Vents, of course, are the locations from which lava flows
and pyroclastic material are erupted. Their forms and orientations
can be used to determine many characteristics of the eruption
with which they were associated. There are two main endmembers
in a spectrum of pyroclastic vents in Hawai'i, spatter vents and
cinder cones. Their differences are due mostly to the gas content
of the magma that is erupted. Additionally, there are satellitic
shields formed during eruptions without fountaining and tuff cones
formed during phreatomagmatic eruptions.
As a dike approaches the surface, it generates a zone of tension
at the surface. This tension is usually manifested as a pair of
cracks with the ground in between sometimes dropped down a little.
The first phase of a Hawaiian eruption is usually characterized
by breaking to the surface of a dike along one of the two fractures
resulting in a line of erupting vents commonly called a "curtain
of fire" (e.g. Macdonald 1972).
After a few hours or few days most parts of the fissure stop erupting
and activity is concentrated at one or more separate vents (e.g. Bruce & Huppert 1989).
It is these vent locations that usually persist long enough (hours
to weeks and sometimes years) to produce significant near-vent
constructs. The change from long continuous erupting fissures
to one or a few vents must be remembered when mapping eruptive
fissures in remote sensing data and relating them to dike dimensions:
The near-surface part of the dike is almost certainly longer than
any line of near-vent constructs (see discussion in Munro 1992).