(3) Rock Cycle and Igneous Rocks Flashcards
Minerals definition
- A naturally occurring inorganic solid with a crystaline structure.
- All minerals crystallize from ‘solutions’ and are in equilibrium with conditions at which they form
Rocks *
Are composed of 1 or more mineral
Magma
- Is hot molten material from the Earth’s interior
- Mixture of liquid elements/compounds/gases
- Temperature range ~600degC - 1800degC
- Created from complete melting of materials
Igneous Rocks
- Form when magma COOLS and CRYSTALLIZES (solidifies)
- Composed of interlocking crystals/grains usually random oriented
- Wide range of compositions
Igneous rocks can be subdivided into 2 groups
- Extrusive/volcanic igneous rocks
- Intrusive/plutonic igneous rocks
Extrusive igneous rocks
- Solidify fast at/near the surface
- Usually fine grained (
Intrusive Igneous rocks
- Solidify slowly beneath the surface
- Usually fine coarse grained (>1mm)
Weathering
- At the surface rocks may break-down by chemical and mechanical processes
- The broken or dissolved materials may then be eroded (removed) and transported to other sites and be deposited (forming)
Sediments
- Unconsolidated materials derived from previously formed rocks
- Fragments = clastic or detrital sediments
- Organic materials = bioclastic sediments
- Inorganic precipitates = chemical sediments
Sedimentary Rocks are caused by:
When sediments are lithified due to compaction and/or cementation
Metamorphic rocks are caused by:
When preiviously formed rocks are subjected to metamorphism (heat, pressure, circulating fluids)
Igneous rocks are formed by:
Magma
Magma in Asthenosphere
- It is partly molten
- Liquid rocks ~10% less dense than solid rock, so it rises towards the surface
- Forceful rising also occurs by ‘squeezing’
Chemical composition of Igneous rocks
- Most common igneous rocks are composed of silicate materials such as olivine and quartz
- Oxygen and silicon are the most abundant elements in the crust, and are the structural/chemical basis of the silicate mineral, therefore SiO2 content is important
Chemical analysis of composition
- Because chemical analysis’s are presented as weight % oxides we use SiO2
- ~40-70% of common igneous are SiO2 by weight
- Quartz is 100% silicate
Categories of igneous rocks based on silica
- Felsic: >65% silica, mainly felspar and silica, light coloured
- Intermediate: ~55-65% silica, mainly Na/K and some quartz
- Mafic: ~50% silica, mainly Mg and Iron, dark coloured
- Ultramafic:
Cooling of Magma
- As magma rises it cools and crystallization begins
- The crystals grow over a range of temperatures
- The type of mineral formed is controlled by the temperature and initial composition of the magma
- Can evolve
- Early formed minerals tend to be UM
- As the magma cools and crystallizes the minerals formed evolve to become more felsic
Melting temperatures of Ultramafic, Mafic, and Felsic
- U: 1800deg
- M: 1200deg
- F: 600 - 800deg
Bowen’s Reaction Series for Discontinuous series
- Mainly the Fe and Mg rich minerals
- Early formed minerals become reabsorbed into the melt as the next mineral is formed
- During resorption the SOT joins together into more complex forms
- Minerals will absorb/recombine as long as the temperature is high enough
- Series: Olivine - Pyroxene - Amphibole - Biotite
- After biotite, all iron and Mg are used up, magma is now enriched with silica
Bowen’s Reaction Series for Continuous series
- Continuous change between Ca and Na rich forms of plagioclase feldspar with decreasing temperature
Last magma to crystallize becomes…
- Enriched in K, Al, and SiO2, so can become quartz
After magma crystallizes
- The composition of the rock should reflect the composition of the magma
- NOT always the case
Differentiation
- Ingredients separate from originally homogeneous mixture
- (A) fractional crystallization: earlier formed crystals are heavier than the magma and may sink to the bottom floor of the magma chamber (crystal settling)
- (B) remove magma from the crystals (filter pressing)
- Magma mixing
Heat sources to melt rocks
- Geothermal gradient (temperature increases with depth)
- Radioactivity
- Friction
- Mantle rock/heat conduction
Diverse compositions of crystallized magma (not differentiation)
- Partial melting: reverse of crytallization (low temperature minerals melt first) (minerals high in K, Al, SiO2)
- Assimilation: melting surrounding rocks (left with xenoliths (unmelted rock) in the magma)
Classification of Igneous rocks based on textures
- Grain size mostly
- Fine grained: due to rapid xtallization most grains are too small to see without magnification
- Coarse grained: slow cooling beneath surface, visible grains
Porphythic: very large grains, two stage cooling or extreme conditions - Glassy: very rapid cooling, no grains visible