An oil refinery in Big Spring, Texas, exploded Monday for as-yet-unknown reasons, injuring four workers and sending large plumes of smoke into the air. The explosion closed schools, shut down the nearby freeway, and shook buildings up to a few miles away. Fires at the facility were apparently extinguished as of Monday night, but the refinery’s 67,000 barrel-a-day output has been slowed. The refinery produces asphalt, jet fuel, ultra-low-sulfur diesel, and unleaded gasoline.