All places on Earth are made of or are on top of rocks. A rock is a naturally occurring solid made up of a mixture of one or several minerals, in varying proportions. The minerals in the rocks vary, making different kinds of rock. The Earth’s crust is made of rock. Rock is often covered by soil or water. Rock is beneath the oceans, lakes, and rivers of the earth, and under the polar icecaps.
Minerals are usually solid, inorganic, crystalline substances formed naturally by geological processes. A mineral is a homogeneous naturally occurring substance with a definite but not necessarily fixed chemical composition. Most minerals are solids with an ordered atomic arrangement made of a single chemical element or more usually a compound. There are over 4,000 types of known minerals.
Rocks are classified by their minerals and chemical make-up.Rocks are aggregates of minerals. Geologists divide rocks into three groups: igneous, metamorphic, and sedimentary. Igneous rocks crystallise from magma. Metamorphic rocks form by the deformation and/or recrystallization of pre-existing rock by changes in temperature, pressure, and/or chemistry. Sedimentary rocks form by weathering and erosion of pre-existing rock to make sediment, which is lithified into rock.
Igneous rocks are formed when molten magma cools, either above or below the surface. They are divided into two main categories: plutonic rock and volcanic rock. Plutonic or intrusive rocks are made when magma cools and crystallises slowly within the Earth’s crust (example granite). Volcanic or extrusive rocks result from magma reaching the surface either as lava or ejecta (examples pumice and basalt).
Sedimentary rocks are the most common rocks on Earth. They form at or near the Earth’s surface. Sedimentary rock is formed in layers which were laid down one by one on top of another. Some of the layers are thin, some are thick. Layers are made by deposition of sediment, organic matter, and chemical precipitates. Deposition is followed by squeezing of sediment under its own weight, and cementation. This process is called ‘consolidation’: it turns the sediment into a more or less hard substance.
Metamorphic rocks are secondary rocks formed by rocks transformed under great pressure and high temperatures under earth’s crust. These conditions change the make-up of the original minerals.
The rock cycle is the process by which rocks of one kind changing into rocks of another kind. Three types of rocks can change into the other kinds by physical processes: cooling, melting, heat, weathering/erosion, compacting (squeezing tightly together), cementing, and pressure. Other substances also can become rocks and enter the rock cycle. When heated deep underground, rocks become magma (liquid rock). Above ground, it is called lava. Sediment, the particles from rock erosion and weathering, is the basis for a sedimentary rock of the future and soil may be reconverted to rock by this process.
These processes can occur in different orders, and the cycle goes on forever. Wind and water can create sediment from rocks, and movement of one tectonic plate against another creates enormous heat and pressure which affects rocks greatly. Subduction converts all kinds into magma, which eventually rejoins the cycle as igneous rock.
Weathering is the breaking down of rocks, soil and their minerals through direct contact with the Earth’s atmosphere, waters, or living things. Weathering occurs in situ (in place, with no movement). It leads to erosion. Erosion is where rocks and minerals are moved downhill (usually towards the sea) by water, ice and the wind.
The two main types of weathering are physical and chemical. Sometimes there are also aspects of biology. Physical weathering is important in very cold or very dry environments. Chemical reactions are most intense where the climate is wet and hot. However, both types of weathering occur together, and each tends to accelerate the other. The materials left after the rock breaks down combined with organic material creates soil.
Mechanical or physical weathering means the breakdown of rocks and soils through direct contact with atmospheric conditions such as heat, water, ice and pressure. It is the mechanical disintegration of rocks without a change in chemical components.
Wind processes are called ‘aeolian’. The Wind erodes the Earth’s surface by removing loose, fine-grained particles, called ‘deflation’. Sand carried by the wind wears down surfaces. Regions which have intense and sustained erosion are called deflation zones. Most aeolian deflation zones are composed of desert pavement, a sheet-like surface of rock fragments that remains after wind and water have removed the fine particles. Almost half of Earth’s desert surfaces are stone deflation zones. The rock mantle in desert pavements protects the underlying material from deflation.
