Rocks are made up of substances called minerals. Any naturally occurring solid substance with a definite chemical composition is called a mineral. For example, a common rock like granite is made up of several minerals put together which are biotite, feldspar, and quartz.
All rocks are formed in the Earth’s lithosphere, which includes the Earth’s crust and the top part of its mantle, where a partially molten rock called magma flows very slowly under the crust.
Rock may be hard or soft and in varied colors. For example, granite is hard, soapstone is soft. Gabbro is black and quartzite can be milky white. Rocks do not have a definite composition of mineral constituents. Feldspar and quartz are the most common minerals found in rocks.
As there is a close relation between rocks and landforms, rocks and soils, a geographer requires basic knowledge of rocks. Petrology is a term used to refer to the study of rocks scientifically. It is a very essential part of geology.
Man has been using rocks throughout his history. Rock metals and minerals have been very important in human civilization. They help us to develop new technologies and are used in our everyday lives. Our use of rocks and minerals includes building material, cosmetics, cars, roads, and appliances.
Some of the most commonly used rocks and their uses from our daily lives:
Mineral grains make up rocks. Rocks are homogenous solids arising from an orderly arrangement of chemical compounds. Chemical bonds are responsible for holding together the rock-forming aggregates. The manner of formation of a rock determines the type of mineral abundance in the rock.
Silica is one of the components contained in very many rocks. It is an oxygen and silicon compound. 74.3% of the earth’s crust is formed by this compound. There is the formation of crystals from this mineral and other rock compounds. The naming of rocks as well as stating their properties is determined by silica and other minerals' proportion.
Classification of rocks is based on factors such as:
These physical properties of rocks result from the rock formation processes. Rocks can change in type with time. This is explained by the rock cycle which is a geological model. This leads to three general rock classes: metamorphic, sedimentary, and igneous.
These classes are further divided into many sub-classes. An increase or decrease in the proportions of the minerals in a rock may make a rock change from one class to another.
There are many different kinds of rocks that are grouped under three families on the basis of their mode of formation. They are:
As igneous rocks are formed out of magma and lava from the interior of the earth, they are known as primary rocks. The igneous rocks (Ignis – in Latin means Fire) are formed when magma cools and solidifies. When magma in its upward movement cools and turns into a solid form it is called igneous rocks. The process of cooling and solidification can happen in the earth’s crust or on the surface of the earth. Igneous rocks that form from red‑hot lava above the Earth’s surface are called extrusive rocks. Igneous rocks that form from the lava spilling from underwater volcanoes are also classified as extrusive rocks. The appearance of all extrusive igneous rocks depends on two major factors - how quickly the lava or magma cooled, and what substances it contains.
Igneous rocks are classified based on texture. The texture depends upon the size and arrangement of grains or other physical conditions of the materials. If molten material is cooled slowly at great depths, mineral grains may be very large. Sudden cooling at the surface results in small and smooth grains. Intermediate conditions of cooling would result in intermediate sizes of grains making up igneous rocks. Granite, gabbro, pegmatite basalt, volcanic breccias, and tuff are some of the examples of igneous rocks.
These rocks are divided into two subgroups:
The size of the crystals that make up extrusive igneous rocks depends on how quickly the lava cooled. When it cools quickly, there is not enough time for large crystals to form. The rocks formed from lava that cools more slowly have larger crystals. Some violent volcanic eruptions shoot out lava filled with gases. The lava cools quickly, while it is still in the air, and traps the gases inside. Rocks that form this way are full of holes. Two examples of this type of rock are pumice and scoria.
The word sedimentary is derived from the Latin word 'sedimentum' which means settling. Rocks (igneous, sedimentary and metamorphic) of the earth’s surface are exposed to denudational agents and are broken up into various sizes of fragments. Such fragments are transported by different exogenous agencies and deposited. These deposits through compaction turn into rocks. This process is called lithification. In many sedimentary rocks, the layers of deposits retain their characteristics even after lithification. Hence, we see a number of layers of varying thickness in sedimentary rocks like sandstone, shale, etc.
Depending upon the mode of formation, sedimentary rocks are classified into three major groups:
The word metamorphic means ‘change of form’. These rocks form under the action of pressure, volume, and temperature (PVT) change. Metamorphism occurs when rocks are forced down to lower levels by tectonic processes or when molten magma rising through the crust comes in contact with the crystal rocks or the underlying rocks are subjected to great amounts of pressure by overlying rocks. Metamorphism is a process by which already consolidated rocks undergo recrystallization and reorganization of materials within original rocks.
Mechanical disruption and reorganization of the original minerals within rocks due to breaking and crushing without any appreciable chemical changes is called dynamic metamorphism.
The materials of rocks chemically alter and recrystallize due to thermal metamorphism. There are two types of thermal metamorphism
In contact metamorphism, the rocks come in contact with hot intruding magma and lava and the rock material recrystallize under high temperatures. Quite often new materials form out of magma or lava are added to the rocks.
In regional metamorphism, rocks undergo recrystallization due to deformation caused by tectonic shearing together with high temperature or pressure or both.
In the process of metamorphism in some rocks grains or minerals get arranged in layers or lines. Such an arrangement of minerals or grains in metamorphic rocks is called foliation or lineation. Sometimes, minerals or materials of different groups are arranged into alternating thin to thick layers appearing in light and dark shades. Such a structure in metamorphic rocks is called banding and rocks displaying banding are called banded rocks. Types of metamorphic rocks depend upon original rocks that were subjected to metamorphism.
Metamorphic rocks are classified into two major groups: foliated rocks and non-foliated rocks.
These rocks are further subdivided into two in relation to their structure:
Gneiss, granite, syenite, slate, schist, marble, and quartzite are some examples of metamorphic rocks.
Rocks do not remain in their original form for long but may undergo a transformation. The Rock Cycle is a continuous process through which old rocks are transformed into new ones. The Rock Cycle is a group of changes. Igneous rock can change into sedimentary rock or into metamorphic rock. Sedimentary rock can change into metamorphic rock or into igneous rock. Metamorphic rock can change into igneous or sedimentary rock.
Igneous rocks are primary rocks and other rocks (sedimentary and metamorphic) form from these primary rocks. Igneous rock forms when magma cools and makes crystals. Magma is a hot liquid made of melted minerals. The minerals can form crystals when they cool. Igneous rock can form underground, where the magma cools slowly. Or, igneous rock can form above ground, where the magma cools quickly.
When it pours out on Earth’s surface, magma is called lava. It is the same liquid rock matter that we see coming out of volcanoes. On Earth’s surface, wind and water can break the rock into pieces. They can also carry rock pieces to another place. Usually, the rock pieces called sediments, drop from the wind or water to make a layer. The layer can be buried under other layers of sediments. After a long time, the sediments can be cemented together to make sedimentary rock. In this way, igneous rock can become sedimentary rock.
All rocks can be heated. But where does the heat come from? Inside Earth, there is heat from pressure (push your hands together very hard and feel the heat). There is heat from friction (rub your hands together and feel the heat). There is also heat from radioactive decay (the process that gives us nuclear power plants that make electricity).
Heat bakes the rock. Backed rock does not melt, but it does change. It forms crystals. If it has crystals already, it forms larger crystals. Because of this rock changes, it is called metamorphic. That change is called metamorphosis. Metamorphosis can occur in the rocks when they are heated to 300 to 700 degrees Celsius.
When Earth’s tectonic plates move around, they produce heat. When they collide, they build mountains and metamorphose the rock.
The rock cycle continues. Mountains made of metamorphic rocks can be broken up and washed away by streams. New sediments from these mountains can make new sedimentary rock.
The rock cycle never stops.
