Igneous rocks

Rocks are formed naturally when the tiny grains of different minerals get compressed due to the pressure exerted on them and also the heat that they are exposed to. There is a different process for different types of rocks. Following are the types of rocks:

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

Igneous rocks

What are Igneous Rocks?

Igneous rocks are defined as types of rocks that are formed when molten rock (rock liquefied by intense heat and pressure) cools to a solid state.

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.

Interested to learn more about other types of rocks, below are the links:

  1. Sedimentary rocks
  2. Metamorphic Rocks

The 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 number 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.


Igneous Rock Example

Granite Mica and quartz
Basalt Diorite 

Types of Igneous Rock

Following are the two types of igneous rock:

  1. Intrusive igneous rock: These rocks crystallize below the earth’s surface resulting in large crystals as the cooling takes place slowly. Diorite, granite, pegmatite are examples of intrusive igneous rocks.
  2. Extrusive igneous rock: These rocks erupt onto the surface resulting in small crystals as the cooling takes place quickly. The cooling rate is for a few rocks is so quick that they form an amorphous glass. Basalt, tuff, pumice are examples of extrusive igneous rock.

Characteristics of 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 the unaided eye
  • Phaneritic: “coarse-grained”; visible crystals; 1 to 10mm
  • Pegmatitic: “very coarse-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 the eruption (e.g. volcanic breccia)
  • Glassy: when lava cools quickly, there is not enough time for large mineral crystals to form (e.g. obsidian)

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