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Debris Flows and Avalanches
Volcanic Debris Flow (Lahar)
• Definition
– Any mass movement involving volcanically
derived, sand-size material or finer.
– This may be total mass moved.
– More commonly this serves as a matrix for the
movement of larger volcanic clasts.
– Water rich
• Thus range from slumped deposits of ash
and hyaloclastites to megabreccias.
• Deposits may be relatively local (few 100
m’s in length and few meters thick)
• To regional (10’s of km’s long and > 100 m
in thickness)
• During eruptions
• Reworking of volcanic debris during times of
quiescence or after volcano has become extinct
• Ways of formation:
Heavy rains
Melting snow or ice (Jokulhaups)
Walls of crater lake collapse
Hydrothermal alteration- clay-rich rocks
Caldera collapse and mass wasting of walls
Pyroclastic flows mix with water
Mass Movement
• Once under way controlling factors for flow
length and speed are:
– Grain size
– Grain size distribution-electrochemical forces-clay
and silt-size
– Solid to water ratio- laminar flow
Thicker near source
Follows valleys and depressions, fans out away from
Can cause erosion by undercutting banks of streams or
those of steep slopes- adds to sediment load (bulking
Osceola Lahar
Mt. Rainer 5600 before present
Valley fill-85 to 200 m
Flow > 120 km into puget sound
Area covered was 350 cubic km
Hundreds of thousands of people live on
Criteria for Recognition
• Heterolithic, clasts
boulders to gravel,
angular to sub round
• Massive and poorly
• Matrix
• Inverse grading at
• Bedding alignment of
Fine fraction commonly clay-rich
Mixed crystal populations
Toe boulder rich
Overlain by bedded sedimentary units (siltstone, sandstone), these
may be lense-like
Laterally into turbidites
Overlain by turbidites
Rip-ups at base, deformation of underlying beds
Debris Avalanche Deposits
• Large scale landslide associated with volcanic
activity- moves under gravity
• Term avalanche used to emphasize the fact that
material has little or no water in it when
emplaced- may have steam, gas, ice, snow but
not liquid water
• Debris Flows or lahars are rapid, watersaturated flows.
• Many debris flows originate as debris
avalanches and become lahars by incorporating
large amounts of water –St. Helen’s, Mt. Rainer
Debris Avalanches
• First recognized and described from 1980
St. Helen’s eruption
• Here DA buried 60 km2 of terrain to an
average depth of 45m, maximum depth
was 200m, deposits 2km wide and 25km
• Became debris flows by sliding into Spirit
Lake and the Touttle river
DA- Divisions
• Block and matrix facies
• Block- large fragment of the volcano with
any internal textures well preserved (> 1m)
• Matrix is a mixture of various kinds of
volcanic fragments derived from various
parts of the source volcano. Soil, gravel,
wood, ice may be incorporated during flow
• Hummocky topography
• Steep sided cliffs at distal and lateral
• Horse-shoe shaped source crater
Composite Volcanoes:
57 active in Japan
28 have DA