Extrusive rock, any rock derived from magma (molten silicate material) that was poured out or ejected at Earth's surface. Both lava flows and pyroclastic debris (fragmented volcanic material) are extrusive; they are commonly glassy (obsidian) or finely crystalline (basalts and felsites). ...

What are 4 extrusive rocks?

Extrusive igneous rocks erupt onto the surface, where they cool quickly to form small crystals. Some cool so quickly that they form an amorphous glass. These rocks include: andesite, basalt, dacite, obsidian, pumice, rhyolite, scoria, and tuff.

What is the most common extrusive rock?

The most common extrusive igneous rock is basalt, a rock that is especially common below the oceans (Figure 4.6). Figure 4.5: Extrusive or volcanic igneous rocks form after lava cools above the surface.