Utente:Mirandolina/Prove

Contents [hide] 1 Eclipses in the Earth-Moon system 1.1 Types of eclipse 1.2 Eclipse phases 1.3 General phases of a solar eclipse 1.4 Local phases of a solar eclipse 1.5 Phases of a lunar eclipse 1.6 The eclipse in mythology 2 Eclipses elsewhere in the solar system 3 See also 4 External links

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[edit] Types of eclipse

1999 Total solar eclipse seen from the Mir space stationThe most dramatic eclipses visible from Earth are:

Lunar eclipses - the Earth obscures the Sun, from the Moon's point of view. The Moon moves through the shadow cast by the Earth. This can only happen at full moon. Solar eclipses - the Moon occults the Sun, from the Earth's point of view. The Moon casts a shadow that touches the surface of the Earth. This can only happen at new moon. Eclipses can be divided into different types:

The ratio between the apparent sizes of the eclipsing body and that of the luminary is called the magnitude of the eclipse. For solar eclipses, the ratio varies around 1, being sometimes more than 1, sometimes less. For lunar eclipses, the magnitude is much larger than 1; they never appear annular (viewed from the Moon).

[edit] Eclipse phases These were used in occult ceremonies.[citation needed]

[edit] General phases of a solar eclipse The general eclipse begins when the Moon's penumbra cone starts to sweep across the Earth's disc. The total or annular eclipse begins when the Moon's umbra starts to sweep across the Earth's disc. The centrality begins when the axis of the Moon's shadow cone starts to sweep across the Earth's disc. The eclipse's maximum occurs when the terrestrial surface within the umbra reaches its largest area. The centrality ends when the axis of the Moon's shadow finishes its sweep across the Earth's disc. The total or annular eclipse ends when the Moon's shadow finishes its sweep across the Earth's disc. The general eclipse ends when the Moon's penumbra finishes its sweep across the Earth's disc.

The French 1999 eclipse[edit] Local phases of a solar eclipse First contact (also called first exterior contact) is the instant when the Moon's disc starts to cover the Sun's. Second contact (also called first interior contact) is the instant when the Moon's disc is entirely surrounded by the Sun's (for an annular eclipse) or the instant when the Sun's disc disappears completely behind the Moon's (for a total eclipse). Third contact (also called second interior contact) is the instant when the Moon's disc starts to come out of the Sun's (for an annular eclipse) or the instant when the Sun's disc reappears from behind the Moon's (for a total eclipse). Lastly, fourth contact (also called second exterior contact) is the instant when the Moon's disc clears the Sun's. [edit] Phases of a lunar eclipse There are three types of lunar eclipses: penumbral, when the Moon crosses only the Earth's penumbra; partial, when the Moon crosses partially into the Earth's umbra; and total, when the Moon crosses entirely within the Earth's umbra.

The progression of a lunar eclipseFirst contact (also called first exterior contact) is the instant when the Moon starts to enter into the Earth's umbra. Second contact (also called first interior contact) is the instant when the Moon enters completely into the Earth's umbra. This is the beginning of totality. The maximum of the eclipse occurs when the angular distance between the centre of the Moon's disc and the centre of the shadow cone is at its smallest value. Third contact (also called second interior contact) is the instant when the Moon starts to come out of the Earth's umbra. This is the end of totality. Lastly, fourth contact (also called second exterior contact) is the instant when the Moon clears the Earth's umbra completely. [edit] The eclipse in mythology

Han Dynasty CarvingBefore modern astronomy arose there were long-standing explanations for eclipses in many cultures. These would typically involve conflicts between mythic forces. For example, in Hindu mythology, the two demons Rahuand Ketu were believed to be the cause of eclipses.However Aryabhata gave an accurate explanation of the eclipse in his scientific treatise Aryabhatiya dated 499 AD .

Similarly in China, at the Imperial observatory in Beijing, is a carved stone with the following explanation:

"This carved stone chart explained the cause of solar eclipses. The center of the golden bird (the symbol of the sun) was covered by the toad (the symbol of the moon). The people of the Han Dynasty called the phenomenon a good combination of the sun and the moon." In this explanation we see a recognition of the celestial realities and a cheerful outlook regarding the event. In other cultures an eclipse could be both a surprising and a terrifying event.

