Revision: magma, lava, volcano shape, volcano style! and how it comes together

So there are many different types of lava type due to the plate boundary. 
The thicker the area of crust that a pluton has to rise through the more enriched in silica the pluton becomes (this is all you need to know really) however, this is how it works-- so to the left is what is known as Bowen's reaction series. the basic principle is that the minerals found at the top crystalise out at higher temperatures- olivive at 1300 degrees; whereas quartz crystalises out at a much lower temperature of 550 degrees...therefore as the the pluton rises further and temperature gets relatvely cooler mafic minerals are the first to crystalise out and thus leaving a higher percentage of silica. (once cooled these are the ligher coloured rocks)

Therefore, the more silicic the magma the more explosive it is as it is sticky and viscous, this high viscocity doesnt allow easy flow therefore the pressure builds up creating some impressive eruptions(plinian,pelean,strombolian and volcanian), whereas basic magma comes directly from the aesthenosphere thus having very little distance to travel, this means very little differentiation can occur and the composition of magma is low in silica this makes for a very tame eruption (unless under an ice cap) the rocks produced are very dark in colour and the eruption less explosive (icelandic and hawaiin)

As the high ratio of silica makes the lava so very viscous the lava cannot move quickly or very far this means that very steep sided volcanoes are formed by silicic magma such as composite volcanoes, strato volcanoes and calderras (as the volcano blowing itself out happens in large explosive eruptions) whereas as shallow sided/flat volcano shapes such as shield volcanoes and fissures are caused by basaltic eruptions.


Other things to note:
-Basic volcanoes are very easy to manage and therefore areas with these attract alot of tourism.
-people live in areas with volcanoes as the soil is very fertile
-the prediction of a volcano is much easier than an earthquake
-you can try and make homes withstand the effecs of a volcano but very little with stand up to a lava flow or pyroclastics!!!

volcanoes

We are now back to volcanoes (: and although todays lessons was not thrilling it was definately a vast improvement upon that of Belfast. We have also been set an essay, nothing like ensuring we don't relax-- it hadnt even been 24 hours after the exam! oh well such as life.

Today was a lesson running through what we were doing before christmas:

There are 2 key types of magma Basaltic and Rhyolitic this is judged upon how much silica (SiO2) is found within the magma, see the table below.

Summary Table
Magma Type	Solidified Rock	Chemical Composition	Temperature	Viscosity	Gas Content
Basaltic	Basalt	45-55 SiO2 %, high in Fe, Mg, Ca, low in K, Na	1000 - 1200 oC	Low	Low

Rhyolitic

Rhyolite

65-75 SiO2 %, low in Fe, Mg, Ca, high in K, Na.

650 - 800 oC

High

High

 

• Hawaiian - These are eruptions of low viscosity basaltic magma. Hawaiian Eruptions are considered non-explosive eruptions.  Very little pyroclastic material is produced.

Strombolian - These eruptions are characterized by distinct blasts of basaltic to andesitic magma from the vent. Strombolian eruptions are considered mildly explosive. Vulcanian - These eruptions are characterized by sustained explosions of solidified or highly viscous andesite or rhyolite magma from a the vent.  Eruption columns can reach several km above the vent, and often collapse to produce pyroclastic flows.  Widespread tephra falls are common.  Vulcanian eruptions are considered very explosive. Pelean - These eruptions result from the collapse of an andesitic or rhyolitic  lava dome. Pelean eruptions are considered violently explosive. Plinian - These eruptions result from a sustained ejection of andesitic to rhyolitic magma into eruption columns that may extend up to 45 km above the vent.  Eruption columns produce wide-spread fall deposits with thickness decreasing away from the vent, and may exhibit eruption column collapse to produce pyroclastic flows.  Plinian ash clouds can circle the Earth in a matter of days. Plinian eruptions are considered violently explosive.

i feel there is a good chance i may add to it soon!