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
