Complex sentence: Pyroclastic flows exploded from the crater and reached the mountain's icecap, melting it and forming lahars, volcanic mudflows and debris flows that cascaded down the mountain into several river valleys below.
Split into simpler sentences: Pyroclastic flows exploded from the crater and reached the mountain's icecap, melting it and forming lahars (volcanic mudflows and debris flows). These lahars cascaded down the mountain into several river valleys below.