ROCK FORMATION AND TYPE OF ROCK

 

1. Definition

Rocks are the solid part of the earth’s crust, consisting of aggregates of minerals. These are believed to be formed from a molten mass called magma at the core of the earth. Rocks are composed of different minerals which are fragments of various sizes forming the source of soil parent materials and therefore transmit their characteristics to the soil. Exposed rocks appear in such places as cliffs and seashore where there are few plants.

2. Types of Rocks and their Formation

Rocks are classified according to the way they are formed into igneous, sedimentary and metamorphic rocks. These are the main rock types from which the soil is formed.

(a) Igneous Rocks

Igneous rocks are formed from molten material (magma or lava) extruded at high temperatures from the interior regions of the earth’s crust to or near the surface where the lava cools and solidifies.

There are two types:

• Intrusive (plutonic) igneous rock: The magma cools and solidifies deep within the earth’s crust.
• Extrusive igneous rock: The lava solidifies on the surface of the earth.

The igneous rocks are derived from which other types of rocks are metamorphic rocks. Examples of igneous rocks include granite, basalt, diorite, andesite, feldspar, mica, quartz and pumice.

(b) Sedimentary Rocks

They are rocks which are formed from already pre-existing igneous or metamorphic rocks. Materials that have been broken off from these rocks by the action of such agents as frost, rain, wind, ice, river or sea action are transported to another place or deposited in layers.

Examples include sandstone, shale, limestone, coal, shale, peat, breccia and lignite.

(c) Metamorphic Rocks

sedimentary rocks that had their structures changed by such agents as heat, pressure or chemically active solution. The change from the original form to another is called metamorphism.

Examples:

• Slate derived from shale
• Marble derived from limestone
• Quartzite derived from sandstone
• Others are graphite, schist and gneiss.

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Types of Rocks

(a) Igneous rock

Labels:

• Feldspar
• Quartz
• Biotite

(b) Sedimentary rock

Labels:

• Fine sand grains
• Bedding (sediment)

(c) Metamorphic rock

Labels:

• Feldspar
• Quartz
• Biotite
• Strata