Monday, 9 July 2012

Partial melting extension idea

We have just published an extension idea for our ELI 'Partial melting - simple process, huge global impact'. This activity explains how partial melting, coupled with plate tectonics, has changed the chemistry of the planet.
The reason why partial melting occurs in different geological environments is sometimes difficult for students to comprehend. This extension idea is a visual method used to demonstrate this concept in the classroom.


Dave, UK said...

The partial melting wax + gravel mix is an adequate analogue for silicate rock, though not brilliant because even the refractory minerals contribute something to the melt under equilibrium conditions. However, for a demonstration of concept at this level I think it's good.

However, the descriptions of what happens in the Earth are a little misleading:

Stage 2: the oceanic crust subducted beneath
oceanic plates partially melts;

Stage 3: the oceanic crust subducted beneath
continental plates partially melts, but the base of the
continental plate also partially melts.

True, but in both cases I think the major contributor to the melt is from hydration melting of the mantle wedge above the subduction zone (between the downgoing ocean crust and the base of the over-riding crust).

In each case that would tend to give primary magma of mafic composition, and the fact that erupted magma is richer in silica than that is because of mixing with the more silica-rich melts that you describe, plus assimilation of crustal rock, plus fractional crystallization during ascent.

ELI Team said...

Many thanks indeed for this clarification, Dave.

The processes involved are pretty complex and difficult to understand by our 'target audience' of 14-16 year old pupils and their teachers, so we're glad to hear that you think the wax/gravel mix is useful at this level.

However, it is also very valuable to have your additional comments for more interested readers. It can be difficult to present complex ideas in simple ways, but this approach of using a simple model with the more accurate and detailed back up material you have provided will be very helpful to all our readers.

ELI Team.