So lets take a look at this one and see if there are volcanoes there.
Since I see the SW rosette as the source of the Nile, and that it should be the southernmost point, then this one would represent things NE of there, which would include the Red Sea, Sea of Aden, Persian Gulf, Arabian Sea, Indian Ocean, and the Gulfs of Gujarat, Kutch and Khambhat.
That is what i think the blue pieces mean. As to the yellow arrow, it points to the northeast when the Nile source rosette is SE, like in the portolan setup. These are mostly places that dont normally show up in a portolan, it usually cuts off in line with the eastern egde of the Black Sea, generally only including the Red Sea at bottom right corner, kind of shoved in, or with just the tip showing, it is not usually shaped correctly.
Here it is not included. Second quarter of 14th century.
Here it is drawn in. Note that it does not even include the Gulfs of Suez and Aqaba, although it hints at Aden. This map is from 1541 but others before it show similar design.
This one from Barcelona 1439 draws it a bit better, and includes Aden and the Persian Gulf as well, although they do not connect, and are not correctly oriented except for the Red Sea.
The yellow arrow points east if the SW rosette points S. On a TO mappa mundi it also represents the east, or top in most cases. Except that of course the red sea should actually face north south, not east west. It is completely removed from the Mediterranean here. I think that might be what the squiggly line is about, correct orientation of the Red Sea in relation to east. Imagine the rosette we are talking about turned so the spray is to the west instead of north. You can sed how thevsquiggly line matches up with both the real life angle of the Red Sea and Persian Gulf in the north up orientation, and in this representation of Hecateaus' map.
In a map of Hecateaus' ecumene, it is the SE quadrant that is represented, and works with the SW rossete being S. However, it is when the SW rosette is SE (Portolan map view) that the S rosette (the one we are looking at) becomes the E rosette, and the positioning of the line is correct looking in that way to indicate the orientation of the red sea, if not the rosette itself, which would be too high up, but that is only because the Nile rosette represents not only the mouth but the source.
When the rosette is looked at as though it is the E rosette, the arrow points NE, which is correct from the point of view of its positioning compared to the most southern source of the Nile, it would cover the Eastern rift, up to the join between the red sea and Aden, then the Persian Gulf, and Gujarat. If it is looked at as the S rosette, as it is here, the arrow correctly points to the SE, to represent the direction of the general area if the Nile rosette is thought of as the mouth. So it morphs and turns like all the other rosettes, in relation to the type of map and what is generally depicted. One day i will make a video to show how these morphs and turns correlate, it is hard to explain in writing or without showing pictures of the rosettes in various orientations, a lot of turning and zooming involved.
The Red Sea has active volcanoes, the Arabian peninsula has volcanos, Persian gulf has mud volcanoes, Gujarat in India has extinct volcanoes. Again the ropes indicate connections, ie the rifts, and indications that there are underground connections between volcanic outlets.