[edit] Eclipses elsewhere in the solar system

A picture of Jupiter and its moon Io taken by Hubble. The black spot is Io's shadow.Eclipses are impossible on Mercury and Venus, which have no moons.

On Mars, only partial eclipses are possible, because neither of its moons is large enough to cover the Sun's disc. Martian eclipses have been photographed from both the surface of Mars and from orbit. See Transit of Phobos from Mars and Shadow of Phobos on Mars.

The gas giants, which have many moons, frequently display eclipses. The most striking involve Jupiter, which has four large moons and a low axial tilt, making eclipses more frequent. It is common to see the larger moons casting circular shadows upon Jupiter's cloudtops.

Pluto, with its large moon Charon, is also the site of many eclipses.

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GrassettoLava From Wikipedia, the free encyclopedia Jump to: navigation, search For other uses, see Lava (disambiguation). Look up lava, Aa, pahoehoe in Wiktionary, the free dictionary.Lava is molten rock expelled by a volcano during an eruption. Magma is molten rock below the earth's surface. Lava, when first exuded from a volcanic vent, is a liquid at temperatures from 700 °C to 1,200 °C (1,300 °F to 2,200 °F). Although lava is quite viscous, about 100,000 times the viscosity of water, it can flow great distances before cooling and solidifying.

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10 m high fountain of lavaContents [hide] 1 Composizione della lava 1.1 comportamento della lava 1.2 Duomi di lava 1.3 Sheeted flows 1.4 I torrenti di lava 1.5 'A'ā 1.6 Pāhoehoe 1.7 lava a cuscini o cuscini di lava 2 Le trasformazini del paesaggio dovute alla lava 2.1 i vulcani 2.2 Cinder and splatter cones 2.3 I duomi di lava 2.4 I tubi di lava 2.5 Le cascate e le fontane di lava 2.6 I laghi di lava 3. Composizione delle rocce vulcaniche 4 Unusual lavas 5 Hazards 6 Città distrutte dalle colate laviche 7 Città perzialmente distrutte dalle colate laviche 8 Collegamenti esterni

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Composizione della lava

In genere la composizione della lava determina il suo comportamanto più della temperatura che essa possiede al momento della eruzione. Le rocce ignee possono essere classificate in tre tipologie a seconda della loro composizione chimica:

• felsiche o acide se contengono il 60-70% di tetraedri silicato;
• intermedie o neutre se contengono il 50% circa di tetraedri silicato;
• mafiche o basiche se contengono meno del 40% di tetraedri silicato.

Sebbene queste tipologie dipendano dalla composizione chimica, vi è una stretta correlazione anche con la temperatura del magma, con la sua viscosità e le modalità di eruzione.

Le lave felsiche come le rioliti e le daciti sono spesso associate ad eruzioni di tipo stromboliano; sono molto viscose. Infatti la composizione del magma è rappresentata da elevate quantità di silicio, alluminio, sodio e calcio che formano un fluido ricco di feldspati e quarzo,che è molto più denso delle altre varietà di magma. I magmi felsici, al momento della eruzione, hanno una temperatura intorno ai 650-750 °C, sebbene talvolta possano essere più caldi.Le lave scorrono lentamente e solidificano formando strutture bulbose. Spesso si solidificano prima di allontanarsi dal condotto vulcanico: in questo caso formano cuspidi che occludono il condotto vulcanico., formando strutture bulbose. Quando solidificano all'interno del condotto.

Le lave intemedie hanno un più basso tenore di alluminio e biossido di silicio, e sono più ricche di magnesio e ferro. Esse formano duomi andesitici e "sheeted flows"; sono generalmente associate ad eruzioni di tipo stromboliano e formano strato-vulcani (composite vulcanos); derivano da magmi con temperature fra i 750 ed i 950° C e sono meno viscose perchè il calore tende a distruggere i legami polimerici, a rendere le lave più fluide e con maggiore tendenza a formare fenocristalli. La maggiore quantità di ferro e magnesio dà alla lava un colore più scuro e provoca anche occasionalmente la formazione di anfiboli e pirosseni.

