submarine environment. The individual columns have a diameter varying from
10 to 20 cm and show hexagonal and cubic cross sections. From field
observations, these flows show different morpho-structural patterns. They
consist of tabular layered flows forming a horizontal and slightly inclined
‘‘staircase’’ with sill-like complexes or with a blocky appearance. They are
dark-grey fine-grained intermediate (SiO 2 = 55–60 %) silicic lava. A sample
(R15) of a massive columnar flow collected at 380 m depth consists of a finegrained andesite containing an assemblage of plagioclase-clinopyroxene-titanomagnetite. The rock’s parallel oriented cracks are filled by a dark chilled
goundmass formed during its cooling prior to the complete solidification of the
rock. More details on the geology and formation of these lava flows are found
in Hekinian et al. (2008).
In summary, Volcano # 1 consists of an oval-shaped collapsed caldera that is
30 km
2 and whose floor is located at 450 m depth. A portion of the caldera wall
was visited and the stratigraphy was studied during one dive (PIV 143). The results
of the dive indicate that the caldera wall’s stratigraphy is the same as an andesitic
strato-cone volcano growing as a result of numerous repeated small eruptions of
pyroclast-ash deposits, alternating with an occasional outpouring of basaltic tubes
and pillow lava (Fig. 10.4b, c, e). In other words, Volcano # 1 was constructed
during successive and short-lived volcanic explosive eruptions of pyroclastic
deposits, which alternated with a quieter outpouring of massive lava flows. The
repeated, violent, explosive volcanic eruptions formed thin (centimeter thick)
layers of ash and pyroclastic debris. The floor and the top of the caldera wall
showed occasional small volcanic constructions covered by consolidated ashlapilli (volcaniclastic) deposits. These volcanic events gave rise to the successive
eruption of silica-rich intrusives and viscous extrusive flows followed by, or
preceded by, volcaniclastic deposits, which represent the sequential explosive
events forming alternate layers of fine-ash beds (5–10 cm thick) and coarser
pyroclastic deposits varying between 50 and 100 cm up to 20 m thick. The
massive flow units forming columnar, tabular and giant radial-jointed (GRJ) flows
represent viscous magma conduits or pipes. The magma conduits are several tens
of meters in diameter and form horizontal columnar flows with ‘‘staircase-like’’
and fan-shaped appearances giving rise to the GRJ flows. The giant radial-jointed
(GRJ) flows are created by magma issued from larger conduits and protrude
outward up to several tens of meters away from the intrusive pipes. The columnar
flows are comparable to volcanic-neck magma intrusions like those encountered in
the subaerial regions of the Coastal Range area of the Sierra Nevada and in Eastern
Taiwan (Juang and Chen 2004; Huber and Rinehart 1967). However, since not all
the massive columnar flows encountered on the caldera wall of Volcano #1 are
seen terminating with a GRJ, it is not excluded that some of the tabular flows were
originally horizontally injected sills.
Volcanic Stratigraphy
361
10 to 20 cm and show hexagonal and cubic cross sections. From field
observations, these flows show different morpho-structural patterns. They
consist of tabular layered flows forming a horizontal and slightly inclined
‘‘staircase’’ with sill-like complexes or with a blocky appearance. They are
dark-grey fine-grained intermediate (SiO 2 = 55–60 %) silicic lava. A sample
(R15) of a massive columnar flow collected at 380 m depth consists of a finegrained andesite containing an assemblage of plagioclase-clinopyroxene-titanomagnetite. The rock’s parallel oriented cracks are filled by a dark chilled
goundmass formed during its cooling prior to the complete solidification of the
rock. More details on the geology and formation of these lava flows are found
in Hekinian et al. (2008).
In summary, Volcano # 1 consists of an oval-shaped collapsed caldera that is
30 km
2 and whose floor is located at 450 m depth. A portion of the caldera wall
was visited and the stratigraphy was studied during one dive (PIV 143). The results
of the dive indicate that the caldera wall’s stratigraphy is the same as an andesitic
strato-cone volcano growing as a result of numerous repeated small eruptions of
pyroclast-ash deposits, alternating with an occasional outpouring of basaltic tubes
and pillow lava (Fig. 10.4b, c, e). In other words, Volcano # 1 was constructed
during successive and short-lived volcanic explosive eruptions of pyroclastic
deposits, which alternated with a quieter outpouring of massive lava flows. The
repeated, violent, explosive volcanic eruptions formed thin (centimeter thick)
layers of ash and pyroclastic debris. The floor and the top of the caldera wall
showed occasional small volcanic constructions covered by consolidated ashlapilli (volcaniclastic) deposits. These volcanic events gave rise to the successive
eruption of silica-rich intrusives and viscous extrusive flows followed by, or
preceded by, volcaniclastic deposits, which represent the sequential explosive
events forming alternate layers of fine-ash beds (5–10 cm thick) and coarser
pyroclastic deposits varying between 50 and 100 cm up to 20 m thick. The
massive flow units forming columnar, tabular and giant radial-jointed (GRJ) flows
represent viscous magma conduits or pipes. The magma conduits are several tens
of meters in diameter and form horizontal columnar flows with ‘‘staircase-like’’
and fan-shaped appearances giving rise to the GRJ flows. The giant radial-jointed
(GRJ) flows are created by magma issued from larger conduits and protrude
outward up to several tens of meters away from the intrusive pipes. The columnar
flows are comparable to volcanic-neck magma intrusions like those encountered in
the subaerial regions of the Coastal Range area of the Sierra Nevada and in Eastern
Taiwan (Juang and Chen 2004; Huber and Rinehart 1967). However, since not all
the massive columnar flows encountered on the caldera wall of Volcano #1 are
seen terminating with a GRJ, it is not excluded that some of the tabular flows were
originally horizontally injected sills.
Volcanic Stratigraphy
361