Igneous rocks

Rocks are mineral aggregates, a material composed of one or more minerals. It can be classified into three types – Igneous, Sedimentary and Metamorphic rocks.`

Igneous rocks

Igneous rocks form when molten rock (rock liquefied by intense heat and pressure) cools to a solid state. Intrusive igneous rocks crystallize or solidify within the earth’s crust. Magma is molten rock (liquid or liquid/crystal “mush”) that exists below earth’s surface (when cooled it forms rocks such as granite, diorite or gabbro). Extrusive igneous rocks are erupted in a molten state on the earth’s surface and then cool and solidify. Volcanic processes associated with this are natural hazards or disasters.

Lava is molten rock flowing out of fissures or vents at volcanic centers (when cooled they form rocks such as basalt, rhyolite, or obsidian). Pyroclastic deposits are accumulations of fragmented material (e.g. ash, bombs, tuffs and volcanic breccias) ejected during volcanic eruptions.

Igneous rocks can be easily identified with their texture, density, color and mineral composition. Its texture depends on upon the shape, size, time period to cool down and solidify and the arrangement of crystals in the rock.

Texture is a description of a rock’s constituent parts in terms of their sizes, shapes and arrangement

Rule of Thumb: The size of mineral crystals in an igneous rock may indicate the rate at which the lava or magma cooled to form a rock. Crystal size can also be affected by the amount of gases or the availability of the chemicals in the molten rock that are required to form the crystals.

Larger crystals generally indicate intrusive igneous rocks. Smaller crystals generally indicate faster cooling associated with extrusive igneous rocks.


1) The igneous form of rocks does not include any fossil deposit. If there are any chances of fossil deep inside the crust, it erupts out of the Earth’s surface and gets destroyed due to sheer heat these rocks produce.

2) Most of the igneous forms include more than one mineral deposit.

3) They can be either glassy or coarse in appearance.

4) These usually do not react with acids.

5) The mineral deposits are available in the form of patches with different size.

Types of igneous rock textures

  • Aphanitic: fine-grained, less than 1 mm, grains not seen with unaided eye
  • Phaneritic: “coarse grained”; visible crystals; 1 to 10mm
  • Pegmatitic: “verycoarse grained”; > 1 cm
  • Porphyritic: composed of both large and fine-grained crystals, and the large crystals are called phenocrysts, and the background is the matrix
  • Vesicular: rocks that have vesicles, resembling a sponge (e.g. scoria and pumice)
  • Pyroclastic: fragmented, angular grains ejected during eruption (e.g. volcanic breccia)
  • Glassy: when lava cools quickly, there is not enough time for large mineral crystals to form (e.g. obsidian)

Practise This Question

Statement 1: When white light ray passes through a prism it disperses into seven different colours, and this phenomenon is called dispersion.

Statement 2: Refraction depends upon the wavelength of light and since different colours have different wavelengths hence they are refracted at different angle and so they are split into the colour spectrum.