Igenous Flashcards
(15 cards)
Types of rocks
- Intrusive rocks = igneous rocks that intruded and cooled beneath the ground.
- Extrusive = igneous rocks that extruded and cooled on the Earth’s surface.
- Plutonic = Rocks which crystallised in fairly large masses at a depth of a few kilometres
- Volcanic = Rocks from erupted magma or a fissure in the crust – will cool more rapidly resulting in smaller crystals or glass but containing larger crystals that formed before the eruption
Generation of Magma?
- Decompression melting – partial melting of mantle peridotite at mantle upwellings
- Aqueous flushing of cooler mantle – water released in mantle which acts as flux to melt rock – subduction zones – dehydration of oceanic crust
Basic composition of igneous rocks?
• Ultramafic
o Less than 45% silica
o Dark or greenish rocks
o Rich in olivine; may also contain pyroxene, amphibole, Biotite
o Higher melting temp, decreased viscosity
• Mafic
o 45-52% silica
o Dark-coloured rocks
o Containing pyroxene, amphibole, olivine, Biotite
• Intermediate
o 52-66% silica
o Greyish to salt and pepper-coloured rocks
o Rich in plagioclase, amphibole, Biotite and quartz
• Felsic
o Light-coloured or red rocks
o Rick in potassium feldspar, quartz, Biotite or muscovite
o Lower melting temp, increased viscosity
• Acidic (silicic)
o Greater than 66% silica
• Nucleation rate (of crystals) dependent on:
– Rate of undercooling
– Availability of ions
– Ease of travel of ions through melt
– Undercooling – when liquids are cooled below the liquidus line.
• Need undercooling because crystal formation bonds release heat which re-melts nuclei.
• Therefore need significant undercooling to allow nuclei to persist and grow as crystals.
Viscosity, bubbles and eruptions?
= More viscous magma = water bubbles are less able to escape magma chamber = pressure builds up = explosive fragmentation
Therefore more SiO2 = more viscous = more explosive magma eruptions
Igneous textures and how they relate to cooling rate
o Fast cooling rate (eruption) = aphanitic = rate of cooling is impeding crystal growth
o Slow cooling rate (intrusion) = Phaneritic = crystals compete for space
o Glassy – chilled margins of intrusions – Crystalisation has been inhibited – rate of cooling is greater than diffusion rate
Trace elements?
• Trace elements have properties that mean that they are either ‘compatible/immobile’ or ‘incompatible/mobile’.
– Compatible elements (eg. Zn, Co, Ni): incorporated in to crystal structures and form strong bonds; do not migrate from crystal structures during melting (immobile).
– Incompatible elements (eg. K, Rb, Sr, Ba, REE): do not easily fit into crystal structures and have weak bonds; migrate easily during melting (mobile).
• Compatible elements are incorporated into mineral phases during crystallisation.
• Incompatible elements remain in the melt for as long as possible.
• By analysing trace element concentrations in igneous rocks we can start to interrogate their magmatic histories and understand magmatic processes.
• Incompatible elements:
– Last elements to enter a crystal lattice on cooling).
– First to exit a crystal lattice on heating).
Anatexis?
• Two components:
– A liquid melt fraction enriched in lower temperature constituents.
– A residual rock (restite) component enriched in higher temperature refractory elements.
• Influencing factors are:
– Composition, temperature and depth of the source rock.
– Percent partial melting.
– Source rock’s previous melting history.
– Processes affecting the magma after it leaves the source
• Melts and solids separate and cannot react with each other.
• Initial partitioning of components :
– Melt is enriched in low temperature components and incompatible elements.
– Restite is enriched in high melting temperature (refractory) components and compatible elements.
• Melt removed so that successive melts of the restite have different compositions.
Magma diversification processes at convergent plate margins:
Controlled by:
- Composition and thickness of overlying plate and rocks undergoing anatexis (mantle, oceanic lithosphere, marine sediments).
- Flux melting.
- Fractionation, assimilation, magma mixing.
- Dip angle of slab.
Volcanic rock settings:
• Island arc Plate subduction o Mafic to intermediate intrusives o Mafic to intermediate extrusives • Plate divergence o Basaltic intrusives o Basaltic extrusives • Hot-spot volcanism o Basaltic extrusives o Basaltic intrusives • Continental Plate Subduction o Mafic to felsic intrusives o Mafic to felsic extrusives
Characterisitics of an eruption?
Pyroclastic eruptions – column’s go far due to the bulk density being less than the surrounding atmosphere due to high temp. – Driven by buoyancy therefore
• Hawaiian eruptions - sustained fire fountains
• Strombolian eruptions – Weak, small volume, transient explosions (no lava)
• A’a’ lavas
• emplaced rapidly; crust continually disrupted into clinker breccia; cascades down front of lava flow as lava moves forward.
Silicic Lava domes:
• Crystalisation, vesiculation, magma viscosity increases up the volcano
• Fragmentation and gas escape near surface
• Compaction and welding of magma at top of volcano (surface)
• On eruption a lava spine and carapace of dome forms
Degree of crystallinity
- Halocrystalline: Composed of just crystals
* Halohyaline: Composed of glass
Grainsize?
• Phaneritic: Crystals visible to the naked eye (1-30mm)
o Can say medium and coarse grained phaneritic
• Aphanitic: Crystals too small to be seen with the naked eye
• Equigranular: Most crystals the same size
• Inequigranular: Crystals of different sizes
• Porphyritic: Large crystals (phenocrysts) in a finer grained groundmass
Grain shape
- Euhedral: Crystal bound by crystal faces
- Subhedral: Crystal faces partially developed
- Anhedral: No crystal faces developed
Textural terms?
- Vesicular: Gas bubbles, may be spherical or irregular
* Amygdaloidal: Gas bubbles filled with secondary mineral (during digenesis)
