UPSC Prelims GS Revision World Physical Geography

KEY REVISION POINTS WORLD PHYSICAL GEOGRAPHY

Introduction:

  • Geography seeks to understand how physical landscapes shape human history.
  • Geography is divided into two main branches: Human geography and Physical geography.
  • Human geography is a social science that focuses on people, where they live, their ways of life, and their interactions in different places around the world.
  • Physical Geography studies the natural features of the Earth’s surface and the processes that shape the Earth’s surface.
  • Eratosthenes is known as the Father of Geography.
  • The Earth is not a perfect sphere. It is slightly flattened at the North and the South Poles and bulges at the equator. Such a shape is known as a Geoid.

Equator, North Pole and South Pole

  • The equator is an imaginary circle that divides Earth into two halves.
  • The northern half is called the Northern Hemisphere.
  • The southern half is called the Southern Hemisphere.
  • The northernmost point on Earth is called the North Pole.
  • The southernmost point on Earth is called the South Pole.

Latitude

  • It is the angular distance of a place north or south of the earth’s equator.
  • Lines of latitude are known as parallels as they are all parallel to each other.
  • All parallels are not in equal length.

Longitude

  • It is the angular distance of a place east or west of the Prime Meridian or 0° longitude.
  • Lines of Longitude are known as meridians.
  • All meridians are of equal length.
  • The zero degree longitude is known as Prime Meridian.
  • This line divided the eastern and western hemispheres of the Earth.

Important lines of latitude based on the sun’s position during Earth’s orbit:

Tropic of Cancer:

  • It is a parallel located at 23.5 degrees north of the equator.
  • Here sun can appear directly overhead at noon during the June solstice.

Tropic of Capricorn:

  • It is a parallel located at at 23.5 degrees south of the equator.
  • Here the sun can appear directly overhead at noon during the December solstice.

Arctic Circle:

  • It is a parallel located at 66½° north of the equator.
  • It marks the latitude above which the sun does not set on the summer solstice, and does not rise on the winter solstice.

Antarctic Circle:

  • It is a parallel located at 66½° south of the equator.
  • Here the sun neither sets on the day of summer solstice (December 22) nor rises on the day of winter solstice (June 21).

Equinox

  • The word equinox literally means “equal length of day and night”.
  • Equinoxes happen directly between the solstices and mark the beginning of the Spring and Fall seasons.
  • The equinoxes occurs in March 21 and September 23.
  • On these dates we will have same length of day and night.
  • At the equator, the sun is directly overhead at noon on these two equinoxes.

Torrid, Temperate and Frigid zones

  • Torrid zone lies in the area between the Tropic of Cancer and the Tropic of Capricorn. This zone receives direct rays of the Sun and is very hot and humid.
  • The region between the Tropic of Cancer to Arctic Circle in the Northern Hemisphere and the Tropic of Capricorn to the Antarctic circle in the Southern Hemisphere falls in Temperate Zone. This zone receives slanting rays of the sun and is neither very hot nor very cold.
  • Frigid zone region extending from the Arctic Circle to the North Pole in the Northern Hemisphere and the Antarctic circle to the South Pole in the Southern Hemisphere fall in this zone. This region is extremely cold and remains covered with snow throughout the year.

Eclipses:

Solar Eclipse:

  • It occurs when the Moon moves in front of the Sun, blocking it out partially or completely.
  • It can only occur at New Moon.
  • During a solar eclipse, the Moon actually casts two shadows towards Earth.
  • One shadow is called the umbra which becomes smaller as it reaches the Earth.
  • The second shadow is called the penumbra which becomes larger as it reaches the Earth

Lunar Eclipse:

  • It happens when the Earth moves between the Sun and the moon, blocking part of the Sun’s light from reaching the moon.
  • During a lunar eclipse, you will see the Earth’s shadow on the moon.

Solar System

  • The Solar System consists of the Sun and the other celestial objects gravitationally bound to it.
  • There are eight planets in our Solar System. The eight planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
  • In these eight planets, four are terrestrial inner planets and other four are gas giant outer planets.

Inner Planets:

  • Mercury, Venus, Earth, Mars are terrestrial or inner planets
  • These planets have dense, rocky compositions, few or no moons, and no ring systems.

Outer Planets:

  • Jupiter, Saturn, Uranus, Neptune are Outer Planets.
  • Also called giant planets or Jovian planets.
  • All four giant planets have rings.

Aphelion and Perihelion:

  • A planet’s distance from the Sun varies in the course of its year.
  • Its closest approach to the Sun is called its perihelion, while its farthest distance from the Sun is called its aphelion.

