Answer:
A Mantle plumes
Step-by-step explanation:
This is caused by the highly fluid (low viscosity) lava erupted, which travels farther than lava erupted from a stratification, and results in the steady accumulation of broad sheets of lava, building up the shield volcano's distinctive form.
(If that makes sense -_- let me know if you get it wrong or right! I had this question before so just wondering)