2 Continental Sediments
2.4 Volcaniclastic Sediments
(Tephra Deposits)
2.4.1 General Aspects and Terms
2.4.2 Tephra Deposits on Land
2.4.3 Marine Tephra Deposits
2.4.4 Volumes and Transport Distances ofTephra
2.4.5 Volcaniclastic Sediments in Various Basin Settings
2.4.6 Production Rates, Duration, and Recurrence
Intervals of Volcanic Activity
2.4.7 Alteration, Diagenesis, and Metamorphism
of Volcaniclastic Rocks
2.4.8 Summary (Tephra Deposits)
2.4.1 General Aspects and Terms
Hardly a depositional system exists in which
volcanielastic beds or some reworked pyroelastic
components in other sediments are absent. In fact, in
several of the basin types described in Chaps. 1 and
12, volcanielastic deposits play a significant part in
the total basin fill. The contribution of volcanielastic
material to the total volume of sediments in various
basin types may be as high as about 25%. For this
reason, a brief review of volcanielastic-producing
phenomena is necessary in the context of this book.
Chapin and Elston (1979), Fisher and Schmincke (1984,
1990, 1994), Cas and Wright (1987), Schmincke (1988),
Fisher and Smith (1991), Schmincke and Bogaard (1991),
Orton (1996) describe volcaniclastic sediments and their
genesis in more detail.
The term volcaniclastic sediments or tephra deposits
refers to all types of volcanic fragments regardless of
grain size, grain shape, composition, specific origin,
and depositional process. V olcanielastic fragments
are subdivided into the following groups:
(1) Juvenile fragments, comprising two subgroups:
- Pyroelasts generated during explosive volcanic
eruptions caused or dominated by degassing of
magma.
- Hydroelasts formed during eruptions due to the
contact of hot magma with external water
(phreatomagmatic eruptions).
(2) Accessory or cognate fragments are derived from
older, co-magmatic rocks.
(3) Accidental fragments originate from underlying
rocks of any composition.
The fragment types of groups (2) and (3) are collectively called lithic fragments or lithoelasts. Based on
the grain size of volcanic fragments, the following
terms are used:
- Blocks and bombs (diameter ~64 mm).
- Lapilli (2-64 mm).
- Volcanic ash (;:;2 mm).
- Volcanic dust (;:;0.063 mm).
The most important group of fragments dealt with
here are juvenile fragments. They represent chilled
sampies of the erupted magma and indicate the nature of the volcanic eruption. Depending on the state
of magma prior to eruption, these fragments may
consist of volcanic glass, or they are partially crystallized. Magmatic explosive eruptions of basalt and
basaltic andesite are dominated by lapilli-sized
pyroelasts which show many vesiculae caused by the
degassing magma. Dark colored pyroelast accumulations of a basic to intermediate composition are
called scoria. Because of the fluid nature of the
erupting basaltic magma, the shape of larger fragments may be affected by their flight through the air
and their impact on the ground surface (angular or
drop-like form, ropy or stringy surface). More viscous, silicic to intermediate magmas (e.g., phonolites
and trachytes) commonly produce highly vesicular
pyroelasts (pumice). These consist of voIcanic glass,
but mayaiso contain crystals formed in the magma
chamber prior to eruption. Pumice elasts are usually
light in color and vary in grain size and shape. Some
are less dense than water and hence float.
Phreatomagmatically fragmented juvenile elasts
(hydroclasts) tend to be more blocky and less vesicular than pyroelasts. F or angular glass fragments the
term shard is frequently used.
Tephra deposits can be grouped into three genetic
types according to their mode of transport and deposition:
- Fallout deposits.
- Flow deposits.
- Surge deposits.
2.4 Volcaniclastic Sediments
(Tephra Deposits)
2.4.1 General Aspects and Terms
2.4.2 Tephra Deposits on Land
2.4.3 Marine Tephra Deposits
2.4.4 Volumes and Transport Distances ofTephra
2.4.5 Volcaniclastic Sediments in Various Basin Settings
2.4.6 Production Rates, Duration, and Recurrence
Intervals of Volcanic Activity
2.4.7 Alteration, Diagenesis, and Metamorphism
of Volcaniclastic Rocks
2.4.8 Summary (Tephra Deposits)
2.4.1 General Aspects and Terms
Hardly a depositional system exists in which
volcanielastic beds or some reworked pyroelastic
components in other sediments are absent. In fact, in
several of the basin types described in Chaps. 1 and
12, volcanielastic deposits play a significant part in
the total basin fill. The contribution of volcanielastic
material to the total volume of sediments in various
basin types may be as high as about 25%. For this
reason, a brief review of volcanielastic-producing
phenomena is necessary in the context of this book.
Chapin and Elston (1979), Fisher and Schmincke (1984,
1990, 1994), Cas and Wright (1987), Schmincke (1988),
Fisher and Smith (1991), Schmincke and Bogaard (1991),
Orton (1996) describe volcaniclastic sediments and their
genesis in more detail.
The term volcaniclastic sediments or tephra deposits
refers to all types of volcanic fragments regardless of
grain size, grain shape, composition, specific origin,
and depositional process. V olcanielastic fragments
are subdivided into the following groups:
(1) Juvenile fragments, comprising two subgroups:
- Pyroelasts generated during explosive volcanic
eruptions caused or dominated by degassing of
magma.
- Hydroelasts formed during eruptions due to the
contact of hot magma with external water
(phreatomagmatic eruptions).
(2) Accessory or cognate fragments are derived from
older, co-magmatic rocks.
(3) Accidental fragments originate from underlying
rocks of any composition.
The fragment types of groups (2) and (3) are collectively called lithic fragments or lithoelasts. Based on
the grain size of volcanic fragments, the following
terms are used:
- Blocks and bombs (diameter ~64 mm).
- Lapilli (2-64 mm).
- Volcanic ash (;:;2 mm).
- Volcanic dust (;:;0.063 mm).
The most important group of fragments dealt with
here are juvenile fragments. They represent chilled
sampies of the erupted magma and indicate the nature of the volcanic eruption. Depending on the state
of magma prior to eruption, these fragments may
consist of volcanic glass, or they are partially crystallized. Magmatic explosive eruptions of basalt and
basaltic andesite are dominated by lapilli-sized
pyroelasts which show many vesiculae caused by the
degassing magma. Dark colored pyroelast accumulations of a basic to intermediate composition are
called scoria. Because of the fluid nature of the
erupting basaltic magma, the shape of larger fragments may be affected by their flight through the air
and their impact on the ground surface (angular or
drop-like form, ropy or stringy surface). More viscous, silicic to intermediate magmas (e.g., phonolites
and trachytes) commonly produce highly vesicular
pyroelasts (pumice). These consist of voIcanic glass,
but mayaiso contain crystals formed in the magma
chamber prior to eruption. Pumice elasts are usually
light in color and vary in grain size and shape. Some
are less dense than water and hence float.
Phreatomagmatically fragmented juvenile elasts
(hydroclasts) tend to be more blocky and less vesicular than pyroelasts. F or angular glass fragments the
term shard is frequently used.
Tephra deposits can be grouped into three genetic
types according to their mode of transport and deposition:
- Fallout deposits.
- Flow deposits.
- Surge deposits.