Complex sentence: During the shield volcano stage of many hotspot islands tholeiitic OIB's builds up most the volcanoes while after the post-erosional stage is usually accompaigned by violents eruptions of alkali basalt and other more evolved volcanic rocks with high alkali content.
Split into simpler sentences: During the shield volcano stage of many hotspot islands, tholeiitic OIBs build up most of the volcano's structure. Following the post-erosional stage this is usually accompanied by violent eruptions of alkali basalt and other more evolved volcanic rocks with high alkali content.