Igneous Rocks - Definition, Examples, Quiz, FAQ, Trivia
Discover how rocks form from molten magma and lava
What Are Igneous Rocks?
Igneous rocks are rocks formed from the cooling and hardening of molten rock material. The word "igneous" comes from the Latin word for fire ("ignis") because these rocks form from extremely hot, melted rock.
There are two main types of igneous rocks:
• Intrusive (plutonic) - form when magma cools slowly beneath Earth's surface (e.g., granite)
• Extrusive (volcanic) - form when lava cools quickly on Earth's surface (e.g., basalt)
Did You Know?
Igneous rocks make up about 95% of Earth's crust, though they're often covered by sedimentary rocks and soil.
How Igneous Rocks Form
Igneous rocks form through the process of crystallization as molten rock cools and solidifies. The key steps are:
Melting
Rocks melt deep underground to form magma (800°C-1300°C)
Movement
Magma rises toward Earth's surface through cracks
Cooling
Molten rock cools either underground or after eruption
Crystallization
Minerals form crystals as the rock solidifies
Rock Formation
Solid igneous rock is created with characteristic texture
The cooling rate determines the rock's texture:
• Slow cooling underground = large crystals (e.g., granite)
• Fast cooling on surface = small or no crystals (e.g., obsidian)
Types & Examples of Igneous Rocks
Igneous rocks are classified by both their composition (mineral content) and texture (crystal size). Here are some common examples:
Granite
Intrusive, light-colored with large crystals of quartz and feldspar
Basalt
Extrusive, dark fine-grained rock forming most oceanic crust
Pumice
Extrusive, lightweight volcanic rock full of gas bubbles
Obsidian
Extrusive volcanic glass that cools too quickly to form crystals
Gabbro
Intrusive equivalent of basalt with visible mineral crystals
Texture Matters
The texture of igneous rocks tells us about their cooling history. Porphyritic texture (mix of large and small crystals) means the magma cooled at two different rates.
Igneous Rocks Quiz
Test your knowledge about igneous rocks with these questions:
Frequently Asked Questions
Common questions about igneous rocks:
Igneous Rock Facts
Discover some amazing facts about igneous rocks:
Moon Rocks
The Moon's surface is mostly made of igneous rocks similar to Earth's basalt. The dark areas (maria) are ancient lava flows that cooled to form basalt plains.
Ocean Floor
The entire ocean floor is made of basalt, an igneous rock that forms at mid-ocean ridges where magma rises to create new crust.
Ancient Rocks
The oldest known rocks on Earth are igneous - zircon crystals from Australia dated at 4.4 billion years old, just 160 million years after Earth formed!
Early Tools
Early humans made tools from obsidian (volcanic glass) because it fractures to form extremely sharp edges - sharper than modern surgical steel!
