Geography optional – Physical Geography
Physical geography of Oceans and Continents
- The continents cover 29% of the total area of the earth and the rest is filled with water.
- The continent that appears today is not the same in the past.
- The continents and oceans in today’s form will appear different in the future.
Continent Drift Theory
- The shape of the Atlantic ocean coastline seems to symmetrical.
- The two Americas that is North and South America, Europe and Africa were once joined together.
- A Dutch Map Maker, Abrahan Ortelius first to propose joining of continents in 1596.
- Antonio Pellegrini drew a picture of the continent in the map showing three-continent were once joined together.
- A German meteorologist, Alfred Wegener proposed the “Continental Drift Theory” in 1912.
- According to Alfred Wegener, “All the continents formed a single continental mass and mega ocean surrounded the same“.
- PANGAEA was named for the supercontinent that means all the whole earth.
- PANTHALASSA was the name given to the ocean when the continents were together that means all water
- As per Continental Drift theory, around 200 million years ago, the continent which was joined together called the supercontinent called as Pangaea began to move from each other or began to split.
- Initially, the Pangaea broke up into two parts of large continents one is called Laurasia and another one Gondwanaland.
- The Laurasia is the northern part and Gondwanaland in the southern part.
- After that, the Gondwanaland and Laurasia were further broken down into many pieces which we see today.
Evidence that supports Continental Drift Theory
Jig Saw Fit Puzzle of Continents
- Just like a Jig-Saw-Fit Puzzle, Africa and South America’s shorelines fit like, once it was joined together.
- The fit of the Atlantic margin was first shown in computer drawing in 1964 by Bullard.
- This fit was done in 1000 fathom, instead of shoreline which we sea.
Age of Rocks Across Oceans
- The age of the rocks was determined by Radiometric dating.
- The rocks of Brazil coast and Western Africa were aged the same about 2000 million.
- The South America and Africa’s earliest marine deposits were from the Jurassic Age, this makes us come to conclusion that ocean is formed before this time.
The Sedimentary called Tillite
- Tillite is a type of Sedimentary rock formed from the glaciers deposits.
- India’s, Tillite from Gondwanana are similar to the Southern Hemispheres six different landmasses.
- There is thick Tillite in the base of the continent that shows extensive and prolonged glaciation, similarly found in Falkland Island, Australia, Madagascar, Africa and Antarctica.
- This makes a conclusion that this Gondawana type sediments of these landmass had similar histories.
- The Tillite provides great pieces of evidence of palaeoclimates and drifting of continents.
- The Ghana coast, there is an occurrence of rich placer deposits of gold and there is a absolute absence of source rock.
- The type is present in gold-bearing veins in Brazil.
- This show Ghana is derived from the plateau of Brazil.
- The distribution of similar type plant and animals found in the landmass which is divided by marine barriers shows there lands were once together.
- For example, the Lemurs found in Madagascar, Africa and India, led some to come to the conclusion that these three lands were once together by landmass called ‘Lemuria’.
- The skeletons of Mesosaurus (a shallow brackish water reptile), found only two places, one is Iraver of Brazil and Southern Cape province of South Africa which was seperated by 4800 km and marine boundaries between them.
Force of Drifting
- The continental Drift is due to the tidal force and pole-fleeing force as per Wegener.
- Pole fleeing force is due to rotation of earth. The earth is not a perfect sphere and it has a bulge at equator which is due to rotation of earth.
- The tidal force is due to sun and moon that developed ocean tides.
- Alfred Wegner believed that these two forces are responsible for the continental drift and some scholars rejects the Alfred Wegners force of Drifting saying these forces are inadequate to move the continents.
- The post-world war -II period added new information regarding the continents.
- In particular, the ocean floor mapping, gave new dimensions in field of study of oceans and continents.
Convectional Current Theory
- The convection currents operating in the mantle portion was discussed by Arthur Holmes in the 1930’s.
- These currents are caused by radioactive elements in the mantle that create Thermal Differences.
