Rocks and Rock Cycle

The Sedimentary rocks are formed from material derived from the pre-existing rocks and from organic sources by the process of denudation. The eroded materials brought by rivers, streams, glaciers or wind are deposited in layers to form Sedimentary rocks. For Example — Limestone, Sandstone.

Igneous rocks are formed by solidification of molten magma that comes out from an active volcano. Hence, it is also known as primary rocks. For Example — Granite, Basalt.

On the basis of occurrence, Igneous Rocks are of two types:

1. Intrusive Igneous Rocks — The rocks that are formed due to solidification of magma below the Earth's surface and remains surrounded by pre-existing rock are called Intrusive Igneous Rocks.
2. Extrusive Igneous Rocks — The rocks that are formed due to solidification of magma above the Earth's surface are called Extrusive Igneous Rocks.

Silicates, Quartz and Feldspars are three rock forming minerals.

Large mass of igneous rocks (chiefly composed of Granite) covering about 100 sq km area is called Batholiths. Its floor is not visible.

Alteration of the minerals and textures of a rock by changes in temperature and pressure and by a gain or loss of chemical components to such an extent that the diagnostic features of the original rocks are modified is called metamorphism.

Following are the causes of metamorphism:

1. volcanic eruptions
2. the mountain building movements
3. endogenetic forces
4. contact with undergoing water

A few important characteristics of Sedimentary Rocks are:

1. The material of sedimentary rock is derived from other pre-existing rocks.
2. They are layered rocks.
3. Sediments are deposited in sequential order i.e. size of sediments decrease from littoral margins to the centre of the water bodies.
4. Most of the sedimentary rocks are permeable and porous.
5. They contain fossils.

The various types of metamorphic rocks are slate, schist, gneiss, quartzite, marble, amphibolite, meta-conglomerate and hornfel.

The agents involved in the formation of sedimentary rocks are rivers, lakes, sea, glaciers, wind and organic materials. The processes of erosion, weathering, dissolution and lithification lead to formation of sedimentary rocks.

Metamorphic rocks formed from igneous rocks — Gniess from granite, Schist from Basalt

Metamorphic rocks formed from sedimentary rocks — Marble from limestone, Quartzite from Sandstone.

Sills— It is sheet like body of igneous rock. It is formed when magma forces its way between two layers of sedimentary rocks and cools and solidifies there.

Dykes— It is sheet like body of intrusive igneous rock. When magma is forced upwards, it fills vertical cracks or fissures in existing rock and it hardens there to form dykes.

Sedimentary rocks are formed by deposition of sediments in layers in beds of water bodies or on earth's surface (if brought by wind). They are deposited in layers, therefore, they are known as stratified rocks and layers are called strata.

The processes involved in the formation of igneous rocks are cooling, solidification and crystallization.

A cyclic process of changing the form of rocks is called Rock cycle. It represents the interrelationships among the three rock-forming processes: igneous, sedimentary and metamorphic.

An assemblage of minerals bound together that form a more or less definite unit of the Earth's crust is called rocks.

On the basis of their origin, rocks are of three types:

1. Igneous rocks
2. Sedimentary rocks
3. Metamorphic rocks

Magma that solidifies below the Earth's surface and remains surrounded by the pre-existing rock is called intrusive rocks. For example, granite.

The rocks that are formed due to solidification of magma on the Earth's surface are called Extrusive Igneous Rocks. For example, Obsidian.

The importance of rocks in industrial development are:

1. Rocks contain minerals called ores which are used for extracting metals and non-metals.
2. They provide fossil fuels like coal and petroleum.
3. The entire automobile industry uses petrol for their transport system.
4. They provide raw material for manufacturing industries like limestone and gypsum are needed by cement industry.
5. They are used as building materials. For example, granite and marble.

Minerals are naturally occuring, homogenous solid inorganic substance that has a definite chemical composition with its own chemical and physical properties.

Rock can be defined as an aggregate of minerals that form a more or less definite unit of the earth's crust.

A sheet like body of igneous rock that is formed when magma forces its way between two layers of sedimentary rocks and cools and solidifies there is called Sills.

A sheet like body of intrusive igneous rock which is formed when magma is forced upwards, it fills vertical cracks or fissures in existing rock is called Dykes.

Conglomerate is a type of mechanically formed sedimentary rock which consists of rounded or sub-rounded fragments, usually water-born cobbles, pebble and gravel, cemented together by a matrix of calcium carbonate, silica, etc.

Igneous rocks are also called primary rocks because they are formed when molten lava solidifies. Sedimentary rocks and metamorphic rocks are formed from other rocks.

Extrusive igneous rocks have generally small crystals because they cool rapidly to solidify. The crystals do not have much time to form until the rock cools all the way, which stops the crystal growth.

Rocks are of great economic significance because they provide us with various minerals, fossil fuels, building materials and precious metals etc.

The Lithosphere is the outermost layer of the Earth's crust that is made up of solid rocks, including igneous and sedimentary rocks. In Greek, Litho means rock or stone. Thus, Lithosphere is also called rocksphere.

Metamorphic rocks are formed due to change in texture and mineral composition of the pre-existing rocks. During this process of change, the fossils of original sedimentary rocks are destroyed and therefore no fossil is found in metamorphic rocks.

Below diagram shows a volcano and igneous rocks:

Below diagram shows the rock cycle:

Igneous Rock

Intrusive Igneous Rock

They have fine crystals

Quartzite

Conglomerates

They are deposited in layers

Sedimentary rocks have pore spaces.

Calcareous rocks

Heat & pressure

Sandstone

Rock cycle

Gneiss

Cooling & solidification of magma

The differences in colour, texture, and hardness of the stones Rina collects are due to how the rocks were formed and the minerals they contain. Since rocks are formed through various geological processes, each type has unique properties based on its origin and composition.

Sample A is Extrusive and sample B is intrusive igneous rock.

Reason: Extrusive igneous rocks cool quickly on the Earth's surface, resulting in fine-grained, smooth textures. In contrast, intrusive igneous rocks cool slowly beneath the surface, allowing larger crystals to form, which gives them a rougher texture.

Both A and R are true and R is not the correct explanation of A.

Explanation — Igneous rocks are indeed called Primary rocks because they are the first type of rocks to form on Earth and serve as the parent rocks for other types. The reason that they form from the solidification of molten magma is also true. However, this process explains their formation, not specifically why they are called Primary rocks. Hence, R is not the correct explanation of A.

Both A and R are true and R is the correct explanation of A.

Explanation — Metamorphic rocks do not have fossils as they are formed by the change of pre-existing rocks due to high heat and pressure. Thus, the fossils of the original sedimentary rocks are destroyed.