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Mechanics of Intrusion

Source of Magma:  Magma is originated from the upper mantle, which lies just under the crust, and is nearly molten. A slight drop in pressure caused by dee faulting etc, completes the melting process. The basaltic magma originates in this manner, while the “granitic magma” can be produced in two ways: (a) from melting of …

Forms of Igneous Bodies

The igneous material is of two types; one is extrusive, and the other is intrusive. The extrusive igneous bodies are formed from the magma poured out at the surface of the earth. The lava flows are examples of extrusive igneous material. While the igneous bodies are formed by the consolidation of magma at some depth …

Structures of Igneous Rocks

Once the molten magma extrudes out, it is affected by several factors. These factors provide varied structures to the rocks on cooling. Some important factors are as under; (1). Flow Structures: Sometimes an igneous rock shows parallel or subparallel bands or streaks which are caused by the flow of magma or lava during cooling and …

Texture of the igneous rocks

The texture of the igneous rocks: The size, shape, and arrangement of mineral grains in a rock are known as their texture. The grain size of the igneous rocks depends on the rate of cooling of magma. In general, slower is the rate of cooling, the coarser is the grain of rock. In the study …

Occurrence of Igneous Rocks

The interior of the earth is very hot. Due to this intense heat, the internal matter of the earth is molten. The molten material of the earth is known as Magma. Magma is produced deep in the earth’s crust where temperatures are very high ranging from 900 degrees Celsius to 1600 degrees celsius. Magma being …

Chemical Composition of Igneous Rocks

Igneous rocks make approximately 90% of the earth’s crust by volume. But their great abundance is hidden on the earth’s surface by a relatively thin layer of sedimentary and metamorphic rocks. Igneous rocks are formed by the cooling and solidification of magma. Magma is a hot viscous, siliceous melt containing water vapour and gases. It …

Introduction to rocks

Rocks: A major part of the earth’s crust is composed of rocks. Rocks may be defined as an aggregate of minerals. Some rocks, such as quartzite (quartz) and marble (calcite), contain grains of one mineral only, but most are composed of a variety of different minerals, The rocks are broadly classified into three groups: (i) …

Common Twinning Laws

Twin Law is a constant crystallographic expression in terms of twin-plane and twin-axis according to which twins of the same substance are found to be related. Following is a brief outline of the most commonly observed twin laws in different crystallographic systems: (1). Twinning Laws in Cubic/Isometric Systems: Spinel Law: It is named after its …

Types of Twinning

There are three basic types of the twinning of crystals; (i) Contact Twins, (ii) Penetration Twins, and (iii) Repeated Twins. (1). Contact Twins: When two halves of a twinned crystal are symmetrical with respect to the twin plane, it is called ”contact twin”. Contact twins have a definite composition plane, which separates the two halves …

Twin Crystals

Twinning: The symmetrical intergrowth of two or more individual crystals are known as the ”twin crystals”. In a twinned crystal the lattice directions of one unit exhibit a definite crystallographic relation to the lattice directions of the other. Twin Plane: This plane exists between two twinned crystals. The twin plane divides the two twinned crystals …