Geomorphological feature identification
The place is called Urðarháls. I could not find much information about it in the scientific literature... It is mentioned in Sigmarsson &amp; Halldórsson (2015): they say it's an interglacial basaltic shield volcano, possibly associated with the Askja volcanic system. The fact that the volcano was eroded by glaciers probably explains the colour difference with the surrounding "fresh", postglacial lavas.

The crater looks like a pit crater to me: it probably collapsed when lava was drained somewhere else, leaving an empty chamber beneath it. Another good example of pit crater is the Dolomieu, on Piton de la Fournaise, which collapsed in 2007. Look at Staudacher et al. (2009): compare figure 3, before the collapse, and figure 11, after.

Note that the authors use the term "caldera", which is loosely defined... What Wikipedia calls a non-explosive caldera would be referred to as a pit crater by some. I guess the two terms kind of overlap.