Igneous Processes Flashcards
Define an igneous rock.
One formed through the cooling and solidification of magma
What is the difference between magma and lava?
Magma = Molten rock below the earth’s surface
Lava = Magma once it reaches the surface
What conditions are required for melting the Earth’s crust and mantle?
Depressurisation
Adding water or other volatiles
Heating to the melting point
What are the three types of magma?
Basaltic, Andesitic, Rhyolitic
Which magma type is most mafic?
Basaltic magma
What type of magma is most felsic?
Rhyolitic lava
What are the typical magma temperatures of the 3 main types?
Basaltic - 1200C +
Andesitic - 800-1000C
Rhyolitic - 600-900C
What does viscosity of magma depend on?
Temperature and composition
The lower something’s viscosity is…..
The easier it flows
Does greater silicate content in magma raise or lower viscosity?
Raise (harder to flow due to silicate chains)
What is magmatic differentiation by fractional crystallisation?
A process by which crystals of certain minerals form, removing this mineral from the magma
- so later crystals have a different composition
What are two ways crystals and magma can become separated?
Compression
Sinking of dense, early minerals
What type of magma did Bowen notice to be more common?
Basaltic magmas.
What did Bowen figure out about andesitic and rhyolitic magmas?
They are derived from basaltic magmas (the primary magma) by magmatic differentiation
What mineral did Bowen study to reach his conclusion?
Plagioclase Feldspars
What did Bowen call the process of continual reaction between magma and crystals?
A continuous reaction series
What did Bowen describe as a ‘discontinuous reaction series’?
Processes where already-formed minerals react with magma to form new minerals
Which is more coarse grained; intrusive or extrusive rocks? Why?
Intrusive, Because the magma crystallises slower
What are phenocrysts?
isolated grains within igneous rocks
What are igneous rocks with phenocrysts called?
Porphyritic
What’s the difference between vesicles and amygdales
Vesicles are bubbles preserved in igneous rocks, when filled with another mineral they are called amygdales
What’s the difference in eruption between high viscosity and low viscosity magmas?
High viscosity (hard to flow) - explosive eruptions with fragments
Low viscosity (easy to flow) - non-explosive eruptions
What are basalt eruptions like?
They produce low, broad, gently sloping shield volcanoes with non-explosive flows
What is tephra?
Airborne volcanic debris
What are the 3 types of tephra? How are they differentiated?
Differentiated by particle size
Bombs >64mm
Lapilli 2-64mm
Ash <2mm
What is the angle of rest of a volcano?
The angle between the slope and flat ground
What are andesitic eruptions like?
The volcano emits viscous lava and tephra
- Usually alternating to form a stratovolcano
What are pyroclastic flows?
Mixture of hot gases and pyroclasts denser than the atmosphere
- Hot highly mobile flow of tephra
What are rhyolitic eruptions like?
- Very silica rich
- Usually form calderas
What are calderas?
Steep walled basin usually >1km in diameter created by volcano collapse
During what type of eruption are calderas common?
Rhyolitic eruptions
What is the order of Bowen’s continuous reactions series?
Peridotite
Gabbro
Diorite
Granite
What is the order of Bowen’s discontinuous reaction series?
Olivine
Pyroxene
Amphibole
Biotite/Quartz
