Rocks are aggregates of minerals. Rocks are of three types i.e., Igneous Rock, Sedimentary Rocks & Metamorphic Rocks

1. ROCKS
TYPES OF ROCKS
12 DECEMBER 2020
PRINCIPLES OF PALEONTOLOGY (ZOL-651)
BY
KASHIF MANZOOR (PHD SCHOLAR, UVAS LAHORE, PAKISTAN)

2. ROCKS
 The solid material forming earth’s crust is called rock. It is an aggregation
of minerals. It may be made up entirely of a single mineral as in the Rock
Salt, but more often it contains two or more types of minerals. Granite
for example, is made up of feldspar, quartz and mica.
 Rocks are of three principle types, namely
i. Igneous Rocks
ii. Sedimentary Rocks
iii. Metamorphic Rocks
 These exhibit a “rock cycle”, a process of transformation of one rock type
into another.

7. 1. IGNEOUS ROCKS
 These rocks are formed by cooling of molten material called
“magma” which originates at depths as great as 200km in the interior
of earth. This process is called Crystallization, may occur either
beneath the earth’s surface, or following an eruption, at the surface.
The magma consists of silicate minerals and gases. The igneous
rocks, thus, may be of two types;
a) Volcanic Rocks
b) Plutonic Rocks

8. 1. IGNEOUS ROCKS
 If magma breaks through the surface it flows in the form of lava. The
lava gradually solidifies forming, the so called, VOLCANIC ROCK.
The magma not able to break through eventually crystallizes below
the surface forming, the so called, PLUTONIC ROCK.
 Lava is similar to magma except that during eruption most of its
gaseous components escape out. Both volcanic and plutonic rocks are
crystalline and do not form layers as in the case of sedimentary
rocks. Examples are Granite.

17. 2. SEDIMENTARY ROCKS
 When igneous or any other pre-existing rocks become exposed to surface they
undergo weathering and erosion in which rocks disintegrate and decompose.
The resulting material is picked up by water, wind, glaciers or waves i.e., eroded
and transported and is deposited in beds of oceans or other large bodies of water
or deserts.
 When water flows down the mountain slopes or through plains, during or after
rain, it carries along with it much suspended matter as sand or soil. When water
is quiet, the sediments begin to settle down slowly on the bottom of oceans or
lakes.
 Later on these sediment hardens into rocks. As more sediments settle down and
hardens, they form successive layers or strata of rocks, a process which may take
millions of layers. Such rocks are called SEDIMENTARY OR STRATIFIED
ROCKS. Common examples of sedimentary rocks are limestone, shale,
sandstone, rock salt, coal etc.

21. 3. METAMORPHIC ROCKS
 Metamorphic rocks mean changed rocks. The sedimentary or igneous rocks
when subjected to intense heat, pressure and weight in the presence of water
and gasses, they change into METAMORPHIC ROCKS. During this process
the texture and mineral composition of rock may be partly or completely altered.
 When metamorphosed rock is subjected to still greater heat and pressure in the
deeper parts of the crust, it will melt to degenerate magma, which will
eventually solidify as igneous rock. Some common metamorphosed rocks are
Slate (formed from clay); Marble (from limestone); Quartz (from sandstone)
etc.
 Rock formation has been going on since the beginning of this earth. Several
methods have been developed to determine the age of different rocks. In the
case of sedimentary rocks, the deepest strata are the oldest while the most
superficial are quite recent. This method only gives the comparative dating of
rocks.

22. 3. METAMORPHIC ROCKS
 It is possible to determine the sequence in which the various strata of the earth’s
surface have been laid down. In the identification of the particular strata, the
fossils that they contain are also helpful. The determination of geological time in
terms of precise years is possible only with the latest techniques of radiometring
methods.

26. THE END