|
( Rock cycle)
The rock cycle is a fundamental concept in geology that describes the dynamic transitions through geologic time among the three main rock types sedimentary, metamorphic, and igneous. As the diagram to the right illustrates, each type of rock is altered or destroyed when it is forced out of its equilibrium conditions. An igneous rock such as basalt may break down and dissolve when exposed to the atmosphere, or melt as it is subducted under a continent. 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. The rock cycle is an illustration that explains how the 3 rock types are related to each other and how processes change from one type to another over time. The original concept of the rock cycle is usually attributed to James Hutton, the eighteenth century father of geology. The rock cycle was a part of Hutton's uniformitarianism and his famous quote no vestige of a beginning, and no prospect of an end, applied in particular to the rock cycle and the envisioned cyclical nature of geologic processes. This concept of a repetitive non-evolutionary rock cycle remained dominant until the plate tectonics revolution of the 1960s. With the developing understanding of the driving engine of plate tectonics, the rock cycle changed from endlessly repetitive to a gradually evolving process. The Wilson cycle (a plate tectonics based rock cycle) was developed by J. Tuzo Wilson during the 1960s and 70s. When rocks are pushed deep under the Earth's surface, they may melt into magma. If the conditions no longer exist for the magma to stay in its liquid state, it will cool and solidify into an igneous rock. A rock that cools within the Earth is called intrusive or plutonic and will cool very slowly, producing a coarse-grained texture. As a result of volcanic activity, magma (which is called Lava when it reaches Earth's surface) may cool very rapidly while being on Earth's surface exposed to the atmosphere and are called extrusive or volcanic rocks. These rocks are fine-grained and sometimes cool so rapidly that no crystals can form and result in a natural glass, such as obsidian. Any of the three main types of rocks (Igneous, Sedimentary, and Metamorphic rocks) can melt into magma and cool into igneous rocks. Rock masses of igneous origin have no sooner cooled than they begin to change. The solids with which the magma is charged are slowly dissipated, lava flows often remain hot and steaming for many years. These gases attack the components of the rock and deposit new minerals in cavities and fissures. The zeolites are largely of this origin. Even before these "post-volcanic" processes have ceased, atmospheric decomposition or weathering begins as the mineral components of volcanic and igneous rocks are not stable under surface atmospheric conditions. Rain, frost, carbonic acid, oxygen and other agents operate continuously, and do not cease until the whole mass has crumbled down and most of its ingredients have been resolved into new products or carried away in aqueous solution. In the classification of rocks these secondary changes are generally considered unessential rocks are classified and described as if they were ideally fresh, though this is rarely the case in nature.
|
Rock cycle Subcategories
Rock cycle Articles
|
|
