First, location of Oxia Palus on Mars.

The MOC map segment is just to the east of of Lunae Palus; to the south is Margaritifer Sinus and the chaos and outflow regions , which in turn connect with the Valles Mariners to the west.

Atlas web page: http://www.msss.com/mars_images/moc/moc_atlas/

This region has long been known telescopically, and under certain conditions the spots -- ie, the craters with black fill and dark streaks -- are visible in a good telescope. Take a look this summer. See if you can see them.

Indeed, the recognition of recent volcanism in Elysium and elsewhere on Mars is an important realization. However, the Elysium complex is thousands of kilometers east (or west, depending on which way you travel) of the Oxia region, and it is unlikely there is a significant connection between the two. However, there are numerous black floored troughs in Cerberus, to the south of Elysium. The origin of this black material is unknown, but it seems to form a layer (or layers) below the surface. The material is friable and can be blown by the wind -- or so it seems. Maybe it's coal! (That's a jest.)

In Oxia, however, the black material does not seem to be easily moved by the wind. In some areas, the black material is firmly cemented, quite solid, and though heavily pitted and cratered it does not show many signs of significant erosion, unlike the white material often found on top of it -- which is always heavily sculpted.

Nevertheless, dark dunes are found in some of the craters. Some kind of erosion must be occurring! In fact, most of the streaks are relatively free of dark material, though there is some. This suggests that whatever cover of dark material there once was has eroded away. The question, however, is how the streaks became deeply etched into the surface. My own thought is that perhaps the dark material was both hot and caustic when it was emplaced, and that it literally ate away the underlying surface.

As for color, well... I defer to Robert Shepherd (whose color calibrations of Viking images are astonishing). "Black" is a relative term, of course. The spacecraft cameras produce images that are usually processed in black and white to "bring out contrast." Thus, light colored areas are frequently seen as a blinding white, and dark areas are seen as sooty black. Neither is likely to be correct. Contrast is often stretched so much that the appearance of the surface is grossly distorted. (Especially in Viking images.)

As for color images... you've seen all the posts from Bob Clark and Barry DiGregorio and others discussing all the problems of color imaging of Mars. From what I've seen, good faith efforts at color rendering of the Oxia region shows a generally mottled ruddy tan surface -- actually rather dark -- on which the "black" material appears to be almost charcoal-gray with a somewhat bluish cast. The "white" material appears to be a fairly neutral beige. But I wouldn't rely too much on anyone's color renderings at this point.

Syrtis Major is also a dark bluish-gray formation, and it is certainly a volcanic shield.

I suggest the dark material in Oxia craters and elsewhere might be an iron-sulfur compound from deep in the planet's interior --it actually could be part of the outer core itself. That's a fairly radical proposition, and I don't expect anyone to accept it, at least not yet! ;-).

But Marcia proposes (below) that the dark material could be volcanic obsidian. I wish there were more signs of volcanism in the area -- unfortunately there are none. That doesn't mean some kind of volcanism couldn't have occurred. Obsidian is possible. Much of the surface mottling appears to be stained as if by a fluid, the source of which may be internal. In Meridiani to the east of this area, the whole surface is heavily carved and etched, deeply channeled. Others have proposed that this appearance is due to erosion by hot water.

As we know, almost the entire surface of Mars is reddish because of the iron content of the surface material. There are some remarkable aspects of this surface iron, however. During the Viking missions, magnets on the collecting arms of the landers retrieved unrusted iron filings from the surface. On Earth, you can use a magnet to pick up tiny iron filings from almost any soil sample, so finding iron filings on the surface of Mars was no surprise.

What was a surprise was the size of these filings: many were more than 2cm long, some 4cm. Imagine this: Iron filings as long as your fingers on Mars. Where did they come from, and how are they preserved on the surface?

More study necessary!

Cheers, BFelix