Complex sentence: Glicken returned to St. Helens to analyze the remains of the volcano's lateral blast, but other CVO employees had already commenced their research at the mountain, so Glicken's offer to help USGS on his own was declined by senior scientists.
Split into simpler sentences: Glicken returned to St. Helens to analyze the remains of the volcano's lateral blast. However, as other CVO employees had already commenced their research at the mountain, Glicken's offer to help USGS on his own was declined by senior scientists.