type ("pyroclastic movements"), the resulting volcanic form is a small ring of pyroclastic fragments around the crater or vent. Rarely exceeds 50 m in height, displays an extensive and shallow crater and deposits, stratified, and are made up of juvenile magmatic material and fragments of preexisting rock.
When the eruption is of magmas rich in silica, consist of pomitic materials. This is the case of pomitic ring Lombo dos Milos - Lagoa Seca, in Furnas (São Miguel Island), formed in the historical eruption 1444(?) and whose initial phase was caused by the interaction with water from the existing pond on site.