Igneous rocks are rocks that form from the cooling and solidification of magma. We can divide them into two types as intrusive and extrusive igneous rocks. The main difference between extrusive and intrusive igneous rocks is their formation.
What is the difference between extrusive and intrusive igneous rocks? Intrusive Igneous rocks form from magma that cools and hardens underground, whereas extrusive rocks form from lava, which is magma that erupts onto the Earth’s surface.
Key Areas Covered
1. What are Intrusive Igneous Rocks
– Definition, Features
2. What are Extrusive Igneous Rocks
– Definition, Features
3. Similarities Between Extrusive and Intrusive Igneous Rocks
– Outline of Common Features
4. Difference Between Extrusive and Intrusive Igneous Rocks
– Comparison of Key Differences
5. FAQ: Extrusive and Intrusive Igneous Rocks
– Answers to Frequently Asked Questions
Key Terms
Extrusive Igneous Rocks, Intrusive Igneous Rocks, Plutonic Rocks
What are Intrusive Igneous Rocks
Intrusive igneous rocks, also known as plutonic rocks, are a type of igneous rock formed deep underground. They form from molten rock, called magma, that pushes its way into cracks and fractures within Earth’s crust. Unlike extrusive rocks, which erupt onto the surface as lava and cool quickly, intrusive rocks solidify beneath the surface. This slow cooling process gives them unique characteristic features.
Slow cooling allows minerals within the magma to crystalize and grow with time. This results in a coarse grain texture, where interlocking crystals are easily seen with the naked eye. A good example of this is granite, a mix of pink, grey, and black crystals. Intrusive rocks are of different shapes and sizes. Batholiths, which are massive, dome-shaped structures, form the cores of mountain ranges. Dikes, long, narrow intrusions, cut through existing rock like a knife. Pegmatite, a type of intrusive rock, can contain crystals as large as your hand.
There are many uses of intrusive rocks including the use of it in the construction industry. They also provide valuable substances that are within them.
What are Extrusive Igneous Rocks
Extrusive igneous rocks form when molten rock, called magma, erupts onto the Earth’s surface, transforming into lava. Unlike their intrusive counterparts that cool deep within the crust, extrusive rocks solidify rapidly under the open sky.
These rocks often have a fine-grained texture, sometimes so fine that individual crystals are invisible to the naked eye, earning them the term aphanitic. Some extrusive rocks, like obsidian, cool so quickly that they form a glassy texture, much like volcanic glass.
Sometimes, gas bubbles get trapped within the magma during eruption in some extrusive rocks. These bubbles freeze with time as the rocks solidify. This results in a frothy or bubbly texture, called vascular. Scoria and pumice are two examples of extrusive rocks known for their vesicular texture.
There is a variety in colour and composition of extrusive rocks. Basalt, which is a dark, fine-grained rock rich in iron and magnesium, is the most common extrusive rock on Earth. Rhyolite, with its light color and high silica content, is another common type.
Similarities Between Extrusive and Intrusive Igneous Rocks
- Both intrusive and extrusive rocks come from the same source – magma underground and lava when it erupts on the surface.
- Both intrusive and extrusive rocks can exhibit igneous textures.
Difference Between Extrusive and Intrusive Igneous Rocks
Definition
- Intrusive rocks are rocks that form deep underground when magma (molten rock) cools and solidifies within the Earth’s crust. Extrusive rocks are rocks that form when lava (molten rock that erupts onto the surface) cools and hardens on the Earth’s surface, like during volcanic eruptions.
Cooling
- Intrusive rocks cool very slowly due to the surrounding rock’s insulation. In contrast, extrusive rocks cool rapidly upon exposure to air and water at the surface.
Texture
- Slower cooling allows intrusive rocks to form large, visible crystals. Extrusive rocks cool so fast that crystals often don’t have time to form, resulting in a fine-grained or even glassy texture.
Gas Bubbles
- Extrusive rocks frequently contain gas bubbles whereas intrusive rocks very rarely contain gas bubbles.
Conclusion
Although both intrusive and extrusive igneous rocks originate from magma, there is a difference between extrusive and intrusive igneous rocks based on their formation locations and subsequent cooling rates. Intrusive rocks, which are formed underground, cool slowly, fostering the growth of large, visible crystals and yielding coarse-grained textures like granite. Conversely, extrusive rocks, formed from lava on the Earth’s surface, cool rapidly, resulting in fine-grained textures or even glassy appearances.
FAQ: Extrusive and Intrusive Igneous Rocks
1. Do extrusive rocks cool fast or slowly?
Extrusive rocks cool very quickly because they form when magma reaches the Earth’s surface and erupts as lava. This fast cooling process does not allow crystals much time to grow, which leads to the characteristic small crystal size in extrusive rocks.
2. Is volcanic glass intrusive or extrusive?
Volcanic glass is an extrusive igneous rock. It forms when lava cools very quickly on the Earth’s surface, resulting in a glassy, non-crystalline texture due to the rapid cooling process.
3. Which rock cools the fastest?
Rhyolite cools quickly, making it one of the fastest-cooling extrusive rocks. It typically forms when high-silica lava erupts and cools rapidly at the Earth’s surface. This quick cooling results in a fine-grained or glassy texture.
4. Why do intrusive rocks cool slowly?
Intrusive rocks cool slowly because they form deep within the Earth’s crust, where the surrounding rock insulates the magma and prevents rapid heat loss. This slow cooling process allows large crystals to form, giving intrusive rocks a coarse-grained texture.
5. Is gabbro extrusive or intrusive?
Gabbro is an intrusive igneous rock. It forms deep from the slow cooling of magma within a volcano, which allows large crystals to grow and gives gabbro a coarse-grained texture.
Reference:
1. “Igneous Rocks.” National Geogrophic.
2. “Extrusive Rock.” Encyclopedia Britannica.
Image Courtesy:
1. “Indian Granite” By Mark A. Wilson (Department of Geology, The College of Wooster) – Original photograph (Public Domain) via Commons Wikimedia
2. “Igneous rock eng modified” By The Editor’s Apprentice – Own work (CC0) via Commons Wikimedia
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