The key difference between igneous rocks and metamorphic rocks is that igneous rocks are the oldest rocks on earth, while metamorphic rocks are derivatives of igneous rocks and sedimentary 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 that formed these rocks. Igneous rocks are formed when melted rock or magma cools and solidifies while sedimentary rocks are formed 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. It is the process by which rocks are formed, degraded and reformed by the internal geological processes like plutonism, volcanism, uplift and/or by external geological processes like erosion, weathering, and deposition.

CONTENTS

1. Overview and Key Difference
2. What are Igneous Rocks
3. What are Metamorphic Rocks
4. Side by Side Comparison – Igneous Rocks vs Metamorphic Rocks in Tabular Form
5. Summary

What are Igneous Rocks?

Igneous rocks are the oldest type of rocks on the earth. All the other types of rocks form from igneous rocks. Igneous rocks form when magma (molten materials) rise from the earth’s interior. We can classify these rocks further according to their depth of formation. The rocks that form below the earth surface are intrusive igneous rocks, while rocks that form on the earth surface are the extrusive igneous rocks (volcanic rocks).

Figure 01: An Igneous Rock

These rocks contain 40% to 80% silica. Magnesium and iron are important components among other components. Granite, pegmatite, gabbro, dolerite, and basalt are some examples of igneous rocks.

Metamorphic rocks form 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 form.

Figure 02: A Metamorphic Rock

Generally, metamorphic rocks form deep in the earth. The heat comes from magma, while pressure comes from the layer of rocks on top of the other layers. We can classify these rocks based on foliation as foliated rocks and non-foliated rocks. Foliation refers to the existence of a series of parallel surfaces. These rocks usually contain crystal. Gneiss, slate, marble, and quartzite are some metamorphic rocks.

The key difference between igneous rocks and metamorphic rocks is that igneous rocks are the oldest rocks on earth, while metamorphic are derivatives of igneous rocks and sedimentary rocks. Furthermore, Igneous rocks make up the major proportion (nearly 95%) of total rocks, while metamorphic rocks are present in a very small percentage.

Moreover, igneous rocks are made up of two or more minerals, while metamorphic rocks are usually made up of only one mineral. So, this is also a significant difference between igneous rocks and metamorphic rocks. Besides, another important difference between igneous rocks and metamorphic rocks is that metamorphic rocks are harder than igneous rocks. However, resistance to weathering and erosion is less in metamorphic rocks compared to igneous rocks. Also, the tendency to react with acids is higher in metamorphic rocks when compared to igneous rocks.

Summary – Igneous Rocks vs Metamorphic Rocks

Igneous rocks, sedimentary rocks, and metamorphic rocks are the major three rock types in the earth’s crust. The key difference between igneous rocks and metamorphic rocks is that igneous rocks are the oldest rocks on earth while metamorphic are derivatives of igneous rocks and sedimentary rocks.

Reference:

1. “Igneous Rock: Definition, Classification, Types and Formation.” Rocks For Kids, Available here.

Image Courtesy:

1. “Pumice” By James St. John (CC BY 2.0) via Flickr
2. “Metamorphic rock talc” By Tiia Monto (CC BY-SA 3.0) via Commons Wikimedia

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