Here's why:

* Shield volcanoes are formed by eruptions of highly fluid, basaltic lava. This lava flows easily and spreads out over large areas, creating a wide, gently sloping cone.

* Hawaii's volcanoes are all basaltic, meaning they are made of magma that is low in silica and very hot. This results in the fluid lava characteristic of shield volcanoes.

* The dome shape comes from the gradual accumulation of lava flows over time. As the volcano grows, its slopes become less steep.

So, the combination of fluid lava and the gradual accumulation of layers leads to the distinctive shield volcano shape seen in Hawaii.