72
vial and lake sediments may be interspersed with basaltic volcanics.
Continental rift zones with initial updoming, as for
example the East African Rift (cf. Sect. 12.1),. ?i~play widespread basic, but in places also more SIllCIC
flood vo1canism. Such lavas are ernpted from large,
laterally continuous fractures in the crnst. The columnar jointed, massive lava flows may be accompanied
by ropy (pahoehoe) and completely fragmented,
rough (aa) lava. The preservation potential of these
features is limited, but their weathered and reworked
products often play a considerable role in the early
filling of the subsequent graben zone. With the onset
of extension and rifting, a petrologically diverse volcanism can be observed in limited areas of the axial
graben. It may range from mafic to silicic rocks of
more or less alkali ne nature. Pyroclastic flows and
ignimbrites are common, and their eroded material is
incorporated into the predominating fluvial and lacustrine sediments of this stage. Later, in an advanced
rifting stage with prevailing marine sediments, th.e
influence of vo1canism in the area of extended contlnental crnst diminishes. If a new spreading center
develops, oceanic tholeiitic basalts are involved.
Intraplate oeeanic volcanism occurs on older oc~anic crnst. Typical representatives are the volcamc
island-seamount chains in the Pacific (Sect. 12.5.4).
The basaltic vo1canoes are built up of pillow lavas,
hyaloclastites (fine-grained sideromelane glass
shards), pyroclastic material, and massive lava. They
eject large volumes of vo1canic debris into the atmosphere and into the sea. The vo1caniclastic material
may mix with reef detritus, shallow-water carbonate,
and pelagic sediments.
The island of Gran Canaria (Canary Islands) in the eastern
Atlantic is an example of such an intraplate volcanic island,
but its late-stage dominant acidic type of magma deviates
from that of the Pacific seamounts and islands. This is explained by differentiation of basaltic magma within the
mantle plume (Funck and Schmincke 1998). The large
volcaniclastic apron around the island has been drilled at
several sites during Leg 157 of the Ocean Drilling Program, which allowed to reeonstruet the evolution of the
island (Schmincke and Sumita 1998). The early stages of
volcanie aetivity are documented by thick hyaloclastite
breccias and debris flow deposits of a shield volcano whieh
began to grow 16.8 Ma aga in deep water and then
emerged above sea level. This and the subsequent stages of
island growth were followed by phases of eollapse and
island destruction. Tephra were transported episodieally
downslope into water dcpths of >4000 m. Late-stage
rhyolitie and trachyphonolitie magma and ash generated
ignimbrites and many ash turbidites whieh were traced 70
km away from the island. Subaerial ash and debris flows
entered the sea and eontinued to flow on the sea floor. In
total, a volume of -50 000 km 3 of magma has been released
from the mantle. The proportion of intrusive magnta, makChapter 2 Continental Sediments
ing up the eore of the island, may amount to twiee the volurne ofthe extruded voleanic material.
Large igneous provinees. In terms of volume, t~e
so-called LIP's, such as the Ontong-Java Plateau 1D
the southwestem Pacific, the Kerguelen Plateau in
the southem Indian Ocean, or the vo1canic passive
margin of the North Atlantic are outstanding. F~rthermore, magma bodies of almost comparable Slze
occur on the continents, e.g. the extensive mid-Cretaceous Dekkan flood basalts in India and the Jurassic
flood basalts of the Parana basin in South America.
The volumes of these vo1canic bodies range from ~ 1
to 36 X 10 6 km 3 (Ontong-Java Plateau), and at least
some of them formed in very short time intervals
(one to a few millions of years; Coffin and Eldholm
1994). The large igneous provinces below the sea
and their accompanying vo1caniclastics are presently
under investigation by the Ocean Drilling Program.
Mid-oeeanie ridge voleanism leads to the accretion
of new oceanic crnst. This mechanism yields the
highest production rate of vo1canic rocks, but only a
minor proportion of the ascending magma is widely
dispersed as vo1caniclastic material. The basaltic
magma extrndes from fissures in the median valley
of the ridges and produces abundant pillow basalts,
other lava flows, vo1canic breccias, and pyroclastic
deposits. These volcanic rocks are character~stic~lly
associated with pelagic sediments such as radlOlanan
chert, siliceous green and red clay, or limestones.
Metalliferous sediments of limited extent can accumulate near hydrothermal vents.
Island are and eontinental margin are voleanism
associated with an active subduction zone (Chap. 1,
Fig. 1.2 and Sect. 12.5) is most effective in producing large volumes of vo1caniclastic materials (see below). The vo1canoes are lined up in a narrow, at least
several hundred kilometers long zone parallel to a
deep-sea trench. The magma production r~te is a
function of the subduction rate and posslbly enhanced by concurrent spreading of a backarc basin.
In the case of an island arc, the volcanoes may be
partially submerged or emerged above sea level.
Their vo1canic products are mainly basaltic or
andesitic, but locally more silicic differentiates including ignimbrites have been observed. Arc vo1canism along Anden-type continental margins exhibits
high proportions of intermediate and ~ilicic, largely
ca1c-alkaline material. They may also eJect large volumes of ignimbrites.
The 1980 eruption ofMount St. Helens in Washington .is a
medium-scale, loeal example of this type of are volcamsm.
Due to westerly winds, the pyroclastie fallout was diSpersed about 1000 km to the east of the voleano. At ~ distanee of 300 km east of the source, the ash fall deposit was
2 cm in thiekness (Lipmann and Mullineaux 1981). Con-
vial and lake sediments may be interspersed with basaltic volcanics.
