Simplify the following text for a Grade 5 native English-speaking person.
Original: On March 20 , 1980 , Mount St. Helens experienced a magnitude 4.2 earthquake ; and , on March 27 , steam venting started .
Simplified: On March 20 , 1980 , Mount St. Helens was the center of a magnitude 4.2 earthquake . Steam venting from the volcano started on March 27 .