The key difference between igneous rocks and sedimentary rocks is that igneous rocks are formed from molten liquid minerals called magma, while sedimentary rocks are formed from lithification of existing rocks.
There are three types of rocks on the earth’s crust as igneous rocks, sedimentary rocks, and metamorphic rocks. Geologist made this classification based on the geological process that formed these rocks. Igneous rocks form when melted rocks cool and solidify while sedimentary rocks form when sediments get solidified. Metamorphic rocks, on the other hand, are rocks that have changed from igneous rocks or metamorphic rocks. Like the water cycle, there exists a rock cycle (geological cycle) in geology. Rock cycle is the process by which rocks are formed, degraded, and reformed by internal geological processes like plutonism, volcanism, uplift and/or by external geological processes like erosion, weathering, deposition, etc. According to the rock cycle, one type of rock can change into another (either of the other two types).
CONTENTS
1. Overview and Key Difference
2. What are Igneous Rocks
3. What are Sedimentary Rocks
4. Side by Side Comparison – Igneous Rocks vs Sedimentary Rocks in Tabular Form
5. Summary
What are Igneous Rocks?
Igneous rocks are the oldest type of rocks on earth. All other types of rocks are formed from igneous rocks. Igneous rocks form when magma (molten materials) rise from the earth’s interior. It is possible to categorize further according to their depth of formation. The rocks that form below the earth surface are ‘intrusive igneous rocks’. Moreover, rocks that form on the earth surface are ‘extrusive igneous rocks’ (volcanic rocks).
Figure 01: Igneous Rock
These igneous rocks contain 40% to 80% of silica. Magnesium and iron are the other important components. Granite, pegmatite, gabbro, dolerite, basalt are some examples of igneous rocks.
What are Sedimentary Rocks?
Rocks break into small pieces due to weathering agents like wind and water. Those small particles are ‘sediments’. These sediments deposit on earth due to various mechanisms. These sediments form as very thin layers. Then these layers become harder over a long period of time. These hardened layers of sediments sedimentary rocks.
Figure 02: Sedimentary Rocks
The texture of sedimentary rocks reflects the mode of sediment deposition and subsequent weathering. Sedimentary rocks are easy to identify because of the visible layers. Most sedimentary rocks form under the water (sea). Sedimentary rocks normally have pores since they form from sediments. Shale, sandstone, limestone, conglomerate, and coal are some examples of sedimentary rocks. These rocks are usually rich in fossils. Fossils are the remains of the animals and plants that are preserved in rocks. Sedimentary rocks are present in a variety of colours.
What is the Difference Between Igneous Rocks and Sedimentary Rocks?
The key difference between igneous rocks and sedimentary rocks is that igneous rocks are formed from magma, while sedimentary rocks are formed from lithification of existing rocks. Igneous rocks are non-porous to water, while sedimentary rocks are porous to water. That is, water cannot penetrate through igneous rocks but can through sedimentary rocks. This is another important difference between igneous rocks and sedimentary rocks. Furthermore, igneous rocks contain fossils very rarely, while sedimentary rocks are rich in fossils.
In addition, igneous rocks are harder than sedimentary rocks. Sedimentary rocks’ tendency to react with acids is higher in comparison to igneous rocks. Moreover, igneous rocks are light or dark coloured, while sedimentary rocks have great colour variety.
The following infographic presents the further difference between igneous rocks and sedimentary rocks.
Summary – Igneous Rocks vs Sedimentary Rocks
Rocks are in three types as igneous rocks, sedimentary rocks, and metamorphic rocks. The key difference between igneous rocks and sedimentary rocks is their formation. Formation of igneous rocks is through magma, while lithification of existing rocks forms sedimentary rocks.
Reference:
1. “Igneous Rock: Definition, Classification, Types and Formation.” Rocks For Kids, Available here.
Image Courtesy:
1. “Basalt” By James St. John (CC BY 2.0) via Flickr
2. “Sedimentary-rock” By Pratheep P S, www.pratheep.com (CC BY-SA 3.0) via Commons Wikimedia