Diamond and Graphite are both giant covalent structures. If they are both made of carbon why does diamond have a higher melting point?

Although both are giant covalent structures, they have different structures. Diamond makes a 3d structure with each carbon attached to 4 other carbons so the only bond to break when melting it is the very strong covalent bond. Graphite forms giant 2d structures made of these strong covalent bonds but then forms layers joined by intermolecular forces. These intermolecular forces are weaker than the covalent bond and so are easier to break with less energy. This explains why the melting point of graphite is lower.