Extrusive rocks, formed from the cooling and solidification of lava on the Earth's surface, possess unique properties due to their rapid cooling process. Here are some key characteristics:
1. Fine-grained Texture:
* Due to rapid cooling, mineral crystals have less time to grow, resulting in small, often microscopic crystals. This gives the rock a fine-grained appearance, sometimes described as aphanitic.
* Some extrusive rocks can have a porphyritic texture, where large crystals (phenocrysts) are embedded in a fine-grained matrix. This occurs when some cooling happens below the surface before eruption.
2. Vesicular Structure:
* Trapped gases during the eruption form bubbles in the solidifying lava, creating holes or cavities called vesicles.
* The presence and abundance of vesicles are significant for identifying extrusive rocks and determining their formation conditions.
3. Glassy Texture:
* Extremely rapid cooling can inhibit crystal growth completely, resulting in a glassy texture called glassy.
* Examples include obsidian (black) and pumice (porous and lightweight).
4. Amygdaloidal Structure:
* Vesicles can be filled with secondary minerals after the lava solidifies, creating almond-shaped structures called amygdules.
* These minerals are often different from the main rock composition.
5. Color and Composition:
* Extrusive rocks vary in color depending on their mineral composition.
* For example, basalts (rich in mafic minerals) are typically dark gray to black, while rhyolites (rich in felsic minerals) are lighter in color, ranging from gray to pink.
6. Weathering:
* Extrusive rocks are generally more susceptible to weathering than intrusive rocks due to their finer grain size and greater surface area.
* This can lead to the development of unique weathering features like slickenside surfaces or honeycomb weathering.
Examples of Extrusive Rocks:
* Basalt: Dark-colored, fine-grained rock formed from the eruption of mafic lava.
* Rhyolite: Light-colored, fine-grained rock formed from the eruption of felsic lava.
* Andesite: Intermediate-colored rock formed from the eruption of intermediate lava.
* Obsidian: Glassy, black rock formed from rapidly cooled lava.
* Pumice: Porous, lightweight rock formed from lava with high gas content.
In Summary:
Extrusive rocks are characterized by their fine-grained texture, vesicular structure, glassy texture, amygdaloidal structure, and diverse colors and compositions. These features provide clues about their formation and help distinguish them from intrusive igneous rocks.
