There is a great variety of rocks. There are varied in their mineral
composition, colour, texture, permeability and grain size. The three main types are
categorized under :
- rocks that are formed by the cooling and solidification of molten
When a volcano erupts, molten rock called magma is ejected onto the Earths
surface. Crystallization, a process in which the magma cools and solidifies forming
mineral crystals called igneous rocks.
Size of the rocks
|depends on the rate at which the molten rock cools. (The longer the cooling, the
larger the crystal formed. ) |
|Magma within the rock cools much slower than the one ejected onto the
Earths surface. This is sobecause the temperatures inside the Earth are high.
Therefore, the igneous rocks on Earth are smaller. |
|Crystalline appearance as they are made up of mineral crystals.
<coarse-grained or fine-grained depending on the rate of cooling> |
|Coarse-grained igneous rock e.g. granite.
|Made up of minerals, quartz, feldspar and mica. The quartz is
grayish and glassy in appearance while feldspar is mostly white. Mica is black and shiny
and are in smaller quantities than quartz and feldspar.|
|Fine-grained igneous rock e.g. Basalt . |
|Consist of feldspar and a high proportion of dark coloured
minerals such as olivine and pyroxene which the rock a dark
- Rocks that are formed by the compaction of sediments or by the
crystallization of the dissolved minerals.
They have a layered appearance because the
materials that form them are deposited in layers.
Plant and animal fossils can be found in many
Sedimentary rocks (compaction) contain rock particles
such as pebbles and sand.
Sedimentary rocks (crystallization) have crystalline
-rocks that are changed into a different one by great heat or
Metamorphic rocks are formed when existing rocks are changed from their
original state by great heat and/or pressure. There is a change in the chemical
composition and physical structure of the existing rocks. e.g. marble and quartzite.
A banded or layered structure
Metamorphic rocks are hard and strong.