Like most Earth materials, rocks are created and destroyed in cycles. The rock cycle is a model that describes the formation, breakdown, and reformation of a rock as a result of sedimentary, igneous, and metamorphic processes. All rocks are made up of minerals. A mineral is defined as a naturally occurring, crystalline solid of definite chemical composition and a characteristic crystal structure. A rock is any naturally formed, nonliving, firm, and coherent aggregate mass of solid matter that constitutes part of a planet.

Igneous rocks

Igneous rocks-form in two very different environments. All igneous rocks start out as melted rock, (magma) and then crystallize, or freeze. Bowen's Reaction Series is a proposed sequence of mineral crystallization from basaltic magma, based on experimental evidence. Volcanic processes form extrusive igneous rocks. Extrusive rocks cool quickly on or very near the surface of the earth. Fast cooling makes crystals too small to see without some kind of magnifier. Basalt is dark rock, gray or black on a freshly broken surface, and weathers brown or red, because it contains lots of dark-colored minerals.

Some basalt contains light-colored crystals. Dacite and andesite are medium in color, and contains medium amounts of dark minerals. Rhyolite is the lightest colored volcanic rock. Rhyolite contains very few dark minerals, but sometimes, rhyolite cools so fast that it quenches and forms volcanic glass instead of crystallizing. Volcanic glass looks dark because of the way light passes through it. Obsidian is volcanic glass. Rhyolite is the most common source of volcanic ash and pumice in Idaho. Intrusive igneous rocks cool in plutons (Pluto was the Roman god of the Underworld.) deep below the surface of the Earth. Slow cooling allows the growth of large crystals. Crystals in intrusive rocks are visible without magnification. Granite has the same minerals as rhyolite, but in much larger crystals. Diorite is the intrusive version of andesite, granodiorite is the intrusive version of dacite, and gabbro is the intrusive version of basalt.

Metamorphic Rocks

Metamorphic rocks form when sedimentary, igneous, or other metamorphic rocks are subjected to heat and pressure from burial or contact with intrusive or extrusive igneous rocks. ("Meta" means change, and "morph" means form.) Heat and pressure from burial cause molecules of flat minerals like mica to line up perpendicular to the direction of greatest compression. Deep burial means higher pressure and hotter temperatures, and very high temperature and pressures cause the formation of new minerals, and mineral grains. Low-grade metamorphic rocks like slate and phyllite break in flat pieces, and have a sheen on the surface. Schist is shiny, and many schists contain garnets, staurolites or other mineral crystals that have grown within the rock. Gneiss is a foliated metamorphic rock. Layers of dark and light minerals stripe the rock, and sometimes it is possible to see how the direction of pressure deep in the Earth changed as the minerals formed. The change in direction forms eye-shaped pods of minerals, called augens ("augen" is German for "eye.") Quartzite is another important metamorphic rock in Idaho. Quartzite is metamorphosed sandstone. Some Idaho quartzite is so pure that it can be used to make computer chips. The most common contact metamorphic rock in Idaho is marble. The Portneuf Gap area provides good examples of Idaho marble. Marble forms when limestone is intruded by a pluton which heats the limestone.

Sedimentary Rocks

Sedimentary rocks are those rocks made up of pieces of other rocks. We call the pieces of rock "clasts" (Clast means "broken piece"). A clast is a piece of rock broken off of another rock. Clasts of rock are eroded from larger rocks, transported (moved) by wind or water and deposited in a basin.After some period of time, the clasts are lithified (lithos is the Greek word for stone). The sedimentary rocks we see today were once gravel, sand, silt, mud, or living things. We decide what to name sedimentary rocks based on the size of the clasts that make up the rock. For most sedimentary rocks, this is easy. Sandstone is made of sand, siltstone is made of silt, mudstone is made of mud and so on. Even volcanic ashcan become sedimentary rock! The only hard ones to remember are conglomerateand breccia. Conglomerates are made up of rounded, gravel-size particles (To a geologist, gravel is anything from 2mm to 4 meters in diameter), and breccia is made up of angular, sharp-edged, gravel-sized clasts. Limestone and chert are classified as sedimentary rocks, but most limestone and chert are grown by living organisms rather than broken from other rocks. Some limestones have fossils, but most limestones and cherts have recrystallized, and the remains of the creatures that made them are no longer visible.