Le lave mafiche sono caratterizzate da un alto contenuto di basalto, che generalmente proviene da magmi che hanno temperature maggiori di 950° C. Hanno un notevole contenuto di ferro e magnesio e sono relativamente povere di alluminio e biossido di silicio, e ciò riduce il grado di polimerizzazione del composto.A causa dell'alta temperatura, la viscosità può essere relativamente bassa, sebbene diverse migliaia di volte maggiore di quella dell'acqua. Il basso grado di polimerizzazione e l'alta temperatura favoriscono la diffusione chimica, cosicchè nelle lave mafiche si possono formare grandi fenocristalli ben formati. I vulcani che si formano da questo tipo di magma sono i vulcani a scudo perchè la lava molto fluida forma colate molto estese.

Le lave ultramafiche, come komatiite e magmi ad alto contenuto di magnesio che formano la boninite, portano la composizione e la temperatura all'estremo. Le Komatiiti contegono più del 18% di ossido di magnesio ed hanno temperature anche fino a 1600° C. A queste temperature non avviene la polimerizzazione dei componenti minerali e si forma un materiale liquido con una viscosità bassa paragonabile a quella dell'acqua. La maggior parte, anche se non tutte le lave ultramafiche, risalgono al Proterozoico , con una piccola quantità di magmi ultramafuici noti che risalgono al Fanerozoico.Non si conoscono lave komatiite moderne, perchè il mantello terrestre si è raffreddato troppo per produrre magmi ad alto contenuto di magnesio.

Il comportamento della lava

La viscosità della lava è importante perchè determina il suo comportamento. Le lave con alta viscosità hanno le seguenti caratteristiche:tendono a scorrere lentamente e formano blocchi semi-solidi che non scorrono. Tendono ad intrappolare gas, che formano bolle nella roccia appena raggiungono la superficie. Sono correlate ad eruzioni espolsive ed associate a "tuff" e flussi piroclastici.Le lave molto viscose non scorrono come scorrono i liquidi ed usualmente formano depositi di lapilli e "cinder". Tuttavia, una lava viscosa privata dei gas o che ha una temperatura più alta della media può formare una colata di lava. Le lave viscose presentano due forme di eruzione non-piroclastica: i duomi lavici e "sheeted flows".

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Le lave poco viscose tendono a scorrere facilmente, formando "puddles", canali e fiumi di roccia fusa. Rilasciano facilmente bolle di gas appena esse si formano. Le eruzioni sono raramente piroclastiche e sono usualmente tranquille. I vulcani prendono la forma di "rifts" non steep conici. Ci sono tre forme di flussi lavici a bassa densità:'a'ā, pāhoehoe e lave a cuscini.Questi sono descritti nel paragrafo seguente. Naturalemente le lave possono avere altri componenti, spesso includono cristalli già formati di vari minerali, frammenti di rocce "estranee" noti come "xenoliti" e frammenti della stessa lava già solidificati.

Duomi lavici=

Talvolta, quando in un vulcano del magma acido risale attraverso il "camino vulcanico" non viene eruttato completamente all'esterno, ma la lava si solidifica formando un "duomo lavico", cioè si forma una struttura a cupola con fratture,fessurazioni e talvolta può eiettare "cHUNKS" FREDDI e "RUBBLE". La cima ed i margini del duomo possono essere ricoperti da frammenti di roccia, brecce e ceneri . Esempi di questo tipo asi possono trovare nel duomo "Novarupta e nei successivi duomi del mont St Helens.

