Last class revision (5.13 PM).

Extrusive Volcanic landforms (continued) (5.25 PM)

• Volcanic cones:
• Diagramatic representation of Volcanic cones:
• 
• Shield volcano:
• It is formed from eruptions of very fluid basaltic magma.
• It is formed along hotspots due to mantle plumes.
• They are the largest of all volcanoes.
• They are not very steep.
• Examples Mauna Leo and Mauna Kea.
• Ash-cinder volcano:
• They are formed due to the accumulation of loose particles around the vent.
• They are dominated by ash, debris, and other volcanic material with less lava.
• An example includes the Taal volcano in the Philippines.
• Composite volcano:
• They are formed due to the accumulation of viscous lava which is of lower fluidity.
• The alternating layers of ash and lava provide for composite structure.
• They are one of the steepest of all types of volcanoes.
• For Example Mount Fuji in Japan, Krakatoa, etc.
• Crater:
• It is a  funnel-shaped depression formed at the mouth of the volcano.
• The crater filled with water results in the formation of a crater lake.
• Caldera:
• It is a greatly enlarged depression resulting from the violent eruption volcano resulting in the subsidence of much of the Volcano.
• When water is filled it results in the caldera lake.
• Flood basalt province:
• When a highly fluid basaltic magma erupts over a plateau region it flows for a long distance covering large areas and creating a flood basalt province
• For example, the Deccan province and the Columbian plateau.
• Hot springs/thermal springs:
• When water sinks deep enough beneath the surface to be heated by a hot spot or a magma chamber or a geothermal belt it rises to the surface continuously without any explosion.
• Water from hot springs contains dissolved minerals with medicinal value.
• For example, the hot springs in USA, Iceland, and India (Rajgir, Manikaran, Manali, etc).
• Geysers:
• They are the fountains of hot water and superheated steam, ejected at regular intervals with explosions.
• For example Geysers of Iceland, New Zealand, USA(Old faithful Geysers)
• Fumarole:
• It involved continuous jet-like emission of steam and other gases.
• For example Fumarole in Iceland.
• Mud volcano:
• It involves the eruption of mud mixed with water and gases.
• They may not contain magma.
• For example Baratang island of Andaman and Nicobar.

Intrusive features of Volcanism(6.19 PM):

• These are the landforms that are formed inside the earth's surface as manga cools down below the surface.
• Diagrammatic representation of Extrusive volcanic landforms:
• 
• Batholiths:
• It is a long irregular dome-shaped structure.
• They are large bodies formed by the cooling of magma along deeper layers of the lithosphere.
• Laccoliths:
• It is a mushroom-shaped feature formed due to the intrusion of magma.
• It maintains contact with the magma chamber.
• Lopoliths:
• When the magma solidifies along a saucer-shaped depression it results in lopoliths.
• Sill:
• These are formed due to the cooling of magma along horizontal bedding planes.
• Dykes:
• These are vertical wall-like structures formed by the cooling of magma.

Geomagnetism(6.49 PM):

• The magnetic field associated with the earth is called geomagnetism.
• Earth has an outer rocky mantle, below which there is the liquid outer core that surrounds the solid inner core.
• It is considered that the motion of iron charges in the liquid parts of the earth's core generates a Magnetic field.
• The motion of iron-charged particles is caused:
• Rotation of earth.
• A convection current of molten material is generated in the outer core due to heat released from the earth's inner core.
• The motion of ions in this molten material produces an electric current through the earth's core that results in a magnetic field around the earth called geomagnetism.
• Palaeomagnetism and Polar wandering:
• It is the historical study of earth magnetism through rock called Paleomagnetism.
• When the ingenious manga crystallizes, the crystal of ferromagnetic minerals such as iron, titanium, etc acquires stable magnetism which becomes frozen as the magma cools down.
• This acquired magnetism is called fossil magnetism.
• Such rocks will have ferromagnetic minerals aligned in the same direction as that of the geomagnetic field at the time of its consolidation.
• When paleomagnetic pieces of evidence for a given region were checked over a long period a gradual change in direction was detected.
• It shows that the position of magnetic poles has moved over of period, this movement is called polar wandering.
• During the study of the oceanic surface, some of the rocks were found pointing toward the north and some toward the south.
• It is inferred that the rocks can not change the polarity but it is the earth's magnetic field that reveres its polarity.
• Such a reversal of magnetic field can be brought by the reversal of convection currents in the outer core.
• The magnetic reversal occurs every 2,00,000-300,000 years.
• Aurora:
• The magnetosphere of the earth's atmosphere blocks the charged particles from solar winds.
• But some of these charged particles get trapped near polar regions where the magnetic lines are straight.
• Due to the interactions of charged particles in the earth's atmosphere near polar regions, colourful lights are released called aurora.
• It is called Aurora Borealis in the northern region and Aurora Australis in the southern region.

Earthquakes(7.52 PM):

• The vibration or oscillation felt near to the earth's surface due to transient disturbance of the elastic or gravitational equilibrium of the rocks at or beneath the surface of the earth.
• The various causes of earthquakes are:
• Plate movements.
• Minning.
• Reservoir-induced seismicity(RIS).

The topic of the next class: Earthquake to continue.