The classification of rocks
The classification of rocks is determined by several factors, including their mineral and chemical composition, the constituent particles of which they are composed, and the processes by which they were created. Using these factors, rocks can be divided into three categories: igneous, sedimentary and metamorphic.
Igneous rocks are composed of cooled, solidified magma. This process occurs in two different ways, dividing igneous rocks into two further classes. When molten magma cools and crystallizes within the Earth’s crust, the resulting rocks are considered plutonic, or intrusive. If the magma reaches the surface and cools, it is considered volcanic, or extrusive rocks.
Sedimentary rocks form when sediments, organic matter, or chemical precipitates (evaporites), are deposited and then compacted into rock. In addition, cementation occurs when minerals fill in the spaces within the particulate matter. These rocks form at or near the Earth’s surface. Included in this category are commonly found rocks such as shale, sandstone, limestone and dolostone.
Metamorphic rocks are formed when any rock type, including already metamorphic rock, is subjected to temperature and pressure conditions that are different from those in which the rock originally formed, and changed into other rock forms. For this to occur, the temperature and pressure must be very high, much higher than at the Earth’s surface.
The three classes of rocks can be divided into many subgroups, but this can lead to difficulties.
The differences between the classes are not hard and fast, and rocks from any one category have the properties of the other two categories. Thus, by increasing or decreasing the amount of particular minerals or the presence of particular structures, one class of rock merges into another. Because of this, geologists frequently recommend that classification be reduced to a few generalized designations, and the simplest criteria.
Rock coloration
The color of sedimentary rocks varies greatly, in large part due to other substances that contaminate them. For example, red sandstone and limestone, both of which are very common, contain small amounts of finely distributed iron oxide, also called hematite. Other substances, such as limonite and goethite, tint sedimentary rocks yellow or brown, and green tints are caused by glauconite or chlorite, or oxidized copper. Black coloration is caused by manganese oxides, asphalt, while bluish coloration can result from contamination with phosphates or the mineral fluorite.
The societal impact of rocks
Rocks have played a key role in human life, and the development of specific cultural practices and technological advances. Technologies that specifically involve the use of rocks are known as Lithic technologies. The mining of metal ore from rock is an example of a Lithic technology. The use of metals has played a major role in human prehistory and history, so much so that the early development of humanity is divided into the Stone Age, Bronze Age, and Iron Age. In addition, rock and products derived from rock continue to play key roles in building construction, road laying, and recreation.
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