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


back-arc basinBalticabanded (ribbon) veinbasaltbasaltic brecciabatholithblended unconformityboudin (vein) ▪ boudin, boudinagebrecciabysmalith

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back-arc forearc

Subduction zone associated with trench, accretionary prism, forearc basin, magmatic front, and back-arc basin at spreading axis. Courtesy of zyzzy2Back-arc basins are associated with tensional forces caused by asymmetric seafloor spreading and oceanic trench rollback at some convergent plate boundaries.

Back-arc basins develop where island arcs are split longitudinally, roughly along the line of the magmatic axis, forming a rift that matures to the point of seafloor spreading, thus allowing a new magmatic arc to form on the trenchward side of the basin. This division strands a remnant arc on the side of the basin away from the trench and subduction zone, and the remnant arc shifts away from the arc axis as it reforms.

Most of the sediment that reaches back-arc basins originates in the active magmatic arc. Back-arc basins usually spread for a few tens of millions of years, then spreading ceases, converting the spreading back-arc basin to a fossil back-arc basin or marginal basin.

The Okinawa Trough is a backarc basin lying between Japan and Taiwan, created by extension within the continental lithosphere behind the Ryukyu trench-arc system. The Okinawa Trough is at an early stage of evolution from arc type to backarc activity [s].

Forearc and backarc basins associated with subduction zones and volcanic island arcs. Courtesy USGSForearc basins are sea floor depressions located between subduction zones and their associated volcanic arc.

Forearc basins receive sediments from the adjacent landmass, the island arc system, and trapped oceanic crustal material. Oceanic crustal fragments may be obducted onto the continent as ophiolites complexes during terrane accretion.

The Central Valley of California developed as a forearc during the late Cretaceous and early Paleogene.

[links: images: maps: swUS in the Late Triassic (215 Ma) as the Cordilleran arc developed, a new back arc basin formed behind the McCloud arc (wp, contrasting hypothesis), by early Cretaceous, the Cordilleran arc had converted to a classic continental (Andean-style) arc comprising a fore arc-trench system, fore arc basin, and Andean arc. (The fore arc-trench was site of famous Franciscan mélange). The Great Valley sequence was deposited in the fore arc basin and the Sierra Nevada batholith complex formed within the magmatic arc; Okinawa trench; diagrams: simple diagram of features of an intra-oceanic island arc subduction zone; the Taigonos segment of the Uda-Murgol Island Arc, and Pekulney segment of the island arc, Russia (wp); hypothetical x-c of Great Slave Craton (wp); webpages: Geological History of the western US: swUS: Precambrian, Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, Permian, Triassic, Jurassic, Cretaceous, Tertiary]

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ancient continent of Baltica (green) in relation to other continental land masses, 550 MaThe continent of Baltica (green) existed from the Late Proterozoic to the Early Palaeozoic and included what is now the East European craton of Northwestern Eurasia. Baltica was created as an entity not earlier than 1.8 Ga. Before this time, the three segments/continents that now make up the East European craton were in different places on the globe.

Laurentia is shown in red, Siberia in peach, embryonic Gondwana is yellow, Australia-East Antarctica grey.

The Baltic Shield (Fennoscandian Shield) now forms the continental core of Europe. The shield is composed of the oldest Precambrian crystalline rock in Europe (Archaean and Proterozoic gneisses and greenstone deformed by rounds of ancient tectonic activity.)

The tectonically stable shield region was unaffected by the Caledonian, Hercynian, and Alpine orogenies, though mountains rose at shield margins. The Baltic Shield is exposed in Finland, Sweden, and Norway as a result of scouring by continental ice sheets during the Pleistocene epoch.

[ more on Baltic Shield ]

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Basalt is a hard gray or black, mafic igneous volcanic rock that is usually fine-grained due to rapid cooling of lava, though it contain larger crystals in a fine matrix (porphyritic), be vesicular, or be a frothy scoria.

aa flows over ropey pahoehoe in Hawaii Basalt magmas form by decompression melting of peridotite in the mantle. The crustal portions of oceanic tectonic plates comprised predominantly basalt, derived from upwelling peridotite in the mantle below ocean ridges. The basalt shield volcanoes of the Hawaiian island chain sit above a mantle plume, or 'hot spot'. (left - click to enlarge - aa flows over ropey pahoehoe in Hawaii - image courtesy of USGS.)

