Igneous rocks are found throughout the world and are the result of volcanic activity. These rocks are formed when magma (molten rock) cools and solidifies. Igneous rocks are divided into two main categories: intrusive and extrusive.
Granite is an intrusive igneous rock that has a granular and phaneritic texture. It is composed of quartz, feldspar, and mica. Granite is the most common rock in the Earth’s crust but is very rare in Pennsylvania.
Diorite is an intrusive igneous rock of felsic to an intermediate composition, usually gray to black. It is usually coarse-grained but can also be phaneritic (containing visibly individual crystals) or even aphanitic (crystalline with fine grain size). Diorite generally has a granular or porphyritic texture.
Gabbro is a dark-colored, fine-grained, intrusive igneous rock that forms from the slow cooling of magnesium-rich and iron-rich magma deep inside the Earth. It is usually found in the form of large masses or dikes.
There are three main types of rock: igneous, sedimentary, and metamorphic. Igneous rocks are made from solidified lava or magma. They are the hardest type of rock. Sedimentary rocks are made from pieces of other rocks broken down by wind, water, or ice. They are not as hard as igneous rocks. Metamorphic rocks are made from sedimentary or igneous rocks that have been changed by heat, pressure, or chemical action.
Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism. Foliation may not correspond to the original sedimentary layering but is in planes perpendicular to the direction of metamorphic compression. It is the finest-grained foliated metamorphic rock.
Schist is a medium-grade metamorphic rock with a grainy, layered structure and is abundant in the Earth’s crust. It has a foliated (layered) structure and is made up of mineral grains that are large enough to be seen with an unaided eye. Schist is usually easy to identify because of these features. The word “schist” comes from the Greek word for “split” because it can easily be split along its layers.
Gneiss (/naɪs/) is a common and widely distributed type of metamorphic rock. Gneiss is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Orthogneiss is gneiss derived from an igneous rock, and paragneiss is derived from a sedimentary rock. Gneisses are usually medium- to coarse-grained; they are largely defined by their mineralogy, composed of feldspar, quartz, and mica. Gneisses occur in high-grade metamorphic terrains where the temperatures and pressures are sufficient to produce important changes in the original rock composition. Gneisses often display a banded structure comprising alternating light and dark-colored bands. The individual bands often vary in thickness from a few millimeters to several centimeters. Gneisses may be foliated (composed of layered minerals) or non-foliated (made up of randomly interlocking minerals). If the structure is foliated, it is called foliation; if not, it is called granoblastic.
There are three types of rocks: metamorphic, igneous, and sedimentary. Metamorphic rocks are made up of other rocks that have been changed by heat and pressure. Igneous rocks are made up of solidified lava or magma. And sedimentary rocks are made of small pieces of other rocks that have been combined.
The conglomerate is a sedimentary rock with large (greater than two millimeters in diameter) rounded clasts. The space between the clasts is generally filled with smaller particles and a finer-grained matrix. Conglomerates may be formed of any rock type, i.e., calcareous, siliceous, or ferruginous material.
Sandstone is a sedimentary rock composed mostly of sand grains. Like conglomerates and breccia, sandstone is defined by its grain size and composition. Sands are small rocks less than 2 mm in diameter. They are often composed of quartz or feldspar because these are the most common minerals in the Earth’s crust. Most sandstones have quartz or feldspar (sometimes both) as their primary constituents. Sands that do not contain either of these minerals are usually called arenites.
Limestone is a sedimentary rock composed mainly of skeletal fragments of marine organisms such as coral, forams, and mollusks. Its major materials are calcite and aragonite, which are different crystal forms of calcium carbonate. About 10% of sedimentary rocks are limestones. The solubility of limestone in water and weak acid solutions leads to karst landscapes, in which water erodes the limestone over thousands of years. Most cave systems are through limestone bedrock.