GARY HINCKS / SCIENCE PHOTO LIBRARY GARY HINCKS / SCIENCE PHOTO LIBRARY
Types of volcanic rock. Table showing different extrusive (volcanic) igneous rocks. Left, from top: scoria, basalt, gabbro. Right, from top: obsidian, rhyolite, granite. Extrusive igneous rocks form when molten rock (magma) is cooled outside the ground, either in the air or underwater. Each rock's properties depend on how fast it cools and how much silica is present. The faster it cools, the finer the grains within the rock. Here the left arrow shows the speed of cooling (size of grain), where red is slow (coarse grained) and blue is fast (fine grained). The top arrow shows increasing silica content from left (mafic) to right (felsic).
Model release not available. Property release not required.