How can I use geographic information to find why a civilization ended

By a mix of human and physical geographic evidence.

Explanation:

Civilizations can end due to natural causes. Volcanic eruptions can be devastating for people living close to it. Checking rock age and strata with any historical references could explain. Data from ice core could indicate changes in the atmosphere. A change, such as increased drought, in regions, might end up causing food shortages and the end of civilizations.

Human evidence can provide the answers too. Migration patterns may explain this, such as the Europeans arrived in the Americas. The land has a carrying capacity. Studying the population might indicate the carrying capacity was exceeded, such as Easter Island.