What are the effects of spark plug overheating?

A Prolonged overheating may induce abnormal combustion such as pre-ignition and detonation resulting in melting of the spark plug electrodes and damage of engine.
When there is overheating, the insulator surface of the spark plug becomes pure white and the burning gases deposit spots. Electrode melting is a more advanced type of overheating and when spark plug temperatures reach 800 °C or more, the tip of the plug may glow red hot and become a source of ignition prior to sparking, causing abnormal combustion that can damage the engine.
Overheating

Causes of Overheating and corrective actions