Pyroclastic density currents and lahars Flashcards
What are the different ways PDCs might form?
Buoyant column collapses
Buoyancy not achieved but there gas “boil over”
Caldera collapse in very large eruptions
Sudden decomproession by a lateral blast
Gravitational collapse of hot effusive material (lava dome collapse)
Collpase of unstable near vent deposits
What are pyroclastic density currents (PDCs)?
Hot, gravity-driven mixtures of gas and solid fragments
Inundating large areas, but with some topographic control
High-velocity (100 km/hr +) and high mobility
What is the composition of the visible part of a PDC?
Low concentration ash cloud which sits above the dense base
What has a study from New zealand shown about how PDCs flow like a fluid?
There is a small layer of gas that sits underneath the dense base that reduces friction and promotes flow
What are co-igimbrites?
when buoyant ash plumes form from the travelling flows
What can lead to the formation of co-ignimbrites?
sufficient air entrapment and heating
What type of eruption are ignimbrites associated with?
Very large explosive eruptions
What are ignimbrites?
Deposits rich in pumice often with a high ash deposit
What can the volumes of the largest explsoive volcanic eruptions be like?
> 1000km*3
WHat are juvinile clasts?
Driving magma - vesiculated pyroclast
What are lithic clasts?
non-juvinile material from the conduit wall/ flow path
How have past interpretations divided PDCs?
According to particle size, velocity and turbulence characteristics: Flow and Surge
What is the particle size, velocity and turbulence of flow PDCs?
Coarse, dense, flow
What is the particle size, velocity and turbulence of surge PDCs?
Lower density, fines-rich turbulent flow
What will th deposits of PDCs be like?
particle concentration gradient
What is the effect of varying temporal factors on PDC deposits?
Currents can be dynamic and long lived meanig the nature of deposits from site to area differ
What is the effect of eruptions experiencing temporal gaps in PDC generation/ deposition?
A PDC will deposit a flow unit (sheet like) and other deposit types (fall deposits) might occur inbetween these events
What might an ignimbrite from one large eruption comrpise of?
Multiple flow units
What can mean that PDCs are structurally complex?
Can potentially both erode and deposit
What are large ignimbrite forming eruptions often associated with?
Caldera formation
What is a caldera?
Subsidence structures associated with the large-scale evacuation of magma from the underlying storage system
What are PDC deposits like compared to fall deposits?
Thick and internally complex
Smaller affected area
Complex array of depositional and erosional features
Rnage of clast types
Diverase apperance - blocky to ashy
What might be the causes of PDCs from smaller eruptions?
Explsoive column collapse/ fountaining
Phreatomagmatic eruptions
Dome collapse/ dome explosion driven PDCs
Lateral blast associated PDCs
What does fountaining occur?
when a buoyant plume isnt achieved
What are Phreatomagmatic eruptions?
interaction with groundwater or surface water
commonly produces high levels of fragmentation and surge-like PDC generation
What migth cause Dome-collapse or explosion driven PDCs?
May originate via (partial) gravitational collapse of lava domes or unstable lava masses
Or
Via explosive disruption of lava domes and upper conduit
What can dome failure result in?
sudden decompression of lava body and a lateral-blast type explosion
What are PDCs the general result of?
Inefficient mixing with the atmosphere
What pathway will PDCs follow?
Pathway of maximum gradient
What are pheonix plumes?
When a column rises again maybe duet to reduced density or heat heat influx or topograpphic
What can the effect on the landscape from large eruptions?
Landscape changing volume can fill basins and overspill with the deposits being easily eroded as its soft and unconsolidated
What time scale will a Mt St Helens eruption be seen on?
1 in 10 year globally
What is an ignimbrite?
A pumice rich PDC/ deposit
What are PDC deposits like?
Interannly complex due to evolution of flow over hours day and months
What are massive volcanic eruptions the product of?
The assumulation of 1000s of years of magma
What is a PDC exaple from the biggest eruption of the last century?
Katmai, Alaska- no human fatalities but glacial landscape (U shaped valley) infilled by PDC deposits
What are small eruotion PDCs like?
More common affecting the local area still having diverse origins
What type of material is common in PDCs forming from lava dome collapse?
Block and ash flow - very dense
What is an example of a devestating small eruption PDC?
Mount Pelee 1902
How many fatalities were there from Mount Pelee?
29000 (2 survivors)
What usually cuases damage to structures and land from PDCs?
Firs generated from intense heat and high dynamic pressure
What is the typical range for smaller PDCs?
5-15km
Why was St pierre so affected by the PDC?
It was at the bottom of a valley fed by the volcano
WHat were the characteristics of the St piere PDC?
200km/h
Low volume
400c
What is the only real way to mitigate for PDCs?
Planning at risk areas before hand and finding a safe place to evacuate to
What are phreatomagmatic eruptions?
Explosive eruption that is fuelled by a water interaction instead of being sub-aerial
How does water affect water explsoive eruptions?
INcreases explosiveness with more fragmentation by increased volatiles
What are lahars?
mixing of water with explsoive volcanic products (fall deposits, PDCs…)
Slurry of volcnic sediment and external water, during eruption or after when reworked by heay rainfall carrying into rivers cauing secondary(post eruption) lahars
When are lahars most devestating?
When they get to low gradients where velocity is reduced and overspill occurs
What are 2 examples of lahars?
Chille, chaiten 2008 - town destroyed new delta of lahar material created
Nevado del ruiz 1985 - aremeo 23000 dead
Why are lahars so destructive?
Due to huge amount of solid material crried but still mobile like a liquid
How far and fast did lahars from nevad del ruiz flow?
60km/hr and 104 km
What are pheratic eruptions?
Explosions driven by hot fluids above a magmatic system. No juvenile magma erupted at surface (i.e. tephra composed of altered lithic grains)
What is an example of a pheratic eruption?
Ontake volcano, Japan 2014
What are pheratic eruptions like?
Small but very explosive
Why are pheratic eruptiosn so hard to predict?
Usual magmatic precursors not present
Why were lahars so devestating from pinatubo?
Occured upto 4 years after ward with worst in 94-95 3 yrs after euption
What were the effects of the pinatubo 91 eruption?
Eruption deposited 5.5 km3 of pyroclastic debris on the volcano, smoothing the terrain
Main eruption: ~200 deaths from roof collapse (tephra fall),
~700 from PDCs, ~100 from immediately post-eruptive lahars
Why are lahars from pinatubo reactivated so readily?
due to intense monsoonal rainfall