Sheeted flows

Sono esempi non comuni di fenomeni eruttivi che si riscontrano in vulcani caratterizzati da magmi felsici o intermedi.La pressione dei gas contenuti nel magma tende a provocare una eruzione piroclastica ed esplosiva. Tuttavia una lava viscosa può scorrere, anche se emolto lentamente , sulla superficie della Terra.La lava solidificandosi nella parte superiore della colata, forma allora delle strutture laminari,o "sheeted" che hanno la parte superiore ed i margini di roccia all'interno delle quali la lava densa e viscosa continua a scorrere. La spessa parte superiore forma una breccia ignea chiamata "autobreccia",as the flow creeps along, churning il margine più esterno a parte. Questo è simile alle lave aa con la differenza che la lava sottostante mostra segni evidenti di stiramenti, deformazioni plastiche e laminazioni. Esempi di sheeted o laminari flows sono gli edifici vulcanici dell'Era terziaria nelle "glasshouse" montagne e le rocce del "Kangaroo Point" presso Brisbane in australia.

=== i "Lava flows" su roccia preesistente. La lava spesso scorre su rocce preesistenti. A causa del riscaldamento , queste roscce tendono a "warp" o metamorfosare.quando il flusso, lavico si interrompe e viene coperto da sedimenti col passare dei milioni di anni, questo strato appare diverso dalle rocce sottostanti, perchè è di natura magmatica e non sedimentaria.Spesso questi flussi lavici generano errori per SillS perchè anche i i sills sono paralleli alla roccia preesistente. Tuttavia essi sono intrusioni magmatiche e si formano dopo le altre rocce sebbene formino il, margine "baked" su entrambi i lati, " as opposed to a lava flow which only does on one side.

Lava 'A'ā

Il nome deriva dalla trsposizione in inglese del vocabolo hawaiano che significa ""stony with rough lava", ma anche "bruciato" o "blaze".é caratterizzato da una superficie rough o rubbly composta da blocchi di lava frammentati chiamati "clinker".La loose, rotta, sharp e spiny superficie di una lava 'A'ā rende difficile caminarvi sopra. La superficie ricopre un cuore denso e massicico, che è stato la parta più attiva della colata. Mentre la lava pastosa scorreva lungo il declivio, i cklinker erano trasportati sulla superficie. Lungo il bordo anteriore di un 'A'ā tuttavia questi frammenti già solidificati, precipitavano davanti al fronte della colata ed erano sepolti dal flusso che avanzava.Ciò dà origine ad uno strato di frammenti di lava sia in fondo che in cima ad un 'A'ā . sugli ?A'a si possono trovare sfere di lava con diametro fino a 3 m. A'ā è usualmente caratterizzato da una maggiore densità rispetto ad un pāhoehoe. pāhoehoe può trasformarsi in un 'A'ā se il flusso lavico ha trovato qualche ostacolòo sul suo corso o un declivio con maggiore pendenza. la superficie liscia e angled dei aa lo rende perfettamnente riflettente al radar e può essere visto da un satellite orbitante (brillante mnelle foto fatte da Magellano)

Pāhoehoe

Trasposizione inglese del vocabolo Hawaiano che significa "smooth" lava è alva basaltica con superficie ondulata o smooth, billowy, undulating, or ropy surface.queste caratteristiche sono dovute al movimaento della lava molto fluida sotto una crosta solidificata. Un tipico flusso di lava Pāhoehoe si forma da una serie di piccol lobi e dita che continuamente fuoriesono dalla rottura della "crosta" superficiale. Si formano anche dei tubi di lava laddove la minima perdita di calore mantiene la bassa viscosiatà. L'aspetto della superficie dei Pāhoehoe varia molto, mostrando tutta una serie di forme spesso bizzarre che assolmigliano a sculture di lava. Aumentando la distanza dal punto dell'eruzione, i Pāhoehoe cambiano in 'A'a perchè diminuendo il calore aumenta la viscosità. I Pāhoehoe non hanno una superficie riflettente e sono difficilmente visibili dai satelliti orbitanti (scuri sulle foto di Magellano)

=== Lave a cuscini===. La lava a cuscini è la tipica formazione rocciosa derivata dal raffreddamento della lava in ambiente subacqueo. La lava viscosa forma, a contatto con l'acqua, una crosta solida are questa crosta si rompe e lascia uscire uleriori "bolle" di lava che nel frattempo continua ad essere espulsa con il flusso susseguente. Poichè la maggior parte della superficie terrestre è ricoperta dall'acqua, e la maggior parte dei vulcani sono situati nelle vicinanze o sotto di essa, la lava a cuscini è molto comune.