Rain is another force that works slowly. The force of raindrops on some rocks makes them wear down. Rain also can make a chemical change in some rocks, because it is usually slightly acidic. The water mixes with the minerals in the rock to break it down.
Changing temperature can make a rock crack. Every day when the sun shines on the rock, its surface is heated. Heat causes the surface to expand (get bigger) a little. The inside of the rock, though, does not heat up as fast as the outside of the rock. The inside of the rock stays cooler. At night, the surface cools down and contracts. The expanding and contracting makes some places on the surface weak, and a crack is made. Also, if water gets into a crack in a rock and the temperature goes below the freezing point, the water will freeze and expand. After some time, the rock may be weak enough to break into pieces.
Ice, which can be miles thick, grinds the surface of the rock below it. The particles are carried with the ice, and if a glacier ends up in the sea, so does all the material carried with it.
Lava or magma can cause weathering as when the molten rock touches rock (either intrusive rock extrusive) it causes the rock to change form to add to the quantity of molten rock causing the rock to have changed form. So the rocks have formed a different crystal structure to before it came in direct contact with the lava or magma.
Chemical weathering is the direct effect of atmospheric or biological chemicals in the breakdown of rocks, soils and minerals. The mineral composition of rock changes due to chemical reaction with water or air. The carbon dioxide cycle is the most important for weathering. CO2 is put into the atmosphere mostly by volcanoes, and it is taken out of the atmosphere by photosynthesis, and by one other process. While it is in the air, CO2 can dissolve in water droplets to form dilute carbonic acid.
When rain hits a rock it does so with mechanical energy and dilute acid. The acid dissolves many types of minerals and rocks though, of course, very slowly. When a mineral like feldspar is dissolved, it lets sodium ions into the sea; chlorine ions come from other minerals. The sea tastes salty because of the elements which have been dissolved out of rocks.
Biological weathering happens when animals mechanically burrow or when plants and trees extend their roots into rock strata breaking it up.
Due to weathering, the particles are loosened up and under the effect of gravity, they are pulled downward. This movement under the influence of gravity is called mass wasting or mass movement.
The mass movement of soil and rock materials gets saturated with water and flows downward with water across a gentle slope, it is called earth flow. Similar flow occurring downwards a steep slope is called mud-flow. Dry soil and rock pieces suddenly flowing downhill across a steep slope is called a landslide.
Weathering is important for soil formation. The soil is a combination of fine rock particles and organic materials called humus. Humus is derived from remains of plants and animals. Soil formation is a slow process taking hundreds of years. Soil formation happens in layers and these layers can be seen distinctly if we dig a pit. The arrangement of horizontal layers of soil is called soil profile.
The topmost layer of soil is called topsoil and contains clay, silt, sand and humus. Most of the plants extend their roots in this layer. The layer below it is called subsoil. It contains coarse clay, sand and minerals. Partially weathered rocks and bedrock are seen below it.
As a consequence of weathering, the rock strata gets disintegrated and transported downwards to get deposited at lower places. Running water, glaciers, winds and sea waves causes this movement and are called agents of gradation. They erode, transport and deposit the earth material along their course of movement. In this process, they continuously change the landscapes and creates new land forms.
Running water in the form of rivulets, streams and rivers is the most important agent of gradation. While waterfalls on the earth’s surface some of them will get soaked into the earth and the remaining flows on the surface of the earth as run-off. Run-off depends on the slope of the land, amount of rainfall and extent of vegetation in the area. The water washes away the topsoil and reduces the fertility. This is called as soil erosion.
During heavy rainfall entire layer of soil is washed off from a large area without plant cover. Such erosion is called sheet erosion. Gully erosion is when rainwater scoops out the soil creating narrow deep channels called gullies while moving down the slope in uneven terrain.