Complex sentence: The Siletzia basalts are tholeiitic, a characteristic of mantle-derived magma erupted from a spreading ridge between plates of oceanic crust, the younger basalts are calc-alkaline, characteristic of magmas derived from a subduction zone.
Split into simpler sentences: The Siletzia basalts are tholeiitic, a characteristic of mantle-derived magma erupted from a spreading ridge between plates of oceanic crust. The younger basalts are calc-alkaline, characteristic of magmas derived from a subduction zone.