Here's why:

* Andesitic lava is a common type of lava found in stratovolcanoes, which is what Mount Tambora is.

* Andesite is a type of volcanic rock that has an intermediate silica content, leading to lava that is typically viscous and flows slowly.

* This type of lava is known for its tendency to build up steep-sided volcanoes like Mount Tambora.

While Mount Tambora's eruption was extremely violent and devastating, its lava composition was typical of the volcanic type it is.