1. Field evidence, experiments, modeling
Collapsing calderas
Field evidence, experiments, modeling

7. Caldera faulting and flexure
From Roche et al 2000

8. Glencoe and Valles
These two examples highlight the important point of simple surface structure contrasting with complex subsurface structure

10. Etive Rhyolites, Glencoe caldera
Lava-like ignimbrites
near-vent
ponded by faults
hot
low fountains
no associated breccia – why ?
Basal phreatomagmatic layers
suggestive of lacustrine environments
Pre-eruptive fault bounded lakes
faults active to form lakes?
possible cause-effect relationship between tectonics and eruptions?

11. Caldera faulting vs tectonic faulting
How does basin development occur at Glencoe ?
Do the basins develop as a result of tectonic faulting?
Or do they develop as true caldera basins?
Or a combination of the two processes?

12. Caldera faulting Thick ignimbrite
Megabreccias and mesobreccias intimately interbedded within and associated with the ignimbrite sequence
This association is probably diagnostic of active syn-caldera faulting, i.e., the development of a caldera
The location of breccia within an ignimbrite sequence can help constrain the timing and nature of caldera collapse

13. Opening of vents and/or crevasses in peripheral extensional zone of caldera?
Early breccias
Moore and Kokelaar 1998

14. Late breccias
Moore and Kokelaar 1998

15. A key question in this regard:
I think a very interesting question is how a caldera subsides, i.e., the timing and nature of the subsidence:
When does subsidence occur: early, middle, or late?
Does it occur incrementally?
And what is the relationship between subsidence mechanics and magma chamber dynamics?
A key question in this regard:
Does a caldera subside passively (response to magma evacuation from chamber) ?
Does a caldera subside actively, i.e., does subsidence of the roof “push” magma out of the reservoir ?

16. Katmai 1912, Pinatubo 1991 Silicic magma (64 hours) (8-9 hours)
Stix and Kobayashi 2008, JGR

17. Basaltic magma
Stix and Kobayashi 2008

18. Basaltic magma
Stix and Kobayashi 2008

19. Subsidence experiments of Kennedy et al. 2008
These experiments examined the settling of the caldera roof into the magma reservoir, modelled by aqueous corn syrup solutions
At high Reynolds numbers:
Flow mainly vertical
Eddy formation
Vorticity at the end…intense stirring
Higher flow velocities and Reynolds numbers:
In wider ring dikes
In magmas with reduced viscosities

20. Two-block experiments (piecemeal collapse)
Kennedy et al 2008

21. Ponding of ignimbrite in tectonically-controlled basins
Perhaps no active faulting during eruptions
Perhaps downsagging (flexure) is important
Lack of megabreccias and mesobreccias ?
Would there be breccias (e.g., fanglomerates) related to tectonic faults, what would they look like, and how might they relate to the volcanic sequence?

22. Caldera faulting and megabreccias
At Glencoe, it appears that faults are re-used during different eruptive and caldera-forming events
This raises the possibility of ignimbrite of older calderas becoming the megabreccia and mesobreccia of younger calderas which are nested
Yellowstone may be a good example
Yellowstone geology, courtesy USGS

23. Glencoe ring fault
It is not clear – at least to me – what the role of the ring fault at Glencoe was
Perhaps it pertains to a later stage of caldera development

24. The caldera “space” problem

25. Roof aspect ratio Aspect ratio = roof thickness / roof width width
Low aspect ratio
High aspect ratio
thickness
reservoir
reservoir

26. Scaling Cohesion of roof
Cohesion needs to be scaled as * = *g*l*, where * is the cohesion ratio, * the density ratio, g* the gravity ratio, and l* the length ratio (most important)
A difficult thing to estimate is the cohesion of natural materials, and its variability in space and time….people typically use values of 106 – 107 Pa
Magma viscosity
Viscosity scales as * = *T*, where T* is the time ratio
The use of water and silicone as magma analogues can result in very different viscosity scaling

27. Caldera faulting aspect ratio = 0.5
Downsagging is observed at early stages
A set of main reverse faults which controls subsidence
Faults propagate upward from margins of reservoir
Fault dips shallow upward (listric)
A zone of peripheral extension which develops as a result of subsidence
this peripheral zone is a region of breccia production
extensional crevasses and vents may develop in this region (see Moore and Kokelaar)
Roche et al 2000

28. Influence of chamber shape
From Roche et al 2000, Kennedy et al 2004, GSA Bull

29. Aspect ratio As the aspect ratio of the roof block increases:
Area of undeformed piston decreases
Area of peripheral extension increases
Intersection of initial reverse faults at depth
This might promote stoping of roof blocks into the chamber
AR=0.2
AR=1

30. Caldera asymmetry – plan view
Note how the circular nature of the caldera decreases as roof aspect ratio increases (magma chamber dimensions are constant)
So if the aspect ratio is low, it is possible to infer the shape of the reservoir, but this becomes increasingly difficult to do at higher aspect ratio
Another influence may simply be the scale of the experiment compared to the scale or grainsize of the sand
AR=0.2
AR=1

31. Experiments at larger scale – greater asymmetry
1 meter
Ossipee ring complex, New Hampshire
From Kennedy et al 2004, Kennedy and Stix 2007

32. Caldera asymmetry – cross-section
Roche et al 2000
Subsidence almost always asymmetric in cross-section
Nucleation and development of first fault – principal fault with greatest throw
A trapdoor-style caldera results
Vents concentrated here
Seen in Roche et al, and also in strike-slip regimes in Holohan et al experiments
Lateral propagation of faults (which is stopped by high-angle regional faults – see Holohan et al)
Kennedy et al 2004

33. Holohan et al experiments
Tangential regional structures important during collapse
Faults within chamber margin: used as caldera reverse faults
Faults outside chamber margin: develop into peripheral normal faults
Non-tangential structures important during tumescence and resurgence ?
Strike-slip faults as preferential pathways for magmas and fluids

34. Repeated uplift and collapse experiments
From Troll et al 2002, Geology, 30,

35. The end