SMALLER SCALE FEATURES
There are many smaller features of Hawaiian volcanoes that
can be used to constrain the type of eruptive activity typical
of a particular volcano, portion of a volcano, or portion of a
volcano's lie. The main ones are vents and lava flows.
VENTS
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).
Spatter vents:
Spatter refers to blobs of lava thrown a little ways into
the air (by expanding gases) that is still molten when it lands.
Spatter ramparts and spatter cones are the vent structures
formed by this type of activity. Spatter ramparts are elongate along
the trace of an eruptive fissure whereas spatter cones occur as
discrete mounds. They range between 1 and 5 m high, are steep-sided,
and are composed of agglutinated (stuck-together) spatter. They
are steep-sided because the hot spatter blobs are able to stick
to each other when they land, and don't flow or roll away. The
fountaining associated with the formation of spatter ramparts
is usually less than 10 m high. At the end of the eruption, lava
often drains back into the fissure, forming prominent drainback
features. Even if nobody actually witnessed a particular eruption,
if you find spatter ramparts or cones associated with it, you
can say that the fountaining that formed the cone or rampart was
not very high.
Cinder cones:
At the high-fountaining end of the spectrum are cinder cones.
Cinder cones can be quite large in Hawai'i; those on the summit
of Mauna Kea (formed during gas-rich alkalic-stage eruptions)
are a few hundred meters high, whereas those on Mauna Loa and
Kilauea usually range between 20 and 100 m high. Pu'u
'O'o on the E rift of Kilauea, which formed between 1983 and 1986 is unusual
in that it reached a height of 255 m above the surrounding surface
(Heliker & Wright 1991).
As their name suggests, cinder cones consist of cinders, more
properly called scoria. Scoria is very vesicular, low density
basalt. Lava fountains are driven by expanding gas bubbles; the
bubbles are trying to expand in all directions but the only way
to relieve the pressure is up out the vent so fountains are usually
directed relatively vertically. The Pu'u
'O'o fountains were at times up to 350 m high, and those during the early stages of the
Mauna Ulu eruption were up to 500 m high. Because the pyroclasts
are thrown so high, they cool before they land and don't stick
together. Cinder cones are therefore composed of loose pyroclastics
at the angle of repose (~33º).
In plan view, cinder cones tend to be roughly
circular. They are usually formed later in an eruption when activity has localized
to one or more discrete vents. If the precise location of the
vent changes during an eruption, the cone loses its simple circular
shape, and becomes more complex. Roadcuts through most cinder
cones expose very complicated crosscutting
relationships relating to the different locations of the fountain centers. Cinder cones
can also be distinctly asymmetric if there was a persistent wind
blowing during the eruption and/or they form at the heads of major
lava flows. In this second instance they are horseshoe-shaped,
with the lava flow issuing out of the open end because during
the eruption any pyroclasts that landed on the flow were rafted
away.
Between the two extremes of spatter ramparts and cinder cones
are all gradations. Some pyroclastic constructs consist of alternating
layers of agglutinate and cinder, indicating that the vigor of
the fountaining varied during the eruption. The early part of
an eruption, often called the "curtain of fire", produces
mostly spatter ramparts and spatter cones. As the activity becomes
localized at one or more points along the fissure, this concentration
of activity usually leads to higher fountaining. Cinder cones
are built at these points, often at the same time that spatter
ramparts are forming at the (less active) ends of the fissure.
During the Mauna Loa eruption of 1984, there was a distinct gradation
from vigorous fountaining at the main vents, progressing to lower
and lower fountaining both up and downrift (Lockwood et al. 1987).
At the farthest uprift end of the fissure, only gas was being
emitted from a spatter cone that had been active earlier in the
eruption.
Satellitic shields:
Approximately 50% of Hawaiian eruptions have no pyroclastic
activity associated with them at all. Instead, lava is quietly
erupted onto the surface. This lava flows away in all directions
forming a miniature shield volcano.
These vents are called "satellitic" or "parasitic" shields, and produce tube-fed flows.
Satellitic shields have diameters of 1-2 km, and can be ~100 m
high with slopes of only a few degrees. While a satellitic shield
eruption is going on, a lava pond usually exists at the summit
of the shield. Overflows from the pond build the shield. There
have been 4 major satellitic shields formed on since the arrival
of Westerners (Mauna Iki 1919-1920, Mauna Ulu 1969-1974, Kupa'ianaha
1986-1992, and the presently-active vent 1992-who knows?). Including
these, 16 satellitic shields have been mapped on Kilauea (Holcomb 1987).
Phreatomagmatic vents:
When external water is involved in a basaltic eruption, the
eruption and vent constructs are markedly different, and such
an eruption would be one to avoid experiencing close up. The water
is usually ground water, but it can also be shallow ocean or lakes.
These phreatomagmatic eruptions have been most closely studied
at Surtsey, Iceland (Thorarinsson 1967).
The interaction of erupted lava and water is explosive, and therefore
the vent constructs are very different from those produced by
"dry" eruptions. During a phreatomagmatic eruption there
are thousands of shallow, closely-spaced, steam explosions that
fragment the lava into ash-sized particles. Because the explosions
are so shallow, much of the explosive energy is directed sub-horizontally.
These sub-horizontal blasts are called base surges and each deposits
a single layer of ash. The resulting vent construct from such
an eruption is more like a ring (much wider compared to its height)
than a cone, and consists of the thousands of individual layers.
The fine-grained phreatomagmatic ash weathers into tuff, and the
constructs are called tuff rings or tuff cones. The best examples
in Hawai'i are those of the Honolulu volcanic series, however,
there is one tuff ring on (Kapoho cone) and one on Mauna Loa (Pu'u
Mahana). Both of the big island examples formed near the coastline
where the water table is within a few meters of the surface.
The very explosive eruption of Kilauea in 1790 was caused
in part by groundwater interaction but was apparently so explosive
that no tuff ring was formed; all the ash was dispersed widely.
In 1924, ground water also came in contact with hot rocks in the
shallow summit region, but this time there was no juvenile magma
involved. Technically these 1924 steam
eruptions were therefore phreatic rather than phreatomagmatic, and all the blocks that
were thrown out were solid pieces of pre-existing lava.
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