Chris Currie, 20, was killed instantly Aug. 19 last year when the eight-kilogram (18-pound) slab smashed through the windshield of the car he was driving in the northern New Zealand city of Aukland.
The High Court jury in Aukland took four hours to reach its verdict on the teenager, identified by Justice Helen Winkelmann as Ngatai Rewiti, at the end of a four-day trial.
The jury of 10 men and two women was told the concrete block smashed into Currie's face and chest, breaking his jaw and breastbone and forcing his broken sternum into his heart, the AP reports.
Three young women passengers suffered minor injuries after the car careered off the highway and crashed.
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