- Holmes suggested that this force existed throughout the Mantle that was responsible for continental drifts.
Mapping of the Ocean Floor
- The ocean floor consists of not only floors or plains and consists of reliefs, mountains etc just like what that in lands.
- The survey of the ocean floor started after the world war-2, that gave results such as the existence of mountains, deep trenches were mostly closer to continent margins.
- The volcanic eruption is mostly active, found at the mid-oceanic ridges.
- From the result of dating, the result is that the rocks from the oceanic crust are much younger than the continental areas.
- The age and constituent of rocks are almost the same for the rocks of two sides of the crest of oceanic ridges and equidistant from the crest location.
Ocean Floor Configuration
- The major division of ocean floor based on type of relief and depth are continental margins, deep-sea basins and mid-ocean ridges.
- The continental margins form the transition between deep sea basins and continental shores.
- The continental margins consists of continental shelf, continental rise and deep oceanic trenches.
- Abyssal Plains lie between the mid-oceanic ridges and continental margins.
- Continental sediments move beyond the margins and get deposited in the Abyssal Plains.
- Mid-Oceanic regions form a chain of mountains inside the ocean.
- Mid-Oceanic ridges are longest on the surface of the earth, which is under the sea.
- “Mid-Oceanic ridges are characterized by a central rift system at the crest, fractionated plateau and flank zone all along its length“.
- The zone of intense volcanic activity is at the rift system at the crest.
Earthquakes and Volcanoes Distribution
- The above map shows the volcanoes and earthquakes distribution across the world.
- The line of dots in the Atlantic ocean is almost parallel to the coastlines
- The line of dots further extends into the Indian Ocean and bifurcates a small part of the Indian subcontinent with two branches, one branch moves into East Africa and another other meeting a similar line from Myanmar to New Guiana.
- “The shaded belt showing another area of concentration coincides with the Alpine-Himalayan system and the rim of the Pacific Ocean”.
- “The foci of the earthquakes in the areas of mid-oceanic ridges are at shallow depths whereas, along the Alpine-Himalayan belt as well as the rim of the Pacific, the earthquakes are deep-seated ones”.
- The volcanoes in the map also has a similar pattern.
- The rim of the Pacific is also called as “Rim of Fire” due to large number of active volcanoes present in this area.
Sea Floor Spreading Concept
The mapping of oceans and palaeomagnetic studies from oceans gives the some facts such as:
- A large amount of volcano eruption brings a lot of lava along the mid-oceanic ridges.
- The characteristics such as age, chemical composition, and magnetic properties are similar among the rocks which are present in the equidistant on either side of the crest of mid-oceanic ridges.
- Rocks which are close to mid-oceanic ridges are youngest and has normal polarity.
- The rock’s age increases as moves away from the crest.
- The ocean crust rocks are younger(200 million years) that the rocks seen in the land (continental rocks – 3200 million years).
- The sediment on the ocean floor is very thin, nowhere the sediment column found are older than 200 million years.
- The deep trenches have deep-seated earthquakes.
- In mid-oceanic ridge, the earthquake foci have shallow depths.
- These facts led Hess (1961) to propose a hypothesis known as the “Seafloor spreading”.
- The plate tectonics was a concept termed by Mckenzie, Parker and Morgan in 1967.
- The lithospheric plate is a massive plate also called a tectonic plate, irregularly shaped slab of solid rock. It is both continental and oceanic lithosphere.
- The tectonic plates move horizontally over asthenosphere.
- The lithosphere includes the crust and upper mantle.
- The thickness of the oceanic parts of the lithosphere is in the range between 5 and 100 km.
- The thickness of continental areas is about 200 km.
- A plate called a continental plant or oceanic plate based on the proportion of area occupied in the continent or in the ocean.
- The Eurasian plate is continental plate as its large portion is part of the land whereas the Pacific plate is oceanic plate as its large portion is the ocean.
- As per plate tectonics theory, the earth lithosphere is divided into seven major plates and some minor plates.
