Key Words: GEOLOGY UNIT 2.2


Magma - molten rock

Lava - molten rock the cools on the surface

Silicic igneous rocks - composition of rich in silica, more than 66% and light coloured

Intermediate igneous rocks - silica content of 66% to 52% and are grey

Mafic igneous rocks - silica content of 52% to 45% and are dark

Ultramafic igneous rocks - silica content of less than 45%

Felsic minerals - light coloured and silica rich

Mafic minerals - dark coloured, silica poor and rich in Mg and Ir

Fine grained crystals - less than 1 mm in size, individual crystals cannot be indentified by the eye

Medium grained crystals - 1 to 5 mm in size, can be seen by the eye

Coarse grained crystals - greater than 5 mm in size, can be seen and identified by the eye

Extrusive - general term for all igneous rocks that cool at the surface, lava and pyroclasts

Intrusive - the term for igneous rocks that cool below the surface

Hypabyssal - when igneous rocks form at relatively shallow depths below the surface

Plutonic - when igneous rocks form deep below the surface

Pegmatites - an igneous rock that has exceptionally large crystals

Equigranular texture - all the crystals are of equal size but may all be fine, medium or coarse

Vesicular texture - where gas bubbles are trapped in lava and as it rapidly cools the holes are left where gas is present, usually found near top of a flow as gas rises

Amygdaloidal texture - where vesicles are later infilled by minerals deposited from groundwater

Flow banding - where layers of dark and light minerals form due to the seperation of minerals within a siliv lava flow

Porphyritic - where a rock has two stages of cooling and results of two distinct sizes of crystals (Large crystals, slow cooling=Phenocrysts and Finer grained groundmass, quicker cooling)

Divergent plate margin - two plates moving apart and magma rising between them

Convergent plate margin - two plates colliding and magma is formed above subduction zone or deep in the crust

Hot spot - formed by a fixed mantle plume bringing magma to the surface

Partial melting - where some of the minerals…


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