2.4 V olcaniclastic Sediments
temporaneous debris flows and mud flows, originating on
the summit and the flanks of the vo1cano, travelled as far as
130 km and filled valleys and pre-existing lakes.
Foreare basin sediments (Seet. 12.5.3) range from
eontinental to shallow-marine and deep-marine. In
the ease of an island are, they tend to be rieh in
voleaniclastie material of basaltie to andesitie eharaeter. Foreare basins assoeiated with eontinental margin ares, on the other hand, aeeumulate more aeidie
to intermediate, ealc-alkaline tephra. Deep foreare
basins typieally display abundant debris flows and
thiek turbidite sequenees rieh in volcaniclasties.
Backarc basins (Seet. 12.5.4) eommonly provide
less deep depositional environments ranging from
fluvial, laeustrine to shallow-marine. If the dominant
winds are direeted toward the eontinent, backare basins ean aeeumulate high volumes of fallout deposits
from ash clouds over the entire basin. Lava flows,
pyroclastie flows, and debris flows, however, enter
the basin from the side of the magmatie are and thus
ereate an asymmetrie basin fill. The backare basins
east of the Andes in South Ameriea provide a prominent example of thiek eontinental volcaniclastie sequenees derived from a volcanie are to the west.
The sediments of mature passive margins and
foreland basins normally eontain only small amounts
of voleaniclasties.
2.4.6 Production Rates, Duration, and
Recurrence Intervals of Volcanic Activity
V olcanie aetivity in eontinental rift zones, at midoeeanie ridges or along magmatie ares ean persist for
eonsiderable time periods, ranging from several tens
to some hundred millions of years.
A simple and purely theoretieal ealculation indieates the volume of volcanie material whieh ean be
produeed by a magmatie are in eonjunetion with
subduetion in a eertain time span needed for the closure of an oeean basin.
Given an ocean of 1000 km width and a subduction rate of
5 cmJa, the elosure of the ocean and hence the volcanic
activity will last for 20 Ma. The volume of ejected volcanic
material depends on the proportion of the subducted crust
and neighboring rocks which is molten and extruded at the
surface. If a plate of I km length (parallel to the subduction
zone), 1000 km wide, and 6 km thick, is subducted and
only 1 % of the volume of this rock body is transformed
into volcanielastic material, a sediment volume of approximately 60 km] (3 km 3 /km arc length per Ma) results.
Evenly distributed over a basin of 120 km width and I km
length, the volcanielastic material can form a layer of 500
m in thickness in a time span of 20 Ma.
This model shows that subduetion-related volcanism
ean produee indeed very large amounts of volcani73
clastie material. In fact, magma produetion of volcanie ares ean be higher, but seerns to be more eomplex
and probably varies eonsiderably along strike.
For oeeanie island ares, Straub and Sehmineke
(1998) have determined a produetion rate of 10-13
kmJ/km are length per Ma. It is therefore possible
that a higher pereentage of the subdueting oeeanie
plate contributes to the volcanie aetivity along magmatie ares.
A comprehensive study of volcanielastic sediments in the
Pacific, which is the world' s oldest and largest ocean basin,
revealed that about 23 vol-% of the existing Cretaceous
and Cenozoic sediments of the Pacific consist of tephra
(Straub and Schmincke 1998). This estimate is based on 65
legs of deep-sea drilling. A major part ofthe drilled tephra
is represented by distal ash falls resulting from explosive
arc volcanism. The percentage of proximal tephra is poorly
known, because sites elose to volcanic arcs were rarely
drilled, or they have been subducted.
Individual volcanie eruptions ejeeting large rock volumes are eommonly less frequent than small eruptions. Likewise, rhyolitie eruptions tend to oeeur less
frequently and to produee larger rock volumes than
do basaltie eruption. Rare eatastrophie volcanie
events are eharaeterized by long reeurrenee intervals
of the order of 100 ka to 1 Ma. They affeet large regions and possibly the entire globe (by eooling), beeause they ejeet extensive and long-persisting dust
clouds into the stratosphere. The reeurrenee intervals
of small eruptions ranges from several to hundreds of
years. Astriet eyclieity of these processes eannot be
expeeted.
Volcanic eruptions in the center of Iceland, located above a
mantle plume and related to rifting and ocean spreading,
appear to have occurred periodically (130-140 year periods) during the past 800 years (Larsen et al. 1998). In periods ofhigh activity, individual eruptions took place at time
intervals of 3 to 7 years; in low-activity periods the intervals were longer. This findings are based on tephrostratigraphy in ice cores and written records. Other parts of the
plate boundary in Iceland show lower eruption frequeneies.
2.4.7 Alteration, Diagenesis, and Metamorphism
of V olcaniclastic Rocks
The maj or part of volcanie minerals and partieularly
glassy materials are thermodynamieally unstable under normal temperatures and pressures near the
Earth's surfaee. In addition, volcaniclastie sediments
have, in eontrast to lava flows, high initial porosities
and permeabilities for eireulating water. As a result,
volcaniclastie sediments are easily altered by
pedogenesis and, under inereasing burial depth, by
diagenesis and metamorphism (cf. Seet. 13.3).
temporaneous debris flows and mud flows, originating on
the summit and the flanks of the vo1cano, travelled as far as
130 km and filled valleys and pre-existing lakes.