- The young fold mountain ridges, trenches and faults surround these major plates.
- Antarctica and the surrounding oceanic plate.
- North American Plate (western Atlantic floor separated from the South American plate along the Caribbean islands)
- South American Plate (western Atlantic floor sperated from the North American plate along the Caribbean Islands)
- Pacific plate
- India-Australia-New Zealand plate
- Africa with the eastern Atlantic floor plate.
- Eurasia and the adjacent oceanic plate.
Important Minor plates
- Cocos plate – Between Central America and the Pacific Plate.
- Nazca Plate: Between South America and Pacific Plate
- Arabian Plate: Mostly the Saudi Arabian Landmass
- Philippine Plate: Between the Asiatic and Pacific Plate, Caroline Plate: Between the Philippine and Indian plate (North of New Guinea)
- Fuji Plate: North-East of Australia
- The plates move constantly, and the continents move as they are part of the plate and all the plates move.
- Alred Wegner though all the continent existed as supercontinent called Pangea, and by later studies, all the plates wander everywhere continuously through the time, and supercontinent called Pangea, was once convergent of all plates.
- Using Palaeomagnetic studies researchers determined the position of the continent from time to time.
- The position of Indian plates was determined from rock of Nagpur area.
The plate boundaries are of three types:
- Divergent Boundaries
- Convergent Boundaries
- Transform Boundaries
- Whenever the plates move from each other, new crust is generated.
- The spreading sites are sites where the plates move away from each other.
- Example: the Mid-Atlantic Ridge divergent boundaries, as a result of American plate separated front he Eurasian and African Plates.
- One plate dived under another where the crust is destroyed, Sub-duction zone is the location where the sinking of a plate occurs.
- The convergence can occur in three ways, these are 1. Between an oceanic and continental plate; 2. between two oceanic plates; 3. between two continental plates
- Plates horizontally slide over each other, the crust is neither created nor destroyed.
- Transform faults are the planes of separation generally perpendicular to the mid-oceanic ridges.
- “The eruption does not take all along the entire crest at the same time, there is a differential movement of a portion of the plate away from the axis of the earth and also the earth rotation has effects on the separated block of the plate“.
Plate Movement Rate
- The rate of the plate movement is determined by the strips of normal and reverse magnetic field that are parallel to the mid-oceanic ridges.
- The slowest plate movement rate is found in Arctic Ridge at less than 2.5cm/yr.
- The fastest plate movement rate is found in East pacific rise near Easter Island in the south pacific about, west of Chile about 3400 km.
Plate Movement Force
- When Alred Wegner proposed the Continental Drift Theory, most scientists believed that earth was static and motion solid object but after various studies such as seafloor spreading, Plate Techtonic, etc, scientists found earth was continuously moving and dynamic one. Now plate movement is accepted by everyone.
- Below the Rigid plate, the mobile rock is believed to moving in a circular manner.
- The heated material is rise above, spreads, and gets cooled and sinks to greater depths and this cycle is repeated continuously and called convection cell or convection flow.
- The two main sources of heat within the earth are Radioactive decay and residual heat.
- This idea is first proposed by Arthur Holmes’s 1930s, later Harry Hess went about Seafloor Spreading.
- The slow movement of hot, softened mantle responsible for the force behind the plate movement.
Indian Plate Movement
- The Indian Plate consists of India Peninsular and Australian continental portions.
- “The subduction zone along the Himalayas forms the northern plate boundary in the form of continent-continent convergence.
- In the east, it extends through Rakinyoma Mountains of Myanmar towards the island arc along the Java Trench.
- In the East, the margin is a spreading site lying to the east of Australia in the form of an oceanic ridge in SW Pacific.
- The Western Margin follows Kirthar Mountain of Pakistan and further extends along the Makrana coast and joins the spreading site from the Red Sea rift southeastward along the Chagos Archipelago.
