The terrain in Icelandic Fissures and the rock arch in Further Down the Coast use Voronoi patterns to mimic cracks far more realistically than Vue's built-in crack patterns (eg Sparse Cracks).
To experiment with this you need a version of Vue that gives you access to the Function Editor (eg any version above Esprit... or Pioneer + the appropriate upgrade package).
Take a Simple Fractal and change the Noise Type from default Perlin to Cellular -> Voronoi (not Voronoi Altitude or Generalised).
Edit the noise parameters and set Profile to "Spike" and Neighbour to one of the "x - y" types. "1st - 2nd" produces cracks less busy and complex than "3rd - 4th" with "2nd - 3rd" in-between.
Add an Opposite filter to the output.
Add a Smooth Clip filter to that and edit the Brightness and Contrast parameters (with the Function Node Preview window open so you can see what you're doing) such that just fissures show through the clipping threshold in V shapes. Brightness and Contrast should be used to change the shape of the cracks from sudden and sharp to ones with soft, complex round edges and, with low contrast and brightness, the bulbous lumpy overall shape normally associated with Voronoi patterns.
If your feeling crazy, add a Filter filter and apply an interesting curve to the crack.
As cracks generally subtract from a level surface, set the Upper Clip of the Smooth Clip filter to 0 instead of 1, then you can Combine/Add Voronoi cracks into any function without the overall height of the output changing.
In the Simple Fractal node, experiment with Metascale, Roughness and the Largest Feature parameters (usually above 10) to get the frequency and roughness of cracks desired (bit of hit and miss there). Ticking the Rotation box generally produces spikier looking cracks.
Experiment and enjoy!
EDIT: improved neighbour type description and a few other things.
Listening to: IamX
Reading: Bill Bryson
Watching: This screen
Playing: Assassins Creed