Page 79 - Journeys Through Society-7
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3. Transform plate movements: These movements
take place when two plates slide past each other
causing immense friction to occur, resulting in
devastating earthquakes.
Crust
Lithosphere
Asthenosphere Folding of the Earth’s Crust
Upper Mantle
Lower Mantle When stress and strain cause rock layers to buckle,
Outer Core fracture and fold, changes occur in the shape and
Inner Core
volume of the crust. These wave-like formations have
alternate upfolds and downfolds. Folds are distinctly
Endogenic and Exogenic Forces visible in rocks that contain specific layers.
Anticlines and Synclines: Anticlines are up-folds where
Tectonic Movement
rocks are pushed upwards into a convex fold. It
According to the scientists, the Earth's crust is divided
resembles an arch with the rock beds sloping away
into seven main lithospheric plates. These float on the
from the centre of the fold.
mantle and are constantly in motion. The movement
A syncline is a down-fold in rocks in which rock layers
of the Earth's lithospheric plates are termed as
are bent downward forming depressions between the
tectonic movements. Tectonic movements are divided
anticlines.
into horizontal (tangential) movements and vertical
(radial) movements.
Horizontal Movements
These movements produce forces of tension and
compression. These forces cause faulting and folding
in the Earth's surface.
Vertical Movements
These movements are caused by forces that uplift the
Earth's crust or cause its subsidence. Anticline and Syncline
Vertical movements cause the crust to be lifted. The Large-scale folding of the crust results in the
part of the crust that is pushed up by these forces formation of a series of mountain ranges called fold
forms continents or plateaus. Sometimes these forces mountains. Some of these mountains achieve great
cause subsidence of land which results in the heights over a period of millions of years. Some
formation of valleys. examples are the Himalayan Mountain ranges in Asia,
Plate movements are categorised into three types. the Andes Mountains of South America, the Rocky
1. Convergent plate movements: These are the results Mountains of North America and the Alps of Europe.
of compressional forces which cause the plates to Fold mountains are formed usually from sedimentary
collide against each other. They result in the folding of rocks which accumulate along the margins of
the Earth's crust to form mountains. continents. When tectonic plates collide, the gathered
2. Divergent plate movements: These are the results layers of rock may crumple and fold just like a
of tensional forces that pull these plates apart. This tablecloth that is pushed across a table. This usually
creates huge faults between plate boundaries through happens when there is a mechanically weak basal
which magma flows out. New oceanic crust forms layer like salt.
when hot molten lava slowly cools and solidifies. Due Wow Fact
to the pull of these plates, the crust in the mid- oceanic
ridges is very thin. The Himalayas are actually the youngest fold
mountain range in the entire world.
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