- The boundary between India and the Antarctic plate is also marked by an oceanic ridge (divergent boundary) running in roughly W-E direction and merging into the spreading site, a little south of New Zealand.”
- India was once a large island off the Australian coast.
- Until about 225 million years ago, the Tethys Sea separated India from the Asian continent.
- About 200 million years ago India started its journey towards the north, at the same time when Pangaea broke.
- About 40-50 million years ago India collided with Asia, producing rapid uplift of Himalayas.
- About 140 million years ago, the Indian subcontinent was located as south as 50 degrees south latitude.
- The two major plates were separated by the Tethys sea and the Tibetian block was close to the Asiatic landmass.
- During the movement of the Indian plate towards the Eurasian plate, a major event that occurred was the outpouring of lava and formation of Deccan traps. This started somewhere around 60 million years ago and continued for a long period of time.
- This started somewhere around 60 million years ago and continued for a long period of time.
- From 40 million years ago and thereafter, the event of formation of the Himalayas took plate and scientist believe that this process is still continuing and the height of the Himalayas is still rising.
Physical Geography of Minerals and Rocks
Minerals and Rocks
- The elements are in solid form in the outer layer and in hot molten form in the interior of the earth.
- Oxygen, silicon, aluminium, iron, calcium, sodium, potassium and magnesium form the 98% of total element on earth crust.
- The rest of 2% of elements are formed by titanium, hydrogen, phosphorous, manganese, sulphur, carbon, nickel and other elements.
- Minerals are substances that are elements in the earth crust that are rarely found combined with other elements to make various substances.
- The mineral can be organic and inorganic substances with definite chemical composition, physical properties and orderly atomic structure.
- A mineral is composed of two or more elements, but sometimes single element minerals like sulphur, copper, silver, gold, graphite etc are found.
- There are 2000 elements have been identified and named and six major mineral groups that are known as major rock-forming minerals.
- The basic source of all minerals is the hot magma in the interior of the earth.
- When magma cools, the crystal of minerals appear and a systematic series of minerals are formed in sequence to solidify to form rocks.
Physical Characteristics of Minerals and Rocks
- External crystal Form- Molecular-level structure such as cubes, hexagonal prism etc
- Cleavage- Tendency to break in a given direction
- Fracture – Crystal will break in an irregular manner
- Lustre – Regardless of colour each mineral has a unique lustre like metallic, silky, glossy etc.
- Colour – the colour is determined by a molecular structure such as malachite, azurite, chalcopyrite etc and some coloured by impurities such quartz which may be white, green, red etc.
- Streak – the color of mineral changes when powdered. Example Malachite is green when powdered gives green whereas fluorite is purple or green but gives a white streak.
- Transparency – Mineral are either Translucent or opaque.
- Structure-Based on the arrangement of crystal it is fine, medium or coarse-grained; fibrous – separable, divergent, radiating.
- Hardness – Degree of Hardness of material is determined by the Mohs scale. The Mohs scale ranges from 1-10. Based on Mohs scale 10 minerals are selected such as Talc-1, Gypsum-2, calcite-3, fluorite-4, apatite-5, feldspar-6, quartz-7, topaz-8, corundum-9, diamond-10.
Metallic minerals is divided into three types:
- Precious Metals – Platinum, Gold, Silver etc
- Ferrous Metals – Iron and other metals usually mixed with Iron to form various types of steel.
- Non-Ferrous Metals – These includes metals like copper, lead, zinc, tin, aluminium etc.
- Sulphur, Phosphates and Nitrates are some of the examples of non-metallic minerals, that does not contain any metal content.
- A Rock is composed of one or more mineral, it will soft or hard and different in colour.
- Example Granite is hard and soapstone is soft.
- Example Gabbro is black and quartzite is milky white
- Feldspar and quartz are the most common minerals found in rocks.
- Petrology is the study of rocks and Petrologist studies the rocks based on characteristics such as mineral composition, texture, structure, origin etc.
- Geographer studies the rocks for landforms, soil etc.
