Social norms are the foundation for social cooperation and productive economic activity, yet we know very little about how and when they emerge. While many different models have been developed to understand the emergence of norms, including linguistic conventions, cultural practices and ideological consensus, studying these dynamics empirically has remained intractable. Using an experimental online environment, we study how the interaction of individual mechanisms with social networks drives the emergence of consensus even in the absence of forces traditionally argued to be necessary, such as centralized authority, common knowledge, and focal points.