Environmental Engineering Reference
In-Depth Information
c volcanic-derived clastic deposits
Clastic deposits composed of ma
1.2 Ma
c volcanic particles
-
in any proportion from
partly to entirely
can be generated by a wide variety of mechanisms spanning
the full range of volcanic to sedimentary processes, and the resultant textures
and morphologies are likewise highly variable. Three genetic categories of ma
-
c
volcanic-derived clastic deposits, based on formation mechanisms, are
primary
and
reworked volcaniclastic deposits
,and
epiclastic deposits
(White and
Houghton,
2006
).
1.2.1 Primary volcaniclastic deposits
Primary volcaniclastic deposits are formed from fragmental material deposited as a
direct result of explosive or effusive eruptions. There are four end-member types of
deposits: autoclastic, pyroclastic, hyaloclastic, and peperitic (White and Houghton,
2006
; White
et al
.,
2009
). The main factors controlling formation of these deposits
are magma eruption rates, concentration of magmatic volatiles, and presence and
relative abundance of external water, either as freestanding bodies or in saturated
sediments. Mobilization of magma-generated particles is unique compared to other
sedimentation processes that depend exclusively on gravity, because primary
volcanic particles may acquire transport energy from their source
-
e.g. explosive
expansion, lava
and may initially be independent of slope or
depositional base level (Fisher and Smith,
1991
).
Autoclastic
deposits are products of auto brecciation, and are generated as
effusive lavas that exceed the viscosity-strain rate threshold and fragment (Peterson
and Tilling,
1980
); rapid cooling promotes groundmass crystallization and crust
disruption (Cashman
et al
.,
1999
). A
'
a lava flow morphology is characterized
by brecciated upper and lower surfaces; slabby or rubbly pahoehoe have broken
or brecciated upper crusts and are transitional between pahoehoe and a
'
a
(e.g. Guilbaud
et al
.,
2005
).
Pyroclastic
deposits form from explosive volcanic eruption plumes and jets or
pyroclastic density currents and can be generated by magmatic or phreatomagmatic
fragmentation mechanisms, or a complicated interplay of both (e.g. Graettinger
et al
.,
2013
). Magmatic fragmentation represents a minor mechanism for generat-
ing ma
flow velocity
-
c volcaniclastic deposits in large igneous provinces, and documented
examples are rare. The Columbia River Basalt vent sites in the Roza Member
have densely agglutinated and welded spatter and highly vesicular scoria fall
deposits (Brown
et al
., 2014).
Involvement of aquifer or surface water in ma
c eruptions leads to large-scale
phreatomagmatic volcanism, and can generate deposits with volumes of up to