The rocks are of three types
- Igneous Rocks
- Sedimentary Rocks
- Metamorphic Rocks
- These are called as Primary rocks formed out of Lava and Magma from the interior of the earth.
- The Magma from interior of earth, solidifies on cooling. This process happens in crust or on the surface of earth.
- Igneous rocks are classified based on Texture, Texture depends upon size and arrangement of grains or other physical characteristics.
- If the magma is cooled slowly in the greater depths, the grain size will be large.
- Rapid cooling of Magma on the surface produces small and smooth grains.
- Intermediate cooling results in intermediate size of grains.
- Example of Igneous rock are Tuff, volcanic breccia, basalt, pegmatite,gabbro, Granite.
- It is derived from Latin word Sedimentum which means settling.
- The rocks of various types are broken up into fragments of various size due to various denudation agents.
- Such fragments are transported to exogenous agencies and deposited and these deposits turn rocks by compaction. This process is called Lithification.
- The number of layers of varying thickness in sedimentary rocks like sandstone, shale etc.
- Sedimentary are classified into three types that are Mechanically formed, Organically formed, Chemically formed.
- Mechanically formed sedimentary rocks are Sandstone, conglomerate, limestone, shale, loess etc
- Organically formed sedimentary rocks are geyserite, chalk, limestone, coal etc
- Chemically formed sedimentary rocks are chert, limestone, halite, potash etc.
- Metamorphic means change of form.
- These rocks are formed by pressure, volume and temperature (PVT).
- When the rocks are subjected to great pressure by tectonic processes or molten magma comes into contact with crustal rocks, Metamorphism occurs.
- Metamorphism is a process by which already consolidated rocks are subjected to re-crystallisation and reorganisation within original rocks.
- Dynamic Metamorphism means without any remarkable chemical changes there occurs a mechanical disruption and reorganisation of the original minerals within rocks sue to breaking and crushing.
- There are two types of thermal metamorphism, contact metamorphism and regional metamorphism.
- The rocks come into the contact with hot intruding molten lava or magma and recrystallise under high temperature is called Contact Metamorphism.
- The rock materials recrystallise due to the deformation caused by tectonics shearing with high temperature or pressure or both. This is a regional metamorphism.
- Foliation or lineation is the term used to mention the arrangement of minerals or grains in metamorphic rocks.
- 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.
- Metamorphic rocks are divided into two groups, foliated rocks and non-foliated rocks.
- Examples of metamorphic rocks are Gneissoid, granite, syenite, slate, schist, marble, quartzite etc.
- Rocks do not remain the same, they transform this called Rock Cycle.
- Igneous rocks are the primary rocks, metamorphic and sedimentary rocks are other forms of these primary rocks.
- The igneous and metamorphic rocks are can be fragmented and turn into sedimentary rocks.
- Sedimentary rocks also fragment further to become new sedimentary rocks.
- The crustal rocks such as Metamorphic and Igneous rocks can sub-duct due to various movement, moves slowly to the mantle and due to high temperature it gets molten down to lava and the rocks cycle continues.
Geomorphic Processes: Physical Geography
- Why the earth surface is uneven?. The answer is earth crust is dynamic and it is subjected to continuous changes. The changes are due to various external and internal forces.
- The external forces such as sunlight etc are called as exogenic forces.
- The internal forces are called as endogenic forces.
- The exogenic forces create degradation or wearing down of relief or elevations and aggradation or filling up of depressions or basins.
- The wearing down of relief variation due to erosion is called gradation.
- The endogenic forces continuously raise or elevate the earth surfaces and exogenic forces fail to make to even surface on the reliefs or surface of the earth.
- Generally, exogenic forces are land wearing forces and endogenic forces are land building forces.
- The endogenic and exogenic forces that bring a change in the configuration of the earth by various physical and chemical action is called as Geomorphic Process.
- Diastrophism and volcanism are endogenic geomorphic processes and erosion, deposition, mass wasting and weathering are exogenic processes.