I paesaggi creati dalle colate laviche

Le colate laviche e le eruzioni sono artefici di formazioni topografiche che vanno da effettei microscopici ad effetti macroscopici. A parte le rocce, niente può resistere all'arrivo di una colata lavica: alberi, costruzioni, tutto viene travolto. Gli alberi, all'avvicinarsi di una colata alvica, si incendiano a causa del calore , e quando la lava li raggiunge, emettono schricchiolii e poi vengono sommersi dalla colata e scompaiono.Anche il mare non può fermare un torrente di lava, ma l'acqua diventa vapore ed il mare si ritira; in questo modo si formanio promontori che si estendono ad una notevole distanza rispetto alla linea di costa as the molten lava hardens into stone.anche : pun suono

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[edit] [edit] Volcanoes

Mount Fuji is a composite volcanic cone formed from basaltic andesite.Volcanoes are the primary landform created by lava eruption and consist of flattish, shallow shield volcanes formed from basalt to steeply-sided ash and lava composite volcanic cones typical of andesite and rhyolite lavas.

Volcanoes can form calderas if they are obliterated by large pyroclastic or phreatic eruptions, and such features typically include volcanic crater lakes and lava domes after the event.

[edit] Cinder and splatter cones Cinder cones and spatter cones are small-scale features formed by lava accumulation around a small vent on a volcanic edifice. Cinder cones are formed from tephra or ash and tuff which is thrown from an explosive vent. Spatter cones are formed by accumulation of molten volcanic slag and cinders ejected in a more liquid form.

[edit] Lava domes

A forested lava dome in the midst of the Valle Grande, the largest meadow in the Valles Caldera National Preserve.Lava domes are formed by the extrusion of viscous felsic magma. They can form prominent rounded protuberances, such as at Valle Calderas.

[edit] Lava tubes Lava tubes are formed when a flow of relatively fluid lava cools on the upper surface sufficiently to form a crust. Beneath this crust, which by dint of being made of rock is an excellent insulator, the lava can continue to flow as a liquid. When this flow occurs over a prolonged period of time the lava conduit can form a tunnel-like apertre or lava tube, which can conduct molten rock many kilometres from the vent without cooling appreciably. Often these lava tubes drain out once the supply of fresh lava has stopped, leaving a considerable length of open tunnel within the lava flow.

Lava tubes are known from the modern day eruptions of Kīlauea, and significant, extensive lava tubes of Tertiary age are known from North Queensland, Australia, some extending for 15 kilometres.

[edit] Lava cascades and fountains

A lava cascade in HawaiʻiThe eruptions of lava are sometimes attended by peculiarities which impart to them much additional grandeur. Instances have occurred in which the fiery stream has plunged over a sheer precipice of immense height, so as to produce a glowing cascade exceeding (in breadth and perpendicular descent) the celebrated Niagara Falls. In other cases, the lava, instead of at once flowing down the sides of the mountain, has been first thrown up into the air as a fiery fountain several hundred feet in height (see Volcanic cone).

[edit] Lava lakes Rarely, a volcanic cone may fill with lava but not erupt. Lava which pools within the caldera is known as a lava lake. Lava lakes do not usually persist for long, either draining back into the magma chamber once pressure is relieved (usually by venting of gases through the caldera), or by draining via eruption of lava flows or pyroclastic explosion.

There are only a few sites in the world where permanent lakes of lava exist. These include:

Mount Erebus, Antarctica Kilauea Volcano, Hawaiʻi Erta Ale, Ethiopia Nyiragongo, Democratic Republic of Congo [edit] Composition of volcanic rocks

ʻAʻā next to pāhoehoe lava at the 'Craters of the Moon'.The sub-family of rocks which form from volcanic lava are called igneous volcanic rocks (to differentiate them from igneous rocks which form from magma, below the surface of the earth, called igneous plutonic rocks).

