The rock mounds are cairns, which are often used as trail markers in many parts of the world. We saw large groups of them all over Iceland, so seems hard to believe they were marking trails and perhaps had a different use there.

I did find some information on this site:

"According to tradition, a farm known as Laufskalar (Icelandic farms have names that stay with them over the years and through changes of ownership) was destroyed in that eruption, which created a lava mound that has since been named Laufskalavarda (“Laufskalar Cairn”). Somehow, it came to be a tradition that travelers passing this spot for the first time would add a stone to the cairn for good luck. Over time, leaving a single stone evolved into building an entire cairn. Some of the hundreds of cairns can be rather large and complex, while others are just a few simple stones. Despite the fact that the Public Roads Administration has reportedly moved a supply of stones to the site “to help continue the tradition,” finding enough loose rock to make a cairn can be challenging." http://epod.usra.edu/blog/2011/10/cairns-of-laufskalavarda.html