Common Characteristics:
* Fine-grained: Extrusive rocks generally have small crystals (less than 1 mm) because they cool quickly, not giving minerals enough time to grow large. This results in a smooth, glassy, or slightly rough texture.
* Porous: Due to the rapid cooling and escape of gases, extrusive rocks often contain numerous holes, vesicles, or cavities.
* Often dark-colored: Many extrusive rocks have a dark gray, black, or reddish-brown color due to the presence of minerals like pyroxene and olivine.
Specific examples and their appearances:
* Basalt: A common extrusive rock, basalt is typically dark gray to black, dense, and fine-grained. It often exhibits a vesicular texture (containing many holes).
* Rhyolite: A silica-rich extrusive rock, rhyolite is usually light-colored (pink, gray, or white), with a glassy or fine-grained texture. It can have a flow-banded appearance with alternating layers of different colors.
* Andesite: An intermediate-composition extrusive rock, andesite ranges in color from gray to dark gray and has a fine-grained texture. It often exhibits a porphyritic texture with larger crystals embedded in a fine-grained matrix.
* Scoria: A dark-colored, vesicular rock with a very porous texture. It forms when lava with high gas content erupts explosively.
* Pumice: A very light, porous, and glassy rock formed from frothy lava with a high gas content. It can float on water due to its high porosity.
* Obsidian: A volcanic glass with a smooth, glassy texture and a dark, black or dark brown color.
Important Note: Extrusive rocks can exhibit a wide variety of appearances within the same type due to the diversity of geological conditions under which they form. Therefore, it's crucial to look at other characteristics, such as mineral composition and texture, to accurately identify them.
