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argillaceous (muddy) ->
arenaceous (sandy) ->
sand/clay mixture ->
Marl (carbonate-rich mud) ->
grains less than 0.1mm in diameter
grains between 0.1 and 1.0mm in diameter
grains over 1.0mm in diameter
Foliation in fine-grained rocks =
cleavage (slaty cleavage)
Foliation in medium-grained rocks =
Foliation in coarse-grained rocks =
bonding (gneissore bonding)
All crystals are approximately the same size and equidimensional
A rock with this texture cannot have a foliation.
Some larger crystals (porphyroblasts) set within a finer-grained matrix
A rock with this texture can have a foliation.
Temperature and pressure progressively increase.
Coarser grains and new mineral assemblages are produced.
As the grade increases, water is released, and rocks become 'drier' up the scale.
Temperature and pressure progressively decrease.
Water is added back to the rock.
Contact (thermal) metamorphism
Hot magma rises from depths and intrudes cooler country rock at a shallow depth.
Heat flows - magma cools and solidifies, and country rock heats up.
Hydrothermal fluids circulate through intrusion and country rock.
Highest-grade rocks form immediately adjacent to pluton, where temperatures are highest.
Progressively lower-grade rocks form farther from the heat.
Band of metamorphic rock around igneous intrusion = metamorphic aureole.
Takes place without stress, so aureoles contain hornfels, a non-foliated metamorphic rock.
A piece of the Earth's crust shears past another.
Upper 10-15km, rock can be fractured/turned to powder.
At greater depths rock is so warm it acts like soft plastic when shearing occurs.
As this happens, minerals recrystallise.
Resultant rock is a mylonite, which has a foliation roughly paralleling the fault.
Mylonites can be found at all plate boundaries (as faulting occurs here).
Regional (dynamothermal) metamorphism
When large mountain ranges develop, large parts of continental crust slip up and over other parts of crust.
Rock that was near surface ends up at great depth beneath mountain range.
Geothermal gradient = protolith heats up.
Weight of overburden increases pressure.
Plate interaction causes differential stress (squashing and shearing).
Protolith transformed into a foliated rock.
Slate forms at shallower depths.
Schist and gneiss form at greater depths.