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A2 Igneous Rocks (TYPES OF MAGMA (ACID- granite and rhyolite >66% SiO2,…
A2 Igneous Rocks
TYPES OF MAGMA
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BASIC- gabbro, dolerite and basalt 45-52% SiO2
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BASIC MAGMA- formed from mantle peridotite through dry partial melting at a constructive plate margin
INTERMEDIATE- formed from mantle peridotite through wet partial melting at a destructive subduction plate boundary
ACID- formed from continental crust through wet partial melting at a destructive convergent plate margin
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Formation of Basic Magma
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composed of olivine, pyroxene and plagioclase which don't contain water
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dry partial melting is caused by decompression melting which occurs when there is a decrease in overlying pressure
decompression melting caused by
1) extension and thinning of the lithosphere
2) mantle begins to rise e.g convection current
Hot Spot Magma
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lie above mantle plumes which remain stationary and lead to the formation of a chain of volcanoes e.g Hawaii (Emperor Chain)
cause eruption of large quantities of basic lava called flood basalts e.g Deccan Traps which cover 1/3 of India and a volume of 5 million km3 or the Siberian Traps with a volume of 15 million km3
release large quantities of So2 and Co2 and have caused mass extinctions e.g the Siberian Traps correspond to the P-T mass exinction
Magma- molten rock contains variable amounts of solid crystals that didn't melt and dissolved gas. Forms in the crust and the upper mantle due to an increase in temperature or decrease in pressure. Surface lava ranges from 1000 to 1300 degrees and in the mantle it is around 1600 degrees
rocks melt when the temperatures exceed the melting temperature of the lowest mineral with the lowest melting temperature
MINERAL CONTENT- melting temperature is controlled by the specific melting temperature of the constituent minerals e.g mafic minerals have a higher melting temperature than silicic
BOWEN'S REACTION SERIES- as the magma cools he observed the types of minerals that crystallised and the temperature at which they formed
OLIVINE- 1240 to 1400
AMPHIBOLE- 940 to 1170
PLAGIOCLASE- 1060 to 1360
BIOTITE- 790 to 1010
QUARTZ- 760 to 960
PRESSURE- as pressure increases, the temperature at which minerals melt increases if the rock is dry
EFFECT OF WATER- presence of water lowers the temperature at which minerals melt. As pressure increases, so does temperature as at high pressure more water can dissolve in magma
Contamination of Magma
- composition of magma is changed because the magma melts or mechanically incorporates material from the surrounding rocks or Xenoliths
1) melting of the contaminate due to conduction of heat from magma. The contaminate then mixes with the magma
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called stoping wwhich occurs when dense and fractured blocks of country rocks fall into a body of buoyant magma
Skaergaard Intrusion
Greenland, Tertiary, intruded in a single injection
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Solidification of Magma
- crystallise from the melt at different temperatures
- newly formed crystals chemically different from magma
- more intermediate than the magma
- MAGMATIC DIFFERENTIATION BY FRACTIONAL CRYSTALLISATION
1) the melt can be squeezed out of the crystal-magma by the weight of the overlying crystals (filter pressing)
2) olivine crystals have greater density so they will sink to the bottom (gravity settling)
PALISADES SILL
- 300km thick
- sandstone
- metaquartzite
- Layered Dunite (No olivine at top and olivine rich at bottom)
- metaquartzite
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MAGMATIC DIFFERENTIATION BY PARTIAL MELTING- minerals with the lowest melting temperatures melt first and creates magmas that are chemically different to the parent rock.
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