Presentation on theme: "Volcanoes. What do you know about volcanoes? What you need to understand about volcanoes Where and why volcanoes occur? Source of lava Composition of."— Presentation transcript:
What do you know about volcanoes?
What you need to understand about volcanoes Where and why volcanoes occur? Source of lava Composition of lava and name of rock associated with each tectonic setting Causes of melting: addition of volatiles; decompression melting –Geothermal gradient –Lithostatic pressure Why and how volcanoes erupt?
Tectonic settingPhysical characteristics Locations on Earth Type of crust Lava sourceComposition of crust Volcanic rock nameVolcanic landforms Convergent with oceanic and continental crust Convergent with two oceanic crusts Divergent Transform
Volcanic rocks formed in subduction zones Water + sediments + oceanic crust + the opposing crust (oceanic or continental) Intermediate composition Andesite
Extension Extension: continental crust thins; magma moves to surface through faults and “weak” crust Tensional stress
Extension: Kilimanjaro Andesite
Long Valley Caldera Basin and Range province Rhyolitic eruption: 760,000 years ago Ash produced by the eruption of rhyolitic lava (melted continental crust
Inyo and Mono Craters: LVC Bimodal volcanism: rhyolite and basalt Basaltic eruptions producing cinder cones
Extension: basalt, andesite and rhyolite may be produced Rhyolite and basalt Andesite
Basaltic lava produced Divergent Plate Boundary Oceanic crust source is the asthenosphere. Asthenosphere
Rift Valley, Iceland Notice the similarity of landforms Shield volcano
Hot spots under oceanic crustoceaniccrust Tectonic Setting Oceanic crust + Asthenosphere
Volcanic rocks formed at mid-oceanic ridges, hot spots under oceanic crust, and areas of extension High % Fe, Mg; Low % Si, O Basalt
PillowPillow Basalts: are formed as basalt is extruded under water. Hawaii
How can the concept of uniformitarianism be applied to the observation of pillow basalts forming? Basalt that contains pillow basalts was originally formed under water. Wales, Great Britain
Hot spot under continental crust Rhyolite is formed Melting of continental crust
Extension Rhyolite Melting of continental crust
Hot spot under continental crust and areas of extension cause melting of continental crustcontinental Rhyolite: highest percentage of Silicon and oxygen
Geothermal Gradient Increase of temperature with depth
Geothermal Gradient Increase temperature with depth Varies depending on tectonic setting Average increase is 25 degrees C/KM Yellow dotted line indicates 500 degrees centigrade
Geothermal Gradient Rocks in the lower crust are near their melting point Any addition heat may induce melting –From superheated or ascending melted rock –What is an example of this process (see above diagram)?
Causes of Melting Addition of volatiles: –Water and carbon dioxide, lowers melting temperature, triggers melting –Where within the plate boundaries we have discussed, does this occur?
Adding water, lowers the rock’s melting temperature.
Basalt is hydrated from processes such as black smokers.black
Addition of volatiles lowers melting temperature
Lithostatic Pressure There is an increase pressure with depth
Decompression Melting As hot material near the melting point ascends, pressure is decreased The lowering of pressure causes the hot material to melt
The decompression of hot water drives a geyser.
Hot water is circulated through the upper crust due to temperature differences.
Hot water remains liquid at depth, due to high pressure.
Geyser Eruption Super heated water is under pressure at depth Remains liquid Rises because it is very hot Gas comes out of solution as pressure is reduced
Geyser eruption Plumbing is correct Allows a balance between liquid and gas to collect
When enough gas is “collected” within the “plumbing” The gas escapes, along with the fluid Geyser Eruption
As super heated water rises, pressure is decreased. Gases come out of solution. When plumbing is correct, gases “drive’ the fluid out of the system.
Gas release drives volcanic Eruptions Eruptions Explosive volcanic eruption
Volcanoes Composition of oceanic and continental crust and how this relates to lava composition Where volcanoes occur and how this relates to lava composition Geothermal gradient and lithostatic pressure Why volcanoes erupt