Basalt is TAS classified according to the relationships between the combined alkali content and the silica content. Basalt typically containts a preponderance of calcic plagioclase feldspar and pyroxene; olivine can also be a significant constituent. Accessory minerals include iron oxides and iron-titanium oxides, providing basalt with a paleomagnetic signature.

Phaneritic, shallow intrusive igneous rocks with a basaltic composition are generally referred to as dolerite (also called diabase) or gabbro.

top-down: basaltic lava; lava field; flow-lines in basalt formation; close-up of vesicular basalt with olivine crystals; surface of basalt hand specimen; basalt columns(image left - click to enlarge - courtesy USGS - top-down: basaltic lava; lava field; flow-lines in basalt formation; close-up of vesicular basalt with olivine crystals; surface of basalt hand specimen; basalt columns.)

[images - roll-over link for preview (where available); large images (well worth a visit) show only as a corner on preview : water-sculpted basalt at Fossil Falls in Yosemite : Basalt Fall unterhalb des Hengifoss, basalt columns, Dverghamrar basaltic columns in Iceland, 2 : cliff of basalt columns : Columbia River basalts, Catherine Creek arch in Miocene columnar basalts : flowing curves of basalt entablature in Yellowstone : basalt columns Armenia : basalt field : basalt and sandstone : 3.7 Ga moon-rock basalt : hand-specimen : hand-specimen vesicular basalt, vesicular basalt with olivine phenocrysts, 2 : hand-specimen diabase : hand-specimen diabase porphyry : hand-specimen diorite : hand-specimen gabbro : hand-specimen scoria : thin-section basalt, 2, 3; thin-sections moon basalts ]

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Half Dome in the 40,000 sq km Sierra Nevada Batholith in Yosemite.Batholiths are complex intrusive bodies composed of plutonic igneous rocks, usually of felsic or intermediate rock-types, such as granite, quartz monzonite, or diorite. Batholiths are also called granite domes.

The exposed surface area is defined (by geographers) as more than 100 square kilometers, though large geological batholiths may have less exposure. Areas smaller than 100 square kilometers are called stocks. The entire batholithic emplacement is typically so large that the bases are rarely exposed, and batholiths have steeply inclined walls that form prominent dome structures when exposed by erosive removal of initially overlying rocks.

Sometimes batholiths arise through several smaller diapiric intrusions (plutons) and have a complex history of magmatic intrusion and crystallization at depths of 5 to 30 kilometers. Batholith formation is commonly associated with lithospheric plate boundaries, where tectonic interactions between plates are associated with large scale melting of crustal rocks and the formation of deep magma chambers. As erosion uncovers the crystalline rock that formed at great depth, crystal structures respond to the decrease in load and expand, rendering the plutonic rocks susceptible to exfoliative weathering.

Half Dome in the 40,000 sq km Sierra Nevada Batholith in Yosemite.Spheroidal weathering produces boulder fields, and sheet-like exfoliative weathering, which is accelerated by frost wedging, creates smooth structures like Half Dome in the 40,000 sq km Sierra Nevada Batholith in Yosemite (upper left, right - click to enlarge). The Coastal Plutonic Complex of British Columbia and Alaska is even larger, extending for 1,800 kilometers and covering 182,500 sq km. Other North American batholiths include the following: The Idaho Batholith is a composite mass of granitic plutons covering approximately 15,400 square miles in central Idaho [map, Castle Peak, Contact Batholith]. Guichon batholith in British Columbia contains several large, low-grade copper deposits. The South Mountain Batholith (SMB) of southwestern Nova Scotia is the largest granite batholith in the Appalachian Orogen with an approximate area of 7300 sq km.

The Cornubian Batholith was intruded into south-west England at the close of the Variscan Orogeny (late Carboniferous - Permian).

