The primary difference between igneous and metamorphic rocks lies in their origin: igneous rocks are formed from cooled and solidified magma or lava, while metamorphic rocks are formed from existing rocks that have been transformed by heat, pressure, or chemically active fluids.
Formation Processes
Igneous Rocks
Igneous rocks are born from fire, quite literally. Molten rock, known as magma when underground and lava when above ground, cools and solidifies. The rate of cooling determines the crystal size:
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Intrusive (Plutonic) Igneous Rocks: Formed from magma that cools slowly beneath the Earth's surface. This slow cooling allows for the formation of large, visible crystals. Granite is a common example.
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Extrusive (Volcanic) Igneous Rocks: Formed from lava that cools quickly on the Earth's surface. The rapid cooling results in small or even glassy crystals. Basalt and obsidian are examples.
Metamorphic Rocks
Metamorphic rocks are the result of change. Existing rocks (igneous, sedimentary, or even other metamorphic rocks) are subjected to intense heat, pressure, or chemically active fluids, causing them to alter their mineral composition, texture, or both. This process, called metamorphism, occurs without the rock completely melting.
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Regional Metamorphism: Occurs over large areas, often associated with mountain building. High pressure and temperature cause widespread changes. Examples include the formation of gneiss and schist.
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Contact Metamorphism: Occurs when magma intrudes into existing rock. The heat from the magma alters the surrounding rock. Hornfels is a common example.
Key Differences Summarized
| Feature | Igneous Rocks | Metamorphic Rocks |
|---|---|---|
| Formation | Cooling and solidification of magma or lava | Transformation of existing rocks by heat, pressure, or fluids |
| Starting Material | Molten rock (magma or lava) | Any pre-existing rock (igneous, sedimentary, metamorphic) |
| Process | Crystallization | Recrystallization, change in mineral composition and texture |
| Typical Texture | Crystalline (coarse or fine-grained) | Foliated (layered) or non-foliated (massive) |
| Examples | Granite, Basalt, Obsidian | Gneiss, Schist, Marble, Quartzite |
Examples
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Igneous:
- Granite: A coarse-grained, intrusive rock often used for countertops and buildings.
- Basalt: A fine-grained, extrusive rock commonly found in lava flows.
- Obsidian: A volcanic glass formed from rapidly cooled lava.
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Metamorphic:
- Gneiss: A foliated rock with distinct bands of minerals, often formed from granite.
- Schist: A foliated rock with visible, platy minerals, such as mica.
- Marble: A non-foliated rock formed from the metamorphism of limestone or dolostone.
- Quartzite: A non-foliated rock formed from the metamorphism of sandstone.
In essence, igneous rocks represent the direct products of volcanic activity and the cooling of molten rock, while metamorphic rocks are the transformed versions of pre-existing rocks altered by intense geological forces.
