By the description, I expected a denser litter of basalt, larger pieces and more angular/less organic.

The gunsight-looking formation around the tower... is that a wall or a walkway? Is it elevated over the ground around it? How high if so?

The shape of the shadow doesn't seem right if the sun is to the east-northeast and about 45 degrees (or any other angle). Is the shadow set to linear burn? You may want to try 75% back shadow (or less) and give it a 1-pixel gaussian blur when you're happy with it to make it look more diffused, and I'd try multiply to see if it eliminates that brown tinge. (Keep in mind that I'm colorblind, so I may be seing a brown tinge that doesn't exist!)

I think this is going to be very cool.