Page 165 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

3. Modern lahar deposit, Rio Lagunillas,
Colombia

The 13 November 1985 lahars formed a thick (-5 m)
muddy bouldery gravel deposit (D) at the mouth of the Rio
Lagunillas valley upstream from Armero. The deposit is
very poorly sorted, unstratified and has a tabular bed
geometry with a gently sloping upper surface. The post-
1985 river (arrow) has cut deeply into the 1985 and older
lahar deposits.

Lahar deposits along the Rio Lagunillas, AD 1985; near
Armero, Colombia.

4. Debris-flow deposit: poorly sorted volcaniclastic
boulder conglomerate
This section exposes a 15 m thick deposit comprising a
very poorly sorted mixture of clay, sand and boulders up to
2-3 m across. Slope wash has nearly concealed the basal
contact (dashed line). Boulders display crude reverse
grading at the base and normal grading towards the top.
The conglomerate is interpreted to be the deposit from
voluminous volcaniclastic debris flows initiated by
avalanches of hydrothermally altered rock from the summit
of Mount Rainier triggered by volcanic explosions
(Crandell, 1969, 1971).

Osceola mud-flow deposit, 5700 a; Inter Fork, Mount
Rainier, Washington, USA.

5. Modern volcaniclastic sandy lahar deposit
Primary pyroclastic and volcanic debris-avalanche
deposits from the May 18, 1980, eruption of Mount
St. Helens have been rapidly resedimented and
reworked by fluvial processes and lahars. The fluvial
deposits (F) in this exposure are dominated by planar
and cross-laminated sand and pebbly sand. They are
interbedded with a 60 cm thick, diffusely stratified,
poorly sorted sand and granule lahar deposit (L). The
bed has a planar, locally erosive base. The top is
irregularly incised by an erosion surface and has been
fluvially reworked. In this case, the lahars were
probably hyperconcentrated flows transitional to
debris flows.

Post-May 1980 fluvial and lahar deposits near Cold
Water Creek, Mount St Helens, USA.

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