Academic Quad-East Academic Quad-West Baker Institute Herring Hall Shepherd, RMC Cohen House and Allen Center Engineering Quad Hamman Hall, Herman Brown, and Mudd Residential Colleges Science Buildings Main Page Contact		An Overview of Rocks   For those of you who are unfamiliar with geology or some of the terms used throughout this website, here are some definitions that may be helpful in understanding the material. Three main types of rock can be definied: IGNEOUS, SEDIMENTARY, and METAMORPHIC.   IGNEOUS ROCKS (from the Latin ignis, for "fire"), form from the direct cooling and solidification of molten material. The molten silicate material, which erupts from volcanos, together with the gasses dissolved in it (volatiles), is called magma. When magma reaches Earth's surface, much of the dissolved gas is released. Magma reaching the surface in liquid form is called lava. The liquid form of a given material is usually less dense than its solid form. Molten magma is less dense than the surrounding material from which it has melted and so it rises. It may erupt at the surface from a volcano as lava and cool to form an extrusive igneous rock. Alternatively, it may crystallize deep in the crust below the surface as an intrusion formed from intrusive igneous rock. Igneous Textures: Phaneritic--a rock in which all the crystals are easily seen with the naked eye. This texture forms when the rock cools slowly. Aphanitic--a fine-grained igneous rock where no distinct crystals are visible to the naked eye. This texture forms when the rock cools rapidly. If cooling is very rapid, a glassy texure may result. (example: obsidian) Porphyritic--a texture in which the bulk of the rock (the groundmass) is fine-grained, but it has a few large crystals, called phenocrysts, that are easily visible by the eye. Such a texture usually results from a two-stage cooling process. The granite used on the majority of Rice buildings is an igneous rock.   SEDIMENTARY ROCKS form from mineral and rock fragments and material of organic origin that is eroded or dissolved and then deposited at Earth's surface. Weathering is the process by which rocks are disaggregated and the constituent fragments or minerals undergo chemical reactions. Transport mechanisms such as wind, ice, water, or gravity, as well as the process of deposition, influence the sorting, grain size, and type of layering (bedding) of the sediment. The site of deposition--whether river, lake, ocean, or desert--gives rise to differences in color, bedding, mineralogy, grain size, and grain-size distribution. Finally, the plate-tectonic setting of sediment deposition plays a significant role in the final product. Sedimentary rocks formed at convergent margins are different in many ways from sedimentary rocks formed at plate interiors. Sedimentary structures: (clues concerning the condition of formation) Cross-bedding--formed as grains are deposited by a moving current, such as wind or water. The sandy bottom of a riverbed, for example, often contains ripples, or regularly spaced ridges of sand, that run roughly perpendicular to the direction of the current. Graded bedding--a continuous variation in grain size from the base of a sedimentary bed to the top. Normal grading consists of coarser grains at the base and finer grains at the top. Mud cracks--dried out mud that has shrunk and cracked which may later be preserved beneath a layer of sand. They tell us that the mud was originally deposited in shallow water and the climate was arid to allow the water to dry out. Lithification is the process of converting loose sediment into rock through compaction or cementation. Sedimentary rocks often contain fossils which tell in what climate the rock was lithified. The limestone which appears all over campus is a sedimentary rock.   METAMORPHIC ROCKS form when the composition, mineralogy, or texture of existing rocks changes in response to high temperatures and/or pressure or by reacting with fluids in the crust. Metamorphic grade is the intensity, or degree, of metamorphism. As pressures and temperatures increase with burial over time, the metamorphic grade increases. The different types of minerals and their abundances in a given rock will change during metamorphism. For a given chemical composition, the metamorphic minerals that form in a rock are a function of temperature-pressure conditions. Types of Metamorphism: Regional--most common because it affects large tracts of crust. Regional Metamorphism generally results from mountain building processes, which, in turn, are caused by collisions between tectonic plates. High-Pressure--rocks that form at high pressures but at low temperatures. The tectonic settings of such high-pressure/low-temperature gradients where high-pressure metamorphism occurs are subduction zones (where a lithospheric plate is subducted, or sinks into the mantle). Contact--a result of high temperatures without accompanying high pressures. Such conditions occur near hot igneous intrusions in the crust. Rocks that are in contact with these intrusions undergo contact metamorphism, or thermal metamorphism. Cataclastic--occurs within and adjacent to fault zones, shear zones, and fractures along which huge masses of rock are thrust during compression. This cataclastic metamorphism involves mechanical processes such as physical crushing and grinding and occasionally even melting at the base of the thrust faults. The marble which decorates many Rice buildings is a metamorphic rock.   Source: Exploring Earth: An Introduction to Physical Geology. Davidson, Reed Davis. Prentice Hall 1997.      back to the top Academic Quad-East Academic Quad-West Baker Institute, Herring Hall, Shepherd and RMC Cohen House and Allen Center Engineering Quad Hamman Hall, Herman Brown, and Mudd Residential Colleges Science Buildings Main Page Department of Geology and Geophysics, Rice University, Houston.