Geomorphology

Magnetism

Earth possesses the property of magnetism. The magnetic field of Earth is similar to that of a bar magnet tilted 11 degrees from the spin axis of the Earth. Origin of earth’s magnetism

attributed to a dynamo effect of circulating electric current in the core of the Earth. The rotation of the Earth plays an integral part in generating the currents which are presumed to be the source of the magnetic field.

At the Earth’s centre is a solid inner core surrounded by a fluid outer core, which is hotter at the bottom. Hot iron rises within the outer core, then cools and sinks. These convection currents, combined with the rotation of the Earth, are thought to generate a "geodynamo" that powers the magnetic field.

Significance of geomagnetism

Atmosphere protection: Magnetosphere deflects most of the solar wind, whose charged particles would otherwise strip away the ozone layer that protects the Earth from harmful ultraviolet radiation

Artificial Satellites: The deflection of solar storms by Earth magnetic field helps in proper functioning of our communication system. The intense solar wind particles can affect geosynchronous satellites.

Rock Dating: The magnetic reversals provide the basis for magneto stratigraphy, a way of dating rocks and sediments.

Aurora: Interaction of the terrestrial magnetic field with particles from the solar wind sets up the conditions for the aurora phenomena near the poles

Navigation: Humans have used Earth’s magnetic field for navigation purpose since ages. Various organisms ranging from bacteria to pigeons use it for navigation and orientation.

Temporal Variation: The geomagnetic field changes can degrade navigation and surveying techniques; it can impede geophysical exploration; it can disrupt electric power utilities, and pipeline operations; and it can influence modern communications systems, spacecraft.

Continental Drift Theory ‐ Wegener
  

• initially all the continents formed a single continental mass named Pangea, which was surrounded by a mega ocean Panthalassa.

• Around 200 million years ago, Pangea broke into two large continental masses ‐ Laurasia and Gondwanaland and subsequently, they broke into smaller continents that exist today.

• The movement responsible for the drifting of the continents was caused by buoyancy and tidal force.

• Zig‐saw fit of coastlines, similarity of age of rocks and similar fossil in different continents were the evidences of the theory.

Seafloor

spreading ‐ Harry Hass

• The expeditions to map the oceanic floor and the study of magnetism of rocks (palaeomagnetic) helped in the development of hypothesis of Sea Floor Spreading.

• It was realised that all along the mid‐oceanic ridges, volcanic eruptions are common and they bring huge amounts of lava to the surface in this area.

• The rocks equidistant on either sides of the crest of midoceanic ridges show remarkable similarities in terms of period of formation, chemical compositions and magnetic properties. Rocks closer to the mid‐oceanic ridges are normal polarity and are the youngest. The age of the rocks increases as one moves away from the crest.

• The ocean crust rocks are much younger than the continental rocks. The age of rocks in the oceanic crust is nowhere more than 200 million years old. Some of the continental rock formations are as old as 3,200 million years.

  

• The sediments on the ocean floor are unexpectedly very thin. Nowhere is Sediment column found to be older than 200 million years.

• The deep trenches have deep‐seated earthquake occurrences while in the midoceanic ridge areas, the quake foci have shallow depths.

• at mid‐ocean ridges, new oceanic crust is formed through volcanic activity, which then gradually moves away from the ridge.

• These ridges in the middle of the oceans are the result of magma breaking through, creating a divergent boundary where the seafloor spread apart

Plate tectonic theory

• Expands the horizon by explaining other important geological formation such as mountains, rift valley, etc

• States that Earth's lithosphere is divided into 7 major and several minor tectonic plates, composed of both continental and oceanic crust.

• Plates move horizontally over the asthenosphere, which is the upper layer of the earth's mantle made up mostly of molten rock, as rigid units.

• Plates have been constantly moving over the globe throughout the history of the earth and will move in the future as well.

• Contrary to Wegener, it is now evident from Platetectonic theory that it is the lithospheric plates and not the continent that move. Rather continents are a part of these plates.

• Tectonic plates are able to move because the Earth's lithosphere has greater mechanical strength than the underlying asthenosphere.

• Heat generated from the radioactive decay of elements deep in the interior of the Earth creates magma (molten rock) in the aesthenosphere.

