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.
The radial pattern of the fracture is related to a blast (probably a dynamite) not an impact. The upper part of the rock had blown out by an internal force. So the surface seen on the photographs is not an impacted surface. It is the surface of the remainder part of the original rock. In the impact crater case you might see extensionally origined tangential fractures parallel to the impact crater not the radial cracks as seen on this case. Here is an link showing a similar deformation pattern origined by a blast. http://www.uoguelph.ca/geology/geol2250/glossary/HTML%20files/shattercone.html Who are interested in impact craters may want to visit the link below. http://www.impact-structures.com/ Thanks for your replies and interests.
The features probably caused by a volcanic bomb (please see my comment above). Then one may able to learn from the shape of the deformation, the location of the eruption center. So, how to determine the kinetic energy amount of the impact, from the dimensions of the deformation features ?
Edit: Though i gave the right answer in another post, I didn't delete this post to let anyone to see the other question in the solving process.