The process of making Mars habitable for humans will start with microbes, and later a world-spanning carpet of lichen and moss. First you’ll need to increase the carbon dioxide in the atmosphere to warm up the planet a bit, but then plants should spread rapidly, happily gobbling all that carbon dioxide and turning into carbon, for the plant, and oxygen, for us. Redwoods and apple trees can come later; for sheer quantity and toughness on volcanic rock, mosses will do the heavy lifting.
I’d been reading about this, in Kim Stanley Robinson’s scientifically careful novel Red Mars, when we landed in Iceland, and drove out onto the volcanic plains surrounding Keflavik airport. Add a red filter, and I’d have been in the landscape of the novel: rock, moss, and lichen to the horizon, plus cold, hard wind.
(Was the Viking lander named for the people who first landed in Iceland, with this parallel in mind? The internets are silent on the question.)
Things move faster on Earth of course. This lava field is from an eruption in the 1300s. Some of it came down very smooth, creating spookily geometric volcanic plains such as the beachside expanse at Reykjarnes where a statue of an extinct great auk contemplates its former habitat …
(It's an impressively remote site for public art, at the end of this long black-gravel road.)
But sometimes new forces blasted apart older layers of lava, flipping them vertical to for a ruins of war effect.
Eventually moss and lichen fully covers the lava, and a while after that, if you throw in some topsoil, you can grow grass on the flat bits, even as the volcano still steams in the distance via a geothermal plant, and hillsides of moss remind you of an earlier stage of history. Finally, you get a terraformed look that might suggest a place for human life.
No trees, though. Accustomed to masses of conifers (as the Norse settlers were) I noticed when rain on the windows of a restaurant turned a barren ridgeline into a mirage of boreal forest. Mirages always show you what you're most desperate for.
How would one live here? How would one build anything here? Stay tuned ...