TYPES OF ROCKS

snow covered mountain during sunrise

A rock is an aggregate of mineral particles found in a soft, solid, or unconsolidated state. The earth’s crust consists of rocks and rocks consist of a combination of different minerals. All
Minerals are formed from one or more of eight main elements. These are oxygen, silicon, potassium, sodium, calcium, magnesium, iron, and aluminum.                                                     

 SEDIMENTARY ROCK

Sedimentary rocks are found in layers; they contain fossils and are very soft. These are weathered particles formed through deposition and lithification processes. Sedimentary rocks are formed when the sediments are accumulated, compacted, and cemented together. The sediments are compacted by compression to form sedimentary rocks. These are called stratified rocks.
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Characteristics of sedimentary rocks

  • They are formed when particles or sediments are accumulated, compacted and cemented together.
  • They contain fossils.
  • They are found in layers (strata).
  • They do not reflect light.
  • They are non-crystallize rocks.
  • They can undergo metamorphic process.

Types of sedimentary rocks

Mechanically-formed sedimentary rocks

These are formed through weathering process. When weathering agents erode and deposit rock particles, they are accumulated, compacted and cemented together to form sedimentary rocks. Examples of mechanically formed sedimentary rocks are clays, gravels and alluviums (all deposited by water),
moraines, boulder clay and gravels (deposited by ice) and loess (deposited

Chemically-formed sedimentary rocks

These are formed through chemical precipitation process. They include carbonate (as it is in stalactite and stalagmite), sulphate, chloride, etc. The main examples are gypsum, rock salt, dolomite, flint, borax, etc.  

Organically-formed sedimentary rocks

These are formed through mineralization process of decaying and decomposition of dead organisms such as animals and plants. The remains of living organisms are accumulated, compacted and cemented together to form sedimentary rocks. The main examples are chalk (limestone) and coral (formed from animals), and peat, coal and lignite (formed from plants).  

Metamorphic rocks

These are rocks that have changed their shape, size, appearance, or chemical composition due to the contact of heat, pressure, or both. This process is referred to as metamorphism. Any rock can be changed into a metamorphic rock. Examples of metamorphic rocks are slate, marble, and granite.
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Characteristics of metamorphic rocks

  • They are very hard due to prolonged action of heat and pressure.
  • Any type of rocks can be subjected to metamorphic rocks.
  • They can undergo weathering process.

The main examples of metamorphism in rocks include the following:

  • Sandstone to quartzite.
  • Coal to graphite.
  • Limestone to marble.
  • Clay to slate.
  • Granite to gneiss.

Igneous rocks

These rocks are formed when molten rock cools and solidifies. All igneous rocks originate inside the earth where they are under great pressure.  Igneous rocks do not occur in layers and they don’t contain fossils which are the chemically-changed remains of ancient plants and animals embedded in rocks. These rocks solidify either within the Earth’s crust and form intrusive features or outside the earth’s surface and form extrusive features.   Igneous rocks are formed when the molten magma is forced out from the upper mantle to the earth’s surface, where it cools and solidifies due to low temperature. Crystals form on cooling and the rocks are called crystalline rocks.  

There are two main types of igneous rocks:

1. Plutonic: these have solidified deep in the crust and they are seen on the surface only after being exposed by prolonged erosion.

2. Volcanic: these have been poured on the earth’s surface where they are called lavas.

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Characteristics of igneous rocks

  • Igneous rocks reflect light.
  • They are not found in layers.
  • They do not contain fossils.
  • They are crystalline rocks.
  • They are formed through cooling and solidification of magma.
  • They can undergo metamorphic and weathering processes.
  • They contain different minerals like iron, magnesium etc.

 Many igneous rocks are found in Dodoma, Iringa and Lake Victoria (Mwanza). The main examples are granite, gabbro, basalt and diorite. Some are found in Kilimanjaro and Rungwe (Mbeya) such as basaltic, pumice, rhorite and peridotite rocks.

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