Asteroids:

  • Asteroids are small, rocky objects that orbit the Sun.
  • Most of them are located in the main asteroid belt – a region between the orbits of Mars and Jupiter.

Comets:

  • Comets are cosmic snowballs of frozen gases, rock, and dust that orbit the Sun.
  • When a comet’s orbit brings it close to the Sun, it heats up and spews dust and gases into a giant glowing head larger than most planets.
  • The dust and gases form a tail that stretches away from the Sun for millions of miles.
  • Coma is a part of a comet consists of long tail of gas and dust often visible to the naked eye.

Earth’s Layers: Crust, Mantle, & Core

Earth consists of three main layers: the crust, the mantle, and the core

Crust:

  • It is the Earth’s outermost layer.
  • It is rocky and rigid.
  • There are two kinds of crust: continental crust and ocean crust.
  • Continental crust is thicker and contains Silica + Alumina (SiAl).
  • Ocean crust is thinner and contains Silica + Magnesium (SiMa).
  • Continental crust less dense than ocean crust.

Mantle:

  • It extends up to a depth of 2900 km. below the crust.
  • Lithosphere is formed from the crust as well as the uppermost layer of the mantle.
  • Upper portion of mantle is called asthenosphere which extends upto 400 km.
  • It is the main source of magma that finds its way to the surface during volcanic eruptions.
  • Mantle density is higher than crust.

Core:

  • Outer core is liquid and inner core is solid.
  • It mostly constituted by nickel and iron (NiFe).

What is a Rock?

  • Any natural mass of mineral matter that makes up the earth’s crust is called a rock. Rocks can be of different colour, size and texture.
  • There are three main Types of Rocks – Igneous rocks, Sedimentary rocks and Metamorphic rocks
  • Igneous rocks are formed by the solidification and cooling of magma.
  • Igneous rocks are divided into two main categories: Plutonic (intrusive) rock and volcanic (extrusive).
  • Plutonic or intrusive rocks are formed when magma cools and crystallizes slowly within the Earth’s crust. Eg: Granite.
  • Extrusive or volcanic rocks are formed when magma exits and cools above (or very near) the Earth’s surface. Eg: Basalt.
  • Sedimentary Rocks formed when sediment becomes cemented into solid rock. Eg: Sandstone is made from grains of sand.
  • Metamorphic rocks on the other hand are formed when pre-existing rocks are exposed intense temperature and pressure. Examples of metamorphic rocks are schist, gneiss, quartzite and marble.

Types of sedimentary rocks:

Depending upon the mode of formation, sedimentary rocks are classified into three major groups:

  • Mechanically formed — sandstone, conglomerate, limestone, shale, loess are examples
  • Organically formed — geyserite, chalk, limestone, coal are some examples
  • Chemically formed — chert, limestone, halite, potashare some examples

Earthquakes:

  • Earthquake occurs when Earth’s surface shakes because of the release of seismic energy following the rapid movement of large blocks of the crust along a fault.
  • A fault is a sharp break in crustal rocks.
  • The focus is the point on the fault surface where motion begins or where energy is released.
  • Focus or hypocentre is the source of the earthquake.
  • The epicenter is the point on Earth’s surface directly above the focus. Epicenter is the first one to experience the waves.
  • A seismograph or seismometer is an instrument used to detect and record earthquakes.
  • Magnitude and Intensity measure different characteristics of earthquakes.
  • Magnitude measures the energy released at the source of the earthquake.
  • The Richter scale is a numerical value used to measure the power or magnitude of earthquakes. The scale ranges between 0-10.
  • The effect of an earthquake on the Earth’s surface is called the intensity.
  • Intensity scale records visible damage caused by an earthquake. The scale ranges between 1 to 12.

Seismic Waves

  • Seismic waves can be divided into Body waves and Surface waves.
  • Body waves travel through Earth’s interior. These are further subdivided into P (primary) waves and S (secondary or shear) waves based upon their vibration direction and velocity.
  • P waves move faster and are the first to arrive at the surface. They can travel through Solid, Liquid and Gas.
  • S waves vibrate perpendicular to their travel direction. They can travel only through Solid.
  • Surface waves travel on Earth’s surface and cause much of the destruction associated with earthquakes.
  • Surface waves can be divided into two types – Love and Rayleigh waves.
  • The first type is called the Love or L wave. L waves only move along the earth’s surface and travel slightly slower than S waves.
  • The second type of surface wave is known as a Rayleigh or R wave. The materials disturbed by a Rayleigh wave move both vertically and horizontally in a vertical plane pointed in the direction in which the waves are traveling.

