Difference Between Igneous Rocks and Metamorphic Rocks

Igneous Rocks vs Metamorphic Rocks

Igneous rocks, sedimentary rocks, and metamorphic rocks are the major three rock types in the earth’s crust. Geologist made this classification based on the geological process, which formed the given rocks. When melted rock or magma cooled and solidified, then it is known as igneous rocks. 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 on 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, and basalt are some examples for igneous rocks.

Metamorphic Rocks

Metamorphic rocks are formed due to metamorphism from existing igneous or sedimentary rocks, or even from existing metamorphic rocks. When existing rocks undergo changes due to high pressure and/or high temperature and/or high shearing stresses, metamorphic rocks are formed. Usually metamorphic rocks are formed deep in the earth. Heat comes from magma, while pressure comes from the layer of rocks on top of the other layers. Metamorphic rocks are classified based on foliation as foliated rocks and non-foliated rocks. Foliation means the existence of series of parallel surface. These rocks usually contain crystal. Gneiss, slate, marble, and quartzite are some of the metamorphic rocks.

What is the difference between Igneous Rocks and Metamorphic Rocks?

- Igneous rocks are the oldest rocks, while metamorphic are being derivative of igneous rocks and sedimentary rocks.

- Igneous rocks are the major proportion (nearly 95%) of the total rocks, while metamorphic rocks are found in a very small percentage.

- Igneous rocks are made up of two or more minerals, while metamorphic rocks are usually made up of only one mineral.

- Igneous rocks have no fossils, while, metamorphic rocks rarely have fossils.

- Metamorphic rocks are harder than igneous rocks.

- Resistance to weathering and erosion is less to metamorphic rocks compared to igneous rocks.

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