CARLSBAD — Carlsbad’s iconic 400-foot-tall smokestack is almost gone, reduced to rubble by the ongoing demolition of the old Encina power plant. San Diego Gas & Electric Co. built the concrete column in the mid-1970s to replace four smaller smokestacks as part of an upgrade of the 1950s-era plant. “The 400-foot stack has been demolished to a height of 135 feet,” said Chris Rimel, communications manager for owner NRG, in an email Monday. “Demolition activities are temporarily on hold but will resume in mid-June with completion by this July,” Rimel said. The work continues inside the power plant building and is scheduled to be completed during the second quarter of 2022.