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Lab Note: Certain slides need to be studied that may not be included in this set list

igneous rocks

formed by the crystallization (or solidification) of melted material

Molten material that emerges at the earth's surface:

LAVA, cools to form VOLCANIC, or EXTRUSIVE igneous rock

Molten material within the earth:

MAGMA. cools to form PLUTONIC, or INTRUSIVE igneous rock


a combination of melted silicate material, volatiles, and possible already solidified crystals


substances (chiefly water) that are gases at low temperatures


resistance of a substance to flow; examples: pine sap (higher) vs. water (lower); increases as the lava starts to cool; the more complex the silicate melted in the magma, the higher the viscosity (complex: quartz, orthoclase)

classification of igneous rocks

mineral composition and texture


igneous rocks rich in light-colored minerals as orthoclase and quartz


igneous rocks rich in minerals such as Na-Ca plagioclase and hornblende


igneous rocks rich in dark-colored ferromagnesian minerals (augite and hornblende) but with an abundant plagioclase feldspar


igneous rocks composed chiefly of dark-colored ferromagnesian minerals, especially olivine and augite, with no plagioclase feldspar

absence of crystals (dense glassy) indicates-

extremely rapid cooling of lava (quenching)

absence of crystals (frothy glassy) indicates-

extremely rapid cooling of lava with high dissolved gas content (see bubbles)

large crystals indicate-

slow cooling

pegmatic texture indicates-

slow cooling in the presence of water

crystal size: fine grained

less then 1 mm

crystal size: medium grained

1-2 mm


larger then 2 mm

crystal size: porphyry

igneous rock with large crystals (phenocrystals) set in a matrix of finer-grained crystals (groundmass)

plutonic rock body

sill, dike, batholith

volcanic landform

fissure flow, shield volcano, composite volcano


(constructional feature) volcanic material accumulates at the surface; i.e. Mt. Rainier


(deconstructional feature) following catastrophic emptying of a magma chamber , a large region collapses; i.e. Yellowstone Caldera, Wyoming

mafic magma

typically has a higher temperature than felsic magma

higher temperature magmas commonly have:

lower viscosity, lower volatile content, less explosive tendency (lava rather then pyroclastic material)

pyroclastic rock

explosively erupted as hot but nevertheless solid particles


explosively erupted, fine-grained volcanic rock whose grains are commonly welded together


cavities in volcanic rocks, once occupies by gas bubbles

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