Chap 4 Magma and Igneous Rocks Flashcards
Geothermal gradient
The rate at which the Earth’s temperature increase with
dept
Formation of Magma
Pressure release = decompression melting= Geothermal gradient
Heat transfer from rising magma
Addition of volatiles
Wet magma = up to 15% volatiles ∴ melts at lower temps
Dry Magma = no volatiles ∴ melts at higher temps
What is magma made of?
- Liquid = melt
- Solids = silicates
- Volatiles ( AND )
Magma types:
Felsic,
Intermediate,
Mafic,
Ultramafic
Felsic magma
%: 66-76%
Density: Very low
Temp: Very low
Viscosity: Very high: Explosive
Plutonic: Granite
(Obsidian
=glass)
Volcanic: Rhyolite
Intermediate magma:
%: 52-66%
Density: Low
Temp: Low
Viscosity: High: Explosive
Plutonic: Diorite
Volcanic: Andesite
Mafic magma:
%: 45-52%
Density: High
Temp: High
Viscosity: Low: thin,
hot, runny
Plutonic: Gabbro
Volcanic: Basalt
Ultramafic magma:
%: 38-45%
Density: Very High
Temp: Very high
Viscosity: Very low
Plutonic: Peridotite
Volcanic: Komatiite
Processes involved in magma variation
Partial melting
Assimilation
Mixing magmas
Fractional crystallisation
Partial melting
= different types of rock melt/partially melt by different amounts
∴ partial melting of: Ultramafic –> Mafic –> Intermediate –> Felsic
Assimilation
= rock surrounding magma may partially melt and mix with original magma
Mixing magmas
= one magma composition can be derived from another during
crystallisation
Fractional crystallisation
= magmas of different compositions can mix to produce new, inbetween compositions
Characteristics of intrusive igneous rock
Baked zone
Chill margin
Xenoliths
Baked zone
= rim of heat
altered country rock
Chill margin
= magma at
contact that cooled rapidly
Xenoliths
= country rock
fragment in magma
∴ magma cooled before xenolith could
be assimilated
Invasion in colder country rock
Thermal metamorphism and melting
Inflation of fractures pushing the rock aside
Incorporation of country rock fragments (xenolith)
Hydrothermal (hot water) alteration
Intrusive rock bodies
Concordant
Discordant
Tabular
Plutonic
Concordant Intrusive rock body
= does not cut across rock layers (sill, locolith, lopolith)
Discordant Intrusive rock body
= cuts across rock layers (batholith, stock, dyke)
Tabular Intrusive rock body
= flat with uniform thickness (sill, dyke)
Plutonic Intrusive rock body
= body/blob of intrusive igneous rock (batholith, stock, lopolith, locolith)
Factors affecting the cooling time of magma
- Depth of intrusion
- Shape and size
- Presence of circulating ground water
Types of textures
- Glassy
- Interlocking mineral crystals/ crystalline
- Fragmental
Glassy texture
= fast cooling OR highly viscous; made of solid glass or solid shards
Interlocking mineral crystals/ crystalline texture
= slow cooling; minerals that fit like jigsaw pieces
Fragmental texture
= pieces of pre-existing rocks
o Aphanitic = finely crystalline, only microscopically visible ∴ rapid cooling
o Phaneritic = coarsely crystalline, mm-cm
o Porphyritic = a mixture of coarse and fine grained (Indicates a 2-stage history ∴ initial slow cooling and subsequent eruption cools magma quickly)
o Pegmatitic = exceptionally coarse mineral crystals; form from water rich melts in
which atoms can move around so rapidly (not very viscous) that large crystals can
grow very quickly
