The Weston Observatory will host a talk, “Trinitite — The Atomic Rock,’’ on Sept. 18 to launch this fall’s colloquium series organized by Boston College’s department of earth and environmental sciences . The 7 p.m. event at 381 Concord Road will feature University of Massachusetts Lowell geologist and geochemist Nelson Eby. The presentation is free, though reservations are required, due to limited seating. Call 617-552-8300 to register; visit www.bc.edu/westonobservatory for more information. Full story for BostonGlobe.com subscribers.