Page 164 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

C. This part of the unit is markedly crystal-rich and
poor in dark grey mud matrix. Coarse quartz and
prismatic feldspar crystals dominate over relict
pumice, volcanic lithic clasts and mudstone
intraclasts.
Mount Read Volcanics, Cambrian; DDH HP2 (117
m), High Point, western Tasmania.
D. Mudstone intraclasts (M) and mud matrix are
conspicuous at this level in the unit. Feldspar-bearing
volcanic lithic fragments (L) and pumice clasts are
also present. Mudstone intraclasts are contorted and
some include crystal fragments near their margins.
Mount Read Volcanics, Cambrian; DDH HP2 (119.6
m), High Point, western Tasmania.
Plate 35 — Deposits from subaerial lahars and volcaniclastic debris flows
1. Modern lahar deposits, Armero, Colombia
Melting of snow and ice by deposits of the 13 November
1985 eruption of Nevado del Ruiz volcano initiated dilute
floods which rapidly transformed to lahars downstream
(Naranjo et al., 1986; Pierson et al., 1990). After flowing
about 74 km the lahars emerged from Rio Lagunillas valley
and branched into three lobes that inundated an area of 34
2
km . The smallest lobe (X) followed the SE trend of the
Rio Lagunillas, the principal lobe continued directly east
(Y) destroying most of Armero and killing 22,000
inhabitants, while the third travelled northward (Z). Armero
is built on part of a volcaniclastic fan constructed by older
lahar deposits.
View of Armero, Colombia, after the AD 1985
eruption of Nevado del Ruiz volcano.
2. Channelised lahar course, Rio Lagunillas,
Colombia
In the Rio Lagunillas valley, vegetation and soil have been
scoured from the river bed and walls, and incorporated in
lahars sourced at Nevado del Ruiz volcano. Bedrock is
exposed in many places, leaving a trim line (T) 20—30 m
above normal river level. Deposits along the channel show
that the debris flows were inhomogeneous, and varied in
rheology and particle concentration both vertically and
laterally during flowage. Most of the scoured material was
deposited after the debris flows reached the mouth of the
valley and spread across the gentle slopes at and beyond
Armero (35.1, 35.3). Note person for scale (arrow).

Rio Lagunillas near Armero, Colombia, after the AD
1985 eruption of Nevado del Ruiz volcano.

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