Igneous Rocks

Lava flows are typically finer grained than intrusive igneous rocks. Why?

The extrusive magma cools quickly so the mineral grains do not have time to grow

Which magma is most likely to quench (congeal) to a natural glass?

highly viscous; cools quickly

The sizes, shapes, and arrangements of mineral grains in an igneous rock are known as _.

Texture

Magma cools and consolidates without growth of mineral grains

aphanitic

mineral grains are of roughly equal size and coarse enough to be seen without a microscope or magnifying glass

phaneritic

rock has two or more, distinctly different-sized populations of mineral grains

porphyritic

a magnifying glass or microscope is needed to see individual mineral grains

glassy

__ is an open cavity in a volcanic rock that was filled by a gas bubble when the lava was still mainly liquid

vesicle

which mineral would you expect to see as a phenocryst in a porphyritic basalt

quartz

granite and __

rhyolite

basalt and __

gabbro

diorite and __

andesite

Which of the following igneous rocks exhibit aphanitic texture

andesite, rhyolite

In a porphyritic volcanic rock, which mineral grains are the last to crystallize?

matrix or groundmass

Visible quartz and potassium feldspar grains are the main constituents in a __

granite

Which of the following igneous rocks have a pyroclastic texture?

rhyolitic tuff

__ is a volcanic rock that is extremely vesicular and glassy

pumice

__ is composed mainly of ferromagnesian minerals

peridotite

Which of the following minerals crystallize early in Bowen’s reaction series?

olivine

__ is the dominant feldspar in basalt

plagioclase

Which of the following are used for studying rocks with a polarizing microscope

thin sections

__ is characterized by very coarse mineral grains

pegmatite

In which of the following igneous rocks and environments would you expect to find unusually high concentrations of rare elements such as lithium, beryllium, and boron?

pegmatite; crystallized from a water-rich, highly differentiated, residual magma