"But that's purely conjecture at this point ... a working hypothesis," Squyres said. "Everything is on the table until we've gotten more data down."
It's been a while since I studied geology, but those rocks definitely look extrusive. They look a lot like aa. I like the high resolution image because you can almost imagine that when it wasn't as weathered that you could see where volcanic gasses were trapped before expanding and escaping at the surface. It should be interesting to see how lava with a high quantity of trapped gases develops on Mars as compared to the Earth. With the lower atmospheric pressure I would expect that gas bubbles in rocks would be much larger and the jaggedness that starts on the surface of a rock would extend much deeper.