Partial Melting

 

Objective: To demonstrate the process believed to be responsible for generating most magmas.

Materials needed:

Demonstration:

Let the grape juice thaw out very slightly (not too much or it will liquefy!) Squeeze the juice concentrate out of the tube, through your hands, squishing it into the plastic tub. Be sure to wear gloves and an apron if you don’t want to end up looking like Barney.

Use the water and the pitcher for clean up. Make grape juice and offer some to the students.

The Principles we are seeing here:

Partial melting takes place in the earth in the mantle, where when minerals with lower melting points, like feldspars and pyroxenes, melt and leave behind olivine crystals, forming basaltic magma. Partial melting also occurs during subduction; when oceanic crust is subducted, it also experiences partial melting. Sometimes the lower continental crust can melt also, when magma coming up from a subducted slab doesn’t get all the way to the surface immediately.

In the grape juice, the sugary juice has the lowest melting temperature (about -40 degrees C or F), whereas the ice crystals in the juice melt at 0 C or 32 F. So if your juice concentrate is at about –5 degrees C, part of the mixture is liquid but the ice crystals are still solid.