greenstone belts

Greenstone belts are zones named for the green hue imparted by chlorite (left) minerals within the rocks, which are variably metamorphosed (greenschist) mafic to ultramafic volcanic sequences associated with sedimentary rocks. Greenstone belts are found between granite and gneiss bodies in Archaean and Proterozoic cratons.

Greenstone belts are interpreted as having formed at ancient oceanic spreading centers and island arc terranes.

◙ greenstone-hosted quartz-carbonate vein deposits ◙ GQCV

Shield greenstones are located in:
● Slave craton of Canada (Archaean and Proterozoic)
● Pilbara craton of Australia (Archaean)
● Barberton greenstone belt in Kapvaal craton of southeastern Africa (Archaean)
● western Africa (Archaean and Proterozoic)
● Madagascar (Archaean and Proterozoic)
● Isua greenstone belt of southwestern Greenland (Archaean)
● shield area in Brazil (Archaean)
● Baltic craton of Scandinavia and the Kola Peninsula (Archaean and Proterozoic).

[links: images: hand-specimens: greenschists; 3.5 Ga komatiite from Lower Onverwacht Group of Barberton Greenstone Belt; formations: greenschists; Storø shear zone; geological maps: world distribution of world class greenstone-hosted quartz-carbonate vein deposits; Canadian greenstone-hosted quartz-carbonate vein districts; northeast-trending "greenstone" belts of ancient gneiss alternating with belts of metamorphosed volcanic and sedimentary rocks in Minnesota and Canada, Superior Province greenstone belts; x-c, and diagramatic model of northeast-trending belts most likely result from structural deformation of the crust following the formation of the volcanic and sedimentary rocks; diagrams: greenstone belts; model for deposition of volcanic and sedimentary rocks; setting of greenstone -hosted quartz-carbonate vein deposit]

volcanism

The 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 lava – Hawaii'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). .

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[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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