• Large convection currents in the aesthenosphere transfer heat to the surface, where plumes of less dense magma break apart the plates at the spreading centers, creating divergent plate boundaries.

• When oceanic plates diverge or converge, stress causes fractures to occur in the lithosphere. This tectonic activity is manifested as earthquakes. Majority of seismicity on the planet occurs at plate boundaries, although intraplate seismicity can occur as well when stresses build up in the plate.

• Seismic data provides us valuable information about whether the plates’ movement is convergent i.e. moving towards, divergent i.e. moving away or transform i.e., slide along each other.

Thus, the theory of plate tectonics is a convergence of Wegener’s Continental Drift theory, Hass’ Sea floor spreading thesis and synthesis of seismic data. It has comprehensively explained the past geography of continents and oceans and the process controlling creation and destruction of landforms.

Lithosphere (crust and upper mantle) has two broad relief features ‐‐ continents & ocean basins, created by the movement of plates along plate‐tectonic boundaries, and consequential volcanic and depositional processes.

Relief features of continents

• active mountain making belts (narrow zones along the margins of lithospheric plates, like the Alps and Himalayas)

• inactive regions of old, stable rock (continental shields and mountain roots).

Great bulk of the continental crust is over 1 billion years old

Ocean basins

• more diverse relief than the continents. oceanic crust is less than 60 million years old.

• The floors of the oceans are rugged with the world’s largest mountain ranges, deepest trenches and largest plains.

Major relief features

Continental margin ‐ 2 parts

1. continental shelf ‐ submerged part of the continent, shallowest part of the ocean with an average gradient of 1o or even less.

• Continental shelves are a rich source of sulphur, monazite sand, calcium, pearls, Petroleum reserves e.g. Bombay High, Persian Gulf.

• Presence of ample sunlight, optimum depth and nutrients deposited from rivers and waves make them flourishing habitat for organisms. Thus they are potential fishing grounds.

2. continental slope‐ it connects continental shelf and the ocean basin. Canyons and trenches are observed here.

2 types of continental margin

• Passive continental margins such as most of the Atlantic coasts have wide and shallow shelves, made of thick sedimentary wedges derived from long erosion of a neighboring continent.

• Active continental margins have narrow, relatively steep shelves, due to frequent earthquakes that move sediment to the deep sea.

Deep sea plains –

• it is the flattest and smoothest regions of the world comprising of fine grained sediments like clay and silt.

• consists older parts of oceanic crust that are smoothened due to sediment deposition. It has deposits from continents (terrigenous), marine life (biogenous) and salts and mineral (inorganic).

• Abyssal plains of Southern Indian Ocean and Eastern Pacific Ocean are rich sources of Poly Metallic Nodules.

Minor Relief features

Mid‐Oceanic ridge –

• ‐it comprises of two chains of mountains separated by a large depression. The peaks can be as high as 2500 m.

• youngest portions of the ocean basins where new ocean crust is generated through mantle upwelling and plate divergence.

• Similarity of constituents, age and magnetic properties of rocks on either side of the ridge helped in understanding sea‐floor spreading.

• Water from hydrothermal vents (along the mid‐ocean ridges) is rich in dissolved minerals and supports organisms like chemoautotrophic bacteria.

Submarine Canyons –

• these are deep valleys sometimes found cutting across the continental selves and slopes, is found near the mouth of rivers. formed due to erosion of continental slope by turbidity currents or mass wasting.

• They act as preferential particle‐transport routes from coastal zone down continental slopes to deep seafloor, enhance carbon sequestration, provide nursery and refuge sites for marine life and they can also be a rich source of genetic resources and chemical compounds. 

 

 

Oceanic deeps / trenches –

• deepest parts of ocean basins formed due to subduction of oceanic crust under continental crust.

• play significant role in the study of plate movements.

• it is the deepest part of the ocean. It is associated with active volcanoes and earthquakes.

Seamounts

1. it is a mountain with pointed summits, which does not reach the surface. These are volcanic in origin.

2. submarine volcanic cones. Seamounts and the water column above them serve as important habitats, feeding grounds and sites of reproduction for many open‐ocean and deep‐sea species.

Guyots – it is a flat topped seamount. They show evidence of gradual subsidence through stages to become flat topped submerged mountains.

Atoll these are low islands found in the tropical oceans consisting of coral reefs surrounding a central depression.



