Page 166 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

6. Modern lahar deposit
The three units (1, 2, 3) evident in this outcrop are
deposits from lahars generated by heavy rain on
unconsolidated fresh ash and colluviurn at the source
volcano. The lowermost unit (1) destroyed and buried
a building at this site, the floor (Fl) of which remains.
The deposits are poorly sorted sand-boulder mixtures
comprising abundant anthropogenic clasts (e.g.
building fragments, Bg) and minor juvenile basaltic
andesite ash. The lahars flowed from right to left
(arrow) and originated about 12 km away from this
site. The section is about 2.5 m thick.
Lahar deposits generated during the AD 1963—64
eruption of Irazu; Cartega, Costa Rica.

7. Modern lahar deposit
The units labelled 1 to 3 are deposits from rain-
triggered lahars, and consist of diverse lithic and
anthropogenic fragments (e.g. house brick, B)
supported in a sandy matrix. The distinctly rounded
clasts in unit 3 have been "recycled" from alluvial
deposits traversed en route. Alluvial deposits also
occur at the base of the sequence. Although
generated on a volcano during an eruptive period,
juvenile components (basaltic andesite ash) are
minor. The lahars ranged from hyperconcentrated
flows to debris flows, and were sourced some 12 km
away.

Lahar deposits generated during the AD 1963-64
eruption of Irazu; Cartega, Costa Rica.
Plate 36 — Subaerial grain-flow and volcanic debris-avalanche deposits

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