Elements combine together to form different minerals and minerals combine to form rocks. There are 92 naturally occurring elements and with exception of a few inert elements like Gold, Platinum etc. , they combine together to form as many as 4000 minerals. Out of this large number of minerals only about two dozens are common (constitutes ~ 99% of Earth crust) and these are composed of about a dozen elements. These rock forming elements can be classified in five groups – silicates, carbonates, oxides and sulfates. In following sections we will briefly describe these minerals, their formation and usage.
Silicates: Silicon is the 2nd most abundant element next to oxygen on the Earth crust. It combines with oxygen to form a tetragonal compound, SiO2, which combines together to form a large tetragonal structure. Besides, SiO2 combines with other metal oxides, to form their silicates. Some examples of common silicates are Olivine, Pyroxene, Amphibole, Micas, Feldspar etc. Carbonates: Carbonates CO32- are linear molecules consisting of carbon and oxygen. These ions are arranged in the form of sheet in minerals like Calcite and Dolomite. Oxides: Metals react with oxygen to form their oxides.
Some important oxide minerals are Hematite (Fe2O3), Magnetite (Fe3O4), Chromite (Cr2O3), Cassiterite (SnO2) etc. Sulfides: These are metal-sulfur compounds. Some important sulfide compounds are known as Pyrites like Chalcopyrite, Fools Gold etc. These minerals have metallic luster. Sulfates: These are composed of metal ions and sulfate ions (SO42-). These minerals have practically no ore value to the date. However, these are used as compounds. One important sulfate mineral is Gypsum (CaSO4. 2H2O). Mineral Formation: Minerals form by crystallization and growth in a liquid.
The liquid can be either a molten rock or an aqueous solution. When temperature of a molten rock or magma falls below freezing point of a mineral, the mineral crystallizes. Minerals do not have a sharp freezing point; instead they solidify over a range, depending on their composition. The first to solidify is the one having highest solidification point and this is Quartz, subsequently other metal silicates solidify. Minerals solidifying towards the end have lowest solidification point. Besides, minerals crystallize in aqueous solution as well.
This happens when an aqueous solution having minerals at some temperature, becomes supersaturated due to change in temperature (generally cooling, but sometimes heating as well). In case of mineral precipitation from an aqueous solution, what is important is solubility and not the melting point of the mineral Properties of Mineral: Minerals are characterized by some of their unique properties like Crystal form, Hardness, Cleavage and Fracture, Color, Specific Gravity etc. crystal form of a mineral depends on the internal arrangement of the constituent atoms / ions.
This leads to well developed faces of the crystal and is a very important clue of crystal identification. Hardness of a mineral depends on the bond strength of the constituent atoms / ions. It is measured on Mohs scale (1 to 10). Higher Mohs value implies higher hardness; 10 is for diamond and 1 for talc. Color: Color of a mineral depends on its chemical composition, structure and also on impurity content. This is also an important clue for mineral identification. Specific Gravity: This is a very important property of minerals and difference in specific gravity is utilized for mineral separation.
Usages of Mineral: We use minerals in many different ways. These are used as such for example, sand as construction material, Limestone for making cement, Gypsum for making Plaster of Paris etc. Besides, metals, the backbone of today’s civilization, are extracted from their respective minerals. Some examples are Iron from hematite / magnetite, Copper from chalcopyrite, Uranium from uranite etc. Thus we can conclude that minerals are invaluable to us. Chapter 26: Rocks Rocks are the building block of the Earth crust. Therefore, we encounter rocks everywhere.
The study of rocks is important as they give us clue of the geological past of the Earth, besides they contain in them minerals, which have great value for us. The Earth can be viewed as the churning ground for rocks, where rocks keep forming and changing their forms. Based on origin, rocks are classified in three categories – Igneous, Sedimentary and Metamorphic. In subsequent paragraphs we will discuss briefly about these rocks. Igneous Rock: These are formed by solidification of magma. Magma is molten rock, which comes out from within the Earth crust during volcanic eruptions.
It is useful to know formation of magma. As we go down the Earth Crust, its temperature rises at a rate of ~ 30 oC per kilometer. Thus at a sufficient depth, temperature of the rock is well above their melting point, but the rock is still solid due to the overhead pressure by the rocks above it. But due to tectonic movements, sometimes a hot solid rock moves up where pressure is less that needed to keep it solid, and then it melts and the molten magma rises above through cracks. The rising magma further reduces the pressure on it and causes formation of more magma.
Also, sometimes, water comes in contact with hot rock through some cracks and water being foreign body or impurity, reduces melting point of the rock, which melts and thus magma is produced. The magma rises up through cracks and heats and melts the rocks in the way and thus creates more magma. The rising magma cools and solidifies and thus igneous rocks are formed. If the magma comes out of Earth’s surface (it is known as Lava) and solidifies then what is formed is Extrusive Rock, an important example is Basalt, which is low silica fast moving magma.
