Vugs (also spelled vugh) are small to medium-sized cavities inside
rock that may be formed through a variety of processes. Most commonly cracks and fissures opened by tectonic activity (
folding and
faulting) are partially filled by
quartz,
calcite, and other secondary
minerals. Open spaces within ancient collapse
breccias are another important source of vugs. Vugs may also result when mineral
crystals or fossils inside a rock matrix are later removed through erosion or dissolution processes, leaving behind irregular voids. The inner surfaces of such vugs are often coated with a crystal
druse. Fine crystals are often found in vugs where the open space allows the free development of external crystal form. The term vug is not applied to veins and fissures that have become completely filled, but may be applied to any small cavities within such veins.
Geodes are a common vug formed rock, although that term is usually reserved for more rounded crystal-lined cavities in
sedimentary rocks and ancient
lavas.
