2006-11-13 21:10:00 · 3 answers · asked by Bill 1 in Science & Mathematics ➔ Earth Sciences & Geology
The identification of igneous rocks is based on COLOR, COMPOSITION (mineralogy) and TEXTURE.
COLOR is based on the relative amounts of iron and magnesium contained within the minerals of the rock. The color is considered light if there is little iron or magnesium (mafic minerals). The color is intermediate if there is an increased concentration of iron and magnesium. The color is considered dark if the rock appears as a dark gray to black. Of course this is only a very rough guide and should not be used as the only method of igneous rock classification. There are many exceptions to this color rule.
COMPOSITION refers to the relative amounts of silica vs. mafic minerals. High silica rocks (silica > 60%) are considered felsic. These rocks, like granite and rhyolite are generally light in color. Rocks rich (>60%) in ferromagnesian minerals (mafic) are generally dark in color.
TEXTURE describe the nature of the grains (crystals) that make up the rock. Rocks are considered COARSE GRAINED if you can distinguish the crystals with the unaided eye. FINE GRAINED igneous rocks have at least a portion of the rock matrix that has no visible crystals to the unaided eye. Porphyritic texture is produced by two distinct cooling stages producing both large and small crystals in the same rock. Slow cooling (generally deep underground) produces large crystals. Rapid cooling (at or near the surface of the Earth) produces smaller crystals. In a PORPHYRY, the crystals are of distinctly different size. The smaller crystals are called the matrix or groundmass. The term PEGMATITE is reserved for igneous rocks that have unusually large crystals. It is a term that is used for any such igneous rock but is normally associated with granites.
2006-11-15 10:28:25 · answer #1 · answered by Sebille 3 · 0⤊ 0⤋
1. study of the minerals... (i got this off a site..the other two I knew off hand)
http://geology.csupomona.edu/alert/igneous/ignrxs.htm
2. Composition of the rock is used..if it was formed under ground or on the surface. under ground is plutonic and I believe surface is volcanic.
3. texture The rocks are usually glossy, they are solidified magma and so they cool ..usually by water.
2006-11-14 05:39:38 · answer #2 · answered by GreyRainbow 4 · 0⤊ 0⤋
the most common charac. are crystal size, color and texture
2006-11-14 05:41:49 · answer #3 · answered by wfxj 2 · 0⤊ 0⤋