If the magma solidifies within the ground itself, then what forms is Intrusive Rock or Plutons, an example is a Dike. Sedimentary Rocks: Weathering of rocks leads to formation of smaller fragments of rocks. This process can be either mechanical or chemical. The weathered rocks erode i. e. they are transported away to a new location by carrier agents like flowing water and wind. During transportation also, these fragments collide with each other and gets smoothened. When the carrying ability of these agents weakens, these fragments settle at those locations. The process continues and new lays keep depositing.
The underlying layer is compressed by the top layers and densifies and thus is formed a layered Rock structure, known as sedimentary rock. If the sediments are composed of small rock fragments, this is called Clastic, while those formed by chemical precipitation are called chemical sediments. Clastic sedimentary rocks are classified according to their constituent particle sizes. Fine seiments are clay, silt and sand and they form rocks like Mudstone Shale, Siltstone and Sandstone respectively. Coarse sediments are pebble, cobble and boulder and they form Conglomerates.
Chemical sedimentary rocks are formed by precipitation from supersaturated aqueous solutions. Some examples of chemical sedimentary rocks are Limestone, Dolomite, and halites; halites are formed by evaporation of lakes or seawater. Many fossils are preserved in the sedimentary rocks and they give clues to the life in past. Metamorphic rocks When a rock mass – igneous as well as sedimentary, is brought in a region (by natural process) where temperature and or pressure is too high for existence of the rock, then is changes its form and the process is termed metamorphism (change in shape) and the resulting rock is metamorphic rock.
The metamorphism can be brought about by recrystallization or mechanical deformation. Some examples of metamorphic rocks are Granite (from lime stone), Diamond (from coal or graphite), etc. Rock cycle refers to a cycle through which these rocks – igneous, sedimentary and metamorphic, keep changing into each other by the different forces of Nature. Chapter 28: Occurrence and Movement of Water Life evolved on the Earth in water (oceans) and water is essential for life. More than 70% of Earth’s surface is covered with water.
Therefore, it is important to understand about this precious material and its cycle. Hydrological cycle refers to the chain of processes through which water moves from different forms (ice, water and water vapor) and different reservoirs (oceans, rivers, glaciers) etc. , while its amount remains more or less constant between different forms and reservoirs. About 97% of Earth’s water is in Oceans which covers more than 70% of Earths surface. About 2% is locked in polar icecaps and glaciers and remaining less than 1% is distributed between water vapors, ground water, rivers and lakes.
The process of evaporation moves water from oceans into the atmosphere as water vapor, approximately 75% of which rains back into the oceans and thus hydrological water cycle is complete for the 75% of the water vapors. Remaining 25% rains on the ground and thus the water cycle gets is prolonged. Because, the water falling on ground is distributed among ground water, river water, glaciers, lakes etc. In subsequent paragraphs we will discuss briefly about these reservoirs. Ground Water About 98. 5% of fresh waters stay beneath the ground in the pores, in the form of Ground water and soil moisture.
Beneath the ground, there is a region where all the pores are filled completely with water. This region is called saturated zone. Above this zone and up to the Earth’s surface there exists unsaturated zone or a zone where the pores are partially filled with water and air. The amount of rainwater that can be absorbed as ground water depends on the nature of the soil i. e. how porous or dense it is. While porous soil absorbs more of rainwater, the dense rocks and soil absorb less of it and let remaining flow as rivers. The water table refers to the level which demarcates the saturated and unsaturated zones.
The water table is not having a flat profile rather it is having a profile of its own. Therefore, water is not stationery in saturated zone; rather it flows under pressure head. The water bearing underground regions where water can flow is known as Aquifers. The flow rate of water through an Aquifer depends on hydraulic conductivity of the aquifer and the hydraulic head. The aquifers are of two types – unconstrained and constrained. In unconstrained aquifer, the sediment over the water table is permeable and allows recharge of aquifer. In confined aquifer, the aquifer is confined between impermeable rocks.
In such aquifers recharge is not from directly above rather the recharge zone is in the unconfined region at a higher elevation. In such aquifers, we have Artesian wells and Artesian springs. Streams are flowing surface water like rivers. They receive water from rains as well as from melting of glacier and transport the same into oceans or sometimes into large lakes. The stream speed depends on stream gradient, stream discharge and channel geometry. The stream gradient is largest near the head and here the channel shape is generally V-shaped and stream speed is very high.
As the stream proceeds towards moth, the stream gradient decreases and therefore, stream speed also decreases and the channel geometry becomes wide and shallow. Drainage basin refers to a network of streams which receives all the rain water in that geographical region and carries it to the oceans. Glaciers are large masses of ice, which move under their own weight due to plastic deformation as well as melting of ice under pressure. A glacier accumulates during winter and it ablates into water by moving down to warmer elevations, where it melts.
Glaciers are the origin of the famous river basins which have water through out the year. The oceans are the largest water mass having more than 97% of water on the Earth. The average depth of oceans is 3800 meters much larger than the average height of the mainland (~800) above the mean sea level. The boundary between the continents and the oceans is called Continental margin. This consists of continental shelf (the submerged portion of the margin) and continental slope and a continental rise. Waves are produced in oceans due to the wind and these waves superimpose with each other and create different patterns.