Here's why:

* Shape: Shield volcanoes are known for their broad, gently sloping, shield-like shape. Skjaldbreiður perfectly fits this description.

* Eruptions: Shield volcanoes are formed by highly fluid, basaltic lava flows that spread out over large areas. Skjaldbreiður's eruptions are known to be effusive, producing large volumes of lava.

* Composition: The lava from shield volcanoes is typically basaltic, which is low in silica and thus very fluid. This is consistent with Skjaldbreiður's volcanic history.

Therefore, based on its shape, eruption style, and lava composition, Skjaldbreiður is categorized as a shield volcano.