Module 7 Flashcards
lahars define and give 2 types
mudflows or debris flows composed mostly of pyroclastic sediments eroded from the flanks of stratovolcanoes
1.“hot” flows (primary lahars)
volcanic eruptions may trigger lahars by rapidly melting snow and ice
pyroclastic flows can generate lahars when extremely hot, flowing rock debris erodes, mixes with, and melts snow and ice as it travels rapidly down steep slopes
2.“cold” flows (secondary lahars)
explosive volcanic eruptions may remove all vegetation around a volcano creating landscapes that are vulnerable to mass movements
intense (i.e., high-volume) or long-duration rainfall events occur during or after an eruption
on steep slopes, rainwater can easily erode and transport pyroclastic sediments generating lahars
-Cold: occur in pyroclastic sediments but long after the eruption occurred
.intense weathering of materials and release of clay minerals
-sediments become saturated by rainfall and cause positive pore water pressure
Nevado del Ruiz in Columbia and Armero ex
series of lahars killed more then 23000 of the 30000 residents of Armero
Problem with these events is that they are NOT part of the lived experience and so areas become fatal many times over a long period
- 30 feet thick
- took 1 hour to arrive at city
Mount Rainer ex
Right below BC in Washington, an example of many large Lahars
Holocene lahar events
Osceola lahar - ~5600 years ago; moved 2-3 km3 of material up to 80 km away from summit; deposits cover 550 km2
National lahar – 2,300-2,200 years ago; moved materials up to 1oo km away from summit
Electron lahar – 500 years ago; moved materials 100 km away from summit
Osceola and National events initiated by explosive eruptions; Electron event initiated in saturated volcanic rocks
-3 main lahar event= Oscola, National, and Electron
Lahars are associated with _______ volcanoes only
stratovolcanoes
Give some info on Lahars:
General speed, distance possible to travel
steep slopes: 200km/h
lowland areas: >30 km/h
run out can be many 10s of km from the source
Mount Baker
on the border with BC and Washington
-prescense of lots of snow and ice could result in a lahar if an eruption occurs
Reverse Grading
Reverse grading: course materials found at top
Mount Meager
-in Southern BC
at least 54 eruptions have occurred at Mount Meager in the last 2,600,000 years
eruptions associated with rock avalanches, ash fall, debris flows and lahars
debris flows and lahars, mainly from Mount Meager, have filled Meager Creek valley to a depth of 250 m
remains a major volcanic hazard capable of producing highly explosive eruptions
Pemberton, BC
Pemberton, B.C., a community 65 km downstream from Mt. Meager, faces high risk
ash fall, debris flows and lahars destroy forest and fishery resources
mass movements will block Lillooet River and Meager Creek resulting in potentially catastrophic flooding
potential road closures require evacuation of population
sediment deposition within Lillooet River raising bed of the river; increased risk of flooding during periods of high rainfall
-True and Present Danger: water being displaced by lahar even after the event
How to minimize the risks to Pemberton,BC for a potential Lahar
monitor the volcano: remotely-sensed data to detect small ground movements that initiate mass movements (e.g., LiDAR)
install a warning system: in situ sensors to detect mass movements; communities lack capacity to install and maintain an early warning system
dredge Lillooet River channel: prevent overtopping of dykes that provide flood protection
additional flood protection: raise level of dykes; raise levels of houses vulnerable to flooding
Dredging the river of a potential Lahar area is an..
EXCEPTIONALLY expensive mitigation tactic
How to monitor for Lahars
Monitoring Lahars
an automated network of small sensors called acoustic flow monitors embedded underground to measure ground vibrations made by passing lahars
upon detection of a lahar, computer base stations alert local 24–hour emergency monitoring and notification centers, who initiate the warning component of the system
warning messages trigger immediate, preplanned emergency-response actions