The lavas of different volcanoes, when cooled and hardened, differ much in their appearance and composition. If a rhyolite lava-stream cools quickly, it can quickly freeze into a black glassy substance called obsidian. When filled with bubbles of gas, the same lava may form the spongy mineral pumice. Allowed to cool slowly, it forms a light-colored, uniformly solid rock called rhyolite.

[edit] Unusual lavas Three types of unusual volcanic rocks have been recognised as erupting onto the surface of the Earth;

Carbonatite and natrocarbonatite lavas are known from Ol Doinyo Lengai volcano in Tanzania, which is the sole example of a carbonatite volcano. Sulfide lavas have been recognised from Chile and Peru Iron oxide lavas are thought to be the source of the iron ore at Kiruna, Sweden, erupted in the Proterozoic [edit] Hazards Lava flows are enormously destructive to property in their path but generally move slowly enough for people to get out of their way, so casualties caused directly by active lava flows are rare. Nevertheless injuries and deaths have occurred, either because people had their escape route cut off, because they get too close to the flow[2] or, more rarely, if the lava flow front travels too quickly.

This notably happened during the eruption of Nyiragongo in Zaire (now Democratic Republic of Congo) on 10 January 1977 when the crater wall was breached during the night and the fluid lava lake in it drained out in less than an hour. Flowing down the steep slopes of the volcano at up to 60 miles per hour (100 km per hour), the lava swiftly overwhelmed several villages whilst their residents were asleep. As a result of this disaster, the mountain was designated a Decade Volcano in 1991[3].

Deaths attributed to lava flows frequently have a different cause, for example pyroclastic flow from a collapsing lava dome, or explosions caused when the flow comes into contact with water[4].

[edit] Towns destroyed by lava flows

Lava can easily destroy entire towns. This picture shows one of over 100 houses destroyed by the lava flow in Kalapana, Hawaiʻi in 1990.Kaimū, Hawaiʻi (abandoned) Kalapana, Hawaiʻi (abandoned) Kapoho, Hawaiʻi (abandoned) Keawaiki, Hawaiʻi (abandoned) Koaʻe, Hawaiʻi (abandoned) San Sebastiano al Vesuvio, Italy (rebuilt) [edit] Towns partially destroyed by lava flows Pompeii, Italy, in the eruption Mount Vesuvius in August 23, 79 AD Catania, Italy, in the eruption Mount Etna in 1669 (rebuilt) Goma, Democratic Republic of Congo, in the eruption of Nyiragongo in 2002 Heimaey, Iceland, in the 1973 Eldfell eruption (rebuilt) Royal Gardens, Hawaiʻi, by the eruption of Kilauea in 1986-87 (abandoned) Paricutin (village the volcano was named after) and San Juan Parangaricutiro, Mexico, by Paricutin from 1943-1952. [edit] External links Wikimedia Commons has media related to: lavaUSGS definition of ʻAʻā USGS definition of Pāhoehoe Volcanic landforms of Hawaiʻi USGS hazards associated with lava flows Hawaiian Volcano Observatory Volcano Watch newsletter article on Nyiragongo eruptions, 31 January 2002 Retrieved from "http://en.wikipedia.org/wiki/Lava" Categories: Volcanology | Igneous rocks

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Ci sono due tipi principali di margine di zolla, a seconda del movimento che compionio le zolle l'una rispetto all'altra. I due tipi sono:

• Margini conservativi, di scivolamento laterale; lo spostamento può avvenire in modo destrorso (a destra dell'osservatore) o sinistrorso ( a sinistra dell'osservatore). In questo caso non si ha distruzione o creazione di nuovo materiale litosferico, ma solo dislocazione (faglia trasforme).

Ne è un esempio la faglia di St.Andreas in California lungo la quale si hanno frequentemente fenomeni sismici e dislocazione di ampie arre di terreno.

• Margini non conservativi, nei quali si forma o si distrugge nuova litosfera.

Fra i margini non conservativi troviamo i margini convergenti ed i margini divergenti.

