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What are the basic characteristics of Shield and cratons.
Why is that they are significant for a country’s geography? Why is that they
are significant for a country’s geography
Shields are a
regional surface of low relief having an elevation within a few hundred metres
above sea level. Shields are broadly convex and relatively immobile regions,
usually constructed of Precambrian metamorphic and igneous rocks.
The gentle,
low relief is broken by resistant rock formations that rise a few tens of
metres above the surrounding less resistant rocks. On a regional basis, shields
are flat, slightly convex and almost featureless.
Shields are
composed of a highly deformed sequence of Precambrian metamorphic rocks and
granitic intrusions known as basement
complexes.
Most of the
rocks are of the Precambrian age and are formed under conditions of high
temperature and pressure, many kilometres below the surface. These rocks are
traversed by faults and joints expressed at the surface by linear depressions.
The
sedimentary volcanic rocks of the shield are highly deformed and as such have
been converted to complex metamorphic rocks.
These rocks
were at a later stage intruded by granitic magmas.
The upper
cover of sedimentary and metamorphic terrain has been removed by erosion,
exposing what we now see at the surface. These complex igneous and metamorphic
rocks form the nucleus of every continent.
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