ROCKS SIMPLIFIED
A rock is an
aggregate of minerals. Sometimes it is entirely composed of a single mineral
such as Quartzite, Rock salt, but more commonly it is composed of two or more
minerals.
The earth
consists of various types of rocks that may be as hard as concrete or as soft
as soap. When heated some melt whereas other remains in their original state.
There are two
major categories of Rocks
PRIMARY
ROCKS
These are the
rocks that are directly produced from the cooling of the molten magma coming
out of the earth. Igneous Rocks fall in this category.
SECONDARY
ROCKS
The rocks that
are derived from preexisting rocks are called secondary rocks. Sedimentary
Rocks and Metamorphic Rocks fall in this category.
Rocks are of
three types
IGNEOUS
ROCKS
Igneous is a Latin
word that means “origin by fire”. These rocks are formed from the cooling of
molten magma or lava. This can happen both inside the earth and on the surface
of the earth. If cooling happen inside earth the resulting rock is called
“Intrusive Igneous Rock” and if it happens outside on earth surface the
resulting rocks are called “Extrusive Igneous Rocks”. 90% of the earth crust is
made up of Igneous Rocks.
SEDIMENTARY
ROCKS
These are the
secondary rocks that are formed from the deposition and compression of rocks
and mineral fragments. The word “sedimentary” is derived from the Roman word
“Sedimentum” that means “Settling”, as these rocks are formed from grains
broken away from existing rocks by the action of wind, water, ice and other
agents. After this their compaction occurs and rock is formed. Most of the
sedimentary rocks started their journey as loose deposits of sand or gravel at
the bottom of sea or desert, later on this sediment is lithified and compressed
into a rock.
METAMORPHIC
ROCKS
These are the
secondary rocks that are created by the transformation of existing rocks by
heat and pressure. The term metamorphic comes from the word “metamorph” meaning
“change”. All rocks may be subjected to metamorphism. Granite can be re-melted
and re-crystallized sand stone can be transformed by heat and pressure into a
much harder rock material. Metamorphic rocks themselves can be changed again
and again.
This phenomenon
usually happens in the tectonic movements zones of the earth crust where due to
the volcanic action when tectonic plates are disturbed and one plate is pushed
up and other slides under. As a result rocks in zones of both plates are
subjected to different temperature and pressure and resultant is the
Metamorphic Rock.
Similarly when
the sheet of lava flows over the surface, the bedrocks of that surface
experience excessive heat and changes its form.
QUARTZ