Igneous Rocks

Describe mafic igneous rocks.

Mafic igneous rocks are rich in magnesium and iron. They are dark-colored: black to very dark gray. They may have pyroxene (augite), plagioclase, and in some cases, olivine.

Describe felsic igneous rocks.

Felsic igneous rocks are rich in feldspar and silica. They are light-colored: white, light gray, and pink to red. They are abundant in light-colored minerals, such as quartz, orthoclase, biotite, muscovite, and plagioclase.

Define aphanitic texture.

Aphanitic texture describes fine-grained rocks. It is characteristic of extrusive (volcanic) rocks and very shallow intrusive rocks, and have crystals indistinguishable with the unaided eye. It indicates fast cooling history.

Define extrusive igneous rocks.

Extrusive igneous rocks solidified at the earth's surface, and therefore had a fast cooling history.

Define glassy texture.

Glassy texture is characteristic of extrusive rocks and forms by very rapid cooling (quenching) of the magma. There are no crystals because the atoms are "frozen" in a random pattern.

In what two ways are igneous rocks classified?

Igneous rocks are classified by texture (the size of mineral grains and/or distribution of grain sizes) and composition (the types of minerals present).

How are igneous rocks formed?

Igneous rocks are formed by the cooling and solidification of magma, and are aggregates of predominantly silicate minerals.

Describe intermediate igneous rocks.

Intermediate igneous rocks are usually medium gray and may contain hornblende, biotite, or plagioclase.

Define intrusive igneous rocks.

Intrusive igneous rocks solidified under the earth's surface, and therefore had a slow cooling history.

Define phaneritic texture.

Phaneritic texture describes coarse grained rocks. They are characteristic of intrusive (plutonic) rocks, and have crystals that can be seen with the unaided eye. Indicates slow cooling history.

Define porphyritic texture.

Porphyritic texture describes large crystals in a finer-grained matrix. These larger crystals are called phenocrysts, while the finer-grained matrix is referred to as the groundmass. A porphyritic rock can have a fine or coarse-grained groundmass. Porphyritic texture indicates two-stage cooling: slow, then fast.

What affects the texture of an igneous rock?

The texture of an igneous rock is determined by the cooling history of the magma from which it formed. Magmas that cool slowly form large crystals, and those that cool quickly form small crystals.

What are the three compositional types of igneous rocks?

The tree compositional types of igneous rocks are felsic (feldspar and silica rich), mafic (magnesium and iron rich), and intermediate rocks.

Define vesicular texture.

Vesicular texture describes some extrusive rocks. It forms when lava with a high gas content solidifies - the gas bubbles become trapped in the rock, creating air pockets.