This is classification of rocks based on their origin and formation. On this basis, rocks are classified as igneous, sedimentary and metamorphic.
Igneous Rock, rock formed when molten or partially molten material, called magma, cools and solidifies. The inner layers of the earth are at a very high temperature causing the masses of silicates to melt. The melted masses of silicates is called magma, which forced up and released on the surface of the earth. This release is called volcanic eruption. The magma that is released cools and solidify into a crystalline rock.
Geologists classify igneous rocks according to the depth at which they formed in the earth’s crust. Using this principle, they divide igneous rocks into two broad categories: those that formed beneath the earth’s surface, and those that formed at the surface.
Rocks formed within the earth are called intrusive or plutonic rocks because the magma from which they form often intrudes into the neighboring rock. Rocks formed at the surface of the earth are called extrusive rocks. In extrusive rocks, the magma has extruded, or erupted, through a volcano or fissure.
Geologists can tell the difference between intrusive and extrusive rocks by the size of their crystals: crystals in intrusive rocks are larger than those in extrusive rocks. The crystals in intrusive rocks are larger because the magma that forms them is insulated by the surrounding rock and therefore cools slowly. This slow cooling gives the crystals time to grow larger. Extrusive rocks cool rapidly, so the crystals are very small. In some cases, the magma cools so rapidly that crystals have no time to form, and the magma hardens in an amorphous glass, such as obsidian.
Sedimentary rocks are formed by the consolidation of particles deposited in any of the three following ways:
- by the mechanical destruction and subsequent deposition of other rocks, usually by water, as in the case of sandstone or lime stone;
- by the action of animals and plants, as in the case of coral;
- by the chemical precipitation of mineral matter from water, as in the case of gypsum. The metamorphic rocks are formed by the transformation, of either igneous or sedimentary rocks through the influence of heat or chemical action. To this class belong marble, gneiss, and slate.
Most sedimentary rocks are characterized by parallel or discordant bedding that reflects variations in either the rate of deposition of the material or the nature of the matter that is deposited.
Sedimentary rocks are classified according to their manner of origin into mechanical or chemical sedimentary rocks.
- Mechanical rocks, or fragmental rocks, are composed of mineral particles produced by the mechanical disintegration of other rocks and transported, without chemical deterioration, by flowing water. They are carried into larger bodies of water, where they are deposited in layers. Shale, sandstone, and conglomerate are common sedimentary rocks of mechanical origin.
- The materials making up chemical sedimentary rocks may consist of the remains of microscopic marine organisms precipitated on the ocean floor, as in the case of limestone. They may also have been dissolved in water circulating through the parent rock formation and then deposited in a sea or lake by precipitation from the solution. Halite, gypsum, and anhydrite are formed by the evaporation of salt solutions and the consequent precipitation of the salts.
Due to the method of formation, sedimentary rocks are naturally soft and can be easily split up along the bedding. Their properties will vary depending on the nature of the sediment and type of bond.
Metamorphic Rock is a type of rock formed when rocky material experiences intense heat and pressure in the crust of the earth.
Metamorphic rock forms when pre-existing rock undergoes mineralogical and structural changes resulting from high temperatures and pressures. These changes occur in the rock while it remains solid (without melting).
The changes can occur while the rock is still solid because each mineral is stable only over a specific range of temperature and pressure. If a mineral is heated or compressed beyond its stability range, it breaks down and forms another mineral. For example, quartz is stable at room temperature and at pressures up to 1.9 gigapascals (corresponding to the pressure found about 65 km [about 40 mi] underground). At pressures above 1.9 gigapascals, quartz breaks down and forms the mineral coesite, in which the silicon and oxygen atoms are packed more closely together.
In the same way, combinations of minerals are stable over specific ranges of temperature and pressure. At temperatures and pressures outside the specific ranges, the minerals react to form different combinations of minerals. Such combinations of minerals are called mineral assemblages.
In a metamorphic rock, one mineral assemblage changes to another when its atoms move about in the solid state and recombine to form new minerals. This change from one mineral assemblage to another is called metamorphism. As temperature and pressure increase, the rock gains energy, which fuels the chemical reactions that cause metamorphism. As temperature and pressure decrease, the rock cools; often, it does not have enough energy to change back to a low-temperature and low-pressure mineral assemblage. In a sense, the rock is stuck in a state that is characteristic of its earlier high-temperature and high-pressure environment.
The size, shape, and distribution of mineral grains in a rock are called the texture of the rock. Many metamorphic rocks are named for their main texture. Textures give important clues as to how the rock formed. As the pressure and temperature that form a metamorphic rock increase, the size of the mineral grains usually increases. When the pressure is equal in all directions, mineral grains form in random orientations and point in all directions. When the pressure is stronger in one direction than another, minerals tend to align themselves in particular directions. In particular, thin plate-shaped minerals, such as mica, align perpendicular to the direction of maximum pressure, giving rise to a layering in the rock that is known as foliation. Compositional layering, or bands of different minerals, can also occur and cause foliation. At low pressure, foliation forms fine, thin layers, as in the rock slate. At medium pressure, foliation becomes coarser, forming schist. At high pressure, foliation is very coarse, forming gneiss. Commonly, the layering is folded in complex, wavy patterns from the pressure.
CLASSIFICATION BASED ON THE PHYSICAL FORM.
Rocks are also classified as stratified and unstratified, depending on their structure. Igneous and metamorphic rocks are unstratified, that is, they are not arranged in any definite form in layers, or strata, but have the constituent parts mingled together.
The sedimentary rocks are stratified, or formed in a series of parallel layers, as they are deposited from water. The layers were originally horizontal, but in most cases they are found more or less inclined and curved on account of the action of disturbing forces. Sedimentary rocks are composed of grains bound together by a cementing medium, and their strength and durability depend on the nature of the cementing material.