Volcaniclastic Rocks Flashcards
(16 cards)
How do hyaloclastic rocks form?
Extruding magma or flowing magmas chilled rapidly in contact with water and explosively fragments
How is a peperite formed?
Mixing and mingling of magma with unconsolidated wet, sediment
What forms fludial facies in peperites, how are they formed and what happens when they cool?
Ductile fragmentation
Magma quenches and fragments, preserved as stable vapour films form around magma preventing direct contact with water in sediment
When magma cools, film boiling stops, so magma is not insulated and becomes blocky
What effect does fluidisation have?
Heats sediment and pore water - causes boiling off to steam
Fluidisation of sediment - dirupts laminae and original sediment
What material can be reworked and how is reworking achieved?
Unconsolidated and lithic material can be reworked
Weathering, erosion, in streams
How are volcaniclastic breccias formed?
Mass wasting, sector and caldera collapse
What triggers sector collapse? 8
Eruptive activity, intrusion, dyke emplacement, faulting, earthquake, gravitational basement spreading, hydrothermal activity, climate change
What are the features of a block facies debris avalanche deposit? 5
Mega blocks, fractured blocks, jigsaw cracks, minor intraclast matrix, hummocky topoography
What are the two features of a matrix facies debris avalanche?
Poorly sorted matrix and fractured blocks
Substrate material
What is a caldera composed of?
Pyroclastic rocks - ignimbrites
How does the caldera floor and walls break down?
Fault scarps, landslide blocks, debris avalanches and debris flows
Name the five caldera end members
Downsag, trapdoor, piston, piecemeal and funnel
Why are kimberlites important?
Carry diamonds up pipes and contain platinum
Why are volcaniclastic rocks not good hydrocarbon reservoirs?
Porosity and permeability reduced by alterations of glass, zeolites, clay formations and veins
What are the three kinds of glass and how do they appear?
Tachylite - opaque, high Fe-Ti, microcrystalline
Sideromelane - isotropic, vesiculated
Palagonite - low temp hydration of sideromelane
Describe the two stages of palagonitisation
Gel palagonite forms - concentrically banded material, translucent, anisotropic
Fibro-palagonite - fibrous, lathlike pr granular structure, bifringent
