Environmental Geosciences I: The Origin of the Earth and its Differentiation, Minerals and Rocks, Tectonic and Climatic Controls on Landforms, Steady State and Equilibrium, the Earth's Energy Budget and Seasons, the Forces that Drive the Wind, the Climates of India with the Monsoon, Western Disturbances, Droughts and ENSO, Residence Time and Geophysical Fields

A mineral is a naturally occurring, inorganic, crystalline solid with a definite chemical composition, or a composition that varies within fixed limits. Minerals are identified by their crystal form, colour, streak, lustre, cleavage and hardness, the last on Mohs' scale from talc (1) to diamond (10), with quartz at 7. Silicates, built of SiO₄ tetrahedra, make up most of the crust, feldspars being the most abundant group and quartz the next. A rock is an aggregate of one or more minerals. The three rock families are linked in the rock cycle: igneous rocks crystallise from magma, sedimentary rocks form from the weathered debris of other rocks, and metamorphic rocks form when any rock is transformed by heat, pressure and fluids without melting.

Bowen's reaction series describes the order in which minerals crystallise from a cooling magma: olivine, pyroxene, amphibole and biotite on the discontinuous branch and calcium-rich to sodium-rich plagioclase on the continuous branch, followed by potassium feldspar, muscovite and finally quartz. Minerals that crystallise first at high temperature are the least stable at the Earth's surface and weather fastest, which links igneous petrology to soil formation. Metamorphism is regional when it accompanies mountain building over large areas under both heat and directed pressure, producing foliated rocks, and contact when heat from an intrusion bakes the surrounding rock, producing non-foliated rocks. Shale becomes slate, then phyllite, schist and gneiss with rising grade; limestone becomes marble; sandstone becomes quartzite; and granite can become gneiss.

Landforms are the outcome of a contest between internal forces that raise and deform the crust and external forces that wear it down. The tectonic control is plate tectonics: the lithosphere is broken into plates that move a few centimetres a year over the weaker asthenosphere. At divergent boundaries plates separate, forming mid-ocean ridges and continental rifts; at convergent boundaries one plate subducts beneath another, forming trenches, volcanic arcs and fold mountains, or two continents collide, as India and Eurasia have done since about 50 million years ago to raise the Himalaya and the Tibetan Plateau; at transform boundaries plates slide past each other, causing earthquakes. The climatic control sets which external process dominates: chemical weathering and deep soils in the humid tropics, frost action and glaciers in cold regions, wind and flash floods in deserts, and rivers almost everywhere.

Earth systems are open systems, exchanging energy and matter with their surroundings. A system is in steady state when its inputs equal its outputs, so that the amount stored does not change even though material flows through it, as a lake whose inflow equals outflow and evaporation keeps a constant level. Equilibrium in the strict sense means no net change in any direction; geomorphologists use dynamic equilibrium for landscapes in which form is maintained while material passes through, as when uplift is balanced by erosion. A system displaced from steady state tends to return to it by negative feedback; positive feedback, such as the ice-albedo feedback, amplifies a change instead.

The Earth receives about 1,361 W/m² of solar radiation at the top of the atmosphere on a surface facing the Sun, the solar constant; averaged over the whole spherical surface this is a quarter, about 340 W/m². About 30% is reflected (the planetary albedo is about 0.3), so about 240 W/m² is absorbed and, in balance, re-emitted as long-wave radiation. The temperature at which a black body would emit that much is the effective temperature, Te = [S(1 − α) ÷ 4σ]^¼ = [1361 × 0.7 ÷ (4 × 5.67 × 10⁻⁸)]^¼ ≈ 255 K, or about −18 °C. The actual mean surface temperature is about 288 K (15 °C); the difference of about 33 K is the natural greenhouse effect. More energy is absorbed than emitted in the tropics and less at high latitudes, and the atmosphere and oceans carry the surplus polewards. The seasons are caused by the tilt of the Earth's axis, about 23.4° from the perpendicular to its orbit, which changes the angle of the Sun and the length of day through the year; they are not caused by the changing distance from the Sun, which is actually least in early January, during the northern winter.

Above the boundary layer, where friction is negligible and the isobars are straight, the pressure gradient and Coriolis forces balance, and the air flows parallel to the isobars as the geostrophic wind, with low pressure on the left in the northern hemisphere (Buys Ballot's law). Where the isobars are curved, a third term, the centripetal acceleration needed to follow the curve, enters the balance, giving the gradient wind: around a low (cyclonic flow) it is slower than the geostrophic value, and around a high (anticyclonic flow) faster. Near the equator the Coriolis force vanishes, so geostrophic balance fails and tropical cyclones cannot form within about 5° of the equator.

India has a tropical monsoon climate modified by relief: from the humid tropical climates of the west coast and the north-east, through the tropical savanna and semi-arid climates of the peninsula and the plains, to the hot desert of the Thar and the cold, dry, high-altitude climates of Ladakh. The south-west monsoon, from June to September, brings about three-quarters of the country's annual rainfall. Its onset over Kerala is normally around 1 June, and it covers the whole country by early July. It is driven by the seasonal reversal of pressure and winds as the land heats far more than the sea, the northward shift of the intertropical convergence zone into the monsoon trough over the Gangetic plain, the heating of the Tibetan Plateau, the tropical easterly jet in the upper troposphere and the cross-equatorial Somali jet near the surface. The north-east monsoon, from October to December, gives Tamil Nadu and south-coastal Andhra Pradesh their main rains.

Western disturbances are extratropical low-pressure systems that originate over the Mediterranean region and travel east along the subtropical westerly jet stream, reaching north-western India mainly between December and March. They bring winter rain to the plains of Punjab, Haryana and western Uttar Pradesh, vital for the rabi wheat crop, and snow to the western Himalaya, which feeds rivers in the dry season; they also trigger cold waves and fog in their wake. Droughts are of four kinds: meteorological (a significant deficiency of rainfall from normal), hydrological (depleted streamflow, reservoirs and groundwater), agricultural (soil moisture inadequate for crops) and socio-economic (when water shortage affects livelihoods and the economy).

El Niño is the periodic warming of the central and eastern equatorial Pacific, every two to seven years, coupled with the Southern Oscillation, a see-saw of air pressure between Tahiti and Darwin; together they are ENSO. In normal years the trade winds pile warm water in the western Pacific and drive the Walker circulation, with rising air over Indonesia; in an El Niño the trades weaken, warm water spreads east, and the rising branch shifts towards the central Pacific, bringing sinking air and suppressed rainfall over the Indian region. El Niño years are therefore often, though not always, years of weak monsoon and drought in India, as in 2009 and 2015. La Niña, the cold phase, tends to strengthen the monsoon. A positive Indian Ocean Dipole, with warmer water in the western than the eastern Indian Ocean, can offset El Niño, as in 1997, when a strong El Niño brought a near-normal monsoon.

The residence time of a substance in a reservoir at steady state is the amount in the reservoir divided by the rate of input or output: τ = M ÷ F. The atmosphere holds about 13,000 km³ of water, and precipitation removes about 500,000 km³ a year, so a water molecule stays in the air for about 13,000 ÷ 500,000 = 0.026 year, roughly 9 days, which is why weather varies so fast and why soluble pollutants are rained out within days. The same arithmetic gives thousands of years for the ocean and millions for the sodium dissolved in it. Residence time sets how fast a reservoir responds to a change in input and how far a pollutant can travel: short-lived sulphur dioxide acidifies regions, while long-lived carbon dioxide and CFCs mix across the whole globe.