how can a asterisk/star like crack occur on a rock?
Radial fracture is fairly common. It can form from cooling, drying mud, chemical shrinkage, impact, or several other means.  Since you are looking at volcanics you are probably looking at cooling shrinkage, The interior cools slower which means it shrinks more and that's how your get radial fractures. Fractures form perpendicular to whatever the cooling front shape is, a round shaped lava flow produces radial fractures. 

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