if rich in silica it is lighter in colour
commonly found in intermediate and silicic rocks
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mafic
darker in colour
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crystal size
look at the average crystal grain size and ignore any phenocrysts
determined by depth below the surface
hyperbassal - close to surface - finer crystals
plutonic - deep - bigger crystals
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phenocryst
larger crystals in an igneous rock, much larger than the groundmass
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size scale
- glass (no crystals)
-fine (<1mm)
-medium (2-5mm)
-coarse (>5mm)
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size and depth
igneous rocks nearer the surface have finer crystals as they cool quicker
igneous rocks at depth have big crystals as they cool slower
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the different magmas
- silicic
-intermediate
-mafic
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silicic magma
- rhyolitic
- lots of chains, harder to break apart
- because of this its more viscous (runs slower)
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mafic magma
- basaltic
- least viscous
- low silica content therefore low chain structure
- forms at the highest temp
- eg Hawaii Chain of Islands
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