Week 2 & 3: Igneous rocks Flashcards
What does magma consist of?
Magma/lava consists of 3 components:
1. Melt - Liquid portion.
2. Solids - Silicate minerals.
3. Volatiles - Dissolved gasses in the melt. e.g. H2O; CO2; SO2.
Name and describe the 4 types of magma based on its silica content.
Felsic magma: 66 - 76% SiO2.
Intermediate magma: 52 - 66% SiO2.
Mafic magma: 45 - 52% SiO2.
Ultramafic magma: 38 - 45% SiO2.
What are the processes that result in a variation of magma composition?
1: Partial melting.
2: Assimilation.
3: Fractional crystallisation.
4: Mixing of magmas.
Describe Bowen’s Reaction Series and how it gives rise to different types of igneous rocks.
Minerals crystalize in a predictable sequence. The series consists of two branches: A discontinuous branch and a continuous branch. As the T of the magma decreases minerals will crystalize along both branches simultaneously.
Discontinuous branch: Olivine –> Pyroxene –> Amphibole Hornblende –> Biotite mica & Potassium Feldspar & Muscovite mica & Quartz.
Continuous branch: Ca rich plagioclase –> Na rich plagioclase & Potassium Feldspar & Muscovite mica & Quartz.
The reactions are not always complete. e.g.not enough Si or too fast T drop.
What are the characteristics of intrusive igneous rocks? What is the evidence of high heat?
Cool slowly and so form large crystals. Baked zone is the rim of heat altered country rock. Chill margin is magma that cooled rapidly. Xenolith are country rock fragments in magma.
What is the influence of magma intrusion on these older country
rocks? What is initiated when magma invades colder country rock.
- Thermal (heat) metamorphism
and melting. - Inflation of fractures pushing the rock aside.
- Incorporation of country rock fragments (xenoliths).
- Hydrothermal (hot water) alteration.
What are the three factors that control the cooling time
of magma?
The depth of intrusion.
The shape and size of a magma body. (The greater the surface area the faster it cools.)
The presence of circulating ground water.
Name and describe all the types of Intrusive rock bodies. Differentiate between concordant and discordant rock bodies.
Magma intrudes into other rocks in two ways: As planar, tabular bodies (dykes and sills) or as balloon shaped blobs. (plutons)
[Tabular]:
Sill - Parallel to beds, concordant.
Dyke - Cross cutting, discordant.
[Plutonic]:
Batholith - discordant.
Stock - discordant.
Laccolith - concordant.
Lopolith - concordant.
Name and describe types of igneous rock textures?
Glassy - solid/shards of glass.
Interlocking mineral crystals - when minerals fit like a puzzle.
Fragmental - pieces of pre-existing rocks.
Crystalline - ingrown mineral crystals.
− Aphanitic: Very fine grain.
− Phaneritic: Course grain.
− Porphyritic: Mixture of course & fine grain.
− Pegmatitic: Exceptionally course grain.
Where fine grain is microscopic crystals and course grain is visible to large crystals.
Name and describe the classification of igneous rocks.
- Glassy igneous rocks (e.g. obsidian, pumice, and scoria).
- Crystalline classification (e.g. granitic, dioritic, gabbroic and ultramafic)
- Pyroclastic rocks
What are the types of crystalline granitic igneous rocks?
Granite, rhyolite, granodiorite,
and daciet.
What are the types of crystalline gabbroic igneous rocks?
Gabbro, basalt, dolerite or diabase, and anorthosite.
What are the types of crystalline ultramafic igneous rocks?
Pyroxenite, peridotite, dunite, komatiite and kimberlite.
How do Igneous rocks form?
Igneous rocks form as molten (or partially molten) rock cools and solidify. This occurs at high temperatures (1,100°C to 650°C).
What is the difference between intrusive, plutonic igneous rocks and extrusive volcanic igneous rocks?
Intrusive - When molten rock cools and solidifies below the Earths surface.
Extrusive - When molten rock cools and solidifies above the Earths surface.
How is magma formed?
Partial melting of crust or upper mantle.
1. Pressure release
(Base of crust hot enough to melt mantle rock, but this rock doesn’t melt as it’s under high pressure from rock above so a decrease in pressure can allow for melting.)
- Volatile addition
(Volatiles are elements/molecules that can evaporate easily and are gaseous at Earths surface.
Mixing volatiles with hot rock helps break up chemical bonds.) - Heat transfer from rising magma
(When mantle derived magma (>1100) intrudes into the crust, it can raise the temperature of the surrounding crust enough (650
-850) to melt it.)
What is the geothermal gradient?
The rate at which the Earth’s
temperature increases with depth.
What is the difference between dry magma and wet magma?
Dry magma has no volatiles and needs high T to melt.
Wet magma has up to 15% volatiles and melts at a lower T.
What minerals are mafic magma composed of?
45% - 52% SiO2.
MAfic - Magnesium
maFIC - Iron
[MgO, FeO and Fe2O3]
What does magma properties depend on?
The properties of magma depend on its composition.
-Viscosity
-Density (SiO2 is less dense than MgO
and Fe so Felsic magmas have lower density, while mafic magmas have higher density)
-Temperature (Felsic magmas can remain liquid at temperatures of only 650o to 800C, while mafic
magmas may reach temperatures of up to 1300C and still remain liquid.)
State the summary of the 3 properties of each of the four main magma types.
Density (D); Temperature (T); Viscosity (V).
Type:
Felsic:
- Very low D.
- Very low T.
- Very high V. (Explosive!)
Intermediate:
- Low D.
- Low T.
- High V. (Explosive)
Mafic:
- High D.
- High T.
- Low V. (Runny eruptions)
Ultramafic:
- Very high D.
- Very high T.
- Very low V.
Explain how partial melting works.
Different types of rock can melt or
partially melt by different amounts.
When you partially melt a rock it produces a 1 step more felsic magma. [The silica rich minerals melt first creating a more silica rich magma - felsic.]
Explain how assimilation works.
When rock surrounding magma may partially be broken up and mix with
the original magma changing its composition.
Explain how fractional crystallisation works.
When one magma composition is derived from another during crystallisation. Dense crystals will settle downward or if not will stick to the walls where it is coolest.
Explain how mixing of magmas works.
Magmas of different compositions
can mix to produce new, in-between compositions.
What is formed when magma/lava cools very fast?
Glass. No time to order the structure.
What is a two stage (cooling) history?
When some large crystals have formed and the remaining magma cools quickly. A porphyritic texture results.
What is the characteristic: vesicle in a rock?
Small holes or cavities.
What are the types of crystalline pyroclastic igneous rocks?
E.g. Tuff, vulcanic brecia and
hyaloclastite.
Where are the two main locations fractionation occurs?
Divergent plate boundaries.
Convergent plate boundaries.
What are these things generally made of?
- Continents.
- Ocean basins.
- Volcanic arcs.
Continents - Felsic igneous rocks.
Ocean basins - Mafic igneous rocks.
Volcanic arcs - Intermediate igneous rocks.
