Questions and Answers
What is an intrusive igneous rock?
An intrusive igneous rock is one that forms when magma cools within Earth.
What is an extrusive igneous rock?
An extrusive igneous rock is one that forms when lava cools on Earth's surface.
How are gabbro and basalt similar? How are they different?
They are similar because gabbro has the same composition as basalt; they are different because basalt is an extrusive igneous rock and gabbro is an intrusive igneous rock.
How does an igneous rock that has both large and small mineral crystals form?
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Would you expect a light gray igneous rock to be rich or poor in silica? Why?
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Why does silica-rich lava tend to build steep volcanoes instead of spreading out?
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What is the main difference between intrusive and extrusive igneous rocks?
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What are two major properties used to classify igneous rocks?
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Why can intrusive igneous rocks be left behind when surrounding rocks are worn away?
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Would you expect extrusive rocks produced by an explosive volcano to be light or dark in color? Why?
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Why are the names intrusive and extrusive appropriate for the two types of igneous rocks?
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Study Notes
Intrusive and Extrusive Igneous Rocks
- Intrusive igneous rocks form when magma cools beneath Earth's surface, resulting in large mineral crystals.
- Extrusive igneous rocks occur when lava cools quickly on the surface, producing small mineral crystals.
Comparison of Gabbro and Basalt
- Gabbro and basalt share the same mineral composition.
- Gabbro is an intrusive igneous rock, while basalt is classified as extrusive.
Formation of Rocks with Mixed Crystal Sizes
- Rocks with both large and small crystals result from magma that cools slowly underground before erupting onto the surface.
- Large crystals develop during slow cooling, while small crystals form from rapid cooling of lava.
Silica Content and Color of Igneous Rocks
- Light gray igneous rocks, like granite and rhyolite, are typically rich in silica.
- Dark-colored igneous rocks, such as gabbro and basalt, contain lower silica levels.
Silica-Rich Lava and Volcano Characteristics
- Silica-rich lava, due to its thick and sticky nature, tends to erupt explosively, forming steep volcanoes.
- Pressure builds in volcanoes until an explosive eruption occurs.
Classification of Igneous Rocks
- Igneous rocks are classified based on mineral grain size and overall composition, which is determined by the specific minerals present.
Durability of Intrusive Igneous Rocks
- Intrusive igneous rocks are often more resilient and can survive erosion, remaining after surrounding rocks have worn away.
Color of Extrusive Rocks from Explosive Eruptions
- Extrusive rocks resulting from explosive volcanic activity are usually dark in color due to the expelled dark ash and smoke.
Naming of Intrusive and Extrusive Igneous Rocks
- The terms intrusive and extrusive accurately describe the formation processes of these rocks, highlighting their distinct origins within or on the surface of Earth.
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Test your knowledge on intrusive and extrusive igneous rocks with this flashcard quiz. Explore definitions and comparisons of key concepts in geology to enhance your understanding. Perfect for students studying Earth science or geology.
