Geology 309 - Lecture 20
Debris Flows
Debris avalanches (dry rock avalanches) and
Lahars
Definition -
Gravity driven fragmental flow which does not
include young magmatic material as essential component. Included volcanic
rocks are typically old and cold.
Usually HUGE compared to block and ash flows (which are also gravity driven)
Three types:
Bezymianny type -
debris avalanche that involves a magmatic eruption. Typical of Mt. St.
Helens, but named after eruption in Kamchatka in 1956.
Bandai type: debris avalanche that involves a non-magmatic explosion.
Bandai volcano in 1888 in Japan. Flank of volcano collapsed, superheated
steam exploded. No new magma, no full blown eruption.
Cold type -
No volcanic activity at all.
Example: Ontake volcano, Japan. Earthquake triggers
collapse of mountain with no steam or lava. At
Ontake,this debris avalanche transformed into a lahar
How do we recognize these slope failures in the past?
- Vast, horseshoe-shaped amphitheater - but can be hidden by later lava
flows or domes. After 10 years, Mt. St. Helens was 1/3 filled with new
dome. Bezymianny after 30 years is now completely filled. Parinacota,
Chile, no trace of avalanche scar remains after 13,000 years
-
Avalanche deposits themselves. Hummocky topography. Thousands of
small hills and closed depressions covering large areas at the base of the
volcano.
Mt. Shasta as an example
Satellite view of Socompa debris avalanche in Chile
Margin of debris avalanche
Debris Avalanche deposits
- Poorly sorted (several km blocks to fine material)
- Smaller clasts resemble angular breccias.
- Maybe reverse grading (nuts in cereal analogy)
Flow mechanism
- Laminar flow, like a plug dome (no deformation inside, only at the
edges). Original stratigraphy often preserved.
- Have high yield strengths (such steep sides!).
- Highly mobile
- How do they move?
Basically just kinetic energy and
"Dynamic fluidization" where rocks banging against each other create a
dispersive force.
Peter Francis' debris avalanche web page
II. Lahars
Definition
of a lahar (from Peter Francis' web page)
Types of lahars
- Lahars during eruptions
- Lahars after eruptions
- Lahars without eruptions
Deposits
- Unsorted (fine mud to huge boulders)
- No bedding
Flow mechanisms
- Gravity
- Small density contrast between slurry and boulders
- Lubrication
Lahars and their effects
Impressive pictures
of lahars from Mt. Pinatubo
More discussion of lahars