- The geomorphic agent is any natural exogenic element such as Ice, wind, water etc that can acquire and transport earth materials.
- These geomorphic agents become mobile due to gradients there remove the material and transport, and deposit them to lower-level over the slopes.
- Geomorphic processes and geomorphic agents are almost the same.
- The geomorphic process is a force that is applied to the earth materials whereas the geomorphic agents are elements such as water, ice, wind etc.
- The gravity is very important for geomorphic process or geomorphic agent is responsible for all process happens on the earth such as carrying slits from the slope using water, water flowing over slopes, stress on rocks etc. The gravitational force plays a vital role.
- All the movement with the earth or outside the side are due to gradients from a higher level to lower levels, from high pressure to low-pressure areas etc.
- The force from inside the earth is responsible for Endogenic process.
- The Endogenic energy is radioactivity, tidal friction, rotational friction and primordial heat.
- This energy due to geothermal gradients and heat flow within induces diastrophism and volcanism in the lithosphere.
- The actions of endogenic forces are uneven due to the difference in geothermal gradients and heat flow from inside, crustal thickness, strength. Therefore the surface is uneven.
- Anything that move, build up or elevate the parts of the earth crust comes under Diastrophism.
- Diastrophism includes i)Orogenic processes that involve mountain building by severe folding and changes long and narrow belts of the earth’s crust, ii) Epeirogenic process that is uplifting or warping large parts of earth’s crust, iii) earth tremors or quakes involving minor movements iv) plate tectonics responsible for horizontal crustal plate movements.
- “Orogeny is a mountain building process whereas Epeirogeny is continental building process”.
- The faulting and fracturing of crust are caused by orogeny, epeirogeny, earthquakes and plate tectonics.
- Each and every of these process cause PVT (Pressure, Volume, Temperature) changes such as metamorphism of rocks.
- It gets the energy from the sun and also the gradients created by tectonic factors.
- Gravitational force acts on all the objects and produce movement slope direction.
- Force applied per unit area is called stress, it is produced by pushing or pulling and induces deformation.
- Forces acting along the faces of earth materials are shear stresses/ separating forces, this stress that breaks rocks and other earth materials.
- The shear stresses result in angular displacement or slippage.
- Apart from that earth materials are subjected to molecular stress due to temperature, changes, crystallisation and melting are most common.
- These chemical reactions make loosening of bonds between grains, dissolving of soluble mineral or cementing materials.
- There we can come to the conclusion that the reason that leads to weathering, mass movements and erosion is the development of stress in earth materials.
- The precipitation and temperature are important climatic elements.
- Weathering, mass movements, erosion and transportation are part of denudation.
- Also, different climatic regions add variations in thermal gradients by latitudinal, seasonal and land and water spread on the surface of the earth.
- The density type and vegetation also depend on precipitation and temperature influence indirectly on exogenic geomorphic processes.
- There is also a variation of insolation received by north and south-facing slopes as compared to east and west-facing slopes.
- Moreover, the velocity and direction of the wind, type of precipitation (snow/water rain), rain intensity, the connection between precipitation and evaporation, daily changes in temperature, freezing and thawing frequency, frost penetration depth, these also affect geomorphic processes.
- The intensity of action of exogenic processes depends upon the structure of rocks such as folds, faults, orientation and inclination of bed, presence or absence of joints, bedding planes, hardness or softness o constituent minerals, chemical of a mineral constituent, permeability or impermeability.
- It is the action of weather and climate the acts on earth materials and reduce them to fragments.
- It is also referred as mechanical disintegration and chemical decomposition of rocks by various element of climate or weather.
- No motion takes place during weathering, It is on site or in-situ process.
- Weathering processes differ from different climatic regions and also the depth of the weathering mantle.
Chemical Weathering Processes
- A group of weathering processes viz; solution, carbonation, hydration, oxidation and reduction act on the rocks to decompose, dissolve, or reduce them to a fine clastic state through chemical reaction by oxygen, surface and /or soil water and other acids.