John Seach, Volcano Live

 Tuff Rings and Maars - John Seach

A tuff ring is a broad flat crater formed by a phreatomagmatic eruption.
Tuff rings have low rims and slopes of 3 to 12 degrees.

A maar is a type of tuff ring which has been affected by sagging so that it lies below the the level of the surrounding surface.
 A maar is often filled with water and surrounded by a rim of ejected material that was probably formed by explosive interaction of magma and groundwater.

Examples of Maars
Lake Nyos (Cameroon)
Suoh (Sumatra, Indonesia)
Karpinsky Group (Kurile Islands)
Ukinrek Maars (Alaska)

Tuff Rings and Maars
Copyright John Seach