igneous textures Flashcards
equigranular texture?
crystals are equally sized (can be coarse, medium or fine)
how is fine grained equigranular texture formed?
rapid cooling at the surface are extruded
usually the deeper the larger the crystals (excluding chilled margins)
how is vesicular texture formed?
where gas bubbles are trapped in lava as it cools rapidly leaving holes where gas was present
vesicular?
rock with ‘holes’ (VESILCLES)
what is a vesicle?
holes are called VESICLES are oval and parralel to flow
where is vesicular texture found?
at the surface of lava flows where gas rises
basalt + pumice
what is amygdaloidal texture?
crystals within vesicles
how does amygdaloidal texure form?
vesicular rocks are very porous so groundwater flows through, containing dissolved minerals such as calcium carbonate so calcite will be precipiatated in the holes. can happen millions of years after rock formed
what rock is amygdaloidal texture commonly found in
basalt
what is an AMYGDALE
an infilled vesicle( crystal in hole)
what is a GEODE
partiallu amygdale (crystal not completly filling hole)
what rock is amygdaloidal texture commonly found in
basalt
what is an AMYGDALE
an infilled vesicle( crystal in hole)
what is a GEODE
partially amygdale (crystal not completly filling hole)
what is flow banding (texture)
dark aligned minerals aligned parallel to flow direction
how does flow banding form in rocks?
layers of dark and light minerals form due to seperation of minerals within a silicic lava flow. usualy seen in rhyolite
what is porphyritic texture
crystalline, large and small crystals
how does porphyritic texture form?
2 stage cooling process
large crystals (phenocrysts) form first from cooling slowly
surrounded by finer groundmass that cooled quickly
how does porhyritic granite form
phenocrysts: often pottasium feldspar form slowly at great depths (around 30km). the magma and feldspar crystals move up with crust by diapiric action to form a batholith ast a deth of around 15km. the rest cools slowly to form the coars grained crystals of the groundmass
how does prorphyritic basalt form
phenocrysts: plagioclase feldspar form in magma chamber below volcano, when eruption occurs, plagioclase crystals and magma are erupted to the surface where rapid cooling forms the groundmass (fine grained)
