However, the volcano is a Stratovolcano/Composite, which would mean a different type of lava typically found on destructive plate boundaries.
Can someone explain how/why this is the case?
Some good answers here from a geologist.
Where the ground is ice-free (around the ice cap and where the ice has melted), the basalt erupts as relatively fluid lava, often as fire fountains through fissures and craters. However, where the basalt erupts beneath the ice cap, the lava interacts with the ice to produce 'phreatomagmatic' eruptions, which are highly explosive and produce large volumes of steam and ash.
These two eruptive styles produce the layers of lava and ash which build up to form a composite volcano.