[images: photos: Baja Batholith spheroidal weathering, Boulder Batholith & spheroidal weathering, Conrnubian batholith Cligga Head, Enchanted Rock batholith & tree at summit, Golden Horn Batholith, Rocky Mtn. Nat Park, Yosemite batholith, batholith, outcrop Granite Mountain; websites: Avalonia, Halifax Pluton, Boulder Batholith, Cornubian Batholith, Peninsular Ranges batholith, Sierra Nevada Batholith & lithography; lithography: conness contact, Enchanted Rock granite, 2, Julianehåb batholith granite, gallery, mingling of microdioritic and granodioritic magmas in the calc-alkaline Pataz Batholith of the northern Peruvian Cordillera Oriental, schlieren banding in the Sausfjellet pluton, rock gallery including Vioolsdrif Batholith; maps: Boulder Creek Batholith, Cornubian Batholith, Enchanted Rock Batholith, Halifax Pluton, Idaho batholith ecoregions, Verkhisetsk batholith, batholiths in Wisconsin; diagram: plutons and volcanic landforms

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typical example of shear boudinage in deformed jasperoid in sheared basalt, Fortnum Gold Mine, Australia. Courtesy of Roland GotthardBoudinage refers to structures deformed by extension in ductile shear zones. Boudinage structures contain a rigid tabular body that has been stretched and deformed where embedded within more deformable (less competent) rocks.

Banded Skagit gneiss with dike of granite orthogneiss; competent banded gneiss is boudinaged by ductile shearCompetent tabular bodies that are susceptible to boudinage include veins and strata such as sandstones. Where conditions favor brittle fracture rather than ductile deformation, imbricate (overlapping) fracturing occurs.

In boudinage, the competent bed break ups into sausage-shaped boudins – forming structures such as ribbon-like boudins or chocolate-tablet boudins (depending upon the axis and isotropy of extension).

boudinaged quartz vein in shear foliation, Starlight Pit, Fortnum Gold Mine, Western Australia. Courtesy of Roland Gotthard.
[links: images: Amphibolite boudins in gneisses; formations: Zoroaster Veining, boudins composed of quartz and plagioclase, boudin of metagabbro (HP mafic granulite) in tonalitic gneiss]

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Bowen's Reaction Series

N. L. Bowen, experimenting on the order in which minerals crystallized from a melt, determined a reaction series for minerals:

discontinuous -------- continuous -------most refractory / least stable at surface
---olivine -------------Ca-rich feldspars------------------mafic
-----pyroxenes -.-------/
---------\------------ plagioclase
-------amphiboles -.--/
------------\------ Na-rich feldspars ------------------intermediate
-- ------------quartz---------------------------------------felsic
--------------------------------------------most fusible / most stable at surface

The feldspars crystallize in a continuous series with the calcium rich endmembers cystallizing out of a melt earliest, the sodium rich members crystallizing later, and the orthoclase feldspar endmember cystallizing last. In the discontinuous branch, the nesosilicate olivine constructed of simple [SiO4]4- tetrahedra crystallizes first, followed in sequence by single-chain pyroxenes, amphiboles, sheet micas, and 3D-framework quartz.

Minerals, such as refractory olivine, which crystallize out of melts at depth are least stable and most prone to weathering, at Earth's surface. Conversely, highly fusible quartz is very stable at the surface.

See magmatic differentiation.


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basalt breccia with epidote groundmassBreccia is a are clastic, sedimentary rocks comprising angular fragments from a previous rock structure that have been cemented in a matrix.

Classification of breccias relates to their constituents, mode of occurrence, constituent fragment size, types of clasts, and source of clasts.
image: basalt breccia with epidote groundmass, courtesy of Siim Sepp

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Bysmaliths are more or less vertical and cylindrical bodies that crosscut (discordant) adjacent sediments and are bounded by steep faults. Bymaliths are commonly associated with the mountain-building (orogenic) processes, and they are typically composed of granites or granodiorites.

Bysmaliths are considered to be conical or cylindrical laccoliths. They develop when highly viscous magma is injected into strata. Because lateral spreading along the bedding is limited by viscosity, the magma moves upward to form the cylindrical shape. Overlying rock layers are fractured. The walls of bysmaliths slope away from each other with depth, which makes their diameter increasingly large at greater depths.

[image Black Mesa bysmalith (Henry Mountains, Colorado Plateau, Utah), close-up of Wolf porphyry, Mixes Baldy-Anderson Peak Bysmalith close to the Blankenship Divide; ref, ref 2, gallery, Henry Mountains Wilderness]

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