Intrusive vs Extrusive Rocks
The key difference between intrusive and extrusive rocks is that the intrusive rocks are formed from magma whereas the extrusive rocks are formed from lava. Before discussing further differences between both rocks, let us see what is a rock and what are intrusive rocks and extrusive rocks. Most of the earth’s surface is covered with rocks of various types. These rocks are formed because of the action of high pressure, high temperatures, and water. Rocks containing different mineral compounds are classified into three major types namely igneous rocks, sedimentary rocks, and metamorphic rocks. This classification also takes into account the manner in which rocks have been formed. Rocks that bear very high pressure and heat within the earth’s crust melt down to form lava. When this liquid lava gets cooled, it solidifies and gets converted into igneous rocks. Intrusive and extrusive are two types of these igneous rocks. It is the difference between intrusive and extrusive rocks that creates confusion in the minds of students.
What are Intrusive Rocks?
These are igneous rocks that are formed by the solidification of hot magma deep inside the earth’s crust. With no air to cool the magma, these rocks are formed very slowly. Composition of these rocks reflects presence of large crystals. These crystals interlock to form the rock. These rocks take a very large amount of time to solidify and they remain buried deep inside the surface of the earth being surrounded by country rocks that have been there already. Very slow cooling means that these rocks remain coarse-grained. The texture of intrusive rocks tells the story of their solidification and crystallization. Some of the perfect examples of intrusive rocks are the diorite, gabbro, and granite. Much of the core of various mountain ranges around the world is made up of these intrusive rocks. These rocks get exposed when their erosion takes place.
What are Extrusive Rocks?
Sometimes, molten rocks find a way to come out of the surface of the earth through cracks and openings. This magma flows in the form of lava and cools down quickly as it comes into contact with air. Igneous rocks that are formed from the magma that pours out of the surface of the earth are called extrusive rocks. As these rocks cool down and solidify very quickly, they do not get sufficient time to form large crystals. Thus, they have small crystals and boast a fine texture. It is difficult to see the crystals of extrusive rocks with naked eyes and you have to make use of a microscope to study their crystals. Water and air come in contact with flowing lava to cool it and solidity in such quick time that they fail to grow large crystals.
What is the difference between Intrusive and Extrusive Rocks?
• Intrusive rocks are formed from magma whereas extrusive rocks are formed from lava.
• Intrusive rocks are formed deep inside the surface of the earth whereas extrusive rocks are formed at the surface of the earth when magma finds a way to eject or pour out of the surface.
• Cooling and solidification of intrusive rocks takes place very slowly whereas contact with air and water causes cooling of extrusive rocks to take place at a very rapid rate.
• Intrusive rocks are made up of enormous crystals whereas extrusive rocks have tiny crystals that can be seen only with a microscope.
• Granite is the best example of intrusive rocks whereas the basalt is a fine example of extrusive rocks.
Images Courtesy:
- Basalt by Zureks (CC BY-SA 3.0)
