Type of Rocks

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Type of Rocks

ROCKS

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 :

Igneous rocks

	Sedimentary rocks
	Metamorphic rocks

IGNEOUS ROCKS

- rocks that are formed by the cooling and solidification of molten rock.

Formation

When a volcano erupts, molten rock called magma is ejected onto the Earth’s 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 Earth’s surface. This is sobecause the temperatures inside the Earth are high. Therefore, the igneous rocks on Earth are smaller.

Features

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 appearance.

SEDIMENTARY ROCKS

- Rocks that are formed by the compaction of sediments or by the crystallization of the dissolved minerals.

Formation

COMPACTION

Sediments such as rock particles & remains of living things are deposited in the sea by rivers and waves. These sediments accumulate layer by layer, over thousands or millions of years, forming thick deposits on land or on the sea floor. The weight of the overlying sediments compact those below. Over time, they form sedimentary rocks. e.g. Sandstone & shale

CRYSTALLIZATION

Sedimentary rocks usually occur in shallow parts of the sea or in lakes in desert areas where evaporation is higher than precipitation. As evaporation takes place, water is lost and the dissolved minerals form crystals. As evaporation continues, more crystals form and accumulate on the sea or lake floor, becoming sedimentary rocks. e.g. coal

FEATURES

	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,
	Sedimentary rocks (compaction) contain rock particles such as pebbles and sand.
	Sedimentary rocks (crystallization) have crystalline appearance.

METAMORPHIC ROCKS

-rocks that are changed into a different one by great heat or pressure

FORMATION

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.

FEATURES

	Crystalline appearance
	A banded or layered structure
	Metamorphic rocks are hard and strong.

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