Mid‐oceanic ridges ‐ an interconnected chain of mountain systems within the ocean.

• a central rift system at the crest, a fractionated plateau & flank zone all along its length.

• They occupy around 33% of the total seafloor area and occur in all the major ocean basins.

• The system of ridges is essentially an oceanic phenomenon but in places it passes laterally into continental rift zones.

  

Topographically, the mid oceanic ridges are very variable and are correlated with spreading rates‐

• fast spreading ridges(like the mid‐Atlantic ridge) having smoother profiles

• slow spreading ridges(like the East Pacific rise and the Galapagos rift) having jagged profiles and an axial rift valley.

Significance

• Paleoclimatic study: They serve as a record of changing Earth's magnetic field with time.

• Reserves of mineral resources: massive deposits of sulfides and thereby could be potential mining targets

• Thermal source‐Heat flow from MOR is many times more than that propagated through Ocean floor.

Interior of Earth with Sesmic Waves

Definition: A seismic wave is an elastic wave generated by an impulse such as an earthquake, volcanic eruptions, magma movement, large landslides or explosions. They are the energy that travels through the earth and is recorded on seismographs.

Seismic waves travel at different speeds when they pass through different types of material, so by studying seismograms, we can learn about Earth's internal structure. Significance of studying seismic waves.

• Analysis of Earthquakes: The release of energy occurs along a fault caused by the movement of the blocks that get deformed and slide past one another abruptly.





• Mid oceanic ridges ‐ formed by ‐ divergent plate boundaries. (plate tectonics)

• This uplifting of the ocean floor occurs when convection currents rise in the mantle beneath the oceanic crust and pour out basaltic magma at the divergent boundary of oceanic plates.

• Ridge‐push occurs when the weight of the ridge pushes the rest of the tectonic plate away from the ridge, often towards a subduction zone.

• At the subduction zone, "slab‐pull" comes into effect. This is simply the weight of the tectonic plate being subducted (pulled) below the overlying plate dragging the rest of the plate along behind it.

   

hypocentre and epicenter are determined.

• Analysis of different type of Earthquake waves: Seismograph records the waves reaching the surface. Earthquakes radiate seismic energy as both body and surface waves.

• Knowledge of Earth’s Interior: This different behavior of P‐ and S‐waves ‐ plastic material beneath the solid crust, called Aesthenosphere. Study of Shadow zones and speed of P‐ and S‐ waves tells that the earth is made up of three layers

• Application of seismology in oil and gas exploration: In oil and gas exploration, seismic waves are sent deep into the Earth and allowed to bounce back. Geophysicists record the waves to learn about oil and gas reservoirs located beneath Earth’s surface.

• Knowledge of Mohorovicic discontinuity, volcanic eruptions etc.

Shield (Plate Techtonics)

It is a large, tectonically inactive mass of Precambrian crystalline rock that underlies most of a continent. A shield’s rocks are igneous and metamorphic in origin and contain some of the oldest rocks on Earth.



Economic significance:

1. Magma contains ferrous and non‐ferrous minerals. Cratons which are blocks of magma when exposed as shield bring these mineral resources to the surface.

• Rich resources propel primary and secondary economic activities like Mining.

• Help in industrialization by providing much needed raw material base for setting up industries.

• Earn forex reserves through exports.

• Employment opportunities and overall economic development in the region by acting as a growth pole. Examples:

• The Canadian Shield is rich in natural resources, including minerals, forests and freshwater. Various minerals and precious stones have been mined or continue to be mined on the Shield, including gold, silver, copper, zinc, nickel, iron, uranium and diamonds.

• The Singbhum Shield is the area where Jharkhand is situated. Since it’s a shield region, there is an abundance of minerals and ores like occurrence of iron, chromite, manganese and silver ores. Hence Jharkhand is a state of Mines, Minerals and Industries.

Based on their location Shields are also known to provide other economic benefits like

• Canadian Shield area: with forest cover there is growth of lumber industry. Its topography with numerous rivers and waterfalls has helped in the generation of hydroelectric power.

• Baltic shield: with glacier retreat shield depressions have turned into lakes supporting inland transport in Sweden and Finland.

• Dharwad Shield with laterite soil has promoted development of building materials like bricks.