Questions and Answers
What is the formation of igneous rocks?
When lava or magma cools and hardens, it forms igneous rocks.
Which type of magma is light colored and rich in silica?
What are intrusive igneous rocks?
Rocks that form from magma below the surface.
What are extrusive igneous rocks?
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How are igneous rocks classified?
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Which of the following is a dark-colored igneous rock composition?
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Match the following textures of igneous rock:
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What characteristic defines glassy igneous rocks?
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What are fine-grained igneous rocks?
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What are coarse-grained igneous rocks?
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How are vesicular igneous rocks formed?
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Study Notes
Formation of Igneous Rocks
- Igneous rocks form when lava or magma cools and hardens.
- Two main categories are Extrusive (cools on the surface) and Intrusive (cools below the surface).
- Magma is less dense than surrounding rock, causing it to rise; when it reaches the surface, it is called lava.
Types of Magma
- Rhyolitic (Granitic): Light colored, silica-rich, contains quartz and feldspar.
- Andesitic: Intermediate color and silica, contains feldspar and amphiboles.
- Basaltic: Dark colored, silica-poor, high in iron and magnesium.
Intrusive Igneous Rocks
- Form from magma that cools and solidifies below the Earth's surface.
- Often exposed at the surface due to erosion of overlying soil.
- Also referred to as plutonic rocks.
Extrusive Igneous Rocks
- Formed when lava cools quickly on the Earth's surface.
- Rapid cooling due to exposure to air and water prevents large crystal growth.
Igneous Rock Classification
- Classification based on composition (types of minerals) and texture (shape, size, arrangement of minerals).
Igneous Rock Composition
- Three basic compositions lead to different colors of igneous rocks:
- Dark (Mafic): Formed from basaltic magma.
- Intermediate: Formed from andesitic magma.
- Light (Felsic): Formed from granitic or rhyolitic magma.
Igneous Texture
- Four primary textures include:
- Fine (Aphantic): Small crystals, typically not visible to the naked eye.
- Coarse (Phaneritic): Large, interlocking crystals visible to the unaided eye.
- Glassy: No crystal structure due to rapid cooling.
- Vesicular: Contain gas bubbles, resulting in a porous texture.
Glassy Igneous Rocks
- Have a shiny appearance resembling glass.
- Crystallization does not occur due to extremely rapid cooling (e.g., Obsidian).
Fine-Grained Igneous Rocks
- Crystals are too small to be seen without a microscope.
- Extrusive in nature, cool quickly on the surface (e.g., Rhyolite, Basalt primarily found in oceanic crust and volcanic islands).
Coarse-Grained Igneous Rocks
- Composed of interlocking mineral crystals of similar sizes, visible without aid.
- Typically formed from slow cooling of magma within the Earth (e.g., Granite).
Vesicular Igneous Rocks
- Formed from lava that contains gas bubbles, leaving open holes in the rock.
- Examples include Pumice and Scoria, characterized by their lightweight and porous nature.
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