Pyroclastic Rocks Flashcards
(23 cards)
What is an ignimbrite?
A deposit of a pyroclastic density current rich in pumice or pumiceous ash shards
Tuff, lapilli-tuff and breccia
What is a pyroclastic density current?
Ground hugging current of pyroclasts and air
Give 4 features of welding
High temperature emplacement of PDC
Pumice and glass still malleable
Fusing together of pumice and shards
Compaction
What is fiamme and how is it formed?
A lens
Forms from flattened pumice shards in a welded ignimbrite
How does temperature effect depositonal features?
Welded or non-welded
Agglutination - rapid syn depostional welding of hot pyroclasts - clasts visible
Coalescence - rapid syn depostional welding of hot pyroclasts - clasts not visible
What lava like features can an ignimbrite have?
Flow banding and folding
How do you distinguish between lava and lava like ignimbrites?
Evidence of fragmentation (vitroclastic textures, broken crystals
Field characteristics - tuff gradations, internal variations in flow laminae, internal breccia zones, no basal autobreccia
Vertical chemical stratification is impossible in lava
Where do fountain-fed lavas come from and what are they typically composed of?
From fissure eruptions and central veins
Variably welded spatter and sporia
Fall deposits coalesce
How do PDCs behave?
Deposits accumulate gradually, reflect material collected at a particular time
Prolonged with waxing and waning over time
How do fully dilute PDCs move?
Collisional momentum has little effect, transport dominated by turbulence of the fluid phase, usually involves traction and saltation
Essentially suspension
Non-stratified
Why is there high shear at flow boundary in fully dilute PDCs?
Individual clasts subjected to lift and drag of fluid
Slide, roll and saltates along substrate before deposition
Impingment of eddies cause entrainment and segregation
How do granular fluid dilute PDCs move?
Collision between grains dominates, little turbulence
Describe granular fluid dilute PDC deposition
Typically massive deposits
Grain interactions dominate clast support
Preferential deposition of fines by overpassing and perlocating
Summarise fluid dilute and granular fluid based conditions
Fully dilute - direct fallout and traction dominated
Granular dilute - granular flow dominated and fluid escape dominated
What do matrix supported lithic blocks show in ignimbrites?
Thickness of aggraded unit before deposition
Where do lithic blocks come from in ignimbrites?
Erosion/collapse of vent walls
Avalanches into PDC
Erosion of substrate by PDC
Ballistic blocks ejected from vent
What does normal grading in lithics show?
Waning current
Decreasing lithic avaliability
What does inverse grading in lithics and pumice show?
Waxing current
Increasing lithic and pumice avaliability
What four features demonstrate sustained eruptions?
No palaeosols, lavas, sedimentary rocks or fall deposits
Why might an ignimbrite only give a partial record of PDC?
Bypassing currents may leave no deposits for part of the deposition
Erosion through groove marks - long linear clasts filled by ignimbrite or gutter clasts - erosional scours
What effect does water have on fragmentation?
Enhances it, produces abundant fine grained ash
How does moisture create ash aggregates?
Hydrostatic and electrostatic agglomeration of ash
What does an ash aggregate do and what forms can it come in?
Simple fall deposits from plume
Ash pellet, cored pellet, coated ash pellet, accretionary lappilus, Cored accretionary lappilus