• • I margini divergenti sono quelli che separano due zolle che si allontanano l'una dall'altra; alcuni esempi:

1- zolla nord-Americana e Euro-asiatica; 2-sud-Americana ed Africana; 3- Africana ed arabica

• • I margini convergenti si hanno quando due zolle si avvicinano progressivamente l'un l'altra; se le zolle sono entrambe costituite da materiale della stessa densità, si avranno corrugamenti crostali, in caso contrario una zolla (la più pesante) sarà spinta sotto l'altra (subduzione) con conseguente risalita di magma (come avviene nel Sud-America in corrispondenza della cordigliera delle Ande o nel Pacifico in corrispondenza degli arcipelaghi delle Filippine e del Giappone.

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Transform boundaries occur where plates slide, or perhaps more accurately grind, past each other along transform faults. The relative motion of the two plates is either sinistral (left side toward the observer) or dextral (right side toward the observer). Divergent boundaries occur where two plates slide apart from each other (examples of which can be seen at mid-ocean ridges and active zones of rifting (such as with the East Africa rift)). Convergent boundaries (or active margins) occur where two plates slide towards each other commonly forming either a subduction zone (if one plate moves underneath the other) or a continental collision (if the two plates contain continental crust). Deep marine trenches are typically associated with subduction zones. Due to friction and heating of the subducting slab volcanism is almost always closely linked. Examples of this are the Andes mountain range in South America and the Japanese island arc. [edit] Transform (conservative) boundaries Main article: Transform boundary The left- or right-lateral motion of one plate against another along a long transform faults can cause highly visible surface effects. Because of friction, the plates cannot simply glide past each other. Rather, stress builds up in both plates and when it reaches a level that exceeds the slipping-point of rocks on either side of the transform-faults the accumulated potential energy is released as strain, or motion along the fault. The massive amounts of energy that are released are the cause of earthquakes, a common phenomenon along transform boundaries.

A good example of this type of plate boundary is the San Andreas Fault complex, which is found in the western coast of North America and is one part of a highly complex system of faults in this area. At this location, the Pacific and North American plates move relative to each other such that the Pacific plate is moving northwest with respect to North America. Other examples of transform faults include the Alpine Fault in New Zealand and the North Anatolian Fault in Turkey. Transform faults are also found offsetting the crests of mid-ocean ridges (for example, the Mendocino Fracture Zone offshore northern California).

[edit] Divergent (constructive) boundaries Main article: Divergent boundary At divergent boundaries, two plates move apart from each other and the space that this creates is filled with new crustal material sourced from molten magma that forms below. The origin of new divergent boundaries at triple junctions is sometimes thought to be associated with the phenomenon known as hotspots. Here, exceedingly large convective cells bring very large quantities of hot asthenospheric material near the surface and the kinetic energy is thought to be sufficient to break apart the lithosphere. The hot spot which may have initiated the Mid-Atlantic Ridge system currently underlies Iceland which is widening at a rate of a few centimeters per century.

Divergent boundaries are typified in the oceanic lithosphere by the rifts of the oceanic ridge system, including the Mid-Atlantic Ridge and the East Pacific Rise, and in the continental lithosphere by rift valleys such as the famous East African Great Rift Valley. Divergent boundaries can create massive fault zones in the oceanic ridge system. Spreading is generally not uniform, so where spreading rates of adjacent ridge blocks are different massive transform faults occur. These are the fracture zones, many bearing names, that are a major source of submarine earthquakes. A sea floor map will show a rather strange pattern of blocky structures that are separated by linear features perpendicular to the ridge axis. If one views the sea floor between the fracture zones as conveyor belts carrying the ridge on each side of the rift away from the spreading center the action becomes clear. Crest depths of the old ridges, parallel to the current spreading center, will be older and deeper (due to thermal contraction and subsidence).

It is at mid-ocean ridges that one of the key pieces of evidence forcing acceptance of the sea-floor spreading hypothesis was found. Airborne geomagnetic surveys showed a strange pattern of symmetrical magnetic reversals on opposite sides of ridge centers. The pattern was far too regular to be coincidental as the widths of the opposing bands were too closely matched. Scientists had been studying polar reversals and the link was made. The magnetic banding directly corresponds with the Earth's polar reversals. This was confirmed by measuring the ages of the rocks within each band. The banding furnishes a map in time and space of both spreading rate and polar reversals.

[edit] Convergent (destructive) boundaries Main article: Convergent boundary The nature of a convergent boundary depends on the type of lithosphere in the plates that are colliding. Where a dense oceanic plate collides with a less-dense continental plate, the oceanic plate is typically thrust underneath due to the greater buoyancy of the continental lithosphere, forming a subduction zone. At the surface, the topographic expression is commonly an oceanic trench on the ocean side and a mountain range on the continental side. An example of a continental-oceanic subduction zone is the area along the western coast of South America where the oceanic Nazca Plate is being subducted beneath the continental South American Plate. While the processes directly associated with the production of melts directly above downgoing plates producing surface volcanism is the subject of some debate in the geologic community, the general consensus from ongoing research suggests that the release of volatiles is the primary contributor. As the subducting plate descends, its temperature rises driving off volatiles (most importantly water) encased in the porous oceanic crust. As this water rises into the mantle of the overriding plate, it lowers the melting temperature of surrounding mantle, producing melts (magma) with large amounts of dissolved gases. These melts rise to the surface and are the source of some of the most explosive volcanism on earth due to their high volumes of extremely pressurized gases (consider Mount St. Helens). The melts rise to the surface and cool forming long chains of volcanoes inland from the continental shelf and parallel to it. The continental spine of western South America is dense with this type of volcanic mountain building from the subduction of the Nazca plate. In North America the Cascade mountain range, extending north from California's Sierra Nevada, is also of this type. Such volcanoes are characterized by alternating periods of quiet and episodic eruptions that start with explosive gas expulsion with fine particles of glassy volcanic ash and spongy cinders, followed by a rebuilding phase with hot magma. The entire Pacific Ocean boundary is surrounded by long stretches of volcanoes and is known collectively as The Ring of Fire.

Where two continental plates collide the plates either buckle and compress or one plate delves under or (in some cases) overrides the other. Either action will create extensive mountain ranges. The most dramatic effect seen is where the northern margin of the Indian Plate is being thrust under a portion of the Eurasian plate, lifting it and creating the Himalayas and the Tibetan Plateau beyond. It has also caused parts of the Asian continent to deform westward and eastward on either side of the collision.

When two plates with oceanic crust converge they typically create an island arc as one plate is subducted below the other. The arc is formed from volcanoes which erupt through the overriding plate as the descending plate melts below it. The arc shape occurs because of the spherical surface of the earth (nick the peel of an orange with a knife and note the arc formed by the straight-edge of the knife). A deep undersea trench is located in front of such arcs where the descending slab dips downward. Good examples of this type of plate convergence would be Japan and the Aleutian Islands in Alaska.

Oceanic / Continental Continental / Continental Oceanic / Oceanic

Plates may collide at an oblique angle rather than head-on (e.g. one plate moving north, the other moving south-east), and this may cause strike-slip faulting along the collision zone, in addition to subduction.

Not all plate boundaries are easily defined. Some are broad belts whose movements are unclear to scientists. One example would be the Mediterranean-Alpine boundary, which involves two major plates and several micro plates. The boundaries of the plates do not necessarily coincide with those of the continents. For instance, the North American Plate covers not only North America, but also far eastern Siberia and northern Japan.

[edit] Driving forces of plate motion As noted above, the plates are able to move because of the relative weakness of the asthenosphere. Dissipation of heat from the mantle is acknowledged to be the original source of energy driving plate tectonics.

Two and three-dimensional imaging of the Earth's interior (seismic tomography), shows that there is a laterally heterogeneous density distribution throughout the mantle. Such density variations can be material (from rock chemistry), mineral (from variations in mineral structures), or thermal (through thermal expansion and contraction from heat energy). The manifestation of this lateral density heterogeny is mantle convection due to buoyancy forces.Tanimoto 2000. How mantle convection relates directly and indirectly to the motion of the plates is a matter of ongoing study and discussion in geodynamics. Somehow, this energy must be transferred to the lithosphere in order for tectonic