Igneous and Metamorphic Rocks

extrusive & intrusive rocks

igneous rocks, no matter the composition, are divided into two groups defined on the basis of where the magma stopped rising and began to cool which are called

crystal size

the most important aspect of texture in igneous rock is

cool slowly

when magmas ______, there will be fewer crystals that are more widely spaced and larger, these magmas solidify within the earth (readily visible)

cool quickly

when magmas _____, there will be many closely spaced crystals and each will be relatively small, these magmas reach earth's surface (not visible to naked eye)

igneous Phaneritic texture

rocks in which the crystals are readily visible to the naked eye

igneous Aphanitic texture

rocks in which the crystals are not visible to the naked eye

igneous Porphyritic texture

rocks with two distinctly different sizes of crystals

larger = phenocrysts

smaller = groundmass

igneous Pegmatic texture

very coarse-grained - ex. glassy rocks that cool almost instantly and no crystals form

igneous Scoriaceous/Vesicular texture

small, round holes (vesicles)

igneous Pumiceous texture

numerous small cavities that give the rock a spongy or frothy appearance - most will float on water

origin (extrusive/intrusive), composition (felsic/mafic), texture

igneous rocks are described by their

contact or regional

most metamorphism takes place in one of two settings

foliated or nonfoliated

metamorphic rocks can be sub-divided into either ____

foliated metamorphic rocks

characterized by the planar arrangement of minerals in the rock (slate, phyllite, schist), or by alternating layers of different minerals (gneiss)

nonfoliated metamorphic rocks

form either when a strongly directed stress field is absent or when the mineralogy of the precursor (parent) rock prevents formation of a foliation - cannot show a preferred orientation (ex. minerals calcite from marble & quartz from quartzite are equidimensional)

felsic igneous rocks

are light colored and have high silica content - rich in feldspar & quartz

mafic igneous rocks

dark colored and have low silica content - rich in magnesium and iron

intrusive

igneous rocks that form from magma cooling slowly beneath the Earth's surface, resulting in large crystals.

extrusive

igneous rocks that form from lava cooling quickly on the Earth's surface, resulting in small crystals.