Foreare basin sediments (Seet. 12.5.3) range from
eontinental to shallow-marine and deep-marine. In
the ease of an island are, they tend to be rieh in
voleaniclastie material of basaltie to andesitie eharaeter. Foreare basins assoeiated with eontinental margin ares, on the other hand, aeeumulate more aeidie
to intermediate, ealc-alkaline tephra. Deep foreare
basins typieally display abundant debris flows and
thiek turbidite sequenees rieh in volcaniclasties.
Backarc basins (Seet. 12.5.4) eommonly provide
less deep depositional environments ranging from
fluvial, laeustrine to shallow-marine. If the dominant
winds are direeted toward the eontinent, backare basins ean aeeumulate high volumes of fallout deposits
from ash clouds over the entire basin. Lava flows,
pyroclastie flows, and debris flows, however, enter
the basin from the side of the magmatie are and thus
ereate an asymmetrie basin fill. The backare basins
east of the Andes in South Ameriea provide a prominent example of thiek eontinental volcaniclastie sequenees derived from a volcanie are to the west.
The sediments of mature passive margins and
foreland basins normally eontain only small amounts
of voleaniclasties.
2.4.6 Production Rates, Duration, and
Recurrence Intervals of Volcanic Activity
V olcanie aetivity in eontinental rift zones, at midoeeanie ridges or along magmatie ares ean persist for
eonsiderable time periods, ranging from several tens
to some hundred millions of years.
A simple and purely theoretieal ealculation indieates the volume of volcanie material whieh ean be
produeed by a magmatie are in eonjunetion with
subduetion in a eertain time span needed for the closure of an oeean basin.
Given an ocean of 1000 km width and a subduction rate of
5 cmJa, the elosure of the ocean and hence the volcanic
activity will last for 20 Ma. The volume of ejected volcanic
material depends on the proportion of the subducted crust
and neighboring rocks which is molten and extruded at the
surface. If a plate of I km length (parallel to the subduction
zone), 1000 km wide, and 6 km thick, is subducted and
only 1 % of the volume of this rock body is transformed
into volcanielastic material, a sediment volume of approximately 60 km] (3 km 3 /km arc length per Ma) results.
Evenly distributed over a basin of 120 km width and I km
length, the volcanielastic material can form a layer of 500
m in thickness in a time span of 20 Ma.
This model shows that subduetion-related volcanism
ean produee indeed very large amounts of volcani73
clastie material. In fact, magma produetion of volcanie ares ean be higher, but seerns to be more eomplex
and probably varies eonsiderably along strike.
For oeeanie island ares, Straub and Sehmineke
(1998) have determined a produetion rate of 10-13
kmJ/km are length per Ma. It is therefore possible
that a higher pereentage of the subdueting oeeanie
plate contributes to the volcanie aetivity along magmatie ares.
A comprehensive study of volcanielastic sediments in the
Pacific, which is the world' s oldest and largest ocean basin,
revealed that about 23 vol-% of the existing Cretaceous
and Cenozoic sediments of the Pacific consist of tephra
(Straub and Schmincke 1998). This estimate is based on 65
legs of deep-sea drilling. A major part ofthe drilled tephra
is represented by distal ash falls resulting from explosive
arc volcanism. The percentage of proximal tephra is poorly
known, because sites elose to volcanic arcs were rarely
drilled, or they have been subducted.
Individual volcanie eruptions ejeeting large rock volumes are eommonly less frequent than small eruptions. Likewise, rhyolitie eruptions tend to oeeur less
frequently and to produee larger rock volumes than
do basaltie eruption. Rare eatastrophie volcanie
events are eharaeterized by long reeurrenee intervals
of the order of 100 ka to 1 Ma. They affeet large regions and possibly the entire globe (by eooling), beeause they ejeet extensive and long-persisting dust
clouds into the stratosphere. The reeurrenee intervals
of small eruptions ranges from several to hundreds of
years. Astriet eyclieity of these processes eannot be
expeeted.
Volcanic eruptions in the center of Iceland, located above a
mantle plume and related to rifting and ocean spreading,
appear to have occurred periodically (130-140 year periods) during the past 800 years (Larsen et al. 1998). In periods ofhigh activity, individual eruptions took place at time
intervals of 3 to 7 years; in low-activity periods the intervals were longer. This findings are based on tephrostratigraphy in ice cores and written records. Other parts of the
plate boundary in Iceland show lower eruption frequeneies.
2.4.7 Alteration, Diagenesis, and Metamorphism
of V olcaniclastic Rocks
The maj or part of volcanie minerals and partieularly
glassy materials are thermodynamieally unstable under normal temperatures and pressures near the
Earth's surfaee. In addition, volcaniclastie sediments
have, in eontrast to lava flows, high initial porosities
and permeabilities for eireulating water. As a result,
volcaniclastie sediments are easily altered by
pedogenesis and, under inereasing burial depth, by
diagenesis and metamorphism (cf. Seet. 13.3).