Volcanism

  • Volcanism is the eruption of molten rock (magma) onto the surface of a planet.
  • A volcano is the vent through which magma and gases are discharged.
  • Magma that reaches the surface is called Lava.
  • A volcanic crater is a circular depression in the ground caused by volcanic activity.
  • A volcanic crater is a bowl or funnel-shaped depression that usually lies directly above the vent from which volcanic material is ejected.
  • A vent enables a volcanic eruption. A crater is formed as a result of a volcanic eruption.

Types of Volcanoes:

  • Fissure volcanoes have no central crater at all. Instead, giant cracks open in the ground and expel vast quantities of lava. This lava spreads far and wide to form huge pools that can cover almost everything around. Eg: Los Pilas volcano
  • Cinder cones are the simplest type of volcano which have a bowl-shaped crater at the summit and steep sides.
  • Shield volcanoes are typically oceanic in origin formed by the gentle outpourings of fluid lavas. They are formed by extensive basaltic lava flows. These are the largest volcanoes in the world. Eg: Hawaiian volcanoes.
  • Strato volcanoes or Composite volcanoes are typically associated with explosive eruptions resulting in stratified layers of ash, cinders and lavas. Eg: Mount Fuji in Japan.
  • Lava domes are formed during volcanic eruptions in which highly viscous magma accumulates in the near-vent region. They are commonly found within the crater of large composite volcanoes.
  • Active volcano is a volcano which is either erupting or is likely to erupt in the future. Examples: Kilauea in Hawaii, Mount Etna & Mount Stromboli in Italy.
  • Dormant volcanoes that have not erupted during historical times, but will probably erupt again. Examples: Mt Kilimanjaro in Tanzania, Mt Fuji in Japan, Mauna Kea in Hawaii.
  • Extinct volcanoes are those volcanoes that are unlikely to erupt again. Examples: Mt Thielsen in Oregon (US).

Volcanic Landforms:

  • Batholiths, Laccolith , Lapolith, Phacolith, Sills and Dyke are volcanic intrusive landforms.
  • Craters and Calderas are volcanic extrusive landforms.

Batholiths:

  • These are large granitic rock bodies formed due to solidification of hot magma inside the earth.
  • They appear on the surface only after the denudation processes remove the overlying materials.

Laccoliths:

  • These are large dome-shaped intrusive bodies connected by a pipe-like conduit from below.
  • These are intrusive counterparts of an exposed domelike batholith.

Lapolith:

  • As and when the lava moves upwards, a portion of the same may tend to move in a horizontal direction wherever it finds a weak plane. It may get rested in different forms.
  • In case it develops into a saucer shape, concave to the sky body, it is called Lapolith.

Phacolith:

  • A wavy mass of intrusive rocks, at times, is found at the base of synclines or the top of the anticline in folded igneous strata.
  • Such wavy materials have a definite conduit to source beneath in the form of magma chambers (subsequently developed as batholiths). These are called the Phacoliths.

Sills and Sheets:

  • The near horizontal bodies of the intrusive igneous rocks are called sill. The thinner ones are called sheets.

Dykes:

  • When the lava makes its way through cracks and the fissures developed in the land, it solidifies almost perpendicular to the ground.
  • It gets cooled in the same position to develop a wall-like structure. Such structures are called dykes.

Terms associated with Volcanism:

  • Pyroclastic material: It is ejected from volcanoes.
  • Ash: Fine particles of pulverized rock that are blown from an explosion vent forms Ash.
  • Fumerole: It is a crack from which volcanic gases, mostly water vapor, escape into the atmosphere.
  • Magma and Lava: Molten rock from Earth’s interior is called Magma. When magma reaches the surface, it’s called lava.
  • Vent: A passage through which the magma travels to the Earth’s surface is known as a vent.
  • Crater: A circular depression of the volcano is called a crater.
  • Caldera: It is the large basin-shaped crater at the top of a volcano. A caldera is formed when the original peak collapses into an empty chamber below.
  • Plate tectonics theory: This theory is explaining the movement of the continents of Earth’s surface.
  • Hot springs or thermal springs are areas where hot water comes to the surface of the Earth.

Three main types of plate boundary:

  • Constructive boundaries (divergent): where plates are moving away from each other as new crust is created between the two plates.
  • Destructive boundaries (convergent): where plates are moving towards each other and old crust is either dragged down into the mantle at a subduction zone or pushed upwards to form mountain ranges.
  • Transform boundaries (conservative): where are plates are moving past each other and crust is neither created nor destroyed.

