  We introduce the concept of a geometric horizon, which is a surface
distinguished by the vanishing of certain curvature invariants which
characterize its special algebraic character. We motivate its use for the
detection of the event horizon of a stationary black hole by providing a set of
appropriate scalar polynomial curvature invariants that vanish on this surface.
We extend this result by proving that a non-expanding horizon, which
generalizes a Killing horizon, coincides with the geometric horizon. Finally,
we consider the imploding spherically symmetric metrics and show that the
geometric horizon identifies a unique quasi-local surface corresponding to the
unique spherically symmetric marginally trapped tube, implying that the
spherically symmetric dynamical black holes admit a geometric horizon. Based on
these results, we propose a suite of conjectures concerning the application of
geometric horizons to more general dynamical black hole scenarios.
