GEOLOGICAL PROCESSES

Geological process - (geology) a natural process whereby geological features are modified.

The Above Figure 2  is an overview of geological/geochemical cycle.

1. IGNEOUS PROCESSES
Pressure and temperature within the earth increase gradually with distance below the surface. At depths of several tens of miles the material which makes up the earth's crust (depending upon its composition) may become partially molten. This molten material is MAGMA, and is normally less dense than the overlying solid rock. As the magma is subjected to tectonic forces, it may be squeezed upward along weak zones in the crust. This process by which magma penetrates the crustal rock is called INTRUSION. The intrusive magma which has cooled and solidified is known as IGNEOUS ROCK (granite, granodiorite, diorite, gabbro).
Some magma reaches the surface of the earth and pours out, explosively or slowly, from volcanoes or fissures in the earth's crust in a process known as EXTRUSION. A magma which pours out over the surface is called LAVA, as long as it is molten, and VOLCANIC ROCK when solidified. When the magma erupts rapidly and explosively into the air, it is fragmented into small particles which solidify into VOLCANIC ASH. Larger particles called CINDERS, VOLCANIC BOMBS, and angular rock fragments comprising BRECCIA and AGGLOMERATE. When the ash settles to the ground and solidifies it may in time be compacted into TUFF, or if welded by contained heat in GLOWING AVALANCHES, it is called IGNIMBRITE.
Heat source for the formation of magmas may be residual heat from the earth's formation, heat generated by tectonic movements, or 'hot spots' from local tectonic heating and/or concentrations of radioactive materials.
2. WEATHERING AND EROSION
Decomposition and disintegration of rocks at or near the surface by physical and chemical processes is called WEATHERING. The products of weathering are normally carried off by EROSION. Water percolating into the ground dissolves some minerals from the rocks and forms acidic or basic solutions which further attack the rock and break it down chemically.
The action of groundwater is selective under certain conditions, and may leave some elements or minerals in place while altering or leaching others. The weathering and subsequent erosion of aluminum or nickel bearing rocks may leave residual or secondary deposits (LATERITES) of materials rich in these metals.
Downward slope movement of soils and rock fragments impelled by gravity, MASS WASTING, is an important part of erosion. Decomposed rock materials are transported by wind (as sand and dust), by water (in streams, rivers, and ocean currents) and by ice (in glaciers).
3. SEDIMENTATION
The process of sedimentation entails the physical and/or chemical movement of the products of weathering to sites of deposition -- through chemical precipitation, evaporation or physical settling. The accumulated weight of sediments, over time, results in compaction and chemical action (DIAGENESIS) that may cement particles together so that sediments are lithified and converted into SEDIMENTARY ROCKS (shale, sandstone, limestone, etc.). Sediments are usually deposited in nearly horizontal layers. Changes in character and rates of deposition may cause development of distinct planes called BEDDING PLANES, which separate the sedimentary layers.
4. METAMORPHISM
Heat and fluids emanating from magma may alter the adjacent rock creating a halo of CONTACT METAMORPHIC ROCKS. Rocks which become deeply buried are subjected to heat and pressure, which with the aid of contained or introduced fluids, and tectonic forces are metamorphosed. Original bedding may be destroyed and a FOLIATED structure developed.
5. TECTONIC PROCESSES
The earth is a dynamic body, undergoing constant movement of both continental and oceanic crust, driven by convection currents and readjustments in the earth's mantle. Different portions of the earth have, at different times, been uplifted above or depressed beneath the seas. Other areas have crumpled rock layers into FOLDS, FAULTS, OVERTHRUSTS, while other areas have been pulled apart forming RIFTS, HORSTS AND GRABENS. The contact between drifting continental masses and spreading oceanic crust is particularly subject to deformation and intrusive/volcanic activity resulting from the SUBDUCTION of the oceanic crust under the continental crust at CONVERGENT MARGINS. Where DIVERGENT MARGINS occur, RIFTS form in the MIDOCEANIC RIDGE (Figure 2).
6. MINERAL DEPOSITION
Groundwater solutions (METEORIC WATER), and/or solutions emanating from a cooling magma (HYDROTHERMAL FLUIDS), and/or fluids ejected from compressing sediments (POREWATER or CONNATE WATER) penetrate along fractures and tiny pore spaces between mineral grains in the rock. Under certain conditions these solutions, which contain different compounds, may dissolve, deposit other minerals, or alter the rock-forming minerals (ROCK ALTERATION).
Leaching, or the decomposition, dissolution and removal of soluble minerals from the rocks may be caused by hydrothermal or meteoric solutions. The dissolved mineral products may be dispersed or re-deposited elsewhere as the same or different minerals, leaving often greater concentrations (ENRICHED). For example, relative content of gold may be increased by the removal of originally associated minerals, leaving an enriched residual gold deposit. And, portions of a copper sulfide deposit in the weathered zone may become enriched through the leaching of the copper content from the surface rocks by downward percolating meteoric waters and re-deposition at the water table.