Environmental Engineering Reference
In-Depth Information
10
2
to 10
5
km
3
(Ross
et al
.,
2005
). Phreatomagmatic pyroclastic and hyaloclastic
deposits, along with peperite, represent the full spectrum of products from magma
-
water interaction (Wohletz,
2002
). External water is integral for formation,
but magmatic volatiles are not precluded, and in the case of phreatomagmatic
eruptions, may also play a role in fragmentation. Clast-forming processes during
hydromagmatism include four primary mechanisms: magmatic explosivity, steam
explosivity, cooling-contraction granulation, and dynamic stressing; all are
dependent on the magma to water ratio (Wohletz,
1983
; Kokelaar,
1986
).
Phreatomagmatic pyroclastic deposits are produced by the optimal fuel (magma)
to coolant (water or sediment-laden water) mixture to generate explosivity
(magma to pure water mass ratio of
0.33: White,
1996
), whereas hyaloclastic
deposits are volumetrically dominated by water and peperite dominated by wet
sediment (wet sediment to magma mass ratios
>
1: Wohletz,
2002
).
Hyaloclastic
deposits are solely generated by quench fragmentation during
magma
~
water interaction, and result from effusive magma contacting abundant
water, in either marine or continental settings. Pillow lavas, pillow
-
-
palagonite
breccias, and hyaloclastites are the most typical products of ma
c magma quench-
ing and spalling in a subaqueous environment.
Peperite
deposits result from magma interaction with unconsolidated, water-
bearing clastic deposits in shallow intrusions, subaqueous or surface environments
(White
et al
.,
2000
; Skilling
et al
.,
2002
). Experimental and theoretical studies
suggest that mechanisms of magma
-
water interaction and magma
-
sediment
-
water
interaction may be similar (Kokelaar,
1986
), and peperites rely on
fluidization and
vigorous injection and mixing of water-saturated sediments and lava (Kokelaar,
1982
). Recognition of peperite indicates contemporaneity of sedimentation and
volcanism (Busby-Spera and White,
1987
). Given the ubiquity of environments
that could generate peperite, there are relatively few documented examples in large
igneous provinces. However, this may be a function of identi
cation rather than
absence. Even in the predominantly arid desert environment that the Paraná
-
Etendeka Large Igneous Province was emplaced into, peperites formed where
pahoehoe lava
flows interacted with wet lacustrine silts and clays, which collected
in low-lying topography of lava
flow surfaces (Waichel
et al
.,
2007
).
1.2.2 Reworked volcaniclastic and epiclastic deposits
Reworked
volcaniclastic deposits are composed of particles sourced from primary
volcaniclastic deposits that have been redeposited by surface processes (wind,
water, ice, gravity) either concurrent with eruption or after a period of immobility.
In reworked volcaniclastic deposits, the volcanic processes that create the particles
are not the same as those that transport the particles to their
final depositional
