Hobbies And Interests
Fast Cooling
Extrusive rocks all cool very quickly. Basalt, andesite, rhyolite, obsidian, pumice, and scoria may all look different, be composed of different minerals, and even feel different, but they all share a quick cooling characteristic. The chilling effect is immediate, given the heated magma comes into contact with the much cooler air surrounding it. Intrusive rocks, such as granite, remain beneath the surface in the high heat for much, much longer. This causes intrusive rocks to remain more fluid, take on more minerals, and create larger crystals.
Small Crystals
All extrusive rocks have small crystals because they cool so rapidly. In most cases, a microscope is required to even see them and, in the case of obsidian, there are no crystals to see regardless. Basalt can also rapidly cool down due to the ocean's influence, as it is the most common igneous rock on earth. The small crystals create a stony texture to basalt, andesite and rhyolite called "aphantic." Though the other types of extrusive rocks can also be considered aphantic, their slightly different creations often add more to their textures.
Holes and Glass
Extrusive rocks also come in two other forms: vesicular (or frothy) and glassy. Obsidian is the only extrusive rock that forms a smooth, glass-like texture all the way through. They are not that common when it comes to volcanic eruptions and wide-scale identification. Two forms of extrusive rocks become frothy in texture, due to air pockets forming and releasing during their creation: pumice and scoria. This turns their surfaces and beyond into a rough, scratchy exterior. Between them, pumice is lighter and has smaller cavities than scoria, mostly due to coming from different lava types. Intrusive rocks tend to be phaneritic; their crystals are visible to the naked eye and are "coarse grained."
Always Form on the Surface
The final and most important characteristic of the extrusive rocks is that they all form near or above the surface of the earth. Two ways exist for the magma to become an extrusive rock: the magma breaks the surface to become a lava flow, or it ruptures explosively into pyroclastic ejections. When lava flows create extrusive rocks, they are smoother and have less holes in them (obsidian, basalt, rhyolite and andesite). When the rocks are pyroclastic, they tend to be the opposite (like pumice and scoria). Rhyolite rests between the two, more often found as pyroclastic material than in a lava flow.