Answer:
Caldera
Step-by-step explanation:
A caldera is defined as a prominent geological feature that is a very large type of depression, which forms right after the eruption of a volcano. The caldera has a diameter of about 1 km and the all the pyroclastic materials, volcanic ashes, blocks and lapillies and harmful gases are ejected through this caldera. As the rocks of the apical portion melts and collapses, the caldera gets is circular shape.