A population-level model of bacterial chemotaxis is derived from a simple bacterial-level model of behavior. This model, to be contrasted with the Keller–Segel equations, exhibits behavior we refer to as the “volcano effect”: steady-state bacterial aggregation forming a ring of higher density some distance away from an optimal environment. The model is derived, as in Erban and Othmer (2004)  R. Erban and H.G. Othmer, From individual to collective behavior in bacterial chemotaxis. SIAM J. Appl. Math, 65 (2004), pp. 361–391. Full Text via CrossRef, from a transport equation in a state space including the internal biochemical variables of the bacteria and then simplified with a truncation at low moments with respect to these variables. We compare the solutions of the model to stochastic simulations of many bacteria, as well as the classic Keller–Segel model. This model captures behavior that the Keller–Segel model is unable to resolve, and sheds light on two different mechanisms that can cause a volcano effect.