Final answer:
A giant volcanic depression, much larger than a crater, with steep sidewalls and flat floors is called a caldera. Calderas are formed when a large volcanic eruption empties the magma chamber beneath a volcano.
The rebound of the crust partially fills in the cavity and creates a flat floor.
Step-by-step explanation:
A giant volcanic depression, much larger than a crater, with steep sidewalls and flat floors is called a caldera.
Calderas are formed when a large volcanic eruption empties the magma chamber beneath a volcano, causing the ground above to collapse and form a depression. The collapse usually creates a steep-walled crater, and the flat floor is the result of the rebound of the crust partially filling in the cavity.
One example of a caldera is the Sif Mons volcano on Venus, which has a volcanic crater about 40 kilometers across and individual lava flows up to 500 kilometers long.