Igneous Rocks 2 Flashcards
What are the 4 mafic minerals?
From higher to lower crystallisation temperature
From higher to lower density
From higher to lower FeMg content
Biotite
Hornblende
Augite
Olivine
What are the 4 silicic minerals?
From lower to higher crystallisation temperature
From lower to higher density
From higher to lower SiO content
Quartz
Muscovite
Orthoclase
Plagioclase
When do rocks melt?
Rocks melt when the temperature exceed the melting temperature of the lowest mineral with the lowest melting temperature
What is magma?
Molten rock that forms with variable amounts of solid crystals and dissolved gas
Why does magma melt?
Increase in temperature or decrease in pressure
What controls what temperature if required to melt rock?
Mineral content of parent rock
Pressure
Effect of water
Mineral content: Order of Bowen’s reaction series from crystallises first to melts first
Olivine Plagioclase F
Pyroxene
Amphibole P F
Biotite KF Quartz Muscovite
Mineral content: Crystallisation temperatures for minerals
Olivine- 1240 to 1400
Amphibole- 940 to 1170
Plagioclase- 1060 to 1360
Biotite- 790 to 1010
Quartz- 760 to 960
Pressure: how does pressure affect temperatures?
As pressure increases the temperature needed to melt also increases
Only applies to dry rock
Effect of water: how does temperature affect melting?
The presence of water vapour in rock lowers the temperature needed to melt the rocks
At higher pressure more water can dissolve into magma temperatures will decrease with more pressure in wet rocks. Opposite to dry rocks
What is magmatic differentiation by partial melting?
Partial melting generates magmas that are more acidic than the parent rock.
When the volume of melt reaches 10-15% it separates from the parent rock and rises
What happens to the content of the magma when rocks melt?
The minerals with the lowest melting temperatures melt first and so the initial melt is enriched with them minerals so it is chemically different to the parent rock
melting of basic magma
Formed from mantle peridotite
Dry partial melting
Constructive plate margin
Melting of intermediate magma
Formed from mantle peridotite
Wet partial melting
Destructive subductive plate margin
Melting of acid magma
Forms from continental crust
Wet partial melting
Divergent convergent plate margin
What rock is formed from basic magma and what is important about the minerals?
Basalt
Contains olivine, pyroxene and plagioclase which do not contain water
Why does dry partial melting occur?
Due to decompression melting when there is a decrease in pressure on materials that are close to their melting temperature
How is decompression melting triggered?
1) extension and thinning of the lithospheric above reduces overburden pressure
2) mantle material begins to rise, such as within a convection current
What are the layers of oceanic crust?
Basalt with pillowed structures
Dolerite with a sheeted dyke complex
Gabbro
Layered peridotite
What happens within the oceanic rocks as they are being formed?
Gravity settling
Olivines sink and form a crystal mush at the bottom
What are hotspots?
Areas of prolonged volcanic activity
Where is andesitic magma found?
In volcanoes along destructive plate boundaries
How is intermediate magma formed?
By wet partial melting of mantle peridotite
Where does the water for wet partial melting come from?
It is carried by the oceanic crust in the subduction zone then given off by metamorphic dewatering
What is significant about the fact that the composition of andesitic magma is the same as continental crust?
Both continental crust and Andesite are formed by wet partial melting
Continental crusts are formed at island arc volcanoes
How is acid magma formed?
Partial melting of rocks in the lower continental crust
What are the three pieces of evidence that support how acid magma is formed?
1) acid volcanoes are confined to continental areas
2) acid magma contains huge amounts of water which formed from the melting of minerals found in continental crust
3) lab experiments
What 3 methods cause continental crust to melt?
Conduction
Slab detachment
Delamination
What temperature does continental crust begin to melt?
780 degrees
Why does the solidus for continental crust decrease with depth?
Cc Must contain water
What is the process called where magma cools and is richer in the first minerals?
Magmatic differentiation by fractional crystallisation
What are the two ways in which crystal-melt can separate?
1) can be squeezed out of the mix by weight of overlying crystals (filter pressing)
2) denser crystals will sink (gravity settling)
What is the case study for magmatic differentiation by fractional crystallisation?
Palisades sill
300m thick
Sandstone Metaquartzite baked margin Basalt chilled margin Layered dunite with no olivine Layered dunite with increasing olivine Layered dunite with rich olivine Basalt chilled margin Metaquartzite baked margin
What is contamination of magma?
Process where the composition of magma is changed as when the magma melts it reacts with or mechanically incorporates material from the surrounding rock /xenoliths
What 2 ways can contamination occur?
1) melting of the xenolith due to the conduction of heat from the magma which then mixes
2) a chemical reaction not involving melting
What is the process of contamination by xenoliths called?
Stoping
What is a continuous reaction series?
When the chemical composition of a mineral changes but it retains its original atomic structure and remains in the same mineral group
What is a discontinuous reaction series?
When the chemical composition and atomic structure of the crystal change resulting in the formation of a new mineral
