igneous rates of cooling Flashcards
(20 cards)
the slower the rate of cooling….
.. the larger the crystal size
crystal size in glassy ign rocks
no crystals
crystal size in fine grain ign rocks
> 1mm
medium grained ign rock
1-5mm
coarse grained ign rock
> 5mm
rate of cooling igneous rock: glassy
very quickly, within hours, usually in sea as water cools magma fast
rate of cooling igneous rock: fine
rapid cooling over weeks or months
rate of cooling igneous rock: medium
fairly slow cooling- few thousand years
rate of cooling igneous rock: coarse
cools very slowly over millions of years
where do ign glassy rocks form?
at surface, ALWAYS volcanic extrusive
where do ign fine rocks form?
- at surface in EXTRUSIVE volcanic rocks as lava flows
2. in chilled margins at the edges of minor intrusions
where to ign medium rocks form:?
below the surface in minor intrusions ~1km down (hypabysall)
where do ign coarse rocks form?
in major inrustions at great depth >10km, as batholoiths or plutonic rocks.
define EXTRUSIVE
general term for all igneous rocks that cool at the surface (both lava and pyroclasts)
define INTRUSIVE
term for igneous rocks that cool below the surface
define HYPABYSALL
when igneous rocks form at relativley shallow depths below the surface
define PUTONIC
when igneous rocks form deep below the surface
why glassy textures form
lava flows into the sea and cools very rapidly, so no time for crystals to form (IE. obsidian)
what is a pegmatite?
an igenous rock with very large crystals can beseveral cms or even metres
how do pegmaities form?
in the last stages of crystallisation, from water rich residual magma
