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.
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