What two characteristics are used to classify igneous rocks?

Answer 1

The first thing one would look at is if there are minerals present that are large enough to either be identified with the naked eye or with a handlense. Most lavas have large volumes of extremely finegrained material or glass whereas plutonic rocks that mostly crystallize in depths of several kilometers are usually composed of millimeter sized minerals. Further differentiation of ejecta can be done like (ash, lapilli, volcanic bombs, pahoehoe-lava etc.).

One can then look at the mineralogical composition of these rocks to define whether they are basic (little silicon) like basalts/gabbros or acid (a lot of silicon) like rhyolites/granites. The usual classification is the QAPF-diagram (quarz, alkali feldspar, plagioclase, feldspathoid), which is composed of two ternary diagrams. If quarz is present the ternary diagram including quarz is used, otherwise the one with an feldspathoid endmember is chosen for classification.

This field description is then usually followed by a series of lab analyses of the chemical and isotopic composition of the rock in order to be classified via their elemental composition. One example is the TAS-diagram (total alkali elements versus silicon dioxide content).

Answer 2

From this often confusing situation, a commonly used classification for igneous rocks has emerged based on two criteria--texture and chemical composition. Texture refers to the size of crystals, presence of glass, and porosity of the rock. Texture is determined primarily by how the magma or lava cooled. Following are common textural terms for igneous rocks.

Answer 3

Intrusive and extrusive factors of an igneous rock classify them. The other factors are if they are vesicular, coarse, fine grained, glassy, or very coarse

Answer 4

Extrusive (cooling of lava/magma above the earths surface) and Intrusive (cooling of lava/magma below the earth's surface)

Answer 5

Texture and crystal size

Answer 6

Texture and composition.

Answer 7

Composition and Texture

Answer 8

Formed by gaseous fire