Porosity Variation of the Major Rock Types

Normally, Rocks are categorized under the three major types, such as; Igneous rocks, Sedimentary rocks and Metamorphic rocks according to the origin.

Igneous rocks: Igneous rocks are formed by the molten rocks which called as magma within the Earth interior and the Lava on the Earth surface. The rock texture is decided by the cooling rate of the molten rock.

Sedimentary rocks: The formation process of the sedimentary rocks is quite different, and formed layered rocks with an excellent porosity. Some sedimentary rocks are originated from the minerals which precipitate directly out of the water ways, and also some from when particles of rocks and minerals settle out from the water ways or air.

Metamorphic rocks: the rocks when undergo metamorphism process formed the metamorphic rocks. Simply, the sedimentary, pre-existing metamorphic rocks and igneous rocks are deviated by the pressure, heat, and chemically reactive waters, and forms metamorphic rocks.

What is the Porosity of a rock?

The porosity of rock can be defined as the measure of its capacity to hold fluids (water, compressed air) in the empty spaces.

Porosity of Sedimentary rocks

The sedimentary rocks are formed by the diagenesis process. A sediment which may be either a product of weathering or chemically precipitated material when buried the weight of overlying materials and the physical, chemical reactions with fluids in the pore spaces can be made changes in porosity; like wise compaction, removal or addition of materials, etc.

Compaction and Deposition of cementing materials – Calcite, Silica are reduced the porosity of the sedimentary rocks, While Dissolution of materials in the pore spaces and Fracturing (Secondary Porosity) will be increased the porosity.

Example: Carbonate Sedimentary rocks

Porosity of Volcanic rocks

Volcanic rocks were solidified on the surface environment with the rapid cooling process. The rapid cooling lava can be trapped degassing products such as something making holes in the rock texture. This texture is called vesicular texture. Such holes can be greatly contributed to the porosity of volcanic rocks. While lava is cooling, the shrinkage cracks can be developed with interconnecting the open spaces.

Example: Porosity of Pumice – Nearly 80%.

Porosity of Igneous and Metamorphic rocks

Both Metamorphic and Igneous rocks have very low porosity with the nature of interlocking crystals. However, the two geological processes, weathering and fracturing can be upsurged the overall porosity of the rocks.

Example: Fracturing (Secondary porosity) can be increased the porosity of the crystalline rocks up to 5%; The physical and chemical weathering can be increased the porosity of Plutonic rocks and Metamorphic rocks up to 30% and 60%.


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