Definitions and images to illustrate geological terms, links to images and website articles


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Boudinaged quartz vein in shear foliation, Starlight Pit, Fortnum Gold Mine, Western Australia. Courtesy of Roland Gotthard.In geology, a vein is often defined as a long, regularly shaped occurrence of an ore (lode). However, more generally, a vein is a finite volume within a rock, which is filled with crystals of minerals precipitated from an (aqueous) fluid. The mineral-laden fluids, often of hydrothermal origin, circulated hydraulically before depositing the minerals by open-space filling or crack-seal growth.

Open space filling occurs at relatively low pressure epithermal vein systems, such as stockworks, in greisens, or in some skarn environments. Crack-seal filling occurs at higher pressures, with reopening of the vein fracture by progressive deposition of minerals on the growth surface.

In a process called boudinage, veins may be pinched and distorted into sausage-shaped bodies called boudins. (image above left - click to enlarge - boudinaged quartz vein in dextral shear foliation, Fortnum Gold Mine, Western Australia.)

An accretion vein is formed by the repeated filling of channels, followed by their opening by pressure-related fracturing in the zone undergoing mineralization.

An asymmetrical vein is a crustified vein of geologic material with unlike layers on each side.

A banded, or ribbon vein is composed of layers of different minerals lying parallel to the walls.

Barren vein matter, or a pinch in a vein, assumed to overlie an ore is called a cap rock.

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Volatiles found in varying amounts in nearly all wall rocks and magmas figure prominently in the magmatic differentiation process of assimiliation. These volatiles include CO2, SO2, O2, Cl2, and particularly H2O.

Water is available in wall rocks of the mid-crust, both as free water and within the hydrated minerals commonly found at depths. Some assimilated water enters hydration reactions with predominantly anhydrous melt components, but most water simply accumulates in the ever-shrinking, residual silicate melt. When a melt has taken on sufficient water, that magma will develop a water-saturated silicate fraction and a separate water-based fluid phase.

Under some conditions, water-saturated silicate fractions can release a whitish fine-grained vein-filling slurry of quartz and feldspars termed aplite. The water-based phase easily assimilates trace elements that do not accommodate well into most silicate crystals. These trace elements include beryllium, lithium, niobium, tantalum, tin, uranium, thorium, tungsten, zirconium and the rare earths. Ore deposits can form when hot, pressurized, mineral-laden hydrous fluid permeates fractured country rock and cools into veins of pegmatite—an very coarse-grained igneous rock containing megacrysts of quartz, feldspars, and sometimes, highly prized minerals. Pegmatite and aplite dikes and veins are common around intrusions.

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diagram of stratovolcanobasaltic lava flowThe term 'volcanism' refers to volcanoes and volcanic processes.

Magma is molten (igneous) rock formed when Earth's radioactivity heats rocks to high temperatures – the deeper in the Earth, the greater the temperature.

When melted rock flows at the surface it is called lavaHawaii's and Iceland's basalt lavas fountain (below center) or flow (above right) freely, while other lavas are sticky and explosive, producing deadly pyroclastic flows (nuee ardentes, such as that which killed victims at Pompeii) and towering clouds (below right, below left; pyroclastic flow, Mount St. Helens, 2, Vesuvius, Pinatubo, Soufrière Hills Volcano on Montserrat, Ischia deposits). .


..Plinian eruption of Klyuchsvskaya in Russiapyroclastic cloud above erupting andesitic stratovolcanolava fountain and lava flow, Kilauea, Hawaii

[links: images: webpages: Vesuvius victims 'died instantly' ]

Video The Ring of Fire

images : USGS : top right, schematic of stratovolcano; top left, basaltic lava; bottom right, Plinian eruption of Mt. Klyuchsvskaya in Russia; lava fountain and lava flow, Kilauea, Hawaii; pyroclastic cloud above erupting andesitic stratovolcano.

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