Igneous rocks get their name from the Latin word for fire, “ignis.” This makes sense, because early naturalists could watch them forming as fiery streams of lava flowing from volcanoes turned into solid rock. The word igneous is now used to describe all rocks that form when molten rock, or magma, solidifies; whether this happens on the ground near a volcano or deep beneath the Earth’s surface. Forming from melted rock is the key difference between igneous rocks and the other two major types of rocks: Metamorphic rocks are formed by heat and pressure that doesn’t melt the rock, and sedimentary rocks form at low temperatures on the surface. Although this type of rock is formed when magma freezes into solid crystals, there are still many different kinds of igneous rocks. Geologists who study these rocks use several different features to divide them into smaller groups, groups that provide hints about how they formed and what geologic activities have taken place in the area. Geologists and rock hounds divide igneous rocks into two general classes based on the size of the mineral crystals that formed when the rock cooled. A rule of thumb is that if the crystals are visible to the naked eye, the rock cooled slowly, usually deep in the ground. If the crystals are too small to see without a magnifier or microscope, the rock cooled quickly, from “lava,” or magma that reaches the surface. The fine-grained varieties are called extrusive igneous rocks and the coarser ones are called intrusive. The most familiar igneous rocks are examples of the two types: Granite is intrusive and basalt is extrusive. Even division on crystal size can have exceptions: The rocks obsidian and pumice solidify so quickly that they seem to have no crystals, much like ordinary glass. At the other extreme, the mineral crystals in pegmatite can be the size of telephone poles, cars or even houses. A porphyry is sort of in between: It’s an igneous rock with large crystals embedded in a matrix of tiny crystals. Crystal size, or intrusive vs. extrusive, only provides a clue about where the rock cooled; so geologists must look more closely at the rock to get more information. Both fine- and coarse-grained rocks can be divided based on the minerals that are present. Minerals that are rich in silicon and aluminum are most common in one broad class of igneous rocks. These are the “felsic” igneous rocks, most of which are light in color, and include granite (coarse-grained) and rhyolite Category:Home › Other • Pomegranates: A newly discovered superfood • Where did the joke why did the chicken cross the road come from and why is it funny? • Can mothers diagnosed with bipolar disorder make good parents? • Spiritual evolution of human consciousness • Tips for getting a college basketball scholarship • Living with Pseudotumor cerebri (PTC) • Caring for the caregiver • Technologys impact on society