Rocks: Mars and Earth compared...

First, from Mars Curiosity's camera. Note the gas bubbles (holes in the rock).
Where have we seen these before? 

Here's a volcano in Iceland, or more specifically, its debris field, and this looks quite similar to scenes from Mars, don't you think?

Here's a sample of pumice, full of gas bubble voids, from somewhere in the U.K.

Or, I could just show you a picture of my Pet Rock, from the West Coast of our Vancouver Island. It's also full of voids from former gas bubbles, and even though it looks quite light, it isn't. 

My point? All these bubble or void-filled rocks were undergoing similar processes at the time of their formation and cooling. The rock involved was undergoing a process of explosive decompression, allowing its compressed gases to expand and enlarge its volume prior to cooling. It could have been volcanic action, or it might have been because the whole planet was exploding from within at the time. The difference is one of scale and magnitude, not process.

And here's the Pet Rock after a bath in hot water and bleach, and then a scrub with laundry detergent, and a rinse-off. Not much change, except it lost some sand from within the voids. If you look in that depression near the upper right, you'll notice a white worm-like object in the bottom of that pocket. Too bad this rock didn't come from Mars - I could probably prove it once had life.