Are you studying rocks and the rock cycle in your classroom this year? If you’re ready to take a deep dive into all things rocks, it’s important to know all about the three main types of rocks: igneous, sedimentary, and metamorphic. In this post, we will take a closer look at igneous rocks. Get ready to learn all about the formation, subtypes, properties, and examples of igneous rocks!
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How do igneous rocks form?
All igneous rocks form from molten rock. When molten rock is underground, we call it magma. Molten rock is only called lava when it’s on Earth’s surface. The intense pressure and heat deep underground keep magma in the molten (liquid) form. However, when the heat and pressure lessen, the molten rock solidifies, forming crystals. This can happen very slowly while the rock is still underground. If the molten rock reaches the Earth’s surface, it cools much more rapidly. This is because the surface of the Earth is very cool compared to the Earth’s insides.
Properties of Igneous Rocks
There are many different igneous rocks, and they do not all look the same. Bubbly pumice, smooth obsidian, and sparkly granite are all igneous rocks! Their varied appearances are influenced by their mineral makeup and the specific conditions during their formation. Igneous rocks come in a wide variety of different textures and compositions. Let’s learn more about both of these properties…
Igneous Textures
As molten rock cools, it forms crystals. The size of these crystals depends on how quickly the rock cools. Slow cooling provides time for big, interlocking crystals to form. Rapid cooling makes tiny, fine-grained crystals. We use special terms to describe these different textures: phaneritic (big crystals), aphanitic (tiny crystals), and porphyritic (a mixture of both). But that’s not all! Some lava rocks, like pumice, have a bubbly texture filled with tiny holes. Others, like obsidian, are smooth and look glassy. There are even rocks made of many pieces of volcanic debris all stuck together, giving them what is known as a pyroclastic texture.
Igneous Compositions
Igneous rocks also come in different colors, which can tell us a lot about the minerals inside them – i.e., their mineral compositions. Light-colored minerals rich in silicon and oxygen are called felsic minerals. Rocks with lots of felsic minerals, like granite and rhyolite, are usually light and warm in color. On the other hand, dark-colored minerals rich in iron or magnesium are called mafic minerals. Rocks with lots of mafic minerals, like gabbro and basalt, are mostly dark gray, black, or green.
Subtypes
There are two main subtypes of igneous rocks: intrusive and extrusive. The main difference between them is where they form. Intrusive rocks form within the Earth while extrusive rocks form on the Earth’s surface.
Igneous Intrusive Rocks
More specifically, an igneous intrusive rock starts as magma, which is what we call molten rock located underground. The magma is insulated deep within the Earth, so it cools very slowly. The slow cooling allows plenty of time for large interlocking crystals to form. Granite, diorite, and gabbro are examples of igneous intrusive rocks.
Igneous Extrusive Rocks
In contrast, igneous extrusive rocks form when hot lava cools on the Earth’s surface such as when a volcano erupts. Lava cools very quickly on the surface, so there is not enough time for big crystals to form. If you are trying to decide if an igneous rock is intrusive or extrusive, texture is your biggest clue! While intrusive rocks are always made of large interlocking crystals, extrusive rocks come in a variety of other textures: fine-grained, glassy, bubbly, and more! Basalt, rhyolite, and pumice are igneous extrusive rocks.
Examples of igneous rocks
Here are some examples of common igneous rocks. If you’re studying rock identification or creating a rock collection, you’ll want to make sure to examine the following rocks:
Aphanitic or Porphyritic Rocks
- Granite
- Diorite
- Gabbro
- Peridotite
Phaneritic Rocks
- Rhyolite
- Andesite
- Basalt
Vesicular Rocks
- Pumice
- Basalt
- Scoria
Others (pyroclastic & glassy)
- Tuff
- Volcanic Breccia
- Obsidian
What’s Next?
For everyone: there’s more to learn about rocks! Continue reading more about the rock cycle or the difference between rocks and minerals.
For educators: check out my posts on how to set up a rock ID lab activity, directions for a fun rock ID review game, and a list of rock and mineral activity ideas that you can try in your classroom!
Study this topic with Wild Earth Lab!
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References and Further Reading
- Earle, S. (2019). Physical Geology. BCcampus Open Education. (Chapters 2-7). Available: https://opentextbc.ca/physicalgeology2ed/
- Johnson, C., Affolter, M.D., Inkenbrandt, P., & Mosher, C. (2017). An Introduction to Geology. Salt Lake Community College. (Chapters 3-6). Available: https://opengeology.org/textbook/
- Jones, C. E. (n.d.). Igneous Textures. University of Pittsburg. Department of Geology and Planetary Science. Available: https://sites.pitt.edu/~cejones/GeoImages/2IgneousRocks/IgneousTextures.html
- Plummer C.C., Carlson D.H., & Hammersley L. (2019). Physical Geology. McGraw-Hill Education. (Chapters 1-7).