2.4 Volcaniclastic Sediments
These primary accumulations of volcanic activity are
frequently reworked both on land and in the sea due
to considerable relief comrnonly created by volcanism. Slopes unprotected by soils and vegetation or,
under water, by early diagenetic processes favor the
erosion and redistribution of primary volcanic material. This is incorporated into other sediments as
epiclasts, causing significant changes in the composition of these rocks.
2.4.2 Tephra Deposits on Land
Fallout tephra deposits result from pyroelastic and
phreatomagmatic eruptions. Their material is ejected
from a volcanic vent. Hot tephra and gas produce a
buoyant plume rising high into the atmosphere (Fig.
2.24a). After a first phase of radial expansion, the
ash cloud is directed down wind and may spread over
distances of several hundreds to thousands of kilometers. With increasing distance from the source, the
fallout layer be comes systematically thinner, finer
grained, and better sorted. Scoria fall deposits are
composed largely of basaltic to andesitic magma.
Pumice fall deposits originate from highly viscous
andesitic, to rhyolitic, phonolitic magmas. They represent the widely dispersed elassic plinian type of
ash eloud fallout.
Ash-fall deposits form under varying conditions
ineluding phreatomagmatic eruptions and coignimbrite processes (see below). They consist of
lapilli and finer grained material. The pyroelasts and
hydroelasts settle not only through air, but also
through lake water and sea water (see below). They
generate widespread thin ash layers which are often
weIl preserved in depressions on land, in swamps
and in lakes. Bentonite layers (tonsteins) in lake sediments and coal-bearing sequences are mostly derived
from former volcanic ash. Ash layers on hilI slopes
may be incorporated into periglacial pro ces ses and be
used to date normal debris covers.
Pyroclastic flows mostly originate from the collapse
of overloaded eruption columns. They are hot, partially fluidized gas-solid mixtures with high partiele
concentrations causing the flow to move downslope
and to fill topographie depressions and pre-existing
valleys (Fig. 2.24b). If the flow enters a lake or the
sea, a second co-ignimbrite ash eloud is generated
which may lead to widespread fallout on top of and
beyond the pyroelastic flow deposits. Subaqueous
pyroelastic flow material may mix with water and
generate tephra-dominated mass flows. Evidence for
the hot emplacement of pyroelastic flow deposits is,
among other criteria, the occurrence of carbonized
wood and welded tephra. Massive, welded silicic and
pumiceous pyroclastic flows are referred to as ignimbrites.
65
An idealized section of a pyroelastic flow unit may
begin with thin, stratified surge deposits (see below),
followed by the main body of the pyroelastic flow or
ignimbrite, and end with volcanic ash from fallout
(Fig. 2.24c). Ignimbrites are generally poorly sorted,
but may show some indistinct layering due to
changes in grain size of the welded primary
pyroelasts. Some grading of the basal zone and inverse grading of the higher part of the flow unit are
frequently observed. The fallout deposit at the top
consists of thin, stratified, graded ash layers. Stacked
flow units may somewhat differ in grain size, color,
and composition.
Three main types of pyroelastic flow deposits are
distinguishable (Fig. 2.24c):
- Block- and ash-flow deposits have an ash matrix
and contain large blocks of the same magma type.
- Scoria flow deposits consist of basaltic to andesitic
ash, lapilli, and larger elasts.
- Pumice-flow deposits or ignimbrites are poorly
sorted, normally welded, massive tephra layers as
described above.
Volcanic mud flows and debris flows (lahars) are
generated in the same way as other gravity rnass
flows (cf. Sect. 5.4), i.e., under normal temperature
with the aid of water. They are, however, similar to
pyroelastic flow deposits in composition and structure. In fact, they may form the distal facies of
pyroelastic flows. Lahars tend to show a more
polymiet composition, better rounded elasts, and
higher proportions of elay-sized matrix than
pyroelastic flows. They form either during eruptions
or independently of specific volcanic activities. They
comprise not only a large proportion of the proximal
facies on the lower slope of explosive volcanoes, but
frequently travel distances of IOto 200 km. Lahars
transport volcanic fragments to the coast and thus
provide the source for volcanielastic submarine debris flows and turbidites.
Pyroclastic surges are high-velocity, low-density
turbulent flows which are caused by various mechanisrns. Most important are base surges initiated by
phreatomagmatic eruptions (interaction of magma
with extemal water). These can create a collar-like
cloud near the Earth's surface expanding radially in
all directions (Fig. 2.25a) as also observed in nuelear
explosions. Such a debris-laden base surge may
reach velocities up to 100 mJs and shatter all trees
and other objects many kilometers away from the
locus of eruption. Phreatomagmatic base surges are
comrnonly "wetl! and have a low temperature. Other
types of surges are associated with pyroelastic flows
(ground surges) and the collapse of an eruption co 1umn (ash-cloud surges).
These primary accumulations of volcanic activity are
frequently reworked both on land and in the sea due
to considerable relief comrnonly created by volcanism. Slopes unprotected by soils and vegetation or,
under water, by early diagenetic processes favor the
erosion and redistribution of primary volcanic material. This is incorporated into other sediments as
epiclasts, causing significant changes in the composition of these rocks.
2.4.2 Tephra Deposits on Land
Fallout tephra deposits result from pyroelastic and
phreatomagmatic eruptions. Their material is ejected
from a volcanic vent. Hot tephra and gas produce a
buoyant plume rising high into the atmosphere (Fig.
2.24a). After a first phase of radial expansion, the
ash cloud is directed down wind and may spread over
distances of several hundreds to thousands of kilometers. With increasing distance from the source, the
fallout layer be comes systematically thinner, finer
grained, and better sorted. Scoria fall deposits are
composed largely of basaltic to andesitic magma.
Pumice fall deposits originate from highly viscous
andesitic, to rhyolitic, phonolitic magmas. They represent the widely dispersed elassic plinian type of
ash eloud fallout.
Ash-fall deposits form under varying conditions
ineluding phreatomagmatic eruptions and coignimbrite processes (see below). They consist of
lapilli and finer grained material. The pyroelasts and
hydroelasts settle not only through air, but also
through lake water and sea water (see below). They
generate widespread thin ash layers which are often
weIl preserved in depressions on land, in swamps
and in lakes. Bentonite layers (tonsteins) in lake sediments and coal-bearing sequences are mostly derived
from former volcanic ash. Ash layers on hilI slopes
may be incorporated into periglacial pro ces ses and be
used to date normal debris covers.
Pyroclastic flows mostly originate from the collapse
of overloaded eruption columns. They are hot, partially fluidized gas-solid mixtures with high partiele
concentrations causing the flow to move downslope
and to fill topographie depressions and pre-existing
valleys (Fig. 2.24b). If the flow enters a lake or the
sea, a second co-ignimbrite ash eloud is generated
which may lead to widespread fallout on top of and
beyond the pyroelastic flow deposits. Subaqueous
pyroelastic flow material may mix with water and
generate tephra-dominated mass flows. Evidence for
the hot emplacement of pyroelastic flow deposits is,
among other criteria, the occurrence of carbonized
wood and welded tephra. Massive, welded silicic and
pumiceous pyroclastic flows are referred to as ignimbrites.
65
An idealized section of a pyroelastic flow unit may
begin with thin, stratified surge deposits (see below),
followed by the main body of the pyroelastic flow or
ignimbrite, and end with volcanic ash from fallout
(Fig. 2.24c). Ignimbrites are generally poorly sorted,
but may show some indistinct layering due to
changes in grain size of the welded primary
pyroelasts. Some grading of the basal zone and inverse grading of the higher part of the flow unit are
frequently observed. The fallout deposit at the top
consists of thin, stratified, graded ash layers. Stacked
flow units may somewhat differ in grain size, color,
and composition.
Three main types of pyroelastic flow deposits are
distinguishable (Fig. 2.24c):
- Block- and ash-flow deposits have an ash matrix
and contain large blocks of the same magma type.
- Scoria flow deposits consist of basaltic to andesitic
ash, lapilli, and larger elasts.
- Pumice-flow deposits or ignimbrites are poorly
sorted, normally welded, massive tephra layers as
described above.
Volcanic mud flows and debris flows (lahars) are
generated in the same way as other gravity rnass
flows (cf. Sect. 5.4), i.e., under normal temperature
with the aid of water. They are, however, similar to
pyroelastic flow deposits in composition and structure. In fact, they may form the distal facies of
pyroelastic flows. Lahars tend to show a more
polymiet composition, better rounded elasts, and
higher proportions of elay-sized matrix than
pyroelastic flows. They form either during eruptions
or independently of specific volcanic activities. They
comprise not only a large proportion of the proximal
facies on the lower slope of explosive volcanoes, but
frequently travel distances of IOto 200 km. Lahars
transport volcanic fragments to the coast and thus
provide the source for volcanielastic submarine debris flows and turbidites.
Pyroclastic surges are high-velocity, low-density
turbulent flows which are caused by various mechanisrns. Most important are base surges initiated by
phreatomagmatic eruptions (interaction of magma
with extemal water). These can create a collar-like
cloud near the Earth's surface expanding radially in
all directions (Fig. 2.25a) as also observed in nuelear
explosions. Such a debris-laden base surge may
reach velocities up to 100 mJs and shatter all trees
and other objects many kilometers away from the
locus of eruption. Phreatomagmatic base surges are
comrnonly "wetl! and have a low temperature. Other
types of surges are associated with pyroelastic flows
(ground surges) and the collapse of an eruption co 1umn (ash-cloud surges).