Continental rift zones with initial updoming, as for
example the East African Rift (cf. Sect. 12.1),. ?i~play widespread basic, but in places also more SIllCIC
flood vo1canism. Such lavas are ernpted from large,
laterally continuous fractures in the crnst. The columnar jointed, massive lava flows may be accompanied
by ropy (pahoehoe) and completely fragmented,
rough (aa) lava. The preservation potential of these
features is limited, but their weathered and reworked
products often play a considerable role in the early
filling of the subsequent graben zone. With the onset
of extension and rifting, a petrologically diverse volcanism can be observed in limited areas of the axial
graben. It may range from mafic to silicic rocks of
more or less alkali ne nature. Pyroclastic flows and
ignimbrites are common, and their eroded material is
incorporated into the predominating fluvial and lacustrine sediments of this stage. Later, in an advanced
rifting stage with prevailing marine sediments, th.e
influence of vo1canism in the area of extended contlnental crnst diminishes. If a new spreading center
develops, oceanic tholeiitic basalts are involved.
Intraplate oeeanic volcanism occurs on older oc~anic crnst. Typical representatives are the volcamc
island-seamount chains in the Pacific (Sect. 12.5.4).
The basaltic vo1canoes are built up of pillow lavas,
hyaloclastites (fine-grained sideromelane glass
shards), pyroclastic material, and massive lava. They
eject large volumes of vo1canic debris into the atmosphere and into the sea. The vo1caniclastic material
may mix with reef detritus, shallow-water carbonate,
and pelagic sediments.
The island of Gran Canaria (Canary Islands) in the eastern
Atlantic is an example of such an intraplate volcanic island,
but its late-stage dominant acidic type of magma deviates
from that of the Pacific seamounts and islands. This is explained by differentiation of basaltic magma within the
mantle plume (Funck and Schmincke 1998). The large
volcaniclastic apron around the island has been drilled at
several sites during Leg 157 of the Ocean Drilling Program, which allowed to reeonstruet the evolution of the
island (Schmincke and Sumita 1998). The early stages of
volcanie aetivity are documented by thick hyaloclastite
breccias and debris flow deposits of a shield volcano whieh
began to grow 16.8 Ma aga in deep water and then
emerged above sea level. This and the subsequent stages of
island growth were followed by phases of eollapse and
island destruction. Tephra were transported episodieally
downslope into water dcpths of >4000 m. Late-stage
rhyolitie and trachyphonolitie magma and ash generated
ignimbrites and many ash turbidites whieh were traced 70
km away from the island. Subaerial ash and debris flows
entered the sea and eontinued to flow on the sea floor. In
total, a volume of -50 000 km 3 of magma has been released
from the mantle. The proportion of intrusive magnta, makChapter 2 Continental Sediments
ing up the eore of the island, may amount to twiee the volurne ofthe extruded voleanic material.
Large igneous provinees. In terms of volume, t~e
so-called LIP's, such as the Ontong-Java Plateau 1D
the southwestem Pacific, the Kerguelen Plateau in
the southem Indian Ocean, or the vo1canic passive
margin of the North Atlantic are outstanding. F~rthermore, magma bodies of almost comparable Slze
occur on the continents, e.g. the extensive mid-Cretaceous Dekkan flood basalts in India and the Jurassic
flood basalts of the Parana basin in South America.
The volumes of these vo1canic bodies range from ~ 1
to 36 X 10 6 km 3 (Ontong-Java Plateau), and at least
some of them formed in very short time intervals
(one to a few millions of years; Coffin and Eldholm
1994). The large igneous provinces below the sea
and their accompanying vo1caniclastics are presently
under investigation by the Ocean Drilling Program.
Mid-oeeanie ridge voleanism leads to the accretion
of new oceanic crnst. This mechanism yields the
highest production rate of vo1canic rocks, but only a
minor proportion of the ascending magma is widely
dispersed as vo1caniclastic material. The basaltic
magma extrndes from fissures in the median valley
of the ridges and produces abundant pillow basalts,
other lava flows, vo1canic breccias, and pyroclastic
deposits. These volcanic rocks are character~stic~lly
associated with pelagic sediments such as radlOlanan
chert, siliceous green and red clay, or limestones.
Metalliferous sediments of limited extent can accumulate near hydrothermal vents.
Island are and eontinental margin are voleanism
associated with an active subduction zone (Chap. 1,
Fig. 1.2 and Sect. 12.5) is most effective in producing large volumes of vo1caniclastic materials (see below). The vo1canoes are lined up in a narrow, at least
several hundred kilometers long zone parallel to a
deep-sea trench. The magma production r~te is a
function of the subduction rate and posslbly enhanced by concurrent spreading of a backarc basin.
In the case of an island arc, the volcanoes may be
partially submerged or emerged above sea level.
Their vo1canic products are mainly basaltic or
andesitic, but locally more silicic differentiates including ignimbrites have been observed. Arc vo1canism along Anden-type continental margins exhibits
high proportions of intermediate and ~ilicic, largely
ca1c-alkaline material. They may also eJect large volumes of ignimbrites.
The 1980 eruption ofMount St. Helens in Washington .is a
medium-scale, loeal example of this type of are volcamsm.
Due to westerly winds, the pyroclastie fallout was diSpersed about 1000 km to the east of the voleano. At ~ distanee of 300 km east of the source, the ash fall deposit was
2 cm in thiekness (Lipmann and Mullineaux 1981). Con-