ROCK CYCLE
The rock cycle is defined as the set of processes that create, change, and reform the rocks of the earth. Rocks are initially formed from molten material referred to as magma, which lies deep in the earth’s crust. As this material moves closer to the surface it cools and solidifies. It can cool slowly and form intrusive igneous rocks, or cool quickly if it rises rapidly to the surface as in a volcano, forming extrusive rocks. As rocks become exposed at the surface they are subjected to the natural forces of weathering, breaking them down into smaller and smaller fragments, which can be transported by the forces of gravity, wind, water, or ice. At their point of eventual deposition these grains accumulate exerting pressure on those beneath causing them to become compacted and eventually lithified as sedimentary rocks. When igneous and sedimentary rocks are subjected to extremes of high temperature and pressure they form metamorphic rocks. All of these rock types, if subsequently reburied deep enough can be melted and create once again igneous rocks, starting the cycle again.
James Hutton, an eighteenth century geologist developed the idea of the rock cycle. He considered this a continually repetitive process with no new material being added to the cycle. The concept was modified in the 1960’s with the presentation by Tuzo Wilson of his continental drift ideas, which suggested the rock cycle was driven by plate tectonics. According to his theory, called the Wilson cycle, the earth’s three rock types, igneous, sedimentary, and metamorphic, are continually changing from one type to the other defining the earth as a rock-recycling machine. Surface rocks are buried to a depth where they form magma through the effects of extreme heat and pressure as a result of the movement of plates overriding each other. This movement also opens huge fissures in the ocean basins where the molten material flows or is ejected to the surface.
When rocks are forced deep within the earth they melt and form magma. The magma will remain molten and in place as long temperature and pressure conditions remain relatively stable. If, however, these conditions moderate, the magma will solidify. Intrusive igneous rocks, which cool slowly, have time for the constituent minerals to form crystals giving rise to coarse-grained rocks such as granite. When the confining pressure release is much more rapid the molten material may rise quickly through vents in the crust forming extrusive rocks such as basalt or obsidian, which can be observed in the vicinity of volcanoes. These rocks are very fine-grained and, in some cases, may cool so rapidly that no mineral crystals can be seen.
Rocks exposed to the atmosphere are variably unstable and subject to the processes of weathering and erosion. Gases originally dissolved in the expelled magma attack other components of the rock forming new minerals. Weathering and erosion breaks the original rock down into smaller fragments and carries away dissolved material. These grains accumulate and are continually being buried by additional material exerting increasing pressure on the underlying sediments eventually forming clastic sedimentary rocks. Fossils are sedimentary rocks formed by the lithification of material from living organisms. Sedimentary rocks are also formed from material that is the result of precipitation from a solution through evaporation. Rock salt and gypsum are examples of chemically precipitated rocks..
The processes of metamorphism involve high temperatures and/or pressures and can alter any igneous, sedimentary, or other metamorphic rock. When these forces act over a large portion of the terrain, such as in mountain building, we have regional metamorphism. Metamorphic rocks typically show banding or foliation. Contact metamorphism defines processes whereby country rocks are intruded by, and in contact with igneous bodies. Within the contact zone recrystallization of the intrusion and the country rocks takes place forming a new unique suite of minerals.
Plate tectonics plays a major role in the rock cycle. Along the ocean floor where plates are moving apart, extensive fissure zones are formed permitting the underlying magma to rise up and form ridges of igneous rocks. Some low-grade metamorphism often occurs during and after the formation of these rocks due to the intrusion of the material by the seawater. As the molten material flows from the fissure, it begins forming ridges adjacent to it. The outpouring of additional magma acts to force the plates further apart, where they are covered by the sediments of continental erosion. When these moving plates encounter a more stable continental plate, the oceanic plate is eventually subducted, or forced under the adjacent continental plate. As this plate is forced deeper it is subjected to higher temperature conditions causing melting of the rocks. As the moving plate and some included sediments are dragged deeper, water and other more volatile materials are driven off and rise into the rock above the subduction zone. The lower pressure, high temperature, and now volatile rich material in this overlying rock melts and the resulting magma rises through the overlying rock to produce submarine and continental volcanoes.
At this stage of the Wilson cycle, two continental or smaller terrains can meet at a convergent zone, where neither can be subducted, as they are both low density. As a result tremendous compressional forces metamorphose the rocks involved, producing folds, faults, and forming mountains. The mountain ranges are immediately subjected to the forces of erosion, which wear them down forming accumulations of sediments in adjacent continental and ocean areas.
Water is an important factor in the rock cycle. Rocks are dissolved by water in the form of precipitation and groundwater. If the water is even slightly acidic it is more effective in breaking down the mineral components that are unstable under surface conditions. Water in streams and rivers transports much of the sediment to the oceans and lakes.
Our Earth’s surface is continually being reshaped by the ongoing processes of the rock cycle. It is a fundamental concept in geology that describes the dynamic transitions through geologic time among the three main rock types. Due to the driving forces of the rock cycle, plate tectonics and the water cycle, rocks do not remain in equilibrium and are forced to change as they encounter new environments. Each type of rock is altered or destroyed when it is forced out of its equilibrium conditions, subsequently re-entering the cycle as different metamorphic rocks or as melted material that can form new igneous rock. The rock cycle never ends.
