Week 4- Volcanics and geothermal resources Flashcards
why do people live near volcanoes
- extremely fertile soils
- geothermal energy resources- free green energy
- mineral resources such as cu, zn, fe, au ores
- tourist and economy
what is a volcano
- an erupting vent through which molten rock surfaces
- surface expression of melting processes
- melts are silicate magmas
- often related to plate boundaries
example of a recent major eruption
2022 Hunga tonga- hunga Ha’apai
- major explosive eruption where plume reached heights of 50 km (HALFWAY TO SPACE)
- blast equivalent to 100 million tons of TNT
- pressure wave picked up globally
- intense ferocity from magma- water interaction
what are products of volcanic eruptions
- lava flows- molten rock moving across the ground
- pyroclastic debris- fragments blown out of a volcano
- volcanic gases- expelled vapour and aerosols
what are the two main types og igenous rocks
intusive
extrusive
what are the temperatures needed to melt a rock
- 650- 1100 degrees
what needs to be done in order to melt the mantle
- the geotherm needs to be disturbed
what is a geotherm
- is a gradient, where the rate of increasing temperature changes with respect to increasing depth in earths interior
what changes are needed to disturb the geothem
- add heat
- reduce pressure (most important)
- change the composition
how does decompression (adiabatic) melting occur
- a geological decrease in the pressure
- molecules have more room to move around
- have a high temperature relative to the amount of room that they have
- melting occurs (the solidus is the T and P where a rock will start to melt)
where does adiabatic melting occur
mid-ocean ridges- forms mid-ocean ridge basalt
continental rifts- east Africa will become an ocean over time
what is volatile assisted melting
- change in composition of the system
- new ions increase the complexity of the system which makes the energy needed to break the chemical bonds less
- adding volatiles to earths mantle lowers its melting temperature
- ## c02 release as a result of carbonate water mixing, and then being subducted into the mantle (IN VOLCANIC ARCS)
how are volcanoes linked to plate techtonics
- they disturb the geotherm
- driver of melting can change over time
- majority of magma is erupted at divergent margins
important points to know for surface volcanism
- is the result of mantle melting
- DONT FORGET
submarine volcanism
volcanism on other planets
how is extrusive volcanism influenced
- rheology (viscosity)
how does the rheology of magma operate
- silica polymerisation- network forming, where liquid silica is the most viscous
- high concentration of network former molecules Mgo, CaO, Na2O, K2O means less bonds between oxygen and therefore less energy required to deform the network (basalt)
vice versa with silica and aliminium
how is viscosity (rheology controlled)
- chemical composition
- temperature
- dissolved gas content
- crystal content
flow style also controlled by these factors
what are the qualities of mafic (basaltic lava flows)
- very hot, low silica, low viscosity
- often thin fluid
- flow rapidly up to 30km an hour
- although most flows cant flow more than 10km, these can flow for several hundred km
- this distance is facilitated by lava tubes
examples of mafic (basaltic lava flows)
- ropy (pahoehoe)- when hot basalt forms skin
-blocky (aa) - basalt solidies jaggy
- lava crumbles into shards and fragments
- what you would say if walking barefoot
- forms further from vent
examples of submarine basaltic lava
- pillow basalts- round blobs of basalts cooled underwater, cools forming a pillow
- surface of pillow is cracked quenched glass
- lava pressure rupturesa pillow to form the next blob
- formed a mound thats common at a mid-ocean ridge
qualities of intermediate lava flows
- higher siO2 content and so more viscous
- dont flow rapidly
- fractures into blocky lava
- remains close to the vent
what are the qualities of silicic lava
- high viscosity
- rarely flows- piles up around the vent and intermittent explosive activity
- lava dome formation and spines
main forms of volcanic gases
- 1- 10 percent may be dissolved gases
- water
- CO2
- sulfur dioxide- precipitates at the edge not stable and ignites (can form crystals)