Network covalent bonds during melting

What happens to the intramolecular bonds when a network covalent substance is melted? I understand that in covalent molecular substances, the intermolecular bonds weaken between the molecules once molten. My confusion is with what happens to the bonds between network covalent substances when melted. 


* Do the intramolecular bonds break, leaving only the individual atoms (carbon for example in graphite or diamond).
* Do the intramolecular bonds weaken
The basis for this query is when I was asked to predict the electrical conductivity of graphite and silicon dioxide in molten states (Graphite sublimates, but under pressure is liquid). I started by researching the atomic and bonding structure of the elements when they are melted but could find very little information.


TLDR: What happens to covalent network bonds when they melt


Thankyou