Distribution of Volcanoes:

  • Scientists have developed a theory called plate tectonics–that explains the locations of volcanoes and their relationship to other large-scale geologic features.
  • Most volcanoes are distributed along the plate boundary.
  • The majority of active volcanoes are located in subduction zones, where one tectonic plate is being forced beneath another. These areas are characterized by intense volcanic activity.
  • Volcanoes are also found along mid-ocean ridges, which are underwater mountain ranges where tectonic plates are spreading apart.
  • Most volcanoes are concentrated on the edges of continents, along island chains, or beneath the sea forming long mountain ranges.
  • More than half of the world’s active volcanoes above sea level encircle the Pacific Ocean to form the circum-Pacific “Ring of Fire.”
  • Volcanoes are also found in the middle of plates (e.g. Hawaii). These are called hot spots.
  • The two types of plate boundaries that are most likely to produce volcanic activity are divergent plate boundaries and convergent plate boundaries. At a divergent boundary, tectonic plates move apart from one another.
  • Volcanoes do not typically occur at transform boundaries. Because there is little or no magma available at the plate boundary.
  • Hotspots are areas of concentrated volcanic activity that are not necessarily associated with plate boundaries. They are believed to result from the upwelling of hot mantle plumes beneath the Earth’s crust.

Volcanoes in India:

  • There are no volcanoes in the Himalayan region or the Indian peninsula.
  • The two volcanic islands in the Indian territory are Narcondam and Barren which are located in the Andaman and Nicobar Islands.
  • Barren Island is the only active volcano in India.
  • Narcondam volcanic island (dormant) is located in the Andaman Sea.

Weathering

Weathering refers to the process by which rocks, minerals, and other geological materials are broken down and altered at or near the Earth’s surface. It is a fundamental part of the rock cycle and plays a significant role in shaping the Earth’s landforms over time. There are three main types of weathering: Physical or Mechanical weathering, Chemical weathering, and Biological weathering.

Physical or Mechanical weathering: It causes rocks to disintegrate. This means the rocks fall apart into smaller pieces. In physical weathering, there is no change in the chemistry of the parent rock.

Chemical weathering: It means transformation of one mineral into another via chemical processes. Chemical weathering causes rocks to decompose. This means the minerals that make up the rock are changed by a chemical reaction.

Biological weathering: Biological weathering is the weakening and subsequent disintegration of rock by plants, animals and microbes. Growing plant roots can exert stress or pressure on rock.

Types of Physical weathering:

  • Freeze-thaw weathering: It occurs when the temperature keeps fluctuating above and below 0°C. It is the crack formation & expansion due to freezing of water into ice.
  • Note: Freezing is when water becomes ice. This happens at a temperature of 0°C. Thawing is when ice turns to water. This happens when the temperature rises above 0°C.
  • Exfoliation: Exfoliation is when pieces of the outer layer of rock breaks away like onion layers. Exfoliation happens in places where there is a very big difference in temperature between the night and day. This is most common in deserts.

Types of Chemical weathering:

  • Hydrolysis: It occurs when water reacts with a mineral to form a new mineral with H20 as part of its crystal structure. Eg: Feldspar to clay
  • Oxidation: Oxidation is a chemical reaction between some minerals in rocks and the oxygen in the air. Oxidation changes iron minerals in rocks from a light grey colour
  • to a brown-red colour. This is called rusting. Eg: Iron + O2 = Rust
  • Dissolution: It occurs when ionic and organic compounds dissolve in water.
  • Carbonation: Carbonation is the chemical weathering of chalk and limestone rocks by rainfall. Carbonation occurs when chalk and limestone are dissolved in rainwater.

What is Erosion?

Erosion is the physical removal and transportation of weathered material by water, winda, glaciera, or mass Wasting (gravity).

Types of Erosion:

  • Water Erosion: Running Water (like Rivers) transport sediments from one place to another. Sediments weathered and eroded by rivers are smooth and round; and the land often has V-shaped valleys
  • Wind Erosion: Wind erosion is the physical wearing of the earth’s surface by wind. It involves transportation and deposition of soil by the wind.
  • Glacial Erosion: Glaciers erode the underlying rock by abrasion and plucking.
  • Mass Wasting (gravity): Mass wasting is the movement of material down a slope under the influence of gravity. Land-slide, Debris avalanche, Earth-flow, Mud-flow and Sheet-flow etc are types of Mass Wasting.

What is Continental Drift Theory?

