2. Assume that a magma chamber has the following kms long kms high. The followin

Question

2. Assume that a magma chamber has the following kms long kms high. The following kms dimensions wide and elements are also present - Si, Na, K, Fe Mg, O and Al. (25 pts) a. If the rate of crystallization is 1 km/million years, hovw long will the whole magma chamber crystallize? b. Most magma chambers change composition as they crystallize through time. If a magma starts crystallizing at~1200°C, arrange the order of these elements as they settle through the magma chamber from bottom to top. What corresponding minerals and rock types do you expect to crystallize? c. Let's say that the magma was able to vent through the surface through a series of volcanic eruptions. The volcanic vent initially tapped the upper parts of your magma chamber and progressively vented the bottom parts of the magma chamber. What changes can you see in the composition, viscosity and eruption style of your volcanic flows? What would be the composition of the oldest and voungest volcanic flows?

Explanation / Answer

Solution:

a. Given rate of crystallization is 1km3/million

Now, according to the question the volume of the given magma chamber is

length*breadth*height = 10*5*2 =100km3

As 1km3 will take 1 million years to crystallize therefore the whole magma chamber having a volume of 100km3 will take 100 million years to crystallize. (ANS)

b. The sequence of minerals crystalling from magma is given by the bowens reaction series. From that series it is known that the high temperature minerals i.e Olivine and Pyroxenes crystallize first. Olivine has a composition bearing Fe and Mg whereas pyroxenes have the composition bearing Mg and Ca in abundance. Thus Fe will be the first to crystallize followed by Mg.

From the same series it is also known that the Plagioclase felspar cystallize first in comparison to Na felspar. Thus K crystallizes first than Na.

Si and O i.e Sio2 is the last member to crystallize. High temperature rocks are the Mafic Rocks whereas low temperature are the felsic ones. The first rocks to crystallize would mostly be BASALTS/GABBRO(on the basis of composition not temperature) as the contain maximum amount of Mg and Fe. The intermediates would be DIORITE and the lower temperature rocks would be GRANITE.

Thus according to me the crystallizing sequence of the given list of minerals from bottom to top would be

Fe>Mg>K>Na>Al>Si/O.(ANS).

Corresponding minerals:- Olivine>Pyroxenes/K felspar>Na felspar>Felspar>Quartz (ANS)

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