(a) Thin ash layers consisting of clay to silt-sand-size (\5 mm in diameter, white
colored layers) alternating with darker and thicker coarse-grained ash beds
(centimeter scale) (Fig. 10.4b, f).
(b) Pyroclastic deposits from only a few cm up 10 cm in thickness, consisting of
angular lapilli (fragments of 5–70 mm in diameters) (Fig. 10.4b). The larger
blocks are gabbroic tonalite and andesite embedded in silt- and sand-sized
material.
(c) Closely alternated sequences of pyroclast and ash layers of a few meters thick
containing cobble-sized accidental debris. These alternating units represent a
more violent eruption than that of the fine sized ash beds (Fig. 10.4c).
(d) Rusted and cemented breccia formed of sand-, gravel- and lapilli-sized debris
embedded in iron-oxyhydroxide hydrothermal precipitates that occur on the
top of the caldera wall and floor (Fig. 10.4a).
We could see that shallow water (\600 m depth) and subaeriel island-arc
volcanism differs from deep-sea volcanism by the production of pumice and due to
the abundance of finely layered dark and light colored deposits. The magma that
generated the light grey and dark ash layers is the result of mixed basaltic (dark
colored) and more silica-enriched (white colored) products. The silica-enriched
material is produced from a more evolved magma residing in the magma chamber
prior to when the eruption started.
The processes giving rise to the mixed layered light and dark colored volcanic
units has been attributed to crystal grinding and to the absorption of a solidified
lava flow on the wall of the magma conduit (Polacci et al. 2005). The change in
viscosity during an upwelling of fluidal basaltic magma plays a role in lubricating
the crystallized and evolved wall rock formations. If the temperature is high
enough, remelted the evolved crystallized products will mix with the basaltic melt.
Thus, a sequential eruption of dark and light layered units will take place. Similar
phenomena could also happen on deep-sea ([1000 m depths) volcanic edifices,
such in those found on the MAR axis at 34°504 N and in intraplate regions of the
Pitcairn, Society and Austral hotspots volcanoes (See Chap. 9).
b IV (Fig. 10.3). The two tracks, one upward and the other one downward, made at a distance of
60 m apart, converge at the summit of the volcanic hill (Fig. 10.3a, b). The bottom photographs
shown in the middle of the two stratigraphic sections illustrates the sample sites and the lithology.
Assuming that each pyroclastic sequence going from coarse-grained and/or unsorted layers to
layered ash deposits represents a single event, then at least twenty eruptions have occurred. The
deposits are mainly oriented horizontally, unless otherwise indicated. In-situ photographs.
a Rusty colored cemented pyroclastic debris exposed on a faulted-step near the top (260 m depth)
of the caldera wall. b Alternating sequence of graded sand-, cobble- and silt- sized pyroclastic
deposits at 350 m depth. c Unsorted and poorly-sorted pyroclastic and ash layers at 360 m depth
overlaying finer grained ash-pyroclast beds. d Vertical dyke near an andesitic lava flow in the
vicinity of sample R9. e Layered ash and silt- to sand-sized pyroclastic deposits with a tilted
unconformity at 435 m depth. f Finely layered ash and sorted fine-grained (silt-sand size)
pyroclasts overlaying coarser ash-pyroclast units at 440–443 m depth near the base of the caldera
wall (21°07.8
0 S–175°43.6
0 W)
Volcanic Stratigraphy
359
colored layers) alternating with darker and thicker coarse-grained ash beds
(centimeter scale) (Fig. 10.4b, f).
(b) Pyroclastic deposits from only a few cm up 10 cm in thickness, consisting of
angular lapilli (fragments of 5–70 mm in diameters) (Fig. 10.4b). The larger
blocks are gabbroic tonalite and andesite embedded in silt- and sand-sized
material.
(c) Closely alternated sequences of pyroclast and ash layers of a few meters thick
containing cobble-sized accidental debris. These alternating units represent a
more violent eruption than that of the fine sized ash beds (Fig. 10.4c).
(d) Rusted and cemented breccia formed of sand-, gravel- and lapilli-sized debris
embedded in iron-oxyhydroxide hydrothermal precipitates that occur on the
top of the caldera wall and floor (Fig. 10.4a).
We could see that shallow water (\600 m depth) and subaeriel island-arc
volcanism differs from deep-sea volcanism by the production of pumice and due to
the abundance of finely layered dark and light colored deposits. The magma that
generated the light grey and dark ash layers is the result of mixed basaltic (dark
colored) and more silica-enriched (white colored) products. The silica-enriched
material is produced from a more evolved magma residing in the magma chamber
prior to when the eruption started.
The processes giving rise to the mixed layered light and dark colored volcanic
units has been attributed to crystal grinding and to the absorption of a solidified
lava flow on the wall of the magma conduit (Polacci et al. 2005). The change in
viscosity during an upwelling of fluidal basaltic magma plays a role in lubricating
the crystallized and evolved wall rock formations. If the temperature is high
enough, remelted the evolved crystallized products will mix with the basaltic melt.
Thus, a sequential eruption of dark and light layered units will take place. Similar
phenomena could also happen on deep-sea ([1000 m depths) volcanic edifices,
such in those found on the MAR axis at 34°504 N and in intraplate regions of the
Pitcairn, Society and Austral hotspots volcanoes (See Chap. 9).
b IV (Fig. 10.3). The two tracks, one upward and the other one downward, made at a distance of
60 m apart, converge at the summit of the volcanic hill (Fig. 10.3a, b). The bottom photographs
shown in the middle of the two stratigraphic sections illustrates the sample sites and the lithology.
Assuming that each pyroclastic sequence going from coarse-grained and/or unsorted layers to
layered ash deposits represents a single event, then at least twenty eruptions have occurred. The
deposits are mainly oriented horizontally, unless otherwise indicated. In-situ photographs.
a Rusty colored cemented pyroclastic debris exposed on a faulted-step near the top (260 m depth)
of the caldera wall. b Alternating sequence of graded sand-, cobble- and silt- sized pyroclastic
deposits at 350 m depth. c Unsorted and poorly-sorted pyroclastic and ash layers at 360 m depth
overlaying finer grained ash-pyroclast beds. d Vertical dyke near an andesitic lava flow in the
vicinity of sample R9. e Layered ash and silt- to sand-sized pyroclastic deposits with a tilted
unconformity at 435 m depth. f Finely layered ash and sorted fine-grained (silt-sand size)
pyroclasts overlaying coarser ash-pyroclast units at 440–443 m depth near the base of the caldera
wall (21°07.8
0 S–175°43.6
0 W)
Volcanic Stratigraphy
359