Final answer:
Obsidian rocks from Yellowstone Park were used as weapons by early civilizations.
Step-by-step explanation:
Obsidian rocks from Yellowstone Park were used as weapons by early civilizations. Obsidian is a type of volcanic glass that can be chipped to create extremely sharp tools and weapons. These tools made from obsidian were traded for other goods with other towns and villages, proving the existence of long-distance trade networks.