Lava cools so quickly that mineral crystals have little or no time to form, so volcanic rocks are normally quite fine-grained, with microscopic crystals. Common volcanic rocks are basalt, rhyolite, obsidian, and pumice. Magma that doesn't reach the surface usually cools more slowly, giving ample time for visible mineral crystals to develop. Granite is the most common coarse-grained igneous rock. Extrusive rock forms lava flows or rounded pillows of rock.
Intrusive rock occurs in a variety of forms.
Vertical pipes of igneous rock are called dykes . Horizontal sheets trapped between other rocks are called sills. Large pods of crystalline rock are called laccoliths, and very large crystalline formations are called batholiths. Glassy texture Obsidian . A shiny, dark rock, which appears to be polished.