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(i) EarthquakEs
Sudden vibrations caused by the movements of lithospheric plates are called earthquakes.
The vibrations are called seismic waves. These vibrations travel over a large area. The
point where the earthquake originates inside the earth’s crust is called the focus while
the point on the earth’s surface, above the focus is called the epicentre. Vibrations travel
outwards from the epicentre in all directions causing vast damage. The maximum damage
is always caused near the epicenter. Earthquake tremors are recorded by an instrument
called ‘seismograph’ and their magnitude or intensity is measured on the Richter Scale.
The Richter Scale has range from 0 to 9. Earthquakes which measure 7 or more on the
Richter Scale are considered of high intensity and cause large scale destructions. Although

earthquakes cannot be predicted accurately, yet their impact can be minimised by adopting
certain preventive measures.

The Richter Scale, designed by an American seismologist Charles
Richter, is used to measure the intensity of the earthquake.

Earthquake Seismograph Richter Scale

The Lithospheric Plate Movement
CasE study

(ii) volcaNoEs

A volcano is a mountain that opens downwards to a pool of molten rock below the surface
of the earth. When pressure builds up, eruptions occur.
Gases and rock shoot up through the opening and spill
over or fill the air with lava fragments. Eruptions can
cause lateral blasts, lava flows, hot ash flours, mud
slides, avalanches, falling ash and floods. Volcano
eruptions have been known to knock down entire forests.
An erupting volcano can trigger tsunamis, flash floods,
earthquakes, mud flows and rock falls. Volcano
Volcanoes are formed when magma from within the earth’s upper mantle works its way to
the surface. At the surface, it erupts to form lava flows and ash deposits. Over time as the
volcano continues to erupt, it will get bigger and bigger.

The earth’s crust is made up of huge slabs called plates. These plates sometimes move.
The friction causes earthquakes and volcanic eruptions near the edges of the plates. The
theory that explains this process is called plate tectonics.

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