AS Igneous Rocks Flashcards
Igneous rocks
are those that have cooled from magma.
Phenocrysts
large or conspicuous crystals in an igneous rock, much larger than the groundmass.
Essential minerals
those minerals used to classify igneous rocks. Other minerals that may be present, but are not used for classification, are accessory or secondary minerals.
Felsic minerals
light coloured and silica rich.
Magma
molten rock, which is beneath the Earth’s surface.
Mafic minerals
dark coloured, silica poor and rich in magnesium and iron.
Flow banding
formed by friction as the magma or lava slows down near an interface, aligning the minerals as it moves.
Conchoidal
a fracture which results in a curved surface.
Vesicular
a textual term for a rock containing vesicles. Vesicles were bubbles of gas which came out of solution as a result of pressure release.
Porphyritic
a texture where large crystals, called phenocrysts, are completely surrounded by smaller crystals.
Equigranular
when crystals in an igneous rock are of approximately the same size.
Amygdaloidal
where there are large vesicles that have been filled with a secondary mineral.
Lava
molten rock that cools at the surface.
Ophiolites
sections of the Earth’s oceanic crust that have been tectonically moved (obducted) onto continental crust.
Extrusion
the emission of magma onto the Earth’s surface where it forms a lava flow.
