Page 80 - Journeys Through Society-7

Faulting of the Earth’s Crust The Himalayan region is one of the most sensitive
Faulting occurs in rocks when they are subjected to regions in the world prone to earthquakes since it is
stress that is stronger than the internal strength of the located between two continental plates: the Indian
rock. This causes it to fracture, thus causing a fault. and the Eurasian plates. The Indian plate continually
When blocks of rocks get dislocated from a plane, a pushes northward a few centimetres each year,
fault or a deep crack occurs. shoving its way under the Tibetan plateau.
Faulting can occur in any direction with the blocks Regions in India prone to earthquakes are Kashmir, the
moving away from each other in both vertical and Western and Central Himalayas, North and Middle
horizontal directions. Bihar, the North-East Indian region, the Rann of Kutch
and the Andaman and Nicobar group of islands.
Earthquakes
Measuring Earthquakes
This sudden shaking of the Earth's crust is called an
Earthquakes spread energy in the form of seismic
earthquake.
waves which are measured by a seismograph which
produces a digital graphic recording of the ground
motion caused by the seismic waves. The digital
recording is called a seismogram.
The Richter scale measures the strength or intensity of
the shock waves produced by an earthquake. Richter
scale magnitudes are based on a logarithmic scale
with base 10. An increase of one numeral on the scale
would represent ten times the magnitude on the
Occurrence of earthquake
seismograph chart.
Earthquakes are most common in regions that are
prone to faulting. Faults occur where two plates meet. Wow Fact
Earthquakes usually originate deep inside the Earth's
On 25 April, 2015, a magnitude 7.8
crust, where the pressure between two plates is so earthquake inflicted severe damage to the
great that they cannot be held in place. Due to the nation of Nepal, causing several Himalayan
release of pressure, the rocks inside the crust snap, mountains to shrink, including Mount
sending shock waves or seismic waves out in all Everest which shrunk by one inch.
directions. The point inside the Earth where the
earthquake originates is called the focus. The point on Volcanoes
the surface that is vertically above the focus is called
A volcano is an opening or a vent in the Earth’s crust
the epicentre.
through which molten magma, ash and gases come
out from below the surface. Volcanoes are generally
found where tectonic plates converge or diverge. The
best example of convergent tectonic plates is the
Pacific Ring of Fire and that of divergent tectonic
plates is the Mid-Atlantic Ridge. Volcanoes can also
form in places where the Earth's crust is subjected to
stretching and thinning, such as in the African Rift
Valley.
Hotspots are usually located where the heat of the
Earth's mantle creates a column of hot, molten
material to rise until it reaches the crust. The
temperature melts the rocks in the crust forming pipes
Focus and epicentre of an earthquake
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Social Studies-7