Classification of Rocks: 3 Types

This article throws light upon the three main types of rocks found in soil. The types are: 1. Igneous Rocks 2. Sedimentary or Aqueous Rocks  3. Metamorphic Rocks.

Type # 1. Igneous Rocks:

Igneous rocks are those that have been formed by the action of heat. These rocks were the first to be formed when the molten mass cooled and consolidated into solid rock. The other rocks viz., metamorphic etc., were formed subsequently. The igneous rocks constitute nearly 25% of the earth’s crust and are about ten miles thick.

These rocks are mostly crystalline and they are therefore, known as crystalline rocks. They are also called massive rocks as they occur in big masses. Igneous rocks consists mainly of primary minerals of which quartz, feldspars, amphiboles, pyroxenes and mica are the most common.

Igneous rocks have been formed in three ways:

(a) Extrusive

(b) Intrusive, and

(c) Plutonic.

Extrusive rocks are those that were formed when the molten mass was poured out on the surface of the earth where it consolidated on cooling. This is generally, of volcanic origin. Rhyolite, basalt etc., are examples of extrusive rocks of volcanic origin.

Sometimes, the molten mass was cooled and consolidated beneath the surface. The rocks so formed are known as intrusive rocks. Among the intrusive rocks, those consolidated at a much a greater depth than others are known as plutonic rocks.

Type # 2. Sedimentary or Aqueous Rocks:

Aqueous rocks have formed through the agency or action of water. They are formed from sediments brought by water. Hence, they are also known as sedimentary rocks. The sediment may consists of various types of substances and of various sizes of particles. The particles are cemented together by substances like silica, iron oxide or lime to give a consolidated form.

Many of these rocks are deposited in layers of strata and hence, they are also known as ‘stratified rocks. All sedimentary rocks are of secondary or derivative origin, as they consist of materials that have previously existed on or beneath the surface of the earth. The transportation, accumulation and consolidation of these materials gave rise to new rocks.

Hence, these rocks consist mainly of secondary minerals together with some quantities of primary minerals. Few important examples are limestone, sandstone, siltstone, shale, calcareous sandstone, arenaceous limestone etc. (See Table 20.2).

Type # 3. Metamorphic Rocks:

Metamorphic rocks have resulted from the subsequent transformation of igneous and sedimentary rocks under the influence of heat, pressure and chemically active liquids and gases. When the change is considerable, the rock is said to have undergone metamorphosis and the new rock is known as a metamorphic rock.

Some important types of metamorphic rocks that are sources of weathered parent material for soil formation are gneiss from granite, slate from shales, marble from limestone, schist from shale and quartzite from sandstone.

Rocks are also classified on the basis of their silica content, such as:

Acid rocks:

Containing 65-75% silica, e.g., granite, rhyolite, sandstone and gneiss.

Intermediate rocks:

With 55-65% silica, e.g., syenite, diorite and andesite.

Basic rocks:

Having 40-55% silica, e.g, gabbroa, basalt, limestone and diabase. These are also rich in iron, calcium, magnesium, and sodium. During weathering processes, it could be generalized that all rocks are subject to losses and gains, the net result being a loss. Rocks having a complex mineral composition weather more easily. Acid igneous rocks weather slowly as compared to basic igneous rocks.

Generally, acid igneous rocks produce soils, with good physical conditions, while soils from basic rocks possess favourable chemical characteristics; for example, soils from basalt are richer in phosphorus and lime than soils from granite.

Sedimentary rocks, in general, do not weather as rapidly as igneous rocks. Sandstone is more resistant than limestone. Soils developing from highly calcareous parent rocks weather rapidly but take much longer time to attain a mature profile.

Rock in order of their decomposition are as follows:

Basalt > Gneiss > Granite > Hornblende > Andesite.