Difference Between Igneous Rocks and Sedimentary Rocks

Igneous Rocks vs Sedimentary Rocks

Rocks in the earth’s crust can be broadly categorized in to three types. Those major rocks types are igneous rocks, sedimentary rocks, and metamorphic rocks. Geologist made this classification based on the geological process, which formed the given rocks. Igneous rocks are formed when melted rocks cool and solidify. Sedimentary rocks are formed when sediments get solidified. Metamorphic rocks are rocks that have changed from igneous rocks or metamorphic rocks. Like water cycle, there exist rock cycle (geological cycle) in geology. Rock cycle means the process by which rocks are formed, degraded, and reformed by the internal geological processes like plutonism, volcanism, uplift etc and/or by external geological process like erosion, weathering, deposition, etc. According to rock cycle one rock type can be changed into another (either of other two types). Out of the volume of the outer 16kms of the earth’s crust, 95% is igneous rocks and 5% is made up of sedimentary rocks. Note that here the metamorphic rocks are included either of the category based on their original rock type, that is, if it is from igneous origin then that is considered under igneous rocks

Igneous Rocks

Igneous rocks are the oldest type of rocks in the earth. All the other types of rocks are formed from igneous rocks. Igneous rocks are formed when magma (molten materials) rise from the earth’s interior. Igneous rocks can be sub classified further according to their depth of formation. The rocks that form below the earth surface are called as intrusive igneous rocks. And rocks that form on the earth surface are called extrusive igneous rocks (volcanic rocks). These rocks contain silica 40% to 80%. Magnesium and iron are important among others. Granite, pegmatite, gabbro, dolerite, basalt are some examples for igneous rocks.

Sedimentary Rocks

Rocks are broken up in to small pieces due to weathering agents like wind, water, etc. Those small particles are known as sediments. These sediments get deposited by various mechanisms. These sediments form very thin layers. Then these layers become harder over a long period of time. Those hardened layers of sediments are called sedimentary rocks. Texture of sedimentary rocks reflects the mode of sediment deposition and subsequent weathering. Sedimentary rocks are easy to identify as layers are visible. Most sedimentary rocks are formed under the water (sea). Sedimentary rocks normally have pores as they formed from sediments. Shale, sandstone, limestone, conglomerate, coal are some of the examples for sedimentary rocks. These rocks are usually rich in fossils. Fossils are the remains of the animals and plants, being preserved in rocks. Sedimentary rocks are found in a variety of colours.

What is the difference between Igneous Rocks and Sedimentary Rocks?

- Igneous rocks are formed from molten liquid minerals called magma, while sedimentary rocks are formed from lithification (cementing, compacting and hardening) of existing rocks.

- Igneous rocks are non porous for water, while sedimentary rocks are porous to the water. That is water cannot penetrate through igneous rocks but can through sedimentary rocks.

- Igneous rocks are having fossils very rarely, while sedimentary rocks are rich in fossils.

- Igneous rocks are harder than sedimentary rocks.

- Tendency to react with acids is higher to sedimentary rocks when compared to igneous rocks.

- Igneous rocks may be light or dark coloured, while sedimentary rocks have great colour variety.