West Marin was on alert last Saturday for a tsunami caused by a powerful volcanic eruption in Tonga. The Marin County Sheriff’s Office told residents to avoid coastal waters, and the Point Reyes National Seashore closed its beaches for several hours. Observers saw fast-moving tidal surges in the Bolinas Lagoon, and on Point Reyes, tide levels fluctuated as many as four feet in a matter of 20 minutes that afternoon, but the waves had little impact.