|Out of universe information|
The Volca were a humanoid species native to the planet Kar'tek. When the Goa'uld came to their world the Volca opposed them but were unable to prevent the world's fall. Although their natural ability to phase shift made them immune to possession by the Goa'uld, they became enslaved by Phobos who used the native Mar'kak as hosts. After the arrival of SG-1, the Tau'ri helped the Volca to overthrow their rulers and form a new government.
The Volca population once thrived on the planet Kar'tek. However, they chose to oppose the Goa'uld when they came to their planet and as such Kar'tek was laid to waste, leaving the Volca to be the only living things on the planet other than the native Mar'kak. They were unique in that they had the natural ability to phase shift through solid objects, similar to the Tollan devices; it was thanks to this gift that they were immune to possession by the Goa'uld. The Volca themselves attributed this ability to Ulysses, who had been exiled to the planet by Ares, and though it is unknown if he did in fact give them this ability, he was in possession of a piece of technology which could mimic the ability for a short amount of time. Ulysses himself was scared that, if the Goa'uld acquired this technology, they would be able to reverse engineer the Volca's immunity to being hosts.
When Phobos was exiled to the planet, he managed to form his own small kingdom by using the native Mar'kak as hosts, and was soon able to enslave the Volca as well though the entire planet eventually lost the location of the chaapa'ai. Though some Volca managed to escape, they were forever hunted by Mars' cybernetic Jaffa. Shortly before SG-1 visited the planet, the Volca discovered the long lost Stargate while they were being chased by Jaffa, which soon meant that Ares knew of its location as well. Though Aemon was captured at this time, Etahy was able to escape and met with SG-1 upon their arrival.
It was shortly after this time that the Tau'ri sent a MALP through to Kar'tek and, upon discovering the Volca's natural ability, they decided to go to the planet to see if the Volca would be able to help in their fight against the Goa'uld. All but Etahy and Daniel Jackson were soon captured by Mars' Jaffa, but Daniel and Etahy both went to meet with Ulysses to see about the rescue of Aemon. Ulysses provided Daniel with a piece of phase shifting technology, which allowed the duo to rescue both Aemon as well as the rest of SG-1 from Mars' grasp. It was at this time that Etahy also used his phase shifting ability to put his arm through Mars' Mar'kak host, supposedly killing him; what none of the group knew was that Mars' symbiote had in fact survived. Nevertheless, the Volca led Mars's former Jaffa to Ulysses—on Daniel Jackson's suggestion—in order to start a new government. (SG1: "Fall of Rome")