  • It explains about how continents shift position on Earth’s surface.
  • The hypothesis that continents ‘drift’ was first put forward by Abraham Ortelius in 1596.
  • It was fully developed by Alfred Wegener in 1912.
  • Wegener was the first to use the phrase “continental drift”.
  • Wegener proposed that Earth must have once been a single supercontinent called PANGAEA (meaning all earth) before breaking up to form several different continents.
  • The mega-ocean called PANTHALASSA (meaning all water) surrounded this supercontinent.
  • This theory explained how similar rock formations and plant and animal fossils could exist on separated continents.
  • Around 200 million years ago, the super continent, Pangaea began to split.
  • Pangaea first broke into two large continental masses as Laurasia and Gondwanaland forming the northern and southern components respectively.
  • Subsequently, Laurasia and Gondwanaland continued to break into various smaller continents that exist today.

Evidences that support Continental Drift Theory:

Matching of Continents (Jig-Saw-Fit):

  • The east coast of South America and the west coast of Africa appeared to fit together like pieces of a jigsaw fit and their rock layers “fit” just as clearly

Distribution of ancient fossils, rocks, and mountain ranges:

  • The plant and animal fossils are found around different continent shores, suggesting that they were once joined. The fossils of the freshwater crocodile, found both in Brazil and South Africa, are one example.
  • Another example is the discovery of fossils of the aquatic reptile Lystrosaurus from rocks of the same age from locations in South America, Africa, and Antarctica.
  • Appalachian Mountains of the eastern United States were geologically related to the Caledonian Mountain systems of Ireland, Wales,Scotland and North Western Europe.

Tillite:

  • Tillite is the sedimentary rock formed out of deposits of glaciers.
  • The Gondawana system of sediments from India is known to have its counterparts in six different landmasses of the Southern Hemisphere.
  • At the base, the system has thick tillite indicating extensive and prolonged glaciation.
  • Counterparts of this succession are found in Africa, Falkland Island, Madagascar, Antarctica and Australia.
  • Overall resemblance of the Gondawana-type sediments clearly demonstrates that these landmasses had remarkably similar histories.
  • The glacial tillite provides unambiguous evidence of palaeoclimates and also of drifting of continents.

Placer Deposits:

  • The occurrence of rich placer deposits of gold in the Ghana coast and the absolute absence of source rock in the region is an amazing fact.
  • The gold bearing veins are in Brazil and it is obvious that the gold deposits of the Ghana are derived from the Brazil plateau when the two continents lay side by side

Locations of ancient climatic zones:

  • Coral reefs and coal-forming swamps are found in tropical and subtropical environments, but ancient coal seams and coral reefs are found in locations where it is much too cold today.
  • Wegener suggested that these creatures were alive in warm climate zones and that the fossils and coal later had drifted to new locations on the continents.

Force for Drifting:

  • Wegener suggested that the movement responsible for the drifting of the continents was caused by pole-fleeing force and tidal force.
  • The polar-fleeing force relates to the rotation of the earth. Earth is not a perfect sphere; it has a bulge at the equator. This bulge is due to the rotation of the earth.
  • The tidal force is due to the attraction of the moon and the sun that develops tides in oceanic waters.
  • Wegener believed that these forces would become effective when applied over many million years. However, most of scholars considered these forces to be totally inadequate.

Post-Drift Studies:

  • The majority of the facts to study continental drift theory were collected from the continental areas. However, a number of discoveries during the post-war era added the latest information to geological literature.
  • Particularly, the information collected from the ocean floor mapping provided new dimensions for the study of the distribution of oceans and continents.
  • Wegener’s idea was far from universally accepted, since it was not clear what would cause large continents to move across the surface of the Earth.
  • It was Arthur Holmes, who suggested the mechanism that the continents are carried by flow of the mantle on which they sit, and that the mantle is flowing because it is convecting. This lead to the foundation of modern plate tectonics.

Convectional Current Theory:

  • It was given by Arthur Holmes.
  • He argued that there are convectional currents operating in the entire mantle portion which could be the driving force of continents.
  • These currents are generated due to radioactive elements causing thermal differences in the mantle portion.
  • Based up on this contemporary scientists discarded the continental drift theory.

Mapping of Ocean floor:

  • Detailed research of the ocean configuration revealed that the ocean floor is not just a vast plain but it is full of relief ( submerged mountain ranges as wells as deep trenches).
  • Mid-oceanic ridges were found to be most active in terms of volcanic eruptions.
  • Rocks on either side of the crest of oceanic ridges and having equidistant locations from the crest were found to have remarkable similarities both in terms of constituents and their age.

Source: Internet, NCERTs