Spherical accumulations of wet, accreted ash, called accretionary lapilli , are also common to many base surge deposits. Maars and Tuff Rings Hydrovolcanic explosions generate maars and tuff rings . These vent types are large circular depressions with low rims of ejected debris. Maars are excavated into the substrate, thus exposing older rocks along their inner walls. Tuff rings, however, are built above the substrate.
Maars contain a greater proportion of fragmented basement rocks in the ejecta blanket.
However, if the magma rises along a linear fracture zone, the interaction with groundwater may result in the reaming out of the fissure. Such eruptions can be even more destructive than those associated with maars and tuff rings, as exemplified by the highly explosive Tarawera eruption which buried three villages and killed 150 people on New Zealand's North Island in 1886. Although littoral cones have a hydrovolcanic orgin, they are not true hydrovolcanic vents. They are composed of basalt tephra that accumulate as ejected debris from the explosive interaction of moving lava and seawater. Although pahoehoe flows generally enter the water in a relatively passive manner, a'a flows often enter the sea explosively to produce cone-shaped piles of ejected debris.