Monday 7 June 2010

Partial melting and plate tectonics

Our latest ELI+ is 'Partial melting - simple process, huge global impact' This simple demonstration leads into an explanation of how partial melting has affected the chemistry of the planet, and the characteristics of igneous rocks and volcanic eruptions. Do try this one - it will answer lots of those puzzling questions!
There are many more Earth-related activities on our website - all free to download.


Anonymous said...

This is a nice visual for partial melting.
However, your stage 2 and stage 3 promote a misconception. Many of us have worked very hard to demonstrate that the subducting plate does NOT melt in a subduction zone. It releases water which lowers the melting T of the overlying mantle.
The parent magma in an arc is usually basalt, and you can't get that by partially melting a basaltic downgoing plate.
The isotherms are also way too cold in most arcs for melting to occur in the downgoing plate.
Most textbooks have revised this now, but some still promote the misconception. Please don't continue that trend.
You can even go to the "Subduction" page on Wikipedia and find the correct explanation.

Thanks for your great work with these Learning Ideas. I usually really like them.

-Sue DeBari

Earth Learning Idea said...

Your observations are largely correct, which is why we have added the caveat
**[Note that island arc volcanic magma is produced by a series of complex processes, and partial melting is only part of this story].
Our understanding, from the experts we have consulted is that a small percentage (perhaps 5%) of the andesitic magma in island arc situations is derived from partial melting in the way described, whilst the rest is derived through the processes that you note in your blog message.
However we find that the concept of partial melting is a very difficult one for students to understand and so we have tried to keep the explanations as simple as possible. Should students go on to further study of plate tectonics, we would hope that the complexities could be
dealt with more fully at that stage.
Earthlearningidea Team