Lahars

24 November 2015

Vallance, 2000, EOV

Downstream lahar transformations

Vallance, 2000, EOV

Downstream transformations of lahars

Vallance, 2000, EOV

Water-saturated debris avalanches transforming to lahars downstream

Vallance, 2000, EOV

Vallance, 2000, EOV

Mt. St. Helens – pumiceous pyroclastic flow from 1980

Lahars – effects on river systems and deposits

Lahars at Armero following the eruption of Nevado del Ruiz, 13 November 1985

Mountain front

Total inundation

Partialinundation

No inundation

Lahars filling rivers following the 18 May 1980

eruption of Mount St. Helens

Lahar-inundated communities following the 15 June 1991 eruption of Mt. Pinatubo, Philippines

Breaching of levees by lahars filling river drainages after the Pinatubo eruption

Pumice-rich debris flow deposit, east side Mt. Pinatubo, 20-21 Aug 1991

Above - Debris flow (unstratified) and hyperconcentrated flow (stratified) deposits, east side Mt. Pinatubo

Lake breakout deposits, east side Mt. Pinatubo

debris flow

debris flow

hyperconc. flow

Rapid erosion of lahar deposits following the 18 May 1980 Mt. St. Helens eruption

Comparison of air photos taken in June 1978 and June 1980 of the North Fork of the Toutle River near Mt. St. Helens

Maximum inundation levels by lahars following the 18 May

1980 eruption of Mt. St. Helens

Lahar inundations and mud-coated debris after the Mt. St. Helens eruption

Normally graded Osceola Mudflow lahar deposit, Mt. Rainier(Vallance and Scott 1997)

Hummocky Osceola Mudflow deposit, Mt. Rainier(Vallance and Scott 1997)

Mobility of debris avalanches vs. debris flows (from Vallance and Scott 1997)

Cotopaxi

Chillos Valley lahar, Cotopaxi, Ecuador(Mothes et al 1998)

~4.5 ka, ~3.8 km3

Note the matrix-supported and ungraded lahar deposit

Debris avalanche and lahar dispersal north of Cotopaxi

Comparison of lahar travel distances:

Cotopaxi, Mt. St. Helens, Nevado del Ruiz

south

north

Grain size parameters as a function of distance from the source

Chillos Valley lahar

Osceola Mudflow

Grain sizes, Chillos Valley lahar