Type of Volcanic Rocks Formed During Explosive
Activity:
Pyroclasts (from the Greek words ‘‘pyros’’ meaning fire and ‘‘klastos’’
meaning broken) are fragmented pieces of material that were ejected
during explosive volcanic eruptions. They refer to the observed scattered
blocky or bomb-shaped ‘‘clasts’’ that are fragments of various sizes from a
few centimeters up to several meters in diameter (See Chap. 5). Pyroclasts
clustered during a volcanic eruption take various forms: (1) isolated bombs
which were blasted from a volcano during explosion or (2) when mixed
with water, these hot ‘‘bombs’’ form ‘‘lahars’’, which are mixed mud and
rock debris that could travel long distances (100 km) from their point of
origin, or (3) avalanche deposits that have rolled down slope.
Volcanic breccias refer to accidental explosive rock debris as well as
mechanically transported rocks caused by gravity slumping along a slope.
Volcanic breccias are cemented with silt and sand size material. This
cementing takes place during diagenesis (physical and chemical changes
undergone by the material after its deposition) and it is due to volatile
exchanges (CO 2 , H 2 O, hydrated silica solutions) and/or during low-temperature hydrothermal fluid circulation.
Hyaloclastites (from the Greek words ‘‘hualos’’ meaning glass and
‘‘klastos’’ meaning broken) usually form flat lying layered deposits or
rubble debris of broken up slabs composed of silt to sand size volcanic
glassy shards cemented by either a sediment matrix and/or by hydrothermal
precipitates. The glass shards are formed during the explosive activity
when magma comes into contact with water. After the shards have fallen on
the sea floor, low temperature hydrothermal precipitates of Fe and Mnoxyhydroxides will cement the glassy shards to form slabs of hyaloclastites.
The cementing products include hydrated material such as goethite, limonite and palagonite (Nayudu 1962). These slabs could be coated with a thin
manganese-enriched crust composed of birnessite as well as crusts of the
clay minerals. Located on top of lava flows and pelagic sediment, hyaloclastites are mainly encountered on seamounts. The hyaloclastite slabs
could extend over hundreds of meters and cover sand-sized loose debris
extruded during explosive volcanism (lapilli).
Volcanic Eruption and Distribution
123
Activity:
Pyroclasts (from the Greek words ‘‘pyros’’ meaning fire and ‘‘klastos’’
meaning broken) are fragmented pieces of material that were ejected
during explosive volcanic eruptions. They refer to the observed scattered
blocky or bomb-shaped ‘‘clasts’’ that are fragments of various sizes from a
few centimeters up to several meters in diameter (See Chap. 5). Pyroclasts
clustered during a volcanic eruption take various forms: (1) isolated bombs
which were blasted from a volcano during explosion or (2) when mixed
with water, these hot ‘‘bombs’’ form ‘‘lahars’’, which are mixed mud and
rock debris that could travel long distances (100 km) from their point of
origin, or (3) avalanche deposits that have rolled down slope.
Volcanic breccias refer to accidental explosive rock debris as well as
mechanically transported rocks caused by gravity slumping along a slope.
Volcanic breccias are cemented with silt and sand size material. This
cementing takes place during diagenesis (physical and chemical changes
undergone by the material after its deposition) and it is due to volatile
exchanges (CO 2 , H 2 O, hydrated silica solutions) and/or during low-temperature hydrothermal fluid circulation.
Hyaloclastites (from the Greek words ‘‘hualos’’ meaning glass and
‘‘klastos’’ meaning broken) usually form flat lying layered deposits or
rubble debris of broken up slabs composed of silt to sand size volcanic
glassy shards cemented by either a sediment matrix and/or by hydrothermal
precipitates. The glass shards are formed during the explosive activity
when magma comes into contact with water. After the shards have fallen on
the sea floor, low temperature hydrothermal precipitates of Fe and Mnoxyhydroxides will cement the glassy shards to form slabs of hyaloclastites.
The cementing products include hydrated material such as goethite, limonite and palagonite (Nayudu 1962). These slabs could be coated with a thin
manganese-enriched crust composed of birnessite as well as crusts of the
clay minerals. Located on top of lava flows and pelagic sediment, hyaloclastites are mainly encountered on seamounts. The hyaloclastite slabs
could extend over hundreds of meters and cover sand-sized loose debris
extruded during explosive volcanism (lapilli).
Volcanic Eruption and Distribution
123