The mantle under the crust (about 2,890 km deep) is composed mostly of silicate rocks rich in magnesium and iron. The elements of the crust have

Transcription

1 The mantle under the crust (about 2,890 km deep) is composed mostly of silicate rocks rich in magnesium and iron. The elements of the crust have derived from the mantle by fractional melting that operates to carry upward Si, O, K, Na, Ca, and leave behind mostly Fe, and Mg.

2 Crustal chemical composition by element weight

3 Mantle chemical composition by element weight

4 Composition of Earth s Crust

5 Physical Earth Atmosphere: 1/1,000,000 of Earth s mass Oceans: 2/10,000 of Earth s mass Crust: 1/250 of Earth s mass Mantle: 2/3 of Earth s mass, 5/6 of volume Core: 1/3 of Earth s mass, 1/6 of volume

6 At divergent plate boundaries ultramafic magma is brought from deep in the mantle toward the surface via convection cells. As it approaches the surface, the reduction in pressure causes the rock to fractionally melt (partially melt). The melt portion is mafic (basaltic) in composition.

7 Bridge across the Álfagjá rift valley in southwest Iceland, that is part of the boundary between the Eurasian and North American continental tectonic plates.

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9 At convergent plate boundaries (subduction zones) oceanic lithosphere descends into the mantle. The sea water it carries with it acts as a flux causing the material above the subducting plate to fractionally melt. This produces first intermediate magmas, and then felsic magmas.

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11 Lava erupts from vents as nonexplosive streams of molten rock that pour or ooze or as explosive lava fountains. The speed at which lava moves depends on its chemical composition its viscosity the steepness of the ground over which it travels whether the lava flows as a broad sheet, through a confined channel, or down a lava tube; and the rate of lava production at the vent. Lava fountain and ʻaʻā flow, Kīlauea Volcano, Hawaiʻi

12 Magma contains dissolved gases that vent as volcanic gases: Water vapor (H 2 O) is the most abundant, followed by Carbon dioxide (CO 2 ) that trapped in low-lying areas can be lethal to people and animals, and lesser amounts of Sulfur dioxide (SO 2 ) that being hydroscopic combines with water vapor to produce Volcanic Smog (aerosol droplets of sulfuric acid, H 2 SO 4 ) irritating to eyes, skin and respiratory system, Hydrogen sulfide (H 2 S), a colorless, flammable gas with a strong rotten-egg smell, and Hydrogen halides (HF, HCl, HBr), which are strong, toxic acids.

13 Volcanic eruptions are violent agents of change, that can drastically alter land and water for tens of kilometers around a volcano damage farther away areas (cities and towns, crops, industrial plants, transportation systems, and electrical grids) by tephra and ash falls, lahars, and flooding. Aerosol droplets of sulfuric acid, (H 2 SO 4 ) erupted into the stratosphere can change our planet's climate temporarily.

14 Pyroclastic flow during August 7, 1980 Mount St. Helens eruption. International Space Station view of Sarychev Volcano (Kuril Islands, northeast of Japan) in an early stage of eruption on June 12, Volcanic ash buries cars and buildings, Papua New Guinea.

15 Relative Explosiveness and resulting Height of Eruption "Plinian" volcanic eruptions, named after Pliny the Elder who described the AD 79 eruption of Mount Vesuvius, Italy, send ash and volcanic gas high into the stratosphere Associated are fast-moving deadly pyroclastic flows ("nuées ardentes") down the volcano s flanks. Ash fallout can affect areas hundreds of miles downwind.

16 Basaltic lavas have lower SiO 2 contents and higher temperatures, and so lower viscosity when compared to more evolved lavas such as dacites and rhyolites. Credit: J. Johnson, U.S. Geological Survey Photoglossary of Volcanic Terms.

17 pahoehoe flow ʻaʻā flow A pressure ridge can form when advancing lava becomes slowed by blockage and pushes up on a hardened lava crust above. A rootless spatter cone (hornito) forms when erupting gasses bubble from a single spot, splashing lava into a cone or cylinder.

18 entablature colonnade The columns form due to cooling contraction (tension stress) forming cracks (columnar jointing) that extend perpendicular to the surface of a hardening lava flow. The colonnade is the result of slow cooling from the base upward and the entablature is the result of rapid cooling from the top downward (and could be due to cooling caused by fresh lava being covered by water). Intrusive sills of magma cool to exhibit colonnade columnar jointing.

19 Pillow lavas, are bulbous, elongate, or tubular shaped masses, commonly up to one meter in diameter, result when lava is extruded under water. Commonly of basaltic composition, they form the upper part of 'Layer 2' of normal oceanic crust.