FOR A DESCRIPTION OF THE TERMS USED IN THIS CHART, SCROLL TO THE BOTTOM OF THE PAGE.
|
IGNEOUS ROCK CLASSIFICATION |
||||
| COLOR | LIGHT COLORED | MEDIUM COLOR | DARK COLOR | |
| CHEMISTRY | FELSIC | INTERMEDIATE | MAFIC | ULTRA MAFIC |
| COARSE GRAINED | GRANITE | DIORITE | GABBRO | PERIDOTITE |
| FINE GRAINED | RHYOLITE | ANDESITE | BASALT | KOMATIITE |
| PEGMATITE | A PEGMATITE is an igneous rock distinguished by its abnormally large crystals. The crystals are normally larger that a few centimeters and can often be dozens of centimeters long or much longer (meters long). Unlike other igneous rocks that develop from the molten state, pegmatites grow from aqueous solutions. The solutions allow for ease of movement of the nutrients to the site of crystal growth. Thus pegmatites can produce large crystals in a short (geologically) period of time. | |||
| PORPHYRITIC | A PORPHYRITIC rock is and igneous rocks that contains two distinct crystal sizes. These distinctly different crystal sizes were produced by different cooling of the liquid rock. Large crystals form slowly beneath the surface of the Earth and small crystals form when rapid cooling takes place (normally at or near the surface). The large crystals in a porphyry are called phenocrysts. The term PORPHYRITIC is used as an adjective to describe this distinct texture of igneous rock, e. g., a PORPHYRITIC basalt. | |||
| GLASSY | Glassy igneous rocks are formed by very rapid cooling. No crystals were formed during the cooling process. Examples are OBSIDIAN and PUMICE. | |||
| FRAGMENTAL | Fragmental igneous rocks are produced when existing igneous rocks are put under stress or moved causing them to fracture. These fragments are then fused to form a new rock. Obviously their is little change in the composition of the rocks. VOLCANIC AGGLOMERATES or VOLCANIC BRECCIAS are examples of the igneous rock type. | |||
You can also follow these thumbnails to the IGNEOUS ROCK pages:
 |
 |
 |
 |
|
| GRANITE | DIORITE | GABBRO | PERIDOTITE | |
 |
 |
 |
no picture | |
| RHYOLITE | ANDESITE | BASALT | KOMATIITE | |
 |
 |
 |
 |
|
| OBSIDIAN | PUMICE | VOLCANIC AGGLOMERATE | PEGMATITE | |
The field identification of IGNEOUS ROCKS is based on COLOR, CHEMISTRY (mineralogy) and TEXTURE. The classification chart below shows how these characteristics are used to identify the basic types of igneous rocks.
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.
CHEMISTRY 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. Pegmatites are unique in that they do not form directly from the igneous melt but instead are formed from fluids derived from or near the igneous rock body. The fluids (generally aqueous and under high temperatures and pressures) allow for a great deal of freedom for the migration of ions (charged atoms or molecules) to the sites of crystallization. The result is the formation of large crystals.