As heating of the lithosphere continues, the degree of partial
melting increases and the absolute volume of magma produced really
increases. A higher degree of partial melting produces tholeiite
basalt, which has a slightly higher % silica than alkalic basalt.
It is during this stage that the plumbing systems within the volcanoes
are the most efficient at transporting magma to the surface. >95%
of each volcano consists of lava erupted during this
main tholeiite
stage (e.g.
Clague 1987).
Both Mauna Loa and Kilauea are in this stage of life, and have
together erupted ~114 times since the arrival of Westerners. These
tholeiite lavas are fluid and can build up only gradual slopes,
producing the classic shield volcano shape.
It is also during this stage that a magma chamber fully develops
to serve as a way-station for ascending magma. A magma chamber
migrates upward as the volcano grows, and the magma chambers of
Mauna Loa and Kilauea are both 2-3 km below the summits. Although
usually depicted as giant balloons, magma chambers are most probably
a complex of smaller interconnected voids (more like a magma chamber
complex). This idea has been confirmed at by geodetic measurements
that show the center of deformation moving around during periods
of inflation and deflation
(Fiske & Kinoshita 1969).
While stored in the magma chamber complex, magma cools and partially
crystallizes. Olivine is usually the first mineral to crystallize
out of Hawaiian tholeiite magma, and olivine crystals will settle
out while the magma is sitting in the magma chamber complex. Olivine-rich
lavas are thus expected if an eruption taps the lower part of
the magma chamber complex. An additional thing that happens during
storage is that gases can escape from the magma and migrate to
the surface. The three main volatiles are water (H2O), sulfur
dioxide (SO2), and carbon dioxide (CO2). CO2 exsolves at a greater
depth so it escapes shortly after a batch of magma reaches the
magma chamber complex. H2O and SO2 stay in solution within the
magma for a longer period of time. This means that scientists
can determine some of the processes going on down in the magma
chamber complex even though they can't actually go
there. For
example, if gases collected at the summit show a high amount of
CO2 relative to SO2; then a fresh batch of magma must have recently
arrived from the mantle. If you are concerned with eruption prediction
this might be something good to know about! On the other hand,
if the ratio of CO2 to SO2 is relatively low, then you know that
the magma that is giving off gases is not new--it is just slowly
releasing the SO2. The final process that goes on while magma
is resting in the magma chamber complex regards pieces of rock
that are picked up while the magma is migrating from its initial
source. Such pieces are called xenoliths ("foreign rock"),
and they are almost always denser than the magma. As soon as the
magma comes into the magma chamber and stops moving upward, these
xenoliths can no longer be supported and they sink to the floor
of the chamber.
The end results of all these processes are that lavas erupted
during this main shield stage of volcanic life are: 1) hot and
fluid because they have an efficient pre-heated plumbing system
to get them to the surface; 2) have already lost some of their
gas when they eventually erupt because it escaped while the magma
was resting in the magma chamber; 3) possibly olivine-rich if
the eruption taps the lower part of the magma chamber; and 4)
unlikely to include xenoliths because the xenoliths sank to the
bottom of the magma chamber.
Another important consequence of the development of a magma chamber
is that it can lead to the formation of a
caldera.
Calderas result
from collapse and/or subsidence into the magma chamber, thus a
caldera is a sign that an active magma chamber is or once was
present, and this in turn implies a high supply to the volcano.
Calderas are very dynamic features, however, and at times they
can be completely filled in, only to re-form again later.
The main products of Hawaiian eruptions are lava flows and pyroclastic
deposits. When lava flows encounter the ocean they may spread
along the coastline because the water provides a cooling barrier
that slows the forward progress. This results in the formation
of a relatively flat lava shelf, even if the slopes directly inland
and offshore are steep. The coalescence of numerous shelves forms
a coastal plain. The frequent eruptions during the
tholeiite stage
mean that construction of the coastal plain is able to keep pace
with the subsidence of the volcano, and indeed the parts of Kilauea
and Mauna Loa that have the most young flows are characterized
by well-developed coastal terraces.
VolcanoWorld