This document discuss the types of rock in detail. A definition is also given. Various examples of different types of each rock is also given/

1. CSEC GEOGRAPHY
O.JOHNSON
Types of Rocks
Rocks
A rock is any hard, naturally occurring substance which is composed of minerals and which
is formed by geological processes. A mineral is a substance which is normally crystalline and
is formed by geological processes. Crystalline means the atoms forming the crystals are
arranged in a definite manner. Some minerals are non-crystalline, i.e. the atoms forming the
mineral are not arranged in any definite order. Most minerals are compounds of several
elements, e.g. Silica (SiO). A few minerals are themselves elements. A geological process is
any natural process which modifies (changes) geological features. People who study rocks
have found it useful to classify them according to the processes by which they were formed.
There are three major ways in which rocks are formed and hence there are three major classes
of rock; igneous rocks, sedimentary rocks and metamorphic rocks.
Igneous Rocks
These rocks are formed when molten rock from deep within the Earth’s crust (magma) finds
its way into or onto the Earth’s crust where it cools and hardens. They do not occur in layers
and most of them are crystalline (contain crystals)
Intrusive igneous rocks
When magma cools and hardens within the Earth’s crust the resulting rocks are called
intrusive igneous rocks. Intrusive igneous rocks are also called plutonic igneous rocks.
Because these rocks were formed by the slow cooling of magma, they often contain large
crystals, making them coarse grained. Examples of intrusive igneous rocks are granite and
gabbro. These igneous are usually made up of large crystals which can be seen with the
naked eye. They only reach the earth surface when the rocks above them are eroded.
Extrusive igneous rocks
When magma reaches the Earth’s surface it is known as lava. This lava cools and hardens
quickly, forming what are known as extrusive igneous rocks or volcanic igneous rocks. These
rocks are usually fine grained as the lava cools too quickly to allow large crystals to develop.

2. Basalt and obsidian are examples of extrusive igneous rocks. The crystals are usually so
small that is almost impossible to see with the naked eye. Obsidian (also known as volcanic
glass) is formed when lava cools so quickly that crystals are unable to form.
Sedimentary rock
These rocks are formed when material which has been deposited by agents such as rivers is
compacted over time until it hardens into rock. This usually happens in bodies of water such
as the sea. The material is deposited in layers and these layers are often clearly visible in the
resulting rock. Examples of sedimentary rocks include limestone and sandstone. All
sedimentary rocks are non-crystalline (contains no crystals). However they usually contain
fossils. There are three main groups:
 Mechanically formed or Clastic sedimentary rocks- They are made from pieces of
other rocks damaged by weathering and erosion. They form when layers of sediment
containing this debris accumulate and cement together into a sedimentary rock.
Examples include: Breccia, siltstone, conglomerate, sandstone and shale
 Chemical sedimentary rocks are made from tiny particles of minerals that precipitate
from a liquid in which they were originally dissolved. Limestone forms when calcium
precipitates out of water to form a layer of tiny particles that eventually cement
together to form rock. Examples include: rock salt, iron ore, chert, flint, some
dolomites and some limestones
 Organic sedimentary rocks are form the accumulation of plant and animal remains.
Examples from animal accumulation include chalk and coral. Examples from plants
are peat, lignite and coal
Metamorphic rock
When igneous or sedimentary rocks are subjected to intense pressure and heat over a long
period of time, their structure and mineral composition may change a great deal. These rocks
metamorphose into a completely different type of rock. The heat and pressure required to
bring about this change in the rock exists far below the earth’s surface. Examples of
metamorphic rocks are marble (which is metamorphosed limestone) Examples

3. -Slate from clay
-Gneiss from granite
-Quartzite from sand
-Graphite from coal