74
These processes are complex and cannot be treated here
(see, e.g., Hughes 1983; Fisher and Schmincke 1984,
1990). Near the Earth's surface and on the sea floor, the
unstable components take up water and are transformed
into clay minerals (e.g., smectites, phyllipsite, and others),
or they form various zeolites. With increasing burial depth,
mechanical compaction and cementation reduce the pore
space and create dense rocks. Chlorite and kaolinite may
replace the early formed clay minerals, but later most
water-bearing minerals are dehydrated and transformed
into more compact mineral phases such as feldspars (e.g.,
albite), epidote, iron oxides, and calcite.
In a strongly altered state, it is hardly possible to discriminate between juvenile components and epiclastic volcanic material. Without some experience and the usage of
special methods, the correct identification and genetic interpretation of such rocks is difficult. Guidelines for the
classification of lithified and metamorphic volcaniclastic
rocks are given by Cas and Wright (1987).
2.4.8 Summary (Tephra Deposits)
Tephra deposits result from volcanic eruptions
and mainly consist of pyroclasts, hydroelasts,
and lithoclasts (fragments ofmagmatic and other
roeks) of different grain sizes.
The transport distances of tephra increase from
pyroclastic flows, debris flows (lahars) and mud
flows to fallout deposits which may oeeur several thousands of kilometers away from the volcanic source.
High-velocity pyroclastic surges on land leave
behind deposits with unusual sedimentary structures. Tephra are frequently reworked and mixed
with other sediment types.
Submarine eruptions generate submarine debris
flows and turbidity currents (ash turbidites).
Chapter 2 Continental Sediments
The frequently used terms spilite and keratophyre designate degraded metamorphic rocks of volcanic origin
(Hughes 1983). These rocks have re-equilibrated under
low-temperature conditions and are recrystallized without
showing much deformation. The term spilite comprises
rocks of predominantly basaltic composition and texture,
while keratophyres represent rocks of intermediate to acid
composition. In both cases, the initial mineral phases are
replaced by a mineralogy corresponding more or less to the
greenschist facies.
For these and other reasons, the appearance (facies)
of ancient volcaniclastic rocks in outcrops, as well as
their petrographie characteristics, may deviate significantly from those of young counterparts. The metamorphic rocks of so-called greenstone belts in Precambrian shields consist, to a great part, of former
volcanic flows and volcaniclastic sediments.
The highest emounts of volcaniclastics are produced by magmatic ares related to subduction
zones. Foreare and backare basins are largely
filled with tephra, but the sediments of wide
ocean areas also contain significant portions of
volcaniclastics.
Individual large-volume eruptions normally
alternate with long time intervals of quiescence.
Smaller, low-volume eruptions may exhibit a
sort of cyclicity.
Continental and marine ash layers can provide
excellent marker horizons for stratigraphie correlation. They perrnit, if not reworked and
strongly altered, precise dating of their host
sediments.
These processes are complex and cannot be treated here
(see, e.g., Hughes 1983; Fisher and Schmincke 1984,
1990). Near the Earth's surface and on the sea floor, the
unstable components take up water and are transformed
into clay minerals (e.g., smectites, phyllipsite, and others),
or they form various zeolites. With increasing burial depth,
mechanical compaction and cementation reduce the pore
space and create dense rocks. Chlorite and kaolinite may
replace the early formed clay minerals, but later most
water-bearing minerals are dehydrated and transformed
into more compact mineral phases such as feldspars (e.g.,
albite), epidote, iron oxides, and calcite.
In a strongly altered state, it is hardly possible to discriminate between juvenile components and epiclastic volcanic material. Without some experience and the usage of
special methods, the correct identification and genetic interpretation of such rocks is difficult. Guidelines for the
classification of lithified and metamorphic volcaniclastic
rocks are given by Cas and Wright (1987).
2.4.8 Summary (Tephra Deposits)
Tephra deposits result from volcanic eruptions
and mainly consist of pyroclasts, hydroelasts,
and lithoclasts (fragments ofmagmatic and other
roeks) of different grain sizes.
The transport distances of tephra increase from
pyroclastic flows, debris flows (lahars) and mud
flows to fallout deposits which may oeeur several thousands of kilometers away from the volcanic source.
High-velocity pyroclastic surges on land leave
behind deposits with unusual sedimentary structures. Tephra are frequently reworked and mixed
with other sediment types.
Submarine eruptions generate submarine debris
flows and turbidity currents (ash turbidites).
Chapter 2 Continental Sediments
The frequently used terms spilite and keratophyre designate degraded metamorphic rocks of volcanic origin
(Hughes 1983). These rocks have re-equilibrated under
low-temperature conditions and are recrystallized without
showing much deformation. The term spilite comprises
rocks of predominantly basaltic composition and texture,
while keratophyres represent rocks of intermediate to acid
composition. In both cases, the initial mineral phases are
replaced by a mineralogy corresponding more or less to the
greenschist facies.
For these and other reasons, the appearance (facies)
of ancient volcaniclastic rocks in outcrops, as well as
their petrographie characteristics, may deviate significantly from those of young counterparts. The metamorphic rocks of so-called greenstone belts in Precambrian shields consist, to a great part, of former
volcanic flows and volcaniclastic sediments.
The highest emounts of volcaniclastics are produced by magmatic ares related to subduction
zones. Foreare and backare basins are largely
filled with tephra, but the sediments of wide
ocean areas also contain significant portions of
volcaniclastics.
Individual large-volume eruptions normally
alternate with long time intervals of quiescence.
Smaller, low-volume eruptions may exhibit a
sort of cyclicity.
Continental and marine ash layers can provide
excellent marker horizons for stratigraphie correlation. They perrnit, if not reworked and
strongly